Type 2 Diabetes and Vegan Diets

Contents

• Observational Studies
  • Prospective Analysis from EPIC-Oxford Cohort (2019)
  • Meta-Analysis of Observational Studies (2017)
  • Prospective Analysis from Adventist Health Study-2 (2011)
  • Cross-Sectional Analysis from Adventist Health Study-2 (2009)
  • Adventist Health Study (1999)
• Clinical Trials
  • PCRM Insulin (2018)
  • Korea (2016)
  • Czech Republic (2011)
  • PCRM Study (2004-05)
  • PCRM Glucose and Insulin (2005)
  • PCRM Pilot Study (1999)
• Conclusion
• More Information
• References

Two prospective studies have found vegans to have drastically lower rates of diabetes. A number of clinical trials have found that a whole foods plant-based diet can lower body weight, reduce blood sugar, and improve other parameters for type 2 diabetes.

Observational Studies

Prospective Analysis from EPIC-Oxford Cohort (2019)

After 17.6 years of follow-up of the EPIC-Oxford cohort, vegans had about half the risk of diagnosis or death from diabetes (19).

Risk of Diabetes in EPIC-Oxford (2019)19
	Meat Eater	Semi-Veg	Pesco	Lacto-Ovo	Vegan
Number	15,181	7,615	7,092	13,645	1,781
Hazard Ratio	1.00	.63 (.65-.75)	.47 (.38-.59)	.65 (.55-.76)	.53 (.36-.79)
Adjusted for sex, method of recruitment, region, age, education, Townsend deprivation index, ethnicity, smoking, alcohol intake, and physical activity.

When further adjusted for body mass index, the association was attenuated (HR 0.99, CI 0.66-1.48 for vegans).

Meta-Analysis of Observational Studies (2017)

A 2017 meta-analysis reviewed 14 studies published in 13 papers (17). Two were cohort studies and the other 12 were cross-sectional.

Vegetarians had a lower incidence of diabetes in eight of the studies while there was no difference in the other five. Based on the pooled analysis of the studies, vegetarians had a 27% lower risk for diabetes compared to omnivores (OR 0.73, 95% CI: 0.61, 0.87).

When the researchers looked at effects in different groups, they found that vegetarian men were less likely to have diabetes than omnivore men, but there was no difference in risk between vegetarian and omnivore women. The researchers also looked at different types of vegetarian diets and found that risk for diabetes was lowest among vegans and lacto-vegetarians. Pesco-vegetarians did not have a lower risk for diabetes compared to omnivores, although semi-vegetarians did.

The main limitation of this meta-analysis is that most of the studies were cross-sectional. In addition, the studies were from diverse populations throughout the world where definitions and composition of vegetarian diets may differ. This might explain why among these studies, the findings were stronger in studies from North America, Europe, and the Western Pacific than in those in Southeast Asia.

Prospective Analysis from Adventist Health Study-2 (2011)

Participants of Adventist Health Study-2 were followed for two years. Vegans had a 60% lower risk of developing diabetes than regular meat eaters. (14).

Risk of Diabetes in AHS-2 (2011)14
	Meat Eater	Semi-Veg	Pesco	Lacto-Ovo	Vegan
Number	17,695	2,404	3,644	14,099	3,545
Rate	1.00	.49 (.31-.76)	.79 (.58-1.09)	.62 (.50-.76)	.38 (.24-.62)
Adjusted for age, BMI, race, sex, education, income, television watching, sleep, alcohol, physical activity, and smoking.

Black ethnicity was associated with an increased incidence of diabetes, as was age, male gender, and BMI. A higher income and more sleep were associated with a lower incidence of diabetes. A separate analysis was done for black participants only and black vegans had a relative risk of .43 (CI .25-.74) compared to meat eaters.

The authors cautioned that because rates were determined by participants reporting a diagnosis of high blood sugar or diabetes, as distinct from measuring each participant’s blood sugar, cases of diabetes might not have been detected.

The authors surmised as to why vegetarians have lower rates of diabetes: “Fruits and vegetables may contribute to a decreased incidence of type 2 diabetes through their low energy density, low glycemic load, and high fiber and macronutrient content. Other features of the vegetarian diet are whole grains and legumes. These foods have been shown to improve glycemic control, slow the rate of carbohydrate absorption and the risk of diabetes.”

