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Running Doc: How much fat do you need in your diet?

Dear Running Doc:

I seem to be reading more and more in athletic magazines lately about fat as a good source of energy. Most nutritionists believe we should eat as little as possible, yet some sports nutrition researchers evidently think we need more to compete well. They both can’t be right. Who is?

Robin G., Bayside, NY

Thanks, Robin. Even some sports doctors recommend fat for replacing lipids in muscles post-race! Most, if not all of us, are not big fans of fat, though the notion seems to have taken on a life of its own. Fat is what it’s always been: a useful fuel for endurance exercise that’s already so plentiful in your body it’s a virtually limitless fuel.

My guess is that fats fraudulent comeback started with a study done about fifteen years ago on cyclists who, after stuffing down a high-fat diet for four weeks, were able to exercise as much as they did previously on a normal diet, but burned much less precious muscle glycogen in the process. Since glycogen is the fuel of choice for your muscles, everyone wants to conserve it. So if extra mayo on the cheeseburger could help shield the supply for a while, why not?

Plenty. First, no group of lab cyclists exempts athletes from the fat-cholesterol-coronary risk equation. Second, the study was in a sense loaded. The tests were done at relatively low exertion levels where the body is happy to burn fat as fuel anyway. If things had been stepped up closer to race pace, the extra-fat athletes might have run out of gas much sooner.

The answer to better long-term endurance is hoarding glycogen, not fat-packing. And if you never race longer than about 90 minutes, you don’t even have to do that: Everyone’s muscles have at least that much glycogen squirreled away. But not many of us do a triathlon, or long cycling race, or marathon, in one-and-a-half hours. We have to pack in as much additional glycogen as we can, usually by carbo-loading (not over-loading please!), and then use it sparingly for fuel.

Your goal is not fat-loading but fat-burning, and there’s a way to do that better. Fat is used for fuel in lower-intensity exercise, say at less than 70 percent of your body’s maximum oxygen capacity. But training exclusively at that level doesn’t help the process go any better: run slow, race slow, burn fat, start it all again.

What you want is for fat to also help you at higher speeds, which means working on a boost to your oxygen capacity, or VO2 max. After all, the higher the “max” from which the 70% is figured, the more fat energy you’ll use in racing. An elite marathoner may zip around that 26.2-mile course at a relatively modest 75% of a lofty VO2 max, and nearly a third of the fuel burned will be fat.

Your second strategy should be to work on your anaerobic threshold — the exercise intensity at which your body, desperate for air, stops using anything but its most efficient fuel, glycogen. At this point you’re no longer burning fat, and you’re also producing lactic acid, a fatigue-enhancing by-product.

Books have been written on boosting your VO2 max and anaerobic threshold, so we’ll steer clear of that here. Just remember: Fat is not, and never was, a training aid. It’s a fact of life. All you need to do is condition yourself to use it. Your body will use what it needs for muscle recovery. Eat your 10-15% and that is plenty.

Enjoy the ride.

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Lewis G. Maharam, MD, FACSM is one of the world’s most extensively credentialed and well-known sports health experts. Better known as Running Doc™, Maharam is author of Running Doc’s Guide to Healthy Running and past medical director of the NYC Marathon and Rock ‘n’ Roll Marathon series. He is Medical Director of the Leukemia & Lymphoma Society’s Team in Training program. He is also past president of the New York Chapter of the American College of Sports Medicine. Learn more at runningdoc.com.

Want your question answered in this column? Write to running doc at runningdoc@nydailynews.com.

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