Army researchers believe they have found an explanation for many of the sudden deaths that occur among black military recruits during strenuous basic training.
The culprit appears to be sickle-cell trait, the abnormal sickle-cell anemia gene that is carried by more than 2 million black Americans.
The Army report provides the first strong evidence that carriers of the sickle-cell trait may be at risk of serious health problems under extreme physical stress. Until now it had been thought that carriers who do not actually have the inherited blood disease did not suffer adverse health effects themselves.
The study concludes that black recruits with sickle-cell trait had a risk of sudden unexplained exercise-related deaths that was 28 times higher than black recruits without the trait and 40 times higher than all other recruits. Their actual risk of death was 1 in 3,200.
"This is the first compelling evidence that sickle-cell trait is related to disease," said Dr. John A. Kark of the Walter Reed Army Institute of Research and the Armed Forces Institute of Pathology in Washington.
But Kark, the principal author of the study, cautioned that the researchers had found only "a statistical association" between the trait and the deaths, not a specific explanation for why the deaths occurred. Nor was the study able to establish exactly how much exercise or what type of exercise was most likely to be responsible.
The new findings, published in today's New England Journal of Medicine, have major potential implications for the 8% of blacks in the United States who carry the sickle-cell trait.
But Kark and other sickle-cell experts said the report was no reason to discriminate against trait carriers in athletics or the workplace or for undue alarm among such individuals.
"The risk is fairly tightly curtailed to very stressful exercise and heavy labor," Kark said in a telephone interview. "I would not expect that the ordinary athlete or ordinary laborer would expose themