Autumn 2005• Volume XIV, Number 4

PCRM’s Study Shows the Weight-Loss Power of a Low-Fat Vegan Diet

A new study shows that a low-fat vegan diet leads to significant weight loss, without requiring dieters to restrict calories, portion sizes, or carbohydrates, or even to exercise. In a controlled trial conducted by PCRM’s Neal Barnard, M.D., Gabrielle Turner-McGrievy, M.S., R.D., and Amy Joy Lanou, Ph.D., along with colleagues at Georgetown and George Washington universities, overweight women lost an average of 13 pounds in 14 weeks. They also trimmed two inches off their waistlines and an inch off their hips. The study is reported in the September 2005 American Journal of Medicine.

The participants ranged in age from 44 to 73. They had been randomly assigned to either a low-fat vegan diet or to a more conventional low-fat comparison diet based on the guidelines of the National Cholesterol Education Program. Because exercise can cause weight loss, they were asked not to alter their exercise patterns for the study’s duration. In weekly meetings, they planned out their menus, learned new recipes, and tried new products.

Both groups quickly adapted to their diets. One participant, whose diet change caused her to lose 45 pounds during and after the study, said the vegan diet is not really a “diet” at all. Rather, it is a different way of choosing foods. “I feel great,” she said. “I’m full of energy. I’m never hungry. I never feel deprived. I haven’t become cranky and irritable as I used to when I was ‘dieting.’ And I love to look at my plate and know that everything on it is truly good for me.”

Losing Weight without Exercise or Calorie-Counting

The control group lost just over a half-pound per week. The vegan group’s weight loss—about 1 pound per week—is similar to that seen with low-calorie diets, but occurred with no limits on calories, portions, or carbohydrates.

If they weren’t cutting calories, trimming portions, or exercising, then why did they lose weight? The most obvious answer is that their foods were filling, but naturally low in calories. Eliminating animal products cuts out every last gram of animal fat, and minimizing the use of oils means that the diet was very low in fat overall. Since fat has more than twice the calories of carbohydrate (that is, 9 calories per gram for fat, compared to only 4 for carbohydrate), cutting out fat means cutting out a lot of calories. And since plants are the only sources of fiber (animal products have none), a vegan diet built from vegetables, beans, fruit, and whole grains is naturally rich in fiber, which tames the appetite. A 2001 meta-analysis showed that every 14 grams of extra fiber in the diet cuts calorie intake by 10 percent. So a low-fat vegan diet tends to reduce calorie intake even when portions are not specifically restricted.

A Bigger After-Meal Burn

However, the PCRM study showed another reason for the weight loss: an increased after-meal calorie burn. In a laboratory at the George Washington University, the investigators tracked the participants’ calorie-burning speed by measuring how much oxygen they consumed minute by minute, and by how much carbon dioxide they exhaled while at rest. Then, each person drank a liquid meal, and her calorie-burning speed was measured over the next three hours. After 14 weeks on the low-fat vegan diet, the women had a noticeable jump in their after-meal calorie burn, while those in the control group had no significant change.

Now, why would a vegan diet do that? The answer came during a second laboratory visit. Each participant had a glucose tolerance test, which meant drinking a syrup containing 75 grams of glucose, followed by blood tests every 30 minutes. Before the study began, the testing had shown a predictable rise in blood sugar over the following three-hour period. However, after 14 weeks on the vegan diet, the tests showed a noticeable change. The participants again drank 75 grams of glucose, but the average blood sugar rise was much more modest. The reason, apparently, was an improvement in insulin sensitivity that caused their cells to be able to pull glucose out of the bloodstream much more quickly.

This latter result suggests that a low-fat vegan diet might also be beneficial in managing diabetes, which is the subject of PCRM’s latest study, whose initial results were recently presented at the American Diabetes Association’s 65th Scientific Sessions in San Diego and the American Association of Diabetes Educators in Washington, D.C.

At first glance, a vegan diet sounds like a challenge. But research participants rate the acceptability of the vegan approach very similarly to that of other therapeutic diets. And while typical diets demand cutting calories and leave the dieter with nothing to assuage hunger pangs, a low-fat vegan approach provides plenty of choices to make up for whatever is missing. Hunger is not part of the equation.

Bottom line: A low-fat vegan diet causes significant weight loss, even without exercise or any limits on calories or portions. The reason, apparently, is that it is naturally modest in calories and increases the after-meal calorie burn.

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