Each chocolaty piece has the same 400 calories, 70 g of carbs, and 20 g of fat. Your friend eats it and stays slim. When you eat it, it goes straight to your hips. It's as though your body has a mind of its own and wants you to get fat.
You've probably beaten yourself up about this. Your friend must work out more—or indulge less. But the fact is, she may have a biological advantage.
It comes down to insulin. Some people secrete more of it than others, and those who secrete more store more calories as fat—and get fatter.
Imagine the fuel gauge in your car. On the right is the F, but it stands for fat instead of full. The E on the left means energy instead of empty. If the needle points to the right—toward the F—it means your body sends more of the calories you eat into storage as fat, rather than having your muscles use them for energy. When the needle points in the other direction—toward the E—you burn more calories as fuel. You have plenty of energy for physical activity, but little will be stored as fat.
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What determines where the needle points? In part, your parents. The genes you're born with play a role in both how much insulin you secrete and how your body responds to the insulin—and therefore how much fat you store and how much you burn.
But you can't blame every extra pound on genetics. What you eat matters too.
The Fat Cycle
Whether you're born predisposed to get fat is beyond your control. But whether your "fat genes" become more active is determined by the foods you eat—specifically, carbohydrate-rich foods. Not all of us get fat when we eat carbs, but for those of us who do get fat, carbs are largely to blame.
Here's why. Refined carbs—which include starchy foods such as bread and pasta, as well as desserts and sodas—cause more insulin to circulate in your body. The more you secrete, the more likely it is that your cells and tissues will become more resistant to that insulin. The pancreas compensates by pumping out still more insulin, which moves the needle on the fuel gauge toward storage. And the result is a vicious fat cycle.
Sensitivity to insulin changes with time—and in response to your diet. If you have been eating too many fattening carbs and sugars, you'll tend to become insulin resistant as you age. The result: weight gain.
In fact, the conventional wisdom that we fatten as we move into middle age because our metabolism slows probably has this cause and effect backward. The more likely scenario is that our muscles become increasingly resistant to insulin as we get older. This sends more of the energy we consume into fat, leaving less available for fuel. Our cells now generate less energy. What appears to be a cause of fattening— the slowing of our metabolism—is really an effect. You don't get fat because your metabolism slows; your metabolism slows because you're getting fat.
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Menopause only compounds the effect. That's because of an enzyme called lipoprotein lipase, or LPL, which sticks out from the membranes of cells. LPL pulls fat out of the bloodstream and into the cells. The female sex hormone estrogen squelches this LPL activity—and therefore works to decrease fat accumulation. But when your estrogen levels drop, the LPL becomes more active—and you store more fat.
The End of Dieting
Whether you want to stay slim or lose excess fat, there is a solution, and it isn't cutting calories in the extreme or exercising to exhaustion. The answer is to stay away from refined carbs and starches. The sweeter the carb-rich food or the easier it is to consume and digest, the more likely it is to make you fat. My break-the-cycle solution:
Avoid "bad carbs." These include refined sugar, flour (bread, cereal, pasta), and starches (potatoes, rice, corn)—the foods that have the greatest effect on our blood sugar and insulin levels.
Reach for "good carbs" instead. Those found in leafy greens such as spinach and kale are bound up with indigestible fiber and take longer to enter the bloodstream, so blood sugar doesn't spike.
And watch out for fructose. It's the carb we convert to fat most readily. Try to avoid anything with added sugar or high-fructose corn syrup—from candy and soda to salad dressing and ketchup.
Adapted from Why We Get Fat: And What To Do About It By Gary Taubes. Copyright 2011 By Gary Taubes. Published By Arrangement With Alfred A. Knopf, An Imprint Of The Knopf Doubleday Publishing Group, A Division Of Random House, Inc.