Messina suggests that the high intake of soy among Seventh-day Adventist vegetarians could play a preventative role by way of its linoleic acid, protein, and isoflavone content (20).

Cross-Sectional Analysis from Adventist Health Study-2 (2009)

In 2009, the first study measuring the type 2 diabetes rates of vegans was released. It was a cross-sectional study and found vegans to have the lowest rates of type 2 diabetes of any diet group (12). Results are in Table 5.

Table 5. Cross-sectional Type 2 Diabetes Rates in AHS-2 (2009)
	Vegan	Lacto-Ovo	Pescoa	Semi-vegetarianb	Non-Veg
Number	2,731	20,408	5,617	3,386	28,761
Diabetes rate ratioc	.51 (.40-.66)	.54 (.49-.60)	.70 (.61-.80)	.76 (.65-.90)	1.00
Diabetes rate ratiod	.32 (.25-.41)	.43 (.39-.47)	.56 (.49-.64)	.69 (.59-.81)	1.00
aVegetarian except for fish. • bAte meat more than once a month but less than once a week. • cAdjusted for age, gender, race, activity, education, TV watching, sleep, alcohol, and BMI. • dAdjusted for all factors above except BMI.

Vegans had a 68% lower risk of type 2 diabetes than did non-vegetarians.

Since this study was cross-sectional, the results could mean that a vegan diet prevents diabetes, or it could mean that people with type 2 diabetes are less likely to adopt a vegan diet – or some combination of both.

Adjusting for BMI diminished the difference between vegans and non-vegetarians to some extent. This should be expected as a high BMI is a risk for type 2 diabetes. However, a lower BMI is possibly caused by a vegan diet, so this should not take anything away from the idea that the diet caused the difference.

Even after adjusting for BMI, the vegans had an advantage. The authors speculated on what else could be beneficial about a vegan diet:

The vegan group consumed about 650 grams/day of fruits and vegetables, about one-third more than the amount consumed by non-vegetarians (data not shown). Observational evidence has shown that these dietary constituents are associated with a reduction in type 2 diabetes of about 40%. Vegetarian diets contain substantially less saturated fat than nonvegetarian diets and saturated fatty acids have been shown to reduce insulin sensitivity, though a recent review concluded that some of the data supporting this idea was flawed. The vegetarian diet typically includes foods that have a low glycemic index such as beans, legumes and nuts. We did not calculate the glycemic load of the diets. Though low glycemic response diets are associated with less type 2 diabetes, cohort studies have not consistently found a relation between dietary glycemic index or load and risk of diabetes; furthermore, whether the glycemic response causes diabetes is not established.

Adventist Health Study (1999)

A 1999 report of the results of the original Adventist Health Study showed vegetarians to have about half the risk of diabetes as non-vegetarians (11). This was a 6-year prospective study and vegans were not separated from the lacto-ovo vegetarians.

Clinical Trials

There have been a number of clinical trials using a vegan or near vegan diet to treat type 2 diabetes. They have been successful in reducing diabetes medication and blood sugar levels.

PCRM Insulin (2018)

The Physicians Committee for Responsible Medicine (PCRM) funded a 16-week, randomized, controlled study to determine the effects of a low-fat, vegan diet on insulin resistance and the function of beta-cells (18).

Adults who were classified as overweight or obese without histories of diabetes were randomly assigned to follow either a low-fat vegan diet (38 participants) or continue their current diets with no changes (37 participants). The vegan diet group was instructed to eat vegetables, grains, legumes, and fruits. Calories comprised of 75% carbohydrate, 15% protein, and 10% fat.

Compared with the control group, the vegan diet group experienced an improvement in beta-cell function and a decrease in fasting insulin resistance, body mass index, body fat, and fat around internal organs (visceral fat).

The loss of visceral fat, in particular, may partially explain the decrease in insulin resistance, as this type of fat releases hormones that affect metabolism. The improvement in beta cell function, represented by meal-induced insulin secretion, was associated with the reduction in body mass index.

