Individuals with sickle-cell trait -- a genetic condition present in about 8 percent of America's black population -- are at increased risk of sudden death during or after vigorous exercise, according to a study of such deaths among military recruits during basic training.

Sickle-cell trait is usually considered harmless to the carrier, although two carriers can produce a child with sickle-cell anemia, a serious blood disorder.

Examining all sudden, unexplained deaths among more than 2 million new recruits, both black and white, over a five-year period, a team of military researchers found that the risk of such deaths in black recruits with sickle-cell trait was 28 times higher than in black recruits without the trait and 40 times higher than in all other recruits.

The frequency of such deaths in black recruits with the trait was still low -- about one death per 3,200 recruits -- but the risk increased with age, according to the study, which was published in today's New England Journal of Medicine. The researchers were unable to determine why carriers of the trait suffered an increase in exercise-related deaths.

Some of the deaths were apparently from cardiac arrest during physical activity and others occurred days to weeks afterward as a result of heat-related illness or muscle breakdown that developed during exertion, according to Dr. John A. Kark, a hematologist at Walter Reed Army Institute of Research and the study's principal author.

The red blood cells of an individual with sickle-cell anemia contain an abnormal form of hemoglobin, the protein that transports oxygen within the circulation. The abnormal protein, called hemoglobin S, tends to clump together within cells when the blood's oxygen content falls or its acidity rises. The clumped protein distorts the normally disc-shaped red blood cells into a crescent or sickle shape and makes them more rigid and fragile, producing clogged vessels and a variety of other complications.

Carriers of sickle-cell trait do not suffer from the disease because their blood contains normal hemoglobin, known as hemoglobin A, as well as hemoglobin S. But the presence of the abnormal protein makes their blood cells vulnerable to sickling if they lack adequate oxygen for long periods, as might occur during the vigorous workouts of basic training, according to an accompanying editorial by Dr. Louis W. Sullivan, president of Morehouse School of Medicine in Atlanta.

"The obvious question is, is there a possibility of increased sickling in a vital center" of the body, such as the heart's pacemaker, which might produce death through cardiac arrest, he said in an interview.

He said he did not know whether the increased risk of sudden death was caused by the presence of sickle hemoglobin in the blood cells of carriers of the trait or by some unrecognized genetic condition that was inherited along with the trait.

Kark said the risk of sudden death among recruits with sickle-cell trait increased with age, and was about eight times higher in 26-year-old recruits than in 17-year-olds. No relationship with age was seen in exercise-related deaths among other recruits. Kark speculated that a minor kidney abnormality, common in carriers of the trait, which impairs the body's ability to conserve water and which worsens with age, could have contributed to the deaths.

Kark said individuals with sickle-cell trait should be especially cautious about undertaking vigorous exercise when starting a new sport or getting used to a hot climate or high altitude. He said fluid intake and limiting activity in hot, humid weather also are extremely important.

Sullivan said he found the study convincing but noted that the increase in sudden deaths seen during basic training does not mean that carriers of sickle-cell trait are at any increased risk during normal daily activity. He emphasized in his editorial that the new findings do not justify discrimination by employers or insurance companies against carriers of sickle-cell trait.

In the study, researchers from the Armed Forces Institute of Pathology, Walter Reed Army Institute of Research and Bethesda Naval Medical Center reviewed all natural deaths that occurred in recruits during basic training from 1977 through 1981.

Of the 42 sudden deaths, 40 were associated with exercise. Twelve of these were sudden, unexplained deaths occurring in black recruits with sickle-cell trait. In five of the 12 cases, medical examiners' reports attributed death to unexplained cardiac arrest and in the others to heat stroke, heat stress or muscle breakdown due to exertion, the study said.

The researchers suggested that the unusual physical demands of basic training -- requiring frequent, prolonged exercise in environmental conditions likely to produce dehydration and heat exhaustion -- put much greater stress on the circulatory system than most carriers of sickle-cell trait would be likely to encounter as civilians.

The report said that in 1982, in response to initial information from the study, the Army Medical Corps alerted military training centers to the possible medical risk for carriers and encouraged modification of basic training regimens to allow more gradual conditioning and to reduce exercise during hot weather.