Active people who get too little zinc in their diets may run out of juice sooner than they should, new research suggests.

The study found that when 14 active young men followed a 9-week diet low in zinc, their cardiovascular fitness dipped in comparison to their performance during 9 weeks on a zinc-fortified diet.

The reason appears to be related to an enzyme in the body called carbonic anhydrase, which relies on zinc for proper functioning. The carbonic anhydrase enzymes in red blood cells help the body expel carbon dioxide, with the demand rising substantially during exercise.

When men in the new study followed a low-zinc diet, these enzymes were less active. The result was that, during exercise, their bodies were less efficient at "getting rid of carbon dioxide," explained study author Henry C. Lukaski, a researcher with the U.S. Department of Agriculture's Human Nutrition Research Center in Grand Forks, North Dakota.

Zinc is an essential mineral that stimulates activity in many of the body's enzymes -- proteins that act as catalysts for the biochemical processes that make the body run. Zinc is already considered an important player in such vital functions as immune system defenses, wound healing and normal growth and development in children.

The new findings point to the importance of adequate zinc intake in a person's capacity for exercise, according to Lukaski.

But that doesn't mean active people should load up on zinc pills.

The best way to get enough zinc is through food, Lukaski stated. Red meat and oysters are rich in the nutrient, while chicken and pork have lesser amounts. People who shun red meat, Lukaski said, should be careful to get enough zinc from sources such as fortified cereals and beans.

The recommended zinc intake is 11 milligrams (mg) per day for men and 8 mg for women; because high levels of the mineral can be toxic, experts advise that adults take no more than 40 mg per day.

The 14 men in Lukaski's study were in their 20s and 30s and regularly active. The low-zinc diet provided 3.5 mg of the mineral per day through food, while the high-zinc diet comprised the same foods, but with a daily 15-mg zinc supplement.

Study participants followed one diet for 9 weeks, during which they underwent two types of exercise tests on a stationary bike. Then, after a 6-week break, they followed the other diet and took the same exercise tests.

Based on the men's exercise performance, the low-zinc diet "made it much more physiologically challenging to perform," Lukaski explained. While on the diet, the exercisers' heart rates climbed, whereas the efficiency of their breathing declined. During one of the tests -- a 45-minute endurance ride -- four of the men had to stop.

Blood tests showed that the low-zinc diet lowered the men's stores of the mineral, and diminished the activity of the carbonic anhydrase enzymes in their red blood cells.

Some past studies have found associations between low zinc intake and poorer muscle strength, a tendency to fatigue easily and diminished speed during exercise. This study, according to Lukaski, suggests that dampened carbonic anhydrase activity may explain these effects.

SOURCE: American Journal of Clinical Nutrition, May 2005.