VEGANISM: A WEAPON TO FIGHT THE OBESITY EPIDEMIC?

John Livesey PhD, Scientific Officer, Department of Endocrinology, Christchurch Hospital, New Zealand

We are all, collectively, getting heavier. Studies in a number of countries show that the prevalence of obesity is increasing. For example, 21% of adult Americans were found to be obese in 2001, up from 20% the year before1, and the incidence of obesity in young (7-15yrs) Australians tripled between 1985 and 19972. New Zealand is no exception, in 1997 a quarter of people aged 45-64 years were obese3. Being obese shortens lifespan by 6-7 years4; even being overweight shortens life by 3 years4. There is increasing alarm around the World at this trend of increasing obesity and in New Zealand an organization called Fight the Obesity Epidemic (FOE) was formed in 2002 to try to do something, particularly about junk food in schools.

Recent data5 from the Seven Countries Study raise the possibility that adopting a vegetarian, or even better, a vegan diet could assist in reducing the incidence of obesity. This study of, among other things, the subscapular skinfold thickness (a measure of body fatness), job-related physical activity and the diets of men in seven countries was conducted between 1958 and 1964. The countries were Finland, Greece, Italy, Japan, Serbia, The Netherlands and the USA. Not perhaps surprisingly it was found that the men with the most physically active jobs tended to have less body fat (R = -0.65), but more interesting were the relationships between diet and body fat. Firstly, and surprising to some, dietary fat intake was not significantly related to body fat. This is in line with other recent evidence that adopting a low fat diet is not of itself much help when trying to lose weight6.

The most interesting result from the Seven Countries Study from the vegetarian/vegan point of view was the finding that body fat was inversely correlated with the amount of fiber in the diet (R = -0.51)5. In other words, the more fiber in the diet, the lower the average level of body fat. This inverse association between dietary fiber and obesity has also been observed in the Framingham Offspring Study7 and in college students8. Studies of people eating a diet high in fruit, vegetables and whole grains show that they put on weight more slowly than those consuming meat and potatoes centred or white bread centred diets9 and in a 10 year study in Quebec an increase in whole fruit consumption was negatively correlated with body-weight changes10. Now fiber (the non-digestible part of food) is only found in plant-derived foods and not at all in animal products; suggesting firstly that the fewer animal products eaten the less the likelihood of becoming obese and, secondly, that plant foods with higher fiber levels should be chosen rather than refined or low fiber plant foods. Indeed, a British study found that non-meat-eaters weighed less than meat-eaters11.

The finding in these these observational studies of an inverse association between dietary fibre and obesity is strongly supported by an experimental study of the relationship between food composition and satiety12. The correlation of fibre content with satiety for 38 common foods was observed to be +0.46, whereas the correlations of the protein and fat contents with satiety were lower, +0.37 and -0.43 respectively. Carbohydrate content did not to correlate significantly with satiety.

The effectiveness of a high fiber diet for weight loss has been demonstrated in a randomised controlled trial where people who ate as much as they wanted of a high fiber diet (60g fiber/day) lost weight whereas the control group eating a low fiber diet (17g/day) did not lose weight13.

In the Seven Countries Study the consistently lowest skinfold thickness' were found in populations where the men consumed on average at 40-60 grams of fiber daily5. In contrast, the average daily fiber consumption in New Zealand is only 24 grams per day for men (and 18g/d for women)3. Thus it seems likely that obesity rates would drop if New Zealanders could be encouraged to at least double their fiber intake. Eating fewer animal products would be an essential part of this process. A high fiber diet is more bulky than a low fiber diet and this is presumably much of the reason why it is associated with lower body fatness - one fills up on fewer calories. In addition, more fibrous food tends to be digested more slowly, thus slowing the return of hunger. Intriguingly, increased dietary fiber also reduces the metabolizable energy content of both fat and protein in the diet14.

It is becoming evident that a high fiber diet has other health benefits apart from weight loss. For example, in a six week study of patients with type 2 diabetes, patients on the high fiber diet (50g/d) had significantly lower blood levels of glucose and cholesterol than patients on a more normal diet (24g fiber/d)15. Indeed, high consumption of cereal fiber seems to reduce the risk of contracting type 2 diabetes in the first place16, and further, a high intake of whole-grain breakfast cereals appears to reduce the risk of dying from all causes, at least in men17. Recent large studies suggest that a diet high in fibre reduces the risks of bowel cancer18, breast cancer19 and coronary heart disease20. Other studies indicates that a high fiber intake may reduce the rate of progression of atherosclerosis21 and reduce estrogen levels in breast cancer patients22.

How do we increase the fiber in our diets? Firstly, minimize foods that contain no fiber at all. These include all animal products and in addition, highly refined plant foods, most notably sugar. Now avoiding animal products entirely is no problem for vegans, but zero consumption of food containing refined sugar would be unacceptable to most people (including your author!). Nevertheless, the less the better. Particularly avoid consuming sugary foods containing no fiber at all such as soft drinks and sweets.

Secondly, minimize consumption of partially refined plant foods. These include white flour, white bread, white pasta and white rice.

Thirdly, cook with wholemeal flour and eat wholemeal bread, wholemeal pasta, brown rice and other whole grains such as oats, barley, amaranth, quinoa and flaxseed (linseed). Wholemeal flour contains about three times as much fiber as white flour and brown rice about four times as much fiber as white rice.

Fourthly, eat lots of high fiber foods such as peas, beans, lentils, green leafy vegetables, fresh fruit and flaxseed. In fact most vegetables, apart from very starchy ones like potatoes, are excellent sources of fiber. Even potatoes (with skins of course) make a modest contribution to fiber intake unless they are combined with fat (as chips).

You may complain that wholemeal bread, pasta and baked goods are less yummy that the white versions. This though I think is part of the reason why high fiber diets are less obesogenic - they are not quite so more-ish so we are not so likely to stuff ourselves with unnecessary calories. A bit of the hair shirt here I'm afraid. There is some good news though - since cocoa is about one third fiber, chocolate cake contains more fiber than plain cake.

In summary, consuming a high fiber vegan diet should assist in weight control, although of course exercise, a healthy combination of foods and probably some caloric restraint are also required.

References

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