We now know why fiber-rich foods can have such a powerful effect on our weight.

The evidence for the role of fiber in weight control started with so-called ecological studies.

These involve comparing population averages.

And researchers noted that populations with extraordinary fiber

intakes tend to have negligible obesity rates. For example, the average Pima Indian in Mexico,

eating their traditional high fiber diet centered around the three sisters, corn, beans, and squash,

is normal weight. But the average Pima on U.S. reservations has obesity. The problem with

dealing with population averages is that we don't know if the individuals eating the higher

fiber diets are themselves necessarily the ones protected from obesity. What do we see in

cohort studies where individuals in their diets are followed over time? A cohort study of overweight youth found that the amount of fiber found in a single half cup

daily serving of beans, about six grams, over about a two-year period, was associated with a

profound 25% difference in abdominal obesity.

In about the same time frame in middle-aged women, each 2-gram increase in daily fiber was associated with a weight decrease of about a pound.

The postpartum period seems to be a critical time for women to be at risk of retaining the weight put on during pregnancy.

A study of hundreds of new moms followed for the first five months found that an adequate fiber intake appeared to increase obesity risk by 24%.

And it's not just women. A cohort that included tens of thousands of men followed for years, concluded that an increase in fiber consumption of just 10 grams a day may prevent about 10% of weight gain within the population.

Overall, the evidence from these kinds of observational studies is strong that increasing

consumption of dietary fiber with fruits, vegetables, whole grains, and legumes across the

life cycle is a critical step in stemming the epidemic of obesity.

These studies can control for non-diotary influences such as physical activity by equipping people with gadgets to measure their movement.

But there may be uncontrolled confounding dietary factors.

Think about the list of high fiber foods.

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