Understanding an epidemic.

How did two-thirds of the population wind up obese or overweight? There's no short answer, but there are several points to ponder.

Humans evolved with their fruit-and-vegetable-heavy diets from primate origins for over 2.5 million years. Only in the last 10,000 years--an evolutionary eye blink--has the human diet changed drastically from these fibrous, roughage-heavy foods. Because our species is capable of manipulating its environment as none other on the planet, we eventually altered our diets severely and unnaturally, causing certain health problems. Genetically, we are not much different from our ancient ancestors, but our diets are. The ability to mill and cultivate cereal grains caused the major substitution of fresh fruits and vegetables with flour-based food. Much less densely packed with essential vitamins and minerals, grains are also completely void of phytochemicals. These plant-based nutrients are powerful antioxidants, and their role in our health--from cancer prevention to faster recovery from workouts--is widely recognized today.

We severely process many of our foods for convenience, and the result is a dangerously increased intake of preservatives, flavor enhancers such as MSG, hydrogenated vegetable oils, artificial colors, and corn syrup. Diseases of affluence like obesity, diabetes, high blood pressure, and heart disease are the result.

As Marion Nestle, PhD, explains in her influential book, "Food Politics: How the Food Industry Influences Nutrition and Health," as the nation became more prosperous, the price of manufacturing food went down as food supplies went up. Competition for dwindling demand became fierce, and so food manufacturing budgets moved from producing to marketing food.

Our culture has since created many neighborhood environments in which highly palatable, incredibly fattening fast foods are accessible at low cost. Some 20% of public schools sell high-fat, brand name fast food. An astounding 58% of public elementary schools allow cola sales through vending machines. Nearly 94% of public high schools allow them. Combine these disconcerting trends with the loss of regular physical education programs in public schools and you have a recipe, so to speak, for an obesity epidemic among the nation's youth. And that's exactly what we're seeing.

(Runner's World Performance Nutrition for Runners by Matt Fitzgerald, 2006, Rodale Press, pp. 25-38; Food Politics by Marion Nestle, PhD, University of California Press, Berkeley, CA, 2002, 469 pp., $15.95; Foods and Beverages Sold Outside School Meal Programs, www.cdc.gov)