Protein Needs of Vegans

by Jack Norris, RD

Contents

• Essential Information
  • Soy
• Additional Tips
• Research on Plant Protein
  • Assessing Protein Needs of a Population
  • Dietary Reference Intakes
    • Setting the DRIs for Protein
    • Women Have the Same RDA as Men
    • Criticism of the Protein RDA
      • Protein Needs for People Over 60
      • Protein Needs of Athletes
  • Protein and Amino Acid Content of Selected Plant Foods
    • Good Sources of Lysine among Plant Foods
  • Plant Protein Balance Research
    • Nitrogen Balance Study on Vegans
    • Nitrogen Balance Studies using Plant Protein
    • Serum Albumin Studies of People on Plant-Based Diets
    • Creatinine Clearance Study on Vegans
    • Summary of Plant Protein Balance Research
  • Amino Acid Intakes and Blood Levels of Vegans
    • Amino Acid Intakes of Vegan Men
    • Amino Acid Intakes of Vegan Women
    • Amino Acid Blood Levels of Vegan Men
    • Arginine and the Vegan Diet
  • Appendix A: Protein Quality Scoring
    • FAO Recommends the PDCAAS in 1989
    • FAO Recommends the DIAAS in 2011
    • Criticism of the DIAAS
  • Appendix B: The DIAAS Reference Protein
  • Appendix C: Muscle Synthesis Studies
  • Muscle Synthesis after a Meal Containing Beef or Plant Protein
  • Bibliography

  Essential Information

  “Where do you get your protein?” is typically the first question vegans are asked. And it’s a bit hard to answer because almost all plant foods contain protein. In other words, vegans get our protein in just about everything we eat!

  Eating a serving of high-protein plant foods at most meals will provide enough protein for most vegans (the exception being some strength athletes who should experiment with varying amounts to find the best results). Anyone who’s regularly been eating meat and cheese is used to high amounts of protein and if you’re craving animal products or feeling fatigued on a vegan diet, adding more protein is often the solution.

  High-protein plant foods include seitan and most vegan meats (about 15-25 grams per serving, but check the label), tempeh (about 15 grams per 1/2 cup), tofu (about 10-15 grams per 1/2 cup), peanuts (about 15 grams per 1/2 cup), beans and lentils (about 8 grams per 1/2 cup), and soymilk (about 7 grams per cup).

  

  Soy

  Speaking of protein, soyfoods have traditionally been a staple of many vegan diets due to their high protein content. Myths abound that soy is harmful and that has made some people shy away, but there’s plenty of scientific evidence that two servings of soyfoods per day is perfectly safe. Higher amounts are probably also safe but they haven’t been studied as thoroughly. As a vegan dietitian, athlete, and someone familiar with the scientific research on soy, I happily eat as much soy as I desire!

  The most robust area of research on soy has been with respect to breast cancer and the overwhelming evidence is that soy can reduce the risk of breast cancer. There’s also evidence to suggest that soy can reduce the risk of prostate cancer and heart disease (by lowering LDL cholesterol). You can read more in our article Soy: Main Controversies.

   

  

  Tofu is an extremely versatile soyfood that has been eaten in some Asian cultures for hundreds of years. You can fry or bake it and add it to just about any savory dish. You can also freeze and then thaw it to give it a chewy texture. Tofu doesn’t have much taste on its own, but it takes on the flavors of the foods it’s mixed with.

  Tofu is normally made with calcium salts and is therefore a rich source of calcium for vegans (check the packaging for “calcium” in the ingredients).

  Another type of tofu, silken tofu, has a smooth texture and is used for making pudding, mousse, and cream-based pies. Silken tofu is often found in the baking or Asian sections of the grocery store.

  While most vegans eat soyfoods, you don’t need to in order to be a vegan as there are plenty of other high-protein foods. But unless you have a specific allergy to soy, there’s no reason why you can’t enjoy it just like millions of other people throughout the world, vegans and meat-eaters alike.

  Additional Tips

  To ensure adequate protein status, vegans should eat 3-4 servings per day of high-protein foods that also are good sources of the amino acid lysine. Below is a list of protein foods from which to choose (weights listed are for one serving of ready-to-eat food):

  • Legumes – 1/2 cup cooked
    • Beans – garbanzo (chickpea), kidney, pinto, navy (125-150 grams)
    • Lentils (100 grams)
    • Peas – split (100 grams) or green (80 grams)
    • Soyfoods – edamame (80 grams), tofu (125 grams), tempeh (165 grams), soy milk (1 cup or 250 mL), soy meats (3 oz or 85 grams)
    • Peanuts – 1/4 cup (35-40 grams)
  • Seitan – 3 oz (85 grams)
  • Quinoa – 1 cup cooked (185 grams)
  • Pistachios – 1/4 cup (30 grams)
  • Pumpkin seeds – 1/4 cup roasted (35 grams)

  It’s hard to design a vegan diet that meets lysine requirements for someone who doesn’t exercise daily without including legumes, seitan, quinoa, pistachios, or pumpkin seeds. People who exercise have higher caloric needs, making it easier to meet lysine needs through other foods.

  Athletes will require somewhat more servings of protein than listed above, but this will be based on their individual sport and training. See Sports Nutrition for more information.

  There’s evidence that as people age, they need a higher percentage of their calories to be protein; people over 60 should focus on making the above high-protein foods a large part of their meals.

  Vegans who don’t eat enough calories to maintain their weight should make an effort to include a higher percentage of high protein foods.

  Research on Plant Protein

  Proteins are made out of chains of amino acids. Some amino acids can be made by the body—generally from other amino acids—but some cannot. The ones that cannot are known as essential or indispensable.

  Because some amino acids are essential, the amino acid requirements are as important as protein needs. But because the essential amino acids are found in fairly consistent amounts in the average diet of Americans, the U.S. Recommended Dietary Allowance (RDA) for protein also covers amino acid needs.

