January 13, 2004 — The figure is staggering: More than 60 percent of Americans are overweight. Despite spending $33 million a year on weight loss products and services, we keep getting fatter. What are we doing wrong? And what's standing in the way of a healthier America?

Let's take a look at some of the hurdles we face and what we can do to encourage healthy changes in our schools, communities, workplaces and homes.
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Who's In Charge Here?
Scientists are only beginning to understand the complex biological systems that control what, when and how much we eat.

By following twins and adoptees in different environments, scientists have determined that genes can indeed play a major role in the development of obesity. Scientists say obesity is just as inheritable as height, and in fact shows more heritability than many disorders we generally think of as having a genetic basis. The Pima Indians of Arizona illustrate the effect of genes on weight. They have a so-called "thrifty gene" that enables them to store fat efficiently during times of scarcity. But, when food's plentiful, the Pimas get too fat.

	Our bodies demand that we eat when we are deprived of food.

To some extent, fat is critical to survival. Our bodies depend on fat as stores of energy, drawing upon these reserves in times of famine. Fertility is closely linked to fat. Women who have unusually low amounts of body fat often stop producing eggs. Two biological drives imperative for our species' survival — not starving and ensuring reproduction — combine to create a weight control system that is designed to protect more against weight loss than weight gain.

Whether we gain or lose weight depends in part on a complex network of interactions between various hormones. Hormone signals are constantly received and processed in the region of the brain called the hypothalamus. Leptin signals how much fat is in the body. Ghrelin indicates hunger and fullness. And peptide YY sends information about the intestines. Somehow these signals combine with higher cognitive thoughts, emotions and sensory information to help determine whether we eat or don't eat.

Friedman studied mice with a propensity toward obesity and learned that the larger mice were leptin defincient.

Leptin, in particular, is not only an important component of our weight-control system, but it is also one of the factors that make it hard to lose weight. Leptin is produced by fat (adipose) tissue. Increased body fat is associated with increased levels of leptin, which in turn tells our body to reduce food intake. A decrease in body fat (through dieting or starvation) results in lower levels of leptin. No matter how big you are, these comparatively lower levels of leptin will make you want to eat more and move less.

This physiological feedback loop protects both obese and lean people against losing weight and helps explain the powerful desire to eat more after one has lost weight. Even an obese body can be tricked into thinking it's on the verge of starvation. Just as we become thirsty and need to drink when we are dehydrated, our bodies demand that we eat when we are deprived of food. In many cases, this primal hunger wins out over our conscious desire to be thin.
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