Ask the Guru

Q. How do ketogenic diets work?

A. Under normal conditions the body burns either glucose or fatty acids for fuel. It can also break down protein, or amino acids, to produce glucose if enough carbs aren’t available. Most tissues in the body burn either glucose or fatty acids, although a few, such as the brain, use glucose only. That’s why people get dopey when their blood sugar is too low. There is, however, an alternative fuel that becomes important under specific conditions, such as starvation or a very-low- carbohydrate diet: ketones, which are made in the liver from the breakdown of fatty acids. Hence the name ketogenic diets. Imagine that you restrict carbohydrates drastically. Hormones, such as insulin, go down, and blood free fatty acids increase. That shifts the body’s fuel use mainly to fatty acids, but don’t forget that your brain uses blood glucose only. Because you’re not eating many carbs, your body has to break down protein, from either the diet or other parts of your body, like your muscles, to manufacture glucose. If that were to continue and your body didn’t adapt, very bad things would happen, and you’d eventually die. Enter ketones, the products of fatty-acid metabolism. When they build up in the bloodstream to a sufficient degree, the body is said to be in ketosis, with many tissues using a combination of ketones and fatty acids for fuel. Even the brain, which normally just uses glucose, will start using ketones, and after a few weeks it may get 75 percent of its energy from ketones. With less need for carbohydrates, the body won’t break down as much protein. So basically, ketosis is a mechanism for sparing protein.

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