David Ludwig, professor of pediatrics at Harvard Medical School and of nutrition at the School of Public Health, who specializes in endocrinology and obesity, rejects the popular belief that overeating causes weight gain. Instead, he asserts, the process of getting fatter causes people to overeat. Even though many biological factors—genetics, levels of physical activity, sleep, and stress—affect the storage of calories in fat cells, he points out that only one has a dominant role: the hormone insulin. “We know that excess insulin treatment for diabetes causes weight gain, and insulin deficiency causes weight loss,” he says. “And of everything we eat, highly refined and rapidly digestible carbohydrates produce the most insulin.”
Ludwig argues that eating a diet high in refined sugars and processed carbohydrates leads to a yo-yo metabolism. When people eat high-glycemic processed fare such as baked goods and white bread, he says, insulin levels spike, causing hormone-sensitive lipase—an enzyme needed for the transfer of triglycerides from blood lipoproteins into tissues—to be turned off. This causes more calories to be stored in fat cells as opposed to the blood, leading the brain to think that the body is hungry.
“Insulin is the ultimate fat-cell fertilizer,” Ludwig says. “When fat cells get triggered to take in and store too many calories, there are too few for the rest of the body—that’s what the brain perceives. We think of obesity as a state of excess, but biologically it’s a state of deprivation, or the state of starvation. The brain sees too few calories in the bloodstream to run metabolism, so it makes us hungry. It activates hunger and craving sensors in the brain, and slows down metabolism.”
This combination of rising hunger and slowing metabolism is a recipe for weight gain, he adds, and explains why only a very small proportion of people on low-calorie diets can keep weight off in the long term.Source: Harvard Magazine
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