Why don’t diets help?
One problem is that when you lose weight on a diet, the amount of energy you expend at rest — part of the need-based system — goes down. Although dieting affects both the need and reward systems, it’s need that can’t be overcome. Built into every one of us, fat or thin, is a metabolic system that drives us to eat when the brain perceives that we don’t have enough energy stores. It’s this perception that can be skewed in an obesity-prone person, whose brain is less sensitive to signals to stop eating.
When you fast, even for as little as 12 hours, levels of the hormone leptin drop like a shot, which is a way for the brain to know the body is hungry. When leptin levels go back up, the brain says, “I’m full.” But obesity-prone people are less sensitive to high levels, and don’t get the message to stop until levels are really, really high.
We saw this when we compared obesity-prone animals to obesity-resistant animals. We fed them both a high-fat diet. Both took in more calories than usual, but after two or three days, something clicked in the brains of the obesity-resistant animals, and they lowered their caloric intake back down to where it was when they were on a low-fat, lower-calorie diet. But it took the obesity-prone animals three or four weeks to make that correction, even though their leptin levels had gone way up.
People probably fall somewhere in between; they don’t overeat quite as much, and a good part of the calories that they put on contribute to their need-based increase in weight, which is what you can’t overcome by changing palatability. In fact, I believe a lot of fad diets work initially, at least, by being boring. People lose interest in eating. But eventually, they start craving their favorite foods, and they can’t stay on the diet anymore.
Tags: obesity
July 10th, 2009 at 3:17 pm
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