Editor’s Note (02/08/18): Scientific American is re-posting the following article, originally published February 19, 2010, in light of the 2018 Winter Games which begin on February 9 in PyeongChang, South Korea.

Having won six medals in his career, Seattle-based speed skater Apolo Ohno stands to make U.S. Winter Olympic history if he wins another one in upcoming short-track competition—the 1,000-meter race this weekend or the 5,000-meter relay on February 26. In various reports, Ohno has said that he's in the best physical shape of his life, weighing five kilograms less than he did for the 2006 Games in Torino, Italy, and nine kilograms less than he did for the 2002 Salt Lake City Olympics. Ohno is now 65 kilograms of almost pure muscle: only 2.8 percent of his body consists of fat.

Elite athletes, of course, are expected to be slimmer than the rest of us. The average amount of body fat in the U.S. is 22 percent for men and 32 percent for women, although most experts believe a healthier body-fat content is 15 percent for men and 22 percent for women, according to The Ultimate Fit or Fat, a book by nutritionist Covert Bailey. Ohno's fat level, though, is down there even for an athlete.

So how low is too low? After all, fat is crucial for normal physiology—it helps support the skin and keep it lubricated, cushions feet, sheaths neurons, stores vitamins, and is a building block of hormones.

Marina Mourtzakis, an assistant professor at the University of Waterloo in Ontario who specializes in nutrition, exercise and metabolism, gives ScientificAmerican.com the skinny on athletes' fat.

[An edited transcript of the interview follows.]

How do athletes achieve such lean physiques?

It takes a long time to achieve and maintain these low levels of body fat. It really comes down to balance. Energy in equals energy out. With increased energy expenditure and lower caloric intake, you can tip the scale to reduce weight.

Is it safe for them to get down to 2 percent body fat?

Athletes have to take in adequate amounts of protein to maintain muscle mass, and they still have to take in adequate amounts of carbohydrate to maintain optimal training intensities. If they do this properly, they can maintain heavy workout sessions and lose fat without compromising their performance.

So 2 percent would be a safe lower limit if athletes eat right. What sort of diet should they have?

Meat, fish, poultry and dairy are good protein sources. But they also need to take in a fair amount of carbohydrates. When you're training at high intensities, you're burning more carbs. When those stores deplete, your body has to use something else—this could be fat, but it could be protein. People don't always appreciate how difficult it is to lose fat without losing protein. For athletes to maintain a high level of performance and low body fat, it means they have to a have a really good balance in their diet in order to maintain their health.

Should athletes continuously strive to lose body fat?

Athletes should not be "dieting" three to four months prior to a major competition. They should be weight stable by the time the competition date arrives. Any changes in weight, if an athlete has that as a goal, would happen much earlier in training to avoid problems with performance and potential injury.

Is it physically desirable to be below normal body fat levels?

It really depends on the sport an athlete is involved with. For example, snowboarders probably have a normal amount of fat, whereas long-distance and endurance athletes [like speed skaters, cross-country skiers and biathletes] likely have less.

Several techniques can estimate body fat composition: Skin-fold measurements; bioelectric impedance tests, which use a small electrical current to estimate the amount of water in the body and then extrapolate a fat figure; and tests based on displacement of water (hydrodensitometry) or air (the "Bod Pod"). How good are they?

They're not very accurate. Dual-energy x-ray absorptiometry [DXA—the same technology used to measure bone mineral density], magnetic resonance imaging and computed tomography are highly precise—[each] can measure fat to within 2 percent accuracy. DXA is probably most common in research, because MRI and CT [which can also determine body composition] are so expensive and inaccessible.

Editor's note (2/19/10): A clarification was added to a question after posting to address a comment.

Editor's note (2/22/10): Professor Mourtzakis wanted to emphasize the risks associated with an extremely low body fat: "While it is possible for some athletes to reach 2 percent body fat, I would certainly not support this approach for athletes. Achieving this range presents health risks, including increased risk of infection and injury. This approach often supports unhealthy eating behaviors and patterns that are reflective of disordered eating behaviors. It is especially important for athletes to maintain adequate intake of all macronutrients (that is, protein, carbohydrates and fats) to achieve optimal performance and maintain their health."