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The Science Behind the Ketogenic Diet

To fully understand how the ketogenic diet works, you must first understand how your body responds to different foods.

When you eat, your body automatically stores part of your meal as fat so it can be released later for energy in times when you’re not eating. When you eat carbs, insulin—a hormone that regulates blood sugar—is released to help take the carbs out of your bloodstream. Since your body can only use a certain amount of glucose (the simple sugar that carbs break down to with digestion) at a time, insulin sees that the excess sugars are shuttled straight into your fat cells for storage. Over time, excess weight, inactivity, and stress can cause your cells to become insulin resistant and make it harder for them to absorb glucose from your blood. If this happens, it becomes difficult to burn stored fat because you’re always refilling your fat cells with glucose.

But you’ve been told since eighth-grade nutrition class that carbs are the best source of energy. Guys who work out have to eat a lot of them, right? “Athletes generally have been told that they need to fuel their exercise with a carbohydrate-based diet,” says Dominic D’Agostino, Ph.D., an assistant professor in the Department of Molecular Pharmacology and Physiology at the University of South Florida. But if you do the math, it’s clear that carbs aren’t the best fuel.

“Your body can only have about 2,500 calories [worth] of carbohydrates stored, but you have almost an unlimited amount of fat for energy,” says D’Agostino. “Even a lean individual has 20 to 30,000 calories [worth] of fat.” In other words, by depriving your body of carbs, you can make it switch to burning fat, which provides more than enough energy for exercise. And when you start burning more fat, you’ll melt more of it away to reveal your abs.

When you begin eating fewer carbs and more fat, your body will slowly begin to produce ketones—energy molecules, like glucose, that are converted from stored body fat. When your body switches over to running mainly on energy from ketones rather than carbohydrate, you are officially in a state of ketosis—hence the name “ketogenic diet.”

The right ratio of macronutrients is essential to achieve ketosis. D’Agostino recommends you get 30% of your calories from protein, 65% from fat and 5% from carbs. (To find a calorie starting point for weight loss, multiply your body weight by 12.) So, a 180-pound man trying to lose fat will eat a total of 2,160 calories consisting of 162 grams of protein, 156 grams of fat and 27 grams of carbs.

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