vegan diet and type 2 diabetes

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June 2007 | Volume 52 | Issue 2

evidence-based ranking system for carbohydrates based

on their effect on blood glucose levels in the first two hours

after eating). Typical meals were:

breakfast – porridge (oatmeal) with cinnamon and raisins,

a slice of melon, and perhaps vegetarian sausage

lunch – a bowl of split-pea soup, beans and rice, or

vegetable chilli

dinner – pasta with tomato sauce, vegetable stew, or a

bean burrito, with plenty of vegetables.

The diet did not limit calories, carbohydrates or portions,

and derived approximately 10% of energy from fat, 15%

from protein, and 75% from carbohydrate.

Standard diet

In contrast, the ADA diet limited protein to between 15%

and 20% of overall intake; saturated fat to below 7%;

carbohydrates and monounsaturated fats to between 0%

and 70%; cholesterol not exceeding 200 mg per day; and

Testing the evidence

Based on evidence from these studies, my research team

put plant-based diets to the test. After a series of favourable

preliminary studies, and with the support of the US National

Institutes of Health, we conducted a randomized trial com-

paring a low-fat vegan diet to a comparison diet based on

the guidelines of the American Diabetes Association (ADA).

Working with the George Washington University, USA, and

the University of Toronto, Canada, 99 adults with type 2

diabetes took part in the 22-week study.

Type 2 diabetes is considerably less

common among vegetarians, compared

to people who habitually eat meat.

Vegan diet

The vegan diet had three components: it excluded all

animal products (meat, eggs, dairy products), was low in

fat, and favoured foods with a low glycaemic index (an

The vegan diet excludes all animal products and favours foods with a low glycaemic index.



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A low-fat vegetarian diet is no

more difficult to follow than

other therapeutic diets.

tion in HbA1c alone – without considering the capacity of

the diet to reduce cholesterol and blood pressure.

People on the vegan diet reduced their LDL cholesterol

by 21% – significantly more than the 9% drop that was

seen with the ADA diet. Although most participants were

on blood pressure-lowering medications and had normal

blood pressure levels, those on the vegan diet showed an

average 4% reduction in systolic blood pressure and 5% in

diastolic pressure. Despite the fact that there was no caloriclimit in the vegan group, weight reduction was 5.8 kg,

compared with 4.3 kg in the ADA group.

People on the vegan diet reduced

their HbA1c

and LDL cholesterol

significantly more than people

following the ‘standard’ diet.

Explaining the mechanisms

Recent studies suggest that insensitivity to insulin is related

to the accumulation of fat within muscle cells (intramyocel-

lular lipid), which is apparently due to a genetically based

reduction in mitochondrial activity – conversion of organic

materials into cellular energy, and other metabolic tasks. This

lipid accumulation seems to be influenced by diet. High-fat

diets appear to inhibit the genes required for mitochondrial

action in skeletal muscle, leading to the accumulation of fat

within the muscle cells. Even a few days on a high-fat diet

is sufficient to increase intramyocellular lipid content. Just

as a key cannot open a lock if someone has maliciously

pushed glue into the mechanism, insulin’s ability to bringglucose into a cell, via intracellular signalling, is impaired

by the build-up of intracellular fat.

Researchers at the Catholic University in Rome, Italy, demon-

strated that intramyocellular lipids can rapidly be reduced,

albeit using a method not recommended for routine use.4

In 2002, eight people underwent gastric bypass surgery,

reducing the stomach to a pouch about the size of an egg

and severing the intestine, attaching the lower portion of the

was individualized, based on body weight and plasma

lipid concentrations. Participants in the ADA group with a

BMI above 25 kg/m2 were prescribed reductions in energy

intake of between 500 kcal and 1000 kcal.

For people in both groups, physical activity was kept con-

stant, and medications were not changed, except in cases

of hypoglycaemia.

Contrasting results

Among people whose medications remained constant, thedrop in HbA

1cwas 0.4% in the ADA group and 1.2% in

the vegan group (from 8.0% to .8% during the 22-week

study). This reduction is greater than that seen with typical

oral diabetes medications.