Although neither group was told to limit their food intake, both groups reduced their caloric intake during the study. Those in the vegan diet group did not reduce their mean calorie intake beyond that of their counterparts, but their fiber and carbohydrate intake increased while their protein, cholesterol, and fat decreased.

These results demonstrate that altering macronutrient composition through a vegan diet, without restricting calories, can improve beta-cell function and fasting insulin resistance in adults who are overweight or obese but with no history of diabetes.

Korea (2016)

The focus of this clinical trial was to compare the effect of a vegan diet to a conventional diabetic diet, as prescribed by the Korean Diabetes Association (KDA), on glycemic control among Koreans (16).

The trial lasted three months. The vegan diet group had 46 people while the KDA diet group had 47. After three months, there was a statistically significant, greater reduction in HbA1c in the vegan group compared to the KDA group (0.5% vs. 0.2%, p = 0.017). When including only participants with high diet compliance, the vegan diet fared even better (0.9% vs. 0.3%, p = .01).

The vegan group ate less calories and saturated fat than the KDA group. Fiber intake for the vegan group and KDA group was 33.7 g and 24.9 g.

The vegan group lost weight while the KDA group didn’t. However, neither group’s blood pressure or LDL-cholesterol went down. The vegan group’s triglycerides went up while the KDA group’s went down; this might indicate the vegan group was eating more simple sugars.

Czech Republic (2011)

In 2011, researchers from the Czech Republic studied a vegetarian (near-vegan) diet compared to a conventional (control) diet for type 2 diabetes (15). The study tested only diet for 12 weeks and then combined diet and exercise for another 12 weeks. Animal products were limited to maximum of one portion of low-fat yogurt a day. The calories in both diets were limited (as distinct from the PCRM studies below in which the vegan diet was unlimited in calories). The vegetarian diet was about 38% fat.

The vegetarian diet group had a greater reduction in diabetes medication (43% vs. 5%), HbA1c, waist circumference, and body fat. LDL cholesterol went down 8% in the vegetarian group only, but HDL cholesterol went up in the control group. Exercise caused the positive differences for the vegetarian diet to be even greater and also raised the HDL in the vegetarian group.

The authors stated:

Several possible mechanisms may explain the beneficial effects of a vegetarian diet: higher intake of fibre, lower intake of saturated fat [and a higher polyunsaturated and saturated fatty acid (P⁄S) ratio], higher intake of non-heme iron and reduction in iron stores, higher intake of vegetable protein in place of animal protein, higher intake of antioxidants and plant sterols. A vegetarian diet was reported to reduce intramyocellular lipid concentrations and this, together with the effect on visceral fat which we observed, might be responsible for a substantial portion of the effect of a vegetarian diet on insulin sensitivity and enzymatic oxidative stress markers.

And:

Especially during exercise, it became evident in our trial that it was easier for subjects to follow a vegetarian diet than a conventional diabetic diet.

PCRM Study (2004-05)

In 2004, researchers affiliated with the Physician’s Committee for Responsible Medicine (PCRM) started a 74-week study with 99 participants, comparing a vegan diet to an American Diabetes Association recommended diet for people with type 2 diabetes.

In this study, the vegan diet was once again a very low-fat diet; less than 10% of calories. Vegan dieters were asked to avoid fatty foods, added oils, fried products, avocados, nuts, and seeds. This time, instead of avoiding all refined grains, they were merely asked to favor low glycemic index foods (see Table 4 for an explanation of the glycemic index).

The control group was put on an American Diabetes Diet (ADA) of 15 – 20% protein, < 7% saturated fat, 60 – 70% carbohydrate and monounsaturated fats, and < 200 mg/day of cholesterol. The diets were individualized, based on body weight and plasma lipids. Those with a BMI > 25 kg/m (2) (all but three) were prescribed energy deficits of 500 – 1,000 kcal.

For this article, I’ve reviewed four papers on this study:

• Medical outcomes at 22 weeks (2)
• Nutrient intakes at 22 weeks (3)
• Medical outcomes and nutrient intakes at 74 weeks (4)
• Acceptability of, and adherence to, the diets at 74 weeks (5)

Once again, the vegan group ate less calories and had significant weight loss. Intake of fiber and carbohydrate went up, while fasting blood sugar and HbA1c went down (only statistically significant for the first 22 weeks). Cholesterol and triglyceride levels also decreased, and many participants were able to reduce their diabetes medications.