  A common belief is that most plant foods are completely devoid of at least one essential amino acid, but the truth is that all plant proteins have some of every essential amino acid. As a general rule, legumes are lower in the essential amino acid methionine while most other plant foods are lower in lysine. In general, though, only lysine is likely to be a concern for most vegans because almost all vegans naturally eat plenty of foods high in methionine.

  In the early 1970s, Frances Moore Lappe wrote a book, Diet for a Small Planet, that popularized the idea of combining plant proteins at each meal in order to get a balance of essential amino acids in order to form a complete protein. In particular, mixing legumes and grains ensures that a vegan is obtaining both lysine and methionine at each meal. But we now know that vegans don’t need to worry about combining proteins at individual meals in order to create a complete protein because our livers store essential amino acids to be used as needed. The 2009 American Dietetic Association’s Position Paper on Vegetarian Diets stated:

  Plant protein can meet requirements when a variety of plant foods is consumed and energy needs are met. Research indicates that an assortment of plant foods eaten over the course of a day can provide all essential amino acids and ensure adequate nitrogen retention and use in healthy adults, thus complementary proteins do not need to be consumed at the same meal.

  In fact, complementary protein combining can be counterproductive for vegans. Dr. Anna Borek explains how the foods highest in protein contain more of all of the essential amino acids than would a combination of foods typically used to complement proteins (Is there value in combining “complementary” plant proteins?, 2024).

  Assessing Protein Needs of a Population

  Discussions about protein needs tend to focus on three different lines of reasoning:

  1. Protein quality scoring
  2. Muscle synthesis studies
  3. Protein balance studies

  Protein quality scoring is mostly relevant to populations at risk for malnutrition while muscle synthesis studies are mostly relevant to athletes. Protein balance studies are the most relevant studies for assessing the protein needs of a well-nourished population.

  We include an in-depth discussion of protein quality scoring in Appendix A: Protein Quality Scoring, including explanations of the protein efficiency ratio (PER), the protein digestibility–corrected amino acid score (PDCAAS), and the digestible indispensable amino acid score (DIAAS).

  We cover muscle synthesis and plant-based diets in our article, Weightlifting for Vegans and one study that wasn’t conducted in a sports nutrition context in Appendix C: Muscle Synthesis Studies.

  Here, we’ll focus on protein balance studies in discussing the protein needs of the general vegan population.

  Dietary Reference Intakes

  The United States Institute of Medicine (IOM) sets the dietary reference intake (DRI). The DRI for protein is given in grams per kilogram per day (g/kg/day) of “body weight” (IOM, 2005). Fat mass doesn’t require much protein for maintenance, so “body weight” is generally interpreted to mean “ideal” or “healthy” body weight even though this isn’t specified in the DRI.

  The table below lists the DRI for protein. The table also includes the DRI for the amino acid, lysine, because lysine tends to be the essential amino acid least plentiful in the average vegan diet; if a vegan is meeting their needs for lysine, they’re most likely meeting their needs for all the essential amino acids.

  U.S. Dietary Reference Intakes for Protein and Lysine
  Age	Protein (g/kg)A	Lysine (mg/kg)A
  7–12 mos	1.20	89
  1–3	1.05	58
  4–8	0.95	46
  9–13 male	0.95	46
  9–13 female	0.95	43
  14-18 male	0.85	43
  14-18 female	0.85	40
  ≥ 19	0.80	38
  Pregnancy	1.1	51
  Breastfeeding	1.3	52
  Aper kg of body weight

  The table below lists the adult RDA for protein and the essential amino acids.

  Adult RDA for Protein and Amino Acids
  	PRO (g)	HIS (mg)	ISO (mg)	LEU (mg)	LYS (mg)	MET+CYS (mg)	PHE+TYR (mg)	THR (mg)	TRP (mg)	VAL (mg)
  RDA per kg of body weight	0.8	14	19	42	38	19	33	20	5	24
  RDA for a 140 lb. person	51	891	1,209	2,673	2,418	1,209	2,100	1,273	318	1,527
  PRO-protein, HIS-histidine, ISO-isoleucine, LEU-leucine, LYS-lysine, MET+CYS-methionine plus cysteine, PHE+TYR-phenylalanine plus tyrosine, THR-threonine, TRP-tryptophan, VAL-valine

  Setting the DRIs for Protein

  In setting the DRSs for protein, the IOM relied on nitrogen balance studies, especially a 2003 meta-analysis by Rand et al. (2003).

  Nitrogen balance studies are used because nitrogen is a component of protein and isn’t found in the other macronutrients (i.e., fat, carbohydrate, and alcohol). Nitrogen balance studies measure how much nitrogen someone eats and then subtracts how much nitrogen they lose through urine, feces, hair, sweat, etc. If they lose more nitrogen than they eat, they’re in negative nitrogen balance and require more protein. If they lose as much as they eat, they’re in nitrogen balance and are obtaining enough protein to meet their needs.

  The IOM determined that the estimated average requirement (EAR) for protein for the average adult aged 19-50 years is 0.66 g/kg/day (IOM, 2005). They increased the EAR to set the recommended dietary allowance (RDA) at 0.8 g/kg/day which is intended to cover the needs of 97–98% of the population.

  Women Have the Same RDA as Men

  Given that women have, on average, a lower percentage of lean body mass than men, it might be surprising that they have the same RDA for protein (and amino acids). In setting the RDAs, the IOM says (IOM, 2005, p. 644):

  Although the data indicate that women have a lower nitrogen requirement than men per kilogram of body weight, this was only statistically significant when all studies were included, but not when the analysis was restricted to the primary data sets. This difference may be due to differences in body composition between men and women, with women and men having on average 28 and 15 percent fat mass, respectively. When controlled for lean body mass, no gender differences in the protein requirements were found. However, in view of the uncertain significance of the difference between the genders, the same protein EAR on a body weight basis for both men and women is chosen.

