Athletes who steer clear of beef and load up on carbohydrates to achieve peak performance may be missing the nutritional mark. The same could be true for wrestlers, gymnasts, and ballerinas who may eat sparingly to maintain a low body weight.
These groups may get only a fraction of the recommended intake of zinc, for which beef is the major source in the U.S. diet.
Trace elements such as iron and copper are known to play an integral role in the ability of body cells to generate energy, or do work. But few studies have looked at the effect of limiting zinc intake, says Henry C. Lukaski, who is at ARS' Grand Forks Human Nutrition Research Center in Grand Forks, North Dakota.
So he recruited 12 athletic men in their twenties to help him assess zinc's impact on the body's performance during exercise. They ate one diet containing 18 milligrams of zinc per day—more than the recommended amount—and another containing only one-fifth the recommended level (3 mg/day) for 9 weeks each.
Lukaski focused on a zinc-containing enzyme, carbonic anhydrase, in red blood cells. More than 20 years ago, scientists reported the enzyme was reduced during zinc deficiency. Carbonic anhydrase helps red blood cells pick up metabolic waste—carbon dioxide—and drops it off in the lungs to be exhaled.
This exchange helps maintain a proper chemical environment in muscle cells for muscle contraction and the energy production so necessary for an athlete's peak performance. "If this exchange is sluggish, the athlete pays the price," says Lukaski.
After each diet period, the men were tested on a cycle ergometer to measure their peak work capacity. Not surprisingly, they had significant drops in peak oxygen uptake and peak carbon dioxide output after the low-zinc diet. Their respiratory exchange ratio dropped as well, indicating energy production during peak exercise was not up to snuff.
The low-zinc diet also depressed these measurements when the men cycled at 75 percent of peak capacity for 45 minutes. Lukaski found that the enzyme's activity was significantly lower after the low-zinc diet. He is now using rats to study other zinc-containing enzymes thought to be involved in energy expenditure.—By Judy McBride, Agricultural Research Service Information Staff.
Henry C. Lukaski is at the USDA-ARS Grand Forks Human Nutrition Research Center, P.O. Box 9034, University Station, Grand Forks, ND 58202; phone (701) 795-8355, fax (701) 795-8395.
"Athletes Need Zinc" was published in the July 1999 issue of Agricultural Research magazine.