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Type 2 Diabetes and the Vegan Diet

by Jack Norris, RD | Last updated February 2013

The only prospective study measuring rates of diabetes in vegans, the Adventist Health Study 2, found them to have a 60% less chance of developing the disease than non-vegetarians after two years of follow-up. Previously, a cross-sectional report from the Adventist Health Study-2 showed vegans to have a 68% lower rate of diabetes than non-vegetarians. A number of clinical trials have now shown that a vegan, or mostly vegan, diet can lower body weight, reduce blood sugar, and improve other parameters for type 2 diabetes.

Vegan Menu for People with Diabetes from the Vegetarian Resource Group

Contents

Observational Studies

Two analyses from the Adventist Health Study 2 and one meta-analysis have looked at rates or risk of diabetes in vegetarians.

2011 Prospective Analysis from Adventist Health Study 2

In 2011, the first prospective study measuring the type 2 diabetes rates of vegans was released. It followed participants of the Adventist Health Study 2 for two years and found vegans to have the lowest rates of diabetes of any diet group (14). The study did not differentiate between type 1 and type 2 diabetes, but it's safe to assume most cases were type 2. Results are in Table 6.

Table 6. Prospective Diabetes rates in Adventist Health Study-2.
  Vegan Lacto-Ovo Pescoa Semib Non-Veg
Number 3,545 14,099 3,644 2,404 17,695
Odds ratio .38 (.24-.62) .62 (.50-.76) .79 (.58-1.09) .49 (.31-.76) 1.00
aVegetarian except for fish. | bAte meat more than once a month but less than once a week. | Adjusted for age, BMI, race, gender, education, income, TV watching, sleep, alcohol, physical activity, and smoking.

This study indicates that vegans had a 60% lower risk of developing diabetes as non-vegetarians. A separate analysis was done for black participants only and black vegans had a relative risk of .43 (.25-.74) compared to non-vegetarians.

Black ethnicity was associated with an increased incidence of diabetes (1.36, 1.09-1.70) as was age, male gender, and BMI. A higher income and more sleep were associated with a lower incidence of diabetes.

The authors cautioned that because diabetes rates were determined by reporting a diagnosis of high blood sugar or diabetes, and not by having each participant's blood sugar measured, cases of diabetes might not have been detected.

As to why vegetarians had lower rates of diabetes, the authors wrote, "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."

2009 Cross-Sectional Analysis from Adventist Health Study 2

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 Adventist Health Study-2.
  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

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.

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, oddly enough:

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 2004-05 Study

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.

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 weeks

74 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. Favor low glycemic-index foods. <10% of energy from fat. | b American Diabetes Diet 2003 guidelines. | aMany 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.

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.

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 .


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 | b NCEP Step II diet | sbStatistically significant effect between diet groups. | swStatistically significant effect within diet groups compared to baseline.

PCRM 2005 Study on Glucose & Insulin

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 butters 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.

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 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.

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 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 is of little use for preventing or treating chronic disease, as foods with very little sugar can have high glycemic indexes.

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 heat 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.

Note that the participants in the PCRM studies were provided with vitamin B12 supplements. All vegans should follow the daily recommendations for vegans.


References

1. Nicholson AS, Sklar M, Barnard ND, Gore S, Sullivan R, Browning S. Toward improved management of NIDDM: A randomized, controlled, pilot intervention using a lowfat, vegetarian diet. Prev Med. 1999 Aug;29(2):87-91.

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

3. Turner-McGrievy GM, Barnard ND, Cohen J, Jenkins DJ, Gloede L, Green AA. 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. J Am Diet Assoc. 2008 Oct;108(10):1636-45. (Nutrient intakes at 22 weeks.)

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

5. Barnard ND, Gloede L, Cohen J, Jenkins DJ, Turner-McGrievy G, Green AA, Ferdowsian H. 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. J Am Diet Assoc. 2009 Feb;109(2):263-72. (Acceptability and adherence at 74 weeks.)

6. Barnard ND, Scialli AR, Turner-McGrievy G, Lanou AJ, Glass J. The effects of a low-fat, plant-based dietary intervention on body weight, metabolism, and insulin sensitivity. Am J Med. 2005 Sep;118(9):991-7.

7. Holt SH, Miller JC, Petocz P. An insulin index of foods: the insulin demand generated by 1000-kJ portions of common foods. Am J Clin Nutr. 1997 Nov;66(5):1264-76. PubMed PMID: 9356547.

8. Foster-Powell K, Miller JB. International tables of glycemic index. Am J Clin Nutr. 1995 Oct;62(4):871S-890S. Review.

9. Mattes RD, Kris-Etherton PM, Foster GD. Impact of peanuts and tree nuts on body weight and healthy weight loss in adults. J Nutr. 2008 Sep;138(9):1741S-1745S. (Asbstract)

10. Davey GK, Spencer EA, Appleby PN, Allen NE, Knox KH, Key TJ. EPIC-Oxford: lifestyle characteristics and nutrient intakes in a cohort of 33 883 meat-eaters and 31 546 non meat-eaters in the UK. Public Health Nutr. 2003 May;6(3):259-69.

11. Fraser GE. Associations between diet and cancer, ischemic heart disease, and all-cause mortality in non-Hispanic white California Seventh-day Adventists. Am J Clin Nutr. 1999 Sep;70(3 Suppl):532S-538S.

12. Tonstad S, Butler T, Yan R, Fraser GE. Type of Vegetarian Diet, Body Weight and Prevalence of Type 2 Diabetes. Diabetes Care. 2009 Apr 7. [Epub ahead of print]

13. Tapsell LC, Batterham MJ, Teuss G, Tan SY, Dalton S, Quick CJ, Gillen LJ, Charlton KE. Long-term effects of increased dietary polyunsaturated fat from walnuts on metabolic parameters in type II diabetes. Eur J Clin Nutr. 2009 Aug;63(8):1008-15. Epub 2009 Apr 8. PubMed PMID: 19352378.

14. Tonstad S, Stewart K, Oda K, Batech M, Herring RP, Fraser GE. Vegetarian diets and incidence of diabetes in the Adventist Health Study-2. Nutr Metab Cardiovasc Dis. 2011 Oct 7. [Epub ahead of print] Link

15. Kahleova H, Matoulek M, Malinska H, Oliyarnik O, Kazdova L, Neskudla T, Skoch A, Hajek M, Hill M, Kahle M, Pelikanova T. Vegetarian diet improves insulin resistance and oxidative stress markers more than conventional diet in subjects with Type 2 diabetes. Diabet Med. 2011 May;28(5):549-59. | Link