Updated July 24, 2006

Getting to Know Ketones

by Richard M. Weil, M.Ed., C.D.E.

People with diabetes, particularly those with Type 1 diabetes, have been at least vaguely aware of the word ketones for a long time. With the recent resurgence of popular interest in low-carbohydrate diets, however, just about everyone seems to be talking about ketones these days. But does anyone really know what a ketone is? Are they a danger to your health (as in diabetic ketoacidosis), or a sign that you have lowered your carbohydrate intake enough to cause weight loss (as some people who follow low-carbohydrate diets believe)?

What are ketones?
Ketones are end-products of fat metabolism in the body. That is, they are formed when fat is burned for energy by the muscles. Chemically, they are acids known as ketone bodies, and there are three types: beta-hydroxybutyric acid, acetoacetic acid, and acetone. But you don’t have to be a chemist to understand what role they play in the body.

To get to know ketones, it’s helpful to understand how your body burns fuel. A simple analogy is that of an automobile. For a car engine to run, the engine must burn fuel (gasoline), and when the fuel is burned, exhaust (carbon monoxide) is created. The carbon monoxide is the end-product of gasoline combustion.

Your body also has an engine that must burn fuel to operate. The engine is muscle, and the fuel is fat, carbohydrate (glucose), and, in certain conditions, protein. When fat is burned, the “exhaust” is ketones, and when glucose is burned, the “exhaust” is lactic acid.

Fat is more desirable as a fuel than glucose because there are more calories in a gram of fat (9 calories per gram) than there are in a gram of glucose (4 calories per gram), so you get more energy per gram of fat burned. In a sense, you could call fat a high-test fuel. But there is one catch to burning fat: To burn it efficiently, with little “exhaust,” you have to burn glucose at the same time. If you don’t have glucose available for fuel (because you are on a low-carbohydrate diet, for example), you will form ketones when your muscles burn fat.

For most people, the ketones that form as a normal product of fat burning and weight loss are nothing to be concerned about because they are simply burned for energy by the body, and any excess are passed out of the body in the urine. In fact, while the brain normally uses glucose for energy, during exercise—and particularly during long-distance events like marathons, when glucose reserves may drop very low—the brain can use ketones for energy. Your liver makes extra ketones when glucose reserves are low so that your brain has enough energy.

For people with Type 1 diabetes, however, having measurable amounts of ketones in the urine (or blood) is cause for concern. Ketones in a person with Type 1 diabetes may be a sign that his diabetes is out of control, he is ill or has an infection, or he is under extreme stress. Because above-normal levels of ketones in the blood can lead to diabetic ketoacidosis, a life-threatening condition, people with Type 1 diabetes who have measurable ketones in their blood or urine should speak with their diabetes educator or doctor promptly.

Low-carbohydrate diets
Low-carbohydrate diets are sometimes called “ketotic” diets because they cause the body to burn mostly fat for energy (since the intake of carbohydrate is so low), which in turn causes the formation of ketones. Some people who follow low-carbohydrate diets periodically test their urine for ketones to see if fat-burning is indeed taking place. However, people who are losing weight on any diet might have a trace of ketones in their urine since a person who is losing weight is almost certainly burning and losing fat.

With all the talk of ketones, some people have gotten the mistaken impression that ketones are a sort of magic bullet that melts fat from the body, no matter how much a person eats. That’s simply not the case. Ketones are only by-products of the metabolism of fat and are markers that show that you are burning fat. They have no active role in burning fat or weight loss. In fact, ketone levels in people who are on low-carbohydrate diets are just barely above baseline, indicating they have no role in producing weight loss. The reason people lose weight on low-carbohydrate diets is not because of ketones; it’s because they have cut out a large food group from what they eat, and as a result, they end up eating fewer calories.

Richard Weil is an exercise physiologist and a consultant to St. Luke’s-Roosevelt Hospital Center in New York City.

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