  Criticism of the Protein RDA

  A group of researchers from The Hospital for Sick Children in Toronto (Elango, 2010) argue that the methods for determining the RDA for protein underestimates needs. One of the authors, Dr. Paul B. Pencharz, was a member of the Panel on DRIs for macronutrients and a member of the Joint WHO/FAO/United Nations University Expert Consultation on Protein and Amino Acid Requirements in Human Nutrition. Elango et al. write:

  The current recommendations for protein intakes in adults are primarily based on the reanalysis of existing nitrogen balance studies. The nitrogen balance technique has inherent methodological limitations, which lead to an underestimation of the requirement estimate. Furthermore, the application of a single linear regression analysis to identify zero nitrogen balance is not appropriate because the nitrogen intake response relationship is not linear. On the basis of these concerns, we reanalyzed published nitrogen balance studies using two-phase linear regression analysis. We also applied the [isotope-based indicator amino acid oxidation (IAAO)] method to determine total protein requirements in adults. The mean and population-safe intakes based on the reanalysis were determined to be 0.91 and 1.0 g protein/kg/day and 0.93 and 1.2 g/kg/day, respectively, based on the IAAO method. These new values are approximately 40% higher than the current recommendations, and therefore, there is an urgent need to reassess recommendations for protein intake in adult humans.

  This group of researchers considers the average protein requirement to be 0.91–0.93 g/kg/day and the amount to cover 97%–98% of the population to be 1.0–1.2 g/kg/day.

  Protein Needs for People Over 60

  Some researchers argue that people over 60 years old are less efficient at maintaining muscle and bone and therefore are healthier on 1.0–1.3 g/kg/day of protein (Gaffney-Stomberg, 2009, Paddon-Jones, 2008, Morais, 2006), with some of this research being supported by or connected to animal agriculture trade organizations (Gaffney-Stomberg, 2009, Paddon-Jones, 2008).

  There is some conflicting research. A randomized trial of 92 men (average age 73 years) found no difference in lean body mass, muscle performance, physical function, well-being, or anabolic response to testosterone between those who consumed 0.8 g/kg/day and those who consumed 1.3 g/kg/day of protein for 6 months (Bhasin, 2018).

  Protein Needs of Athletes

  The RDA doesn’t include a higher protein recommendation for athletes. However, in a joint position paper on nutrition and athletic performance, the American College of Sports Medicine, the Academy of Nutrition and Dietetics, and Dietitians of Canada recommend 1.2 to 2.0 g/kg/day for athletes (Thomas, 2016). They don’t differentiate between strength and endurance athletes.

  A meta-analysis of protein intake among strength athletes concluded that a daily protein intake of 1.6 g/kg/day, separated into 0.25 g/kg doses, is sufficient for muscle protein synthesis (Morton, 2018).

  Protein and Amino Acid Content of Selected Plant Foods

  See our online spreadsheet, Protein and Amino Acid Content of Selected Plant Foods.

  

  Good Sources of Lysine among Plant Foods

  The United States Food and Drug Administration (FDA) defines a “good source” of a nutrient as 10% to 19% of the DRI or Daily Value (FDA, 2023). Because vegans might have slightly higher protein needs than non-vegans, we’ll use the midpoint (14.5%). Using a reference body weight of 70 kg for men and 57 kg for women (IOM, 2005, p. 680), a “good source” of lysine contains 350 mg per serving. Here’s a list of good sources of lysine (weights are for one serving of ready-to-eat food) and listed in ours section above, Additional Tips.

  Plant Protein Balance Research

  How much protein do vegans need?

  The IOM says, “In conclusion, available evidence does not support recommending a separate protein requirement for vegetarians who consume complementary mixtures of plant proteins (p. 662, IOM, 2005).”

  But what is the evidence?

  Nitrogen Balance Study on Vegans

  There has been only one nitrogen balance study conducted on vegans.

  Bartholamae and Johnston (USA, 2023) performed a nitrogen balance study on 18 minimally active, healthy male vegans aged 31.6 ± 6.2 years. They fed them a diet with a protein content of 0.8 g/kg/d. Nitrogen balance was determined after 5 days using the equation:

  nitrogen balance (g/d) = (protein intake (g/d) / 6.25) − urine urea nitrogen (UUN) (g/d) − 4 g/d

  The mean absolute nitrogen balance was −1.38 ± 1.22 g/d, and was statistically lower than the nitrogen equilibrium score of zero (95% CI: −2.00 to −0.75), indicating that the average vegan wasn’t in nitrogen balance. Only 3 vegans appeared to be in positive nitrogen balance.

  There were some limitations to this study. Instead of measuring non-UUN nitrogen losses directly, they used a factor of 4 g/d. They also allowed only 5 days for an adaptation feeding period which is on the short end of standard protocol.

  Another limitation is that the equation for nitrogen balance used the typical protein nitrogen conversion factor of 6.25. The protein nitrogen conversion factor is the amount of protein typically represented from an amount of protein. Using a protein nitrogen conversion factor of 6.25 for plant proteins has been criticized (Mariotti, 2008, Craddock, 2021); Mariotti et al. recommends using a protein nitrogen conversion factor of 5.6 rather than 6.25 for mixed diets using animal and plant protein.

  In our spreadsheet, Bartholomae & Johnston’s nitrogen balance study on vegans, we recalculated the average nitrogen balance results using a conversion factor of 5.6 which results in a nitrogen balance of -0.25 g/d (95% CI: -0.87 to 0.38). Because Bartholamae and Johnston provide a sample menu used in the study, we were also able to create a conversion factor of 5.435 based on data provided by Mariotti, 2008. Using a conversion factor of 5.435 results in a nitrogen balance of 0.08 g/day (95% CI: -0.54 to 0.71). Although these are just estimates, they indicate that the vegans, on average, were likely close to nitrogen balance.

  The RDA for protein is supposed to cover the needs of 97–98% of a population and so while the average vegan in this study was probably close to nitrogen balance on 0.8 g/kg/d of protein, this amount of protein probably isn’t enough to cover the needs of 97-98% of non-athletic vegans.