The ADA diet group also made improvements in weight loss, HbA1c, total cholesterol, and medications.

Among participants in the vegan group with no changes to diabetes medications, HbA1c had fallen 1.2% points at 22 wk and 0.8% points by 74 wk. Among medication-stable participants in the conventional diet group, the HbA1c reduction was 0.3% points at 22 wk, and 0.2% points at 74 wk.

The only statistically significant differences between diet groups for medical outcomes was that vegan dieters lost more weight than did the ADA diet group. See Table 2 for more details.

Table 2. 2004 Study on People with Type 2 Diabetes
	Vegan Dieta	Control Dietb
Calorie restriction	No	Yes
Number at baseline	49	50
Adherence at: 22 weeks 74 weeks	33 25	22 24
Calories Baseline 22 weeks 74 weeks	1745 1432sw 1366sw	1844 1458sw 1422sw
Carbohydrates (g) Baseline 22 weeks 74 weeks	202 244sb, sw 226sb, sw	210 170 165
Fiber (g) Baseline 22 weeks 74 weeks	11 35sb, sw 22sb, sw	11 18sw 13sw
Fat (g) Baseline 22 weeks 74 weeks	71 32sb, sw 34sb, sw	74 56 54
Saturated fat (g) Baseline 22 weeks 74 weeks	24 7sb, sw 8sb, sw	22 16 16
BMI (kg/m2) Baseline 22 weeks 74 weeks	33.9 31.8sb, sw 32.3sb, sw	35.9 34.3sw 34.8sw
Fasting glucose (mmol/L) Baseline 22 weeks 74 weeks	9.1 7.1sw 8.0sw	8.9 7.0sw 8.1
HbA1c Baseline 22 weeks 74 weeks	8.0% 7.1%sw 7.7%	7.9% 7.4%sw 7.8%
Total cholesterolc (mg/dl) Baseline 22 weeks 74 weeks	187 159sw 166sw	198 174sw 184sw
Triglycerides (mg/dl) Baseline 22 weeks 74 weeks	148 120sw 114sw	158 133 150
Diabetes medications 22 weeks74 weeks	43% reduced; 8% increased 35% reduced; 14% increased	26% reduced; 8% increased 20% reduced; 24% increased
aNo oils, fried foods, avocados, nuts, or seeds • Favored low glycemic-index foods • < 10% of energy from fat. bAmerican Diabetes Diet 2003 guidelines cMany subjects were on lipid-lowering medications and adjusted them during the study. sbStatistically significant effect between diet groups swStatistically significant effect within diet groups compared to baseline

The acceptability of the vegan diet was rated about the same as the acceptability of the ADA diet. Although the participants in the vegan group rated the difficulty of preparing the foods as higher, they reported less dietary restraint, probably due not to having to limit calories or count carbohydrates. Vegan dieters’ cravings for sweets and fats also decreased.

At week 22, 67% of the vegans were adhering to the diet. By week 74, it was down to 51%. This drop in adherence was probably influenced by the change in the program from weekly meetings for diet and cooking instruction, to bi-weekly optional meetings for weeks 23 to 74.

PCRM Glucose and Insulin (2005)

In 2005, PCRM researchers published results from a study performed on postmenopausal women who were overweight or obese, but did not have type 2 diabetes (6). This study put half the women on a very low-fat vegan diet of less than 10% calories as fat and no oils, avocados, olives, nuts, nut butter, or seeds. No mention was made of refined carbohydrates or high glycemic foods. This diet was compared to a National Cholesterol Education Program (NCEP) Step II Guidelines diet, which was less than 30% fat, less than 7% saturated fat, protein about 15% of calories, and less than 200 mg/day of cholesterol.

After 14 weeks on the vegan diet, fasting glucose decreased and insulin sensitivity increased. The NCEP Step II diet did not result in such changes. Table 3 includes more results from this study.

