Potential Explanations For The Antiobesity Effect Of A Vegetarian Diet

While data indicate that vegetarians may weigh less than other population subsets, it does not necessarily follow that it is their avoidance of meat that is responsible. Vegetarians are also more likely to adopt other healthy life-style habits, such as regular exercise and reduced alcohol consumption, that also impact their lack of obesity. There is some evidence, however, that eating more vegetables and abstaining from meat does play a significant role in their leaner profiles. A study by Kahn and others of 79,000

individuals followed for over 10 years showed that the one habit that seemed to prevent the development of abdominal obesity was eating vegetables. Those who consumed 19 or more servings of vegetables per week did not succumb to an increase in central girth, while those who ate meat (beef, pork, or lamb) more that seven times per week did.26

Vegetarian diets can be lower overall in total energy intake. Also, the macronutrient composition and sources of macronutrients tend to be different in vegetarian diets, as compared to the omnivore diet. They are overall higher in dietary fiber and complex carbohydrates and lower in protein and fat. Their nutritional differences may partially account for the potential anti-obesity effect of vegetarian diets.

A. Dietary Fiber and Total Energy Intake

The fiber intake of the average American adult amounts to around 15 gm per day,27 while a modest 1800 kcal Western vegetarian diet contains about 45 gm of fiber. In more primitive vegetarian regimens, fiber content has been reported to be as high as 80 gm per day.Wolever and Jenkins have suggested that diets naturally high in fiber content may be beneficial in maintaining weight or preventing weight gain.28 Since the presence of non-nutritive fiber in fruits and vegetables increases the bulk of the ingested food, a vegetarian diet may positively impact obesity due to its generally lower energy density. On a volume-for-volume basis, vegetarian fare contains fewer calories than omnivorous diets. Thus, even when the volume of food intake in a vegetarian diet is greater, the total calories ingested may be less. Levin et al.29 compared vegetarians with omnivores and reported that the average weight of the vegetarians was significantly lower than that of the omnivores (60.8 kg vs. 69.1 kg), but that the vegetarian diet supplied a significantly greater number of calories than the non-vegetarian diet (3,030.5 cal/day vs. 2,626.8 cal/day).

Fiber apparently also affects short-term food intake. Levine et al.30 in a study of the consequence of a high fiber breakfast indicated that fiber reduced subsequent hunger, resulting in less snacking at mid-morning breaks and a lessened food intake at lunch time. Unfortunately, data from Delargy et al.31 reveals that most subjects consuming a high fiber breakfast compensated for these initially lower calories by eating more later in the day, so that the total daily energy intake remained unchanged. It can be speculated that including fiber-rich foods at the other meals may allow the appetite depression effect to continue throughout the day. Even small amounts (5 grams) of pectin, a soluble type of fiber found mainly in fruit, increases satiety when fed with other food.32 This may indicate an important role for increased fruit consumption as a way to curb appetite.

Besides decreasing caloric density and curbing appetite, fiber may have an additional obesity-preventive benefit. Baer et al.33 have demonstrated that fiber interferes with the absorption of fat and protein from the intestinal tract and, while the effect is modest (less than 4% of the energy intake), it will provide a significant advantage over time. On a 2500 kcal daily diet, this would amount to the equivalent of a 100 kcal per day reduction. During the course of a year, this means that over 36,000 kcal would be eliminated, theoretically preventing about a 10-pound (at 3500 kcal per pound) weight gain.

B. Carbohydrate Intake

Vegetarian diets tend to contain significantly more carbohydrates, particularly complex carbohydrates. A high carbohydrate meal may actually speed up the resting metabolic rate, while a high fat meal seems to have little effect on metabolism. There is some evidence that overfeeding with carbohydrates — but not with fat — provokes an insulin-mediated ther-mogenesis that acts to retard weight gain.34 Toth and Poehlman35 found that young male vegetarians had an 11% higher resting metabolic rate than non-vegetarians in spite of similar energy intakes. The major dietary difference between the two groups was an increased ingestion of carbohydrate and a reduction in fat intake by the vegetarians.

C. Protein Intake

Campbell36 has proposed that excess protein intake may be responsible for excess weight. Vegetarian diets tend to provide a much lower protein intake than diets that include meat products. The China study, involving dietary records on thousands of Chinese, indicated that protein intake paralleled body fatness. Campbell has shown that animals on low-protein, low-fat diets burn extra energy through very slight increases in thermogenesis. The energy is thus released as heat, instead of becoming body fat. His studies in rats demonstrate that, when protein intake is low, thermogenesis increases and thus more of the ingested energy is required for heat production. Animals on a low protein, high carbohydrate diet gained less body fat even though the actual caloric intake was higher. Interestingly, animals on the low protein diet also voluntarily exercised more.

Slattery et al.,37 in a study of over 5000 young adults, also supports a protein connection. Protein intake was positively related to BMI in all age groups, while carbohydrate ingestion showed a negative association with body fatness. Rolland-Cachera et al.38 have shown that children who are fed high protein diets before the age of 2 are more likely to become obese in later life. Since elevated protein intake may occur more easily when meat is included in the diet, vegetarian children might have an advantage. These results are somewhat ironic, in view of the recent fad promoting high protein diets for weight loss.

D. Dietary Fat Intake

Although studies on the effect of dietary fat on obesity are somewhat equivocal, in general, a high fat diet promotes obesity to a greater extent than an isocaloric low fat intake.39 As body fat increases, so too does the percentage of food energy reported as derived from fat.40 When there is covert manipulation of dietary fat, subjects on a high fat program consistently eat more than those on a low fat regimen.41 This effect may be particularly pronounced in obese subjects,42 and extends even to children. Maffeis et al.,41 in a study comparing obese with non-obese children, found that, in spite of a similarity in reported overall energy intake, obese children ingested significantly more fat. It is also much easier to consume extra energy on a high fat diet since fat does not induce as potent a satiety signal or produce a compensation effect on subsequent energy intake as do diets rich in carbohydrates or proteins.44

Restricting fat, however, is certainly not the total answer to the problem of obesity. In fact, Hervey-Bernio in a recent study indicated that those on a calorie-restricted program lose twice as much weight as those who simply restrict their fat intake.45 The quality and type of fat may also be important. The P/S ratio of the diet may be a consideration in weight loss. A small, randomized crossover study by van Marken Lichtenbelt et al.46 using six subjects found that the resting metabolic rate was elevated by 3.6% in subjects after a 2-week diet with a P/S ratio of 1.67, as compared with a low P/S ratio diet of 0.19.

The recent identification of the appetite regulating hormone leptin also provides some intriguing clues. Plasma concentrations of leptin tend to parallel the amount of adipose tissue, providing an inhibitory feedback that tends to reduce energy intake when body fat increases. Cha et al.47 has documented that leptin levels (in rats at least) also respond to dietary cues, with polyunsaturated fats being more leptingenic than saturated fats. This may give vegetarians an advantage in appetite control even when dietary fat levels are similar.

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