  Nitrogen Balance Studies using Plant Protein

  Doyle et al. (1965) conducted a study in two stages. In the first stage, eight young men were fed a vegan diet with 0.50 g/kg/day of protein and an amino acid profile matching milk. Unsurprisingly, on such a low protein intake, the men weren’t in nitrogen balance. In the second stage, protein was increased to 0.75 g/kg/day using 0.25 g/kg/day of soy protein and the men were, for the most part, in nitrogen balance.

  Register et al. (1967) fed a vegan diet of 0.91 g/kg/day of protein, of which 0.55 g/kg/day was legume protein, to participants for 3 weeks. On the vegan diet, 9 out of 12 of the participants were in nitrogen balance. They also found that the protein on a vegan diet was 2.6% less digested than the protein on a non-vegetarian diet.

  A third study placed young men on a near-vegan diet (except for 41 g of dried, skim milk) for 90 days, using 1 g/kg/day of protein(Yáñez, 1986). Some of the protein was from legumes (amount not reported). Seven out of the 8 subjects were in nitrogen balance.

  Serum Albumin Studies of People on Plant-Based Diets

  Albumin is a protein in the blood that responds to different amounts of dietary protein and can be used to measure protein needs.

  Caso et al. (2000) found a 12% reduced rate of albumin synthesis among men eating a diet for 10 days of 63% plant protein compared to 26% plant protein; total protein was 78 g/day. Albumin synthesis returned to normal after 18 g/day of soy protein was added, increasing plant protein to 78% and total protein to 96 g/day. Participants were eating about 1.09 g/kg/day without the soy and 1.34 g/kg/day with the soy. The synthesis of two other proteins, prealbumin and transferrin, were also lower on the diet with 63% plant protein. The lower synthesis of transferrin, an iron transport protein, is interesting because lysine supplements have been found to increase iron absorption.

  In a cross-sectional study, Haddad et al. (1999) found vegans (10 men and 15 women) to have significantly higher serum albumin levels than non-vegetarians. We estimated the protein intake of vegans to be 0.93 g/kg/day (calculations); 10 of the 15 vegan women didn’t meet the protein RDA of 0.8 g/kg/day. The authors stated, “Although serum albumin may not be a sensitive indicator of protein nutriture, the higher concentrations suggest that the diets of the vegan participants were adequate in protein.”

  Creatinine Clearance Study on Vegans

  Using data collected during the 1980s, Andrich et al. (2011) found similar muscle mass among vegan and non-vegan middle-aged women despite differences in protein intake of 1.0 g/kg/day for vegans and 1.3 g/kg/day for omnivores. Muscle mass was estimated using formulas based on creatinine clearance (a byproduct of muscle metabolism).

  The vegan women’s lysine intake of 30 mg/kg/day didn’t meet the RDA of 38 mg/kg/day. However, the study estimated vegan women to be consuming an average of only 1,511 kcal/day vs. 1,866 kcal/day for the omnivores despite similar body mass indexes (20.0 and 20.7 respectively), suggesting that food intake for the vegans was underestimated possibly due to a lack of data on vegan foods.

  Summary of Plant Protein Balance Research

  The results of protein balance studies of people on plant-based diets is compiled in the table below. It’s not obvious what they indicate for the protein needs of vegans, but an estimate is that vegans might benefit from at least 1.0 g/kg/day of protein.

  Protein Balance Studies Using Plant-Based Diets
  Study	Protein (g/kg/day)A	Legume Protein	Result
  Doyle, 1965	0.5	Amino acids matched milk	Subjects not in nitrogen balance
  Doyle, 1965	0.75	Amino acids matched milk	100% in nitrogen balance
  Bartholamae, 2023	0.8	Protein was mostly legumes	Average vegan was possibly in nitrogen balance but some probably were not
  Register, 1967	0.91	0.55 g/kg/day	75% in nitrogen balance
  Haddad, 1999	0.93	1.2 servings per day	Normal albumin levels
  Yáñez, 1986	1.0	Small amount	7 out of 8 in nitrogen balance
  Andrich, 2011	1.0	Lysine intake 79% RDA	Muscle mass similar to omnivores
  Caso, 2000	1.09	Not reported	12% lower albumin synthesis than controls
  Caso, 2000	1.34	At least 0.25 g/kg/day	Normal albumin synthesis
  Agrams per kg of body weight per day

  Amino Acid Intakes and Blood Levels of Vegans

  Researchers from EPIC-Oxford analyzed the dietary intake and blood levels of amino acids in various diet groups in adult men (Schmidt, 2015). The study included 98 men for each diet group (vegan, lacto-ovo-vegetarian, pescatarian, and meat-eater). On average, participants had followed their diet for several years.

  Protein and Amino Acid Intakes of Vegan Men

  The vegan men in Schmidt et al.’s analysis of EPIC-Oxford were eating 0.85 g/kg/day of protein (calculations). Although the men were apparently healthy, protein balance wasn’t measured. It should also be noted that the protein and amino acid intakes were measured by a food frequency questionnaire which the authors say might have underestimated the intakes of vegans.

  The table below compares vegan amino acid intakes in EPIC-Oxford to the United States RDA for amino acids.

  Essential Amino Acid Intakes of Vegan Men in EPIC-Oxford
  Amino Acid	Intake (g/day)	RDA (g/day)	Percentage of RDA
  Isoleucine	2.47	1.29	191
  Leucine	4.33	2.85	152
  Valine	2.95	1.63	181
  Histidine	1.52	0.95	160
  Lysine	2.82	2.58	109
  Methionine+Cysteine	1.72	1.29	133
  Phenylalanine+Tyrosine	4.79	2.24	214
  Theronine	2.19	1.36	161
  Tryptophan	0.77	0.34	226
  Weight of the participants wasn’t reported, but vegans had an average body mass index of 22.1 kg/m2. If we assume an average height of 175.3 cm for a British male (BBC, 2010), the average weight of the vegans was 67.9 kg.