Table 3. 2005 PCRM Study on Postmenopausal, Overweight Women
	Vegan Dieta	Control Dietb
Calorie restriction	No	No
Number of participants	29	30
Calories Baseline 14 weeks	1774 1408	1762 1424
Carbohydrates (g) Baseline 14 weeks	232 274	231 221
Fiber (g)	30 sb	21
Fat (g) Baseline 14 weeks	62 18sb	58 31
Saturated fat (g) – 14 weeks	3sb	9
BMI (kg/m2) Baseline 22 weeks	33.6 31.5sb	32.6 31.2
Fasting glucose (mmol/L) Baseline 14 weeks	5.4 5.0sw	5.6 5.5
Insulin sensitivity Baseline 14 weeks	4.6 5.7sw	4.3 4.6
aNo oils, avocados, olives, nuts, nut butters, or seeds, < 10% fat bNCEP Step II diet sbStatistically significant effect between diet groups swStatistically significant effect within diet groups compared to baseline

In total, these studies suggest that a very low-fat, primarily whole foods vegan diet is an appropriate diet for people with type 2 diabetes, and as effective as an American Diabetes Association diet. I want to emphasize the primarily whole foods aspect of this diet. If someone is eating a vegan diet with a lot of refined grains and sugars (including breads, white rice, and juices) they might not realize many of these benefits.

White potatoes are the one whole, plant food that might be harmful for people with type 2 diabetes if eaten in more than small amounts. White potato starch has a chemical structure that makes the sugar quickly digested and absorbed, and white potatoes produce a large glycemic load and insulin response (see Table 4) (7).

Table 4. Definitions
Glycemic index – Measurement of how quickly glucose from a specific, solitary food is released into the blood. Glycemic load – Measurement of how much glucose from a specific, solitary food is released into the blood over the course of a certain amount of time (e.g., two hours). Insulin load – Measurement of how much insulin is released into the blood over a certain amount of time, in response to eating a specific food.
Glycemic index alone is of little use for preventing or treating chronic disease, as foods with very little sugar can have high glycemic indexes.

PCRM Pilot Study (1999)

The first was a pilot study of only 12 weeks and 11 participants, published in 1999 (1). The study participants all had type 2 diabetes.

The study put some of them on a (very) low-fat vegan diet. They could not eat added oils, sugars, or refined carbohydrates such as white bread or pasta. The diet was designed to have less than 10% of calories as fat. The control diet was designed to be less than 30% of calories as fat, and prohibited red meat but substituted fish and poultry. There was no limit on calories or portion sizes on either diet.

Despite not being required to limit calories, the vegan diet participants did reduce calories and lost weight. They also reduced fat and saturated fat, and their cholesterol and triglyceride levels went down.

The vegan dieters increased carbohydrates and fiber. Despite increasing carbohydrates, their fasting blood sugar levels went down, and their HbA1c (a measure of how much sugar has been in your blood over the previous three to four months) also went down.

The differences in medical outcomes between groups were that vegan dieters lost more weight and reduced blood sugar levels further. Many of the vegan dieters reduced or eliminated medications, while only one of the control dieters was able to. See Table 1 for more details.

Table 1. 1999 Pilot Study on People with Type 2 Diabetes
	Vegan Dieta	Control Dietb
Calorie restriction	No	No
Number of participants	7	4
Calories Baseline 12 weeks	1683 1409	1430 1526
Carbohydrates (g) Baseline 12 weeks	194 264	164 194
Fiber (g) Baseline 12 weeks	14 26	12 20
Fat (g) Baseline 12 weeks	64 17	50 53
Saturated fat (g) Baseline 12 weeks	19 5	18 14
Weight (lbs) Baseline 12 weeks	213 197sb	213 205
Fasting glucose (mmol/L) Baseline 12 weeks	10.7 7.8 sb	9.7 8.6
HbA1c Baseline 12 weeks	8.3% 6.9%	8.0% 7.0%
Total cholesterol (mg/dl) Baseline 12 weeks	203 179	215 190
Triglycerides (mg/dl) Baseline 12 weeks	188 165	203 164
Oral hypoglycemic agents	1 of 6 discontinued 3 of 6 reduced	0 of 4 reduced
Insulin	2 of 2 reduced	None were taking
Blood pressure medication	2 of 5 discontinued	1 of 4 reduced
Lipid-lowering	no changes	no changes
aNo oils, sugars, refined carbs, < 10% fat bNo red meat < 30% fat sbStatistically significant effect between diet groups.

