Ask the Doctors: The skinny on keto and paleo diets

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Q: I've heard that the keto or paleo diets are good for weight loss and controlling blood sugar. Now I'm reading that a new study says they're bad for your heart. What's actually true?

A: You're referring to the ketogenic diet, also known as keto, and the Paleolithic diet, or paleo. Both are low-carb approaches to eating that are quite popular right now.

The paleo diet, also known as the caveman diet, draws from the foods that were presumably available to our Paleolithic ancestors. Since the Paleolithic era lasted 2.5 million years and ended about 12,000 years ago, the specifics of those diets are basically guesswork. But the core tenet of the paleo diet is that if a caveman didn't eat it, neither should we. The result is a low-carb diet heavy on meat, with a limited amount of high-fiber fruit and vegetables. Since the Paleolithic era predates agriculture, things like grains, beans, cereals, legumes and dairy products, each of which require farming, don't make it onto the plate.

The keto diet is even more restrictive. Its roots go back to the 1920s, when researchers found that a diet containing a very low proportion of carbohydrates and a very high proportion of fat appeared to reduce the number or severity of seizures in people with epilepsy. The goal of the diet is to put the body into a metabolic state known as ketosis. In ketosis, the body has depleted its stores of carbohydrates, its first go-to for energy, and instead begins to burn fat. This results in the creation of metabolic byproducts, including certain fatty acids, which are believed to ease the symptoms of epilepsy. Another side effect of ketosis is steady weight loss. The keto diet is heavy on protein and fat, including meat, fish, sausages, bacon, oils, butter, cheeses, nuts and seeds. The diet allows high-fiber carbohydrates, but in limited amounts.

It's true that studies have associated both the paleo and keto diets with improved blood glucose control. They each also result in better appetite control and speedy weight loss. However, we don't yet have enough reliable data about whether these diets promote long-term weight loss, or what health risks may be attached over time. The study you mentioned in your letter found a sharp increase in a certain blood biomarker linked to heart disease risk in participants who followed the paleo diet. At the same time, they had an increase in high-density lipoprotein, or HDL, the so-called "good cholesterol."

Another study tied high-meat, low-carb diets to a drop in the diversity of the gut microbiome. An increase in the number of harmful bacteria was also observed. A separate analysis of a number of existing studies also found links between meat-heavy diets and an increase in cardiovascular risk. Bottom line — we're still sorting things out.

We understand the allure of the keto and paleo diets, but our advice continues to be a focus on whole rather than processed foods, lean proteins, a wide variety of fresh fruits and vegetables, whole grains, nuts and legumes, and healthful oils. Good for the heart, good for the gut and good for the planet.

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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