   
  Vegan men met the RDA for all essential amino acids.

  Lysine was the limiting amino acid in the diets of the vegan men; they surpassed the RDA by only 9%. The 95% confidence interval for lysine intake was 2.69-2.95 g/day, with a lower margin of 104% of the RDA.

  The vegan men surpassed the RDA for methionine by the next lowest amount of 33%. Methionine is traditionally the amino acid of second most concern in vegan diets.

  Vegans in the United States might eat higher amounts of protein than those in the United Kingdom. The Adventist Health Study-2 from the United States found an average protein intake of 71 g/day for men and women combined (Average Nutrient Intakes), considerably more than the 58 g/day of this subset of men in EPIC-Oxford (Schmidt, 2015).

  Amino Acid Intakes of Vegan Women

  As a percentage of body weight, the RDA for protein and amino acids is the same for women as for men.

  Vegan women in EPIC-Oxford ate 56 g of protein per day (Average Nutrient Intakes), only marginally less than vegan men Schmidt et al.’s subset of EPIC-Oxford, and so their amino acid intakes are likely similar.

  As with men, Seventh-day Adventist vegan women in the United States most likely eat more protein than vegan women in the United Kingdom (Average Nutrient Intakes), suggesting that Seventh-day Adventist vegan women are surpassing the RDA for lysine and other amino acids.

  A study of Danish vegans (Aaslyng, 2023) found that a high percentage of the participants (36 women, 4 men) didn’t meet the recommendations for protein and some essential amino acids. Although 3-day diet records were used, which are typically a reliable method to estimate intakes, energy appeared to be significantly underestimated with 55% of the participants not meeting recommendations (despite an average body mass index of 24.8). Had food intake not been underestimated, a high percentage of participants would have likely been shown to meet recommendations given that only one participant who met energy recommendations failed to also meet protein recommendations.

  Amino Acid Blood Levels of Vegan Men

  Schmidt et al. (2015) compared the blood levels of amino acids between diet groups. Vegans had lower levels of lysine, methionine, tryptophan, and tyrosine, and higher levels of alanine and glycine. The researchers didn’t seem concerned by these differences.

  The table below compares the plasma levels of amino acids of vegan men in Epic-Oxford to the reference ranges given by the U.S. National Library of Medicine (NLM). The NLM notes that these numbers are dependent on the specific laboratory methods used (NLM, 2016).

  Plasma Amino Acid Levels in Vegan Men
  Amino Acid	Plasma µmol/l (95% CI)	Reference Range µmol/l
  Alanine	621 (595, 648)	230-510
  Arginine	44 (39, 48)	13-64
  Asparagine	98 (95, 102)	45-130
  Aspartate	69 (66, 72)	0-6
  Citrulline	40 (38, 42)	16-55
  Glutamate	262 (248, 277)	18-98
  Glutamine	547 (529, 566)	390-650
  Glycine	452 (434, 470)	170-330
  Histidine	117 (113, 120)	26-120
  Isoleucine	96 (92, 100)	42-100
  Leucine	191 (184, 199)	66-170
  Lysine	210 (201, 219)	150-220
  Methionine	27 (26, 28)	16-30
  Ornithine	205 (197, 215)	27-80
  Phenylalanine	97 (93, 101)	41-68
  Proline	244 (233, 256)	110-360
  Serine	197 (190, 205)	56-140
  Theronine	165 (159, 171)	92-240
  Tryptophan	65 (63, 68)	n/a
  Tyrosine	73 (70, 76)	45-74
  Valine	217 (209, 225)	150-310
  The NLM provides a reference range for cystine (two cysteine molecules combined), but Schmidt et al. didn’t list plasma levels for cystine or cysteine. The NLM provides no reference range for tryptophan, with no explanation.

   
  Vegans had levels higher than the reference range for alanine, glutamate, glycine, leucine, ornithine (a non-protein amino acid), phenylalanine, and serine; aspartate was also higher, but the reference range is surprisingly low. It’s unclear if these elevated levels have clinical significance.

  Arginine and the Vegan Diet

  People with herpes are often advised to limit their intake of the amino acid arginine in order to reduce symptoms. EPIC-Oxford found vegans to have a non-significant trend toward lower levels of arginine in the blood and also a lower arginine intake than meat-eaters (3.92 g/day for vegans, 4.13 g/day for meat-eaters); lacto-ovo vegetarians had the lowest intake at 3.36 g/day (Schmidt, 2015).

  Appendix A: Protein Quality Scoring

  One of the earliest ways to assess protein quality was the protein efficiency ratio (PER) which determined a score based on the growth of rats fed different proteins. The PER was criticized both for using rats and for not taking into account protein maintenance needs which led to the Food and Agriculture Organization (FAO) of the United Nations recommending other ways to determine protein quality.

  FAO Recommends the PDCAAS in 1989

  In 1989, the FAO recommended replacing the PER with the protein digestibility–corrected amino acid score (PDCAAS) as the “most suitable regulatory method for evaluating protein quality of foods and infant formulas (p. 42, FAO, 1991).” The FAO considered the PDCAAS to be an improvement over the PER because the PDCAAS based protein scores on the needs of humans, specifically preschool-age children (p. 8, FAO, 1991).

  The PDCAAS ranks a food’s protein quality based on its limiting amino acid which is the essential amino acid that the protein contains the least amount of in comparison to the reference protein, typically the milk protein casein (p.5, FAO, 1991). The PDCAAS scores each essential amino acid in a protein and then uses the lowest PDCAAS score for the overall protein, truncated at 1.00 (p. 35, FAO, 1991):

  (mg of limiting amino acid in 1 g of test protein / mg of same amino acid in 1 g of reference protein ) x fecal true protein digestibility percentage

  Table 11 (p. 39, FAO, 1991) provides the PDCAAS scores known by the FAO at the time. Only three animal foods are listed: casein (1.00), egg white (1.00), and beef (0.92). There are just over a dozen familiar plant proteins listed and their PDCAAS scores range from 0.26 to 0.69.