White pastas, on the other hand, have a low glycemic load and insulin response (7) and although they are processed they might be about the same as many whole plant foods in terms of their affects on blood sugar. Spaghetti, especially if it hasn’t been overcooked, has a low glycemic and insulin load (8).

Presumably, the researchers from the PCRM studies picked such a low fat vegan diet, without nuts or avocados, because they wanted to guarantee weight loss and cholesterol improvements. However, most of the research on nuts has suggested at least moderate amounts are beneficial to heart disease and weight loss (9). It seems safe to eat moderate amounts of nuts if you have type 2 diabetes, such as 1 – 2 oz per day. A 2009 study of people with type 2 diabetes showed that 30 g of walnuts per day resulted in a greater reduction in insulin levels (13).

There has not been much research on avocados and their effects on diabetes or weight loss. It is reasonable to project that small amounts, about 1/4 an avocado per day, would be beneficial both for heart disease and weight loss. A half a cup of cubed avocados has 120 calories and 5 grams of fiber which is a reasonable amount.

There are concerns about the long-term effects of a diet with less than 10% of calories from fat. Fat is important for cell membranes, nerve tissue, and aids in the digestion of fat-soluble vitamins. Unsaturated fats keep HDL (good cholesterol) at healthy levels. Omega-3 fats are also important for reducing heart disease and nerve tissue. The average vegan eats about 27-29% (10) of their calories from fat. That much may not be necessary, but a diet of less than 20% fat might not be best over the long term. That doesn’t mean you have to get your fat from French fries, but nuts, avocados, and small amounts of olive and canola oil can be a healthy addition to the diet.

Conclusion

In conclusion, it appears safe to say that:

• A whole foods vegan diet is safe for people who have type 2 diabetes and is as beneficial, if not moreso, than a typical ADA diet.
• Vegans have a lower risk of type 2 diabetes than non-vegetarians.

In their 2017 Standards of Medical Care, the American Diabetes Association says, “A variety of eating patterns are acceptable for the management of type 2 diabetes and prediabetes including Mediterranean, DASH, and plant-based diets.”

More Information

• Vegan Menu for People with Diabetes from the Vegetarian Resource Group

References

Last updated March 2018

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2. A low-fat vegan diet improves glycemic control and cardiovascular risk factors in a randomized clinical trial in individuals with type 2 diabetes. Barnard ND, Cohen J, Jenkins DJ, Turner-McGrievy G, Gloede L, Jaster B, Seidl K, Green AA, Talpers S. Diabetes Care. 2006 Aug;29(8):1777-83. PubMed PMID: 16873779. (Medical outcomes at 22 weeks.)

3. Changes in nutrient intake and dietary quality among participants with type 2 diabetes following a low-fat vegan diet or a conventional diabetes diet for 22 weeks. Turner-McGrievy GM, Barnard ND, Cohen J, Jenkins DJ, Gloede L, Green AA. J Am Diet Assoc. 2008 Oct;108(10):1636-45. (Nutrient intakes at 22 weeks.)

4. A low-fat vegan diet and a conventional diabetes diet in the treatment of type 2 diabetes: a randomized, controlled, 74-wk clinical trial. Barnard ND, Cohen J, Jenkins DJA, Turner-McGrievy G, Gloede L, Green A, and Ferdowsian H. Am J Clin Nutr 2009;89(suppl):1S-9S. (Medical outcomes and nutrient intakes at 74 weeks.)

5. A low-fat vegan diet elicits greater macronutrient changes, but is comparable in adherence and acceptability, compared with a more conventional diabetes diet among individuals with type 2 diabetes. Barnard ND, Gloede L, Cohen J, Jenkins DJ, Turner-McGrievy G, Green AA, Ferdowsian H. J Am Diet Assoc. 2009 Feb;109(2):263-72. (Acceptability and adherence at 74 weeks.)

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