Ask the Doctors: How does the keto diet work?

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Q: Can you please explain the keto diet? A bunch of my friends are on it, but it seems like they're all doing it a different way.

A: Your friends are among the millions of Americans who have jumped onto the keto bandwagon, making it the most popular diet in the United States right now.

The basic idea is simple: You cut carbohydrates to about 10 percent of your total food intake, typically fewer than 50 grams per day, and sometimes as low as 20 grams per day. This changes the way your body obtains energy. Instead of burning glucose, also known as blood sugar, your body is forced into Plan B — burning stored fat. This is a metabolic state called ketosis. The name comes from compounds produced by the liver, known as ketone bodies, which the body burns for energy when glucose, its favorite energy source, is not available.

Any eating plan that causes this shift from burning glucose to burning stored fat is a ketogenic diet. You can check whether you are in ketosis with special test strips, available at your local drugstore, which detect the presence of ketones in your urine.

The high-fat, low-protein, low-carbohydrate formula may be the most widely accepted version of a keto diet right now, but it's not the only one of its kind. Depending on your age, some of you may remember the Stillman diet from the 1960s, which eliminated carbs completely and focused instead on animal proteins such as beef, chicken, fish, eggs and cottage cheese. Part of the secret to the rapid weight loss on that diet? Ketosis. Ditto for the Atkins diet, which uses ketosis in certain phases of its eating plan, as do the paleo and Zone diets, both of which restrict carbs.

Today's keto diet, in which up to 90 percent of calories come from fat, dates back to the 1920s. Sometimes referred to as "classic keto," it was originally developed to help manage epilepsy, but soon fell out of favor with the advent of effective anti-seizure drugs. Other versions of the keto diet vary the percentages of fat, protein and carbohydrates, the trio of macronutrients our bodies require.

In addition to rapid weight loss, people in ketosis report decreased appetite. This makes sticking to such a restrictive way of eating a bit easier. The diet is also associated with improved insulin metabolism. However, while many people show improvements in blood lipids levels, the diet can raise levels of LDL cholesterol — the so-called "bad" cholesterol — in some people. Other challenges include headache, fatigue and irritability, particularly at the start of the diet. Many keto adherents also report dealing with chronic constipation, as well as food boredom.

A ketogenic diet can be a good way to jump-start weight loss. That means a focus on high-quality whole foods, including olive oil, nuts, avocados and fatty fish. But since this diet eliminates entire food groups, it makes getting needed nutrients a challenge. We think it's wise to check with a registered dietician, or with your family doctor, for advice and guidance.

Eve Glazier, M.D., MBA, is an internist and associate professor of medicine at UCLA Health. Elizabeth Ko, M.D., is an internist and assistant professor of medicine at UCLA Health.Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.

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