  Although the PDCAAS ranks proteins according to the needs of humans, it’s been criticized for calculating amino acid absorption, aka digestibility, based on a protein fecal analysis of rats (Schaafsma, 2000). Rat digestion is different from humans and a fecal analysis doesn’t take into account that bacteria in the colon can change the amino acid composition of the feces. The FAO recommended changing from fecal digestibility to ileal digestibility when agreed upon procedures and sufficient data are available (p. 32, FAO, 1991).

  The FAO implied that some of the PDCAAS might be based on human digestibility studies; they say “when human balance studies cannot be used, the standardized rat faecal-balance method of Eggum or McDonough et al. is recommended (p. 31, FAO, 1991).” But the abstracts for the citations of the PDCAAS scores given in Table 11 (p. 39, FAO, 1991) don’t indicate that any human data was used.

  The PDCAAS has also been criticized for truncating protein scores at 1.00 which prevents the score from reflecting a food’s ability to complement other foods in the diet (Schaafsma, 2000). Only animal foods have PDCAAS scores high enough to be truncated.

  The 1991 FAO report relied mostly on data by Sarwar and colleagues in listing the PDCAASs of foods (p. 39, FAO, 1991). Sarwar later published a paper suggesting that the PDCAAS method overestimates the quality of proteins for foods containing anti-nutritional factors (Sarwar, 1997) which are primarily plant foods.

  FAO Recommends the DIAAS in 2011

  In 2011, the FAO Expert Consultation on Protein Quality Evaluation in Human Nutrition was held to review the adequacy of the PDCAAS (FAO, 2013). The panel recommended changing how proteins are scored from the PDCAAS to the digestible indispensable amino acid score (DIAAS). The DIAAS uses the digestibility of specific amino acids rather than of entire proteins.

  The DIAAS for a food is the lowest score for any of the essential amino acids (p. 3, FAO, 2013):

  100 x (mg of digestible dietary indispensable amino acid in 1 g of the dietary protein / mg of the same dietary indispensable amino acid in 1 g of the reference protein)

  DIAAS scores are not truncated at 100%. The ileal digestibility of each amino acid is preferably determined in humans, followed by growing pigs, and then growing rats (p. 4, FAO, 2013).

  For infants, the DIAAS reference protein is the amino acid pattern found in human breast milk. For non-infants the DIASS uses a reference protein based on the needs of humans. Appendix B: The DIAAS Reference Protein explains how they calculated the reference protein.

  The FAO provides few examples of DIAAS scores. Table 6 (p. 44, FAO, 2013) provides a DIAAS score for whole milk powder (122), wheat (40), and peas (64).

  Criticism of the DIAAS

  Craddock et al. (2021) published a list of critiques of the DIAAS with respect to plant-based diets.

  Much of Craddock et al.’s criticism focuses on the digestibility of plant foods which they argue is underestimated by the DIAAS. They cite Tomé D. (2013) whose review paper provides a table of ileal food digestibility in humans. Rather than being markedly lower in digestibility than animal protein, Craddock et al. point out that “soy protein isolate, pea protein isolate, pea protein flour, wheat flour, and lupin flour exhibited 89–92% digestibility, approximating values of 90–95% for eggs, meat, and milk proteins.”

  Here’s a list of Craddock et al.’s criticisms with brief explanations:

  • Nitrogen-to-protein conversion factors influence animal and plant foods differently.
    • The ratio of essential amino acids to total protein can be underestimated for plant foods when PDCAAS and DIAAS scores are calculated because the FAO doesn’t require food-specific nitrogen-to-protein conversion factors when determining the amount of protein in a food.
  • There’s a limited number of fruits and vegetables within the DIAAS.
    • Many plant foods won’t be included in an assessment of an entire diet because most don’t have DIAAS scores.
  • A DIAAS diet score is of questionable relevance to those following plant-based dietary patterns in developed nations.
    • Plant-based diets in food-secure regions will easily meet the needs for total essential amino acids. (In this criticism, Craddock et al. treat total essential amino acids as a whole and don’t analyze individual amino acids.)
  • DIASS scores are based on animal digestion.
    • Digestibility of amino acids in rats and pigs cannot be extrapolated to humans. (The FAO admits that rats are not an ideal model, but says that pig digestion approximates humans while obtaining human data for a wide range of foods is impractical.)
  • DIAAS scores for foods change when they’re part of a mixed diet.
    • In mixed diets, the digestibility of proteins and amino acids changes, while DIAAS scores are based only on single foods and give animal foods an inflated advantage.
  • Digestibility of plant foods improves with heat treatment and processing.
    • Raw foods are used for most DIAAS scoring, whereas protein-rich plant foods are typically cooked or processed making them easier to digest.
  • The DIAAS scores were never intended for athletes consuming a plant-based diet.
    • The FAO endorsed the DIAAS as a tool to help the global population avoid malnutrition by meeting protein requirements but hasn’t endorsed the DIASS for ranking proteins on the basis of their muscle-building potential. Craddock et al. cite research suggesting that proteins with higher DIASS scores don’t increase muscle hypertrophy.

  Craddock et al. conclude that “emphasizing protein quality in developed nations is unproductive.” They say that if protein quality is to be assessed, the PDCAAS is the most suitable tool for use in Western adults who eat a plant-based diet.

  It’s not clear why Craddock et al. believe that the PDCAAS is superior to the DIASS given that most of their critique of the DIASS would similarly apply to the PDCAAS. Perhaps they prefer the PDCAAS because it truncates animal protein scores at 1.00.

  Appendix B: The DIAAS Reference Protein

  Because it’s hard to follow how the FAO determined the DIAAS reference protein for non-infants, I’ve included more details here which require following along with the tables in their document.