To put this in perspective, the landmark UK Prospective

Diabetes Study demonstrated that a 1% drop in HbA1c for

people with type 2 diabetes lowers the risk of eye or kidney

complications by about 37%. This is the effect of the reduc-



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small intestine directly to the tiny stomach pouch. Because

fat is absorbed in the upper part of the small intestine, the

procedure results in a massive malabsorption of fat. The

result was not only weight loss (from an average of 137 kgto 104 kg over months), but also an 87% reduction in

intramyocellular lipid – and even though the people re-

mained overweight, their insensitivity to insulin had largely

disappeared.

A low-fat vegan diet approaches the same goal by rear-

ranging the menu, rather than the gastrointestinal anatomy.

Devoid of animal fat and most vegetable oils, the diet

greatly reduces fat intake, presumably leading to a reduc-

tion in intramyocellular lipids. Supporting evidence comes

from a case-control study at Imperial College, London, UK:intramyocellular lipid concentrations were about 30% lower

in a group of vegans, compared with omnivores matched

for age and body weight.5

The lipid-lowering effect of a low-fat vegan diet stems from

the absence of animal fat and cholesterol, and the abun-

dance of soluble fibre. This aspect is particularly important,

given that cardiovascular complications are the primary

cause of death and disability in people with diabetes.

Transient increases in triglyceride levels may occur in peo-

ple who consume refined carbohydrates – foods that have

undergone a process to remove high-fibre content, such

as white rice, white bread, sugary cereals, and pasta and

noodles made from white flour. The diet used in our study

led to the opposite result – a significant drop in average

triglyceride levels. We attribute this to the fact that the

vegan diet was not only low in fat, but also high in fibre

and low-glycaemic-index foods.

The weight change is evidently due to the fact that the re-

duced fat intake and increased fibre intake lower the energydensity of the diet. The reduction in caloric intake is similar

to that achieved with the ADA diet, but is accomplished

without the hunger that caloric limitations typically cause.

In order to reassure healthcare providers who might ques-

tion the acceptability of the diet, we have studied this

aspect with quantitative measures of diet acceptability in

several clinical populations, finding that a low-fat vegetar-

ian or vegan diet is no more difficult to follow than other

Neal Barnard

Neal Barnard is Adjunct Associate Professor of Medicine

at George Washington University School of Medicine,

Washington DC, USA. He is President of the Physicians

Committee for Responsible Medicine (www.pcrm.org).

References1 Kuzuya T. Prevalence of diabetes mellitus in Japan compiled from literature.

Diabetes Res Clin Pract 1994; 24 (Suppl): S15-21.

2 Snowdon DA, Phillips RL. Does a vegetarian diet reduce the occurrence

of diabetes? Am J Publ Health 1985; 75: 507-12.

3 Jenkins DJA, Kendall CWC, Marchie A, et al. Type 2 diabetes and the

vegetarian diet. Am J Clin Nutr 2003; 78 (Suppl): S10-.

4 Greco AV, Mingrone G, Giancaterini A, et al. Insulin resistance in morbid

obesity: reversal with intramyocellular fat depletion. Diabetes 2002;

51: 144-51.

5 Goff LM, Bell JD, So PW, et al. Veganism and its relationship with insulin

resistance and intramyocellular lipid. Eur J Clin Nutr 2005; 59: 291-8.

therapeutic diets. In fact, just as, for many people, giving

up smoking is easier than attempting to moderate cigarette

use, simply avoiding certain foods can be easier than trying

to moderate their intake.

Recommendations

To assist people in beginning a low-fat vegan diet, we

recommend two steps:

Exploration

For 1 to 2 weeks prior to beginning the diet change, a

person can try out recipes, aiming to find several that

are appealing and easy to prepare. People who favour

convenience foods or who dine at restaurants will explore

their options, which are more numerous than they are likelyto imagine a priori.

Test drive

Once a person feels ready, the next step is to begin a

3-week period during which every meal is drawn from those

selected in step one. After 3 weeks, the person is likely to

have lost weight, seen an improvement in blood glucose,

and, most importantly, begun to experience a taste change,

rather like the adjustment that occurs with the switch from

whole milk to non-fat varieties – most people readily adapt

and have little desire to return to previous habits.

vegan diet and type 2 diabetes

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