  For non-infants, the reference protein used in DIAAS scoring is a calculation of the amino acid needs, represented in Table 3 (p. 27, FAO, 2013) and narrowed down in Table 5 (p. 29, FAO, 2013).

  The numbers in the bottom section of Table 5, Scoring pattern mg/g protein requirement, are calculated by multiplying the sum of the Maintenance and the Growth factor by the Amino acid requirements. The numbers derived in Table 3 (p. 27, FAO, 2013) are taken from a 2007 FAO report which explains how they’re calculated (FAO 2007).

  In the 2007 FAO report, Table 23 (p. 150) lists the mean adult requirement for each amino acid as determined by the committee based on a variety of methods and sources, typically human nitrogen balance studies in which specific amino acids are limited. The FAO’s 2007 amino acid requirements for adults don’t match the IOM’s estimated average requirement (EAR), but are relatively close.

  Appendix C: Muscle Synthesis Studies

  This appendix covers a muscle synthesis study that wasn’t conducted in a sports nutrition context.

  Muscle Synthesis after a Meal Containing Beef or Plant Protein

  One study comparing proteins raises concerns about relying solely on whole-foods plant protein, although there’s not enough information to draw conclusions.

  Pinckaers et al. (2023, The Netherlands) conducted a crossover trial which provided 8 older males and 8 older females (average age of 72 years) a meal containing beef or a meal containing plant protein from whole plant foods. Each meal contained 0.45 g per kg of bodyweight (roughly about 36 g of protein); the meals had relatively similar essential amino acid (EAA) contents.

  The animal-based meal resulted in a higher increase of EAAs in the blood, higher leucine levels, higher markers of muscle protein and whole body protein synthesis. Markers of muscle protein synthesis, on average, actually decreased during the six hour post-meal period for the plant-based meal (as indicated by Figure 7 of the paper).

  The authors summarize their concern:

  Though we support the concept of moving toward a more plant-based diet for ethical, environmental, and health considerations, there are some concerns with regards to maintaining muscle health later in life. A more plant-based diet will likely provide health benefits, with many of those secondary to a low(er) energy intake due to the high fiber content and satiating effect of consuming more plant-based whole foods. However, the anabolic properties of each main meal may be of key relevance to stimulate muscle protein synthesis rates and, as such, to support muscle maintenance. This may be of particular importance for older adults, as the age-related loss in muscle mass is at least partly attributed to the attenuated postprandial muscle protein synthetic response to acute feeding in older compared with younger subjects.

  There are some significant caveats to their findings. Pinckaers et al. didn’t measure changes in muscle mass which are more important than markers of muscle synthesis. For one thing, overall muscle synthesis has to take into account muscle breakdown which Pinckaers et al. didn’t measure. The study also only measured a 6-hour time period and cannot account for any adaptation to plant proteins that might occur over time on a plant-based diet.

  Other research, by Michell et al. (2023, Canada) found no association between post-meal markers of muscle synthesis and muscle mass after resistance exercise. Michell et al. write, “Clearly, however, acute early measures of [muscle protein synthesis] are not proxy measures for hypertrophy or hypertrophic potential within the same individual.”

  The situation is complex and there isn’t enough research to sort it out at this time. The study by Pinckaers et al. raises some concerns about the ability of whole foods vegan diets to stimulate muscle synthesis in older adults. While more research is needed, this study might indicate a benefit from more processed plant-based proteins for vegans, especially older vegans.

  Last updated April 2024

  Bibliography

  Aaslyng MD, Dam AB, Petersen IL, Christoffersen T. Protein content and amino acid composition in the diet of Danish vegans: a cross-sectional study. BMC Nutr. 2023 Nov 15;9(1):131. Erratum in: BMC Nutr. 2024 Apr 16;10(1):58. The erratum notes that the original paper used the terms “requirements” and “recommendations” in an ambiguous way.

  Andrich DE, Filion ME, Woods M, Dwyer JT, Gorbach SL, Goldin BR, Adlercreutz H, Aubertin-Leheudre M. Relationship between essential amino acids and muscle mass, independent of habitual diets, in pre- and post-menopausal US women. Int J Food Sci Nutr. 2011 Nov;62(7):719-24. Epub 2011 May 16.

  Bartholomae E, Johnston CS. Nitrogen Balance at the Recommended Dietary Allowance for Protein in Minimally Active Male Vegans. Nutrients. 2023 Jul 16;15(14):3159.

  BBC News. Statistics reveal Britain’s ‘Mr and Mrs Average’. 2010 Oct 13.

  Bhasin S, Apovian CM, Travison TG, et al. Effect of Protein Intake on Lean Body Mass in Functionally Limited Older Men: A Randomized Clinical Trial. JAMA Intern Med. 2018 Apr 1;178(4):530-541.

  Caso G, Scalfi L, Marra M, Covino A, Muscaritoli M, McNurlan MA, Garlick PJ, Contaldo F. Albumin synthesis is diminished in men consuming a predominantly vegetarian diet. J Nutr. 2000 Mar;130(3):528-33. Protein per body weight calculations: Average healthy body weight of the men based on a BMI of 22 and average height of 1.74 m was 66.6 kg (78 g protein per 66.6 kg = 1.17 g/kg, 96 g protein per 66.6 kg = 1.44 g/kg). Actual average body weight of the men was 77 kg (78 g protein per 77 kg = 1.01 g/kg, 96 g protein per 77 kg = 1.25 g/kg). Averaging the healthy body weight with the actual body weight gives a range of 1.09 and 1.34 g/kg.

  Ciuris C, Lynch HM, Wharton C, Johnston CS. A Comparison of Dietary Protein Digestibility, Based on DIAAS Scoring, in Vegetarian and Non-Vegetarian Athletes. Nutrients. 2019 Dec 10;11(12):3016. Not cited. Used DIAAS scores listed in the FAO 2013 report to calculate diet scores of lacto-ovo-vegetarians and meateaters.

  Craddock JC, Genoni A, Strutt EF, Goldman DM. Limitations with the Digestible Indispensable Amino Acid Score (DIAAS) with Special Attention to Plant-Based Diets: a Review. Curr Nutr Rep. 2021 Mar;10(1):93-98.

  Doyle MD, Morse LM, Gowan JS, Parsons MR. Observations on nitrogen and energy balance in young men consuming vegetarian diets. Am J Clin Nutr. 1965 Dec;17(6):367-76.

  Elango R, Humayun MA, Ball RO, Pencharz PB. Evidence that protein requirements have been significantly underestimated. Curr Opin Clin Nutr Metab Care. 2010 Jan;13(1):52-7.

  Evans WJ. Protein nutrition, exercise and aging. J Am Coll Nutr. 2004 Dec;23(6 Suppl):601S-609S. Not cited.

  Food and Agricultural Organization (FAO) of the United Nations. Report of the Joint FAO/WHO Expert Consultation. FAO Food and Nutrition Paper 51. Proceeds of meeting held in Bethesda, MD, USA 4-8 December 1989. Report published in 1991.

  Food and Agriculture Organization of the United Nations, World Health Organization & United Nations University. Protein and amino acid requirements in human nutrition: report of a joint FAO/WHO/UNU expert consultation. 2007.

  Food and Agricultural Organization (FAO) of the United Nations. Food and Nutrition Paper 92. Dietary protein quality evaluation in human nutrition. Report of an FAO Expert Consultation. Proceeds of meeting held in Auckland, New Zealand 31 March–2 April, 2011. Report published in 2013.

  Food and Drug Administration. United States Department of Health and Human Services. Code of Federal Regulations. Title 21, Volume 2. 21CFR101.54. Current as of Jan 17, 2023.

  Gaffney-Stomberg E, Insogna KL, Rodriguez NR, Kerstetter JE. Increasing dietary protein requirements in elderly people for optimal muscle and bone health. J Am Geriatr Soc. 2009 Jun;57(6):1073-9.

  Haddad EH, Berk LS, Kettering JD, Hubbard RW, Peters WR. Dietary intake and biochemical, hematologic, and immune status of vegans compared with nonvegetarians. Am J Clin Nutr. 1999 Sep;70(3 Suppl):586S-593S.

  Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. National Academy of Sciences. Food and Nutrition Board. 2005.

  Mariotti F, Tomé D, Mirand PP. Converting nitrogen into protein–beyond 6.25 and Jones’ factors. Crit Rev Food Sci Nutr. 2008 Feb;48(2):177-84.

  Mitchell CJ, Churchward-Venne TA, Parise G, Bellamy L, Baker SK, Smith K, Atherton PJ, Phillips SM. Acute post-exercise myofibrillar protein synthesis is not correlated with resistance training-induced muscle hypertrophy in young men. PLoS One. 2014 Feb 24;9(2):e89431.

  Morais JA, Chevalier S, Gougeon R. Protein turnover and requirements in the healthy and frail elderly. J Nutr Health Aging. 2006 Jul-Aug;10(4):272-83.

  Morton RW, Murphy KT, McKellar SR, Schoenfeld BJ, Henselmans M, Helms E, Aragon AA, Devries MC, Banfield L, Krieger JW, Phillips SM. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018 Mar;52(6):376-384.

  National Library of Medicine. United States. Plasma amino acids. Medline Plus. Accessed January 30, 2016.

  Paddon-Jones D, Short KR, Campbell WW, Volpi E, Wolfe RR. Role of dietary protein in the sarcopenia of aging. Am J Clin Nutr. 2008 May;87(5):1562S-1566S.

  Pinckaers PJ, Domić J, Petrick HL, Holwerda AM, Trommelen J, Hendriks FK, Houben LH, Goessens JP, van Kranenburg JM, Senden JM, de Groot LC, Verdijk LB, Snijders T, van Loon LJ. Higher Muscle Protein Synthesis Rates Following Ingestion of an Omnivorous Meal Compared with an Isocaloric and Isonitrogenous Vegan Meal in Healthy, Older Adults. J Nutr. 2023 Nov 15:S0022-3166(23)72723-5.

  Rand WM, Pellett PL, Young VR. Meta-analysis of nitrogen balance studies for estimating protein requirements in healthy adults. Am J Clin Nutr. 2003 Jan;77(1):109-27.

  Register UD, Inano M, Thurston CE, Vyhmeister IB, Dysinger PW, Blankenship JW, Horning MC. Nitrogen-balance studies in human subjects on various diets. Am J Clin Nutr. 1967 Jul;20(7):753-9.

  Sarwar G. The protein digestibility-corrected amino acid score method overestimates quality of proteins containing antinutritional factors and of poorly digestible proteins supplemented with limiting amino acids in rats. J Nutr. 1997 May;127(5):758-64.

  Schmidt JA, Rinaldi S, Scalbert A, Ferrari P, Achaintre D, Gunter MJ, Appleby PN, Key TJ, Travis RC. Plasma concentrations and intakes of amino acids in male meat-eaters, fish-eaters, vegetarians and vegans: a cross-sectional analysis in the EPIC-Oxford cohort. Eur J Clin Nutr. 2015 Sep 23.

  Schaafsma G. The protein digestibility-corrected amino acid score. J Nutr. 2000 Jul;130(7):1865S-7S.

  Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. J Acad Nutr Diet 2016;116:501-528.

  Tomé D. Digestibility issues of vegetable versus animal proteins: protein and amino acid requirements–functional aspects. Food Nutr Bull. 2013 Jun;34(2):272-4. Table 2: Fecal versus ileal digestibility (%) of proteins in humans.

  Yáñez E, Uauy R, Zacarías I, Barrera G. Long-term validation of 1 g of protein per kilogram body weight from a predominantly vegetable mixed diet to meet the requirements of young adult males. J Nutr. 1986 May;116(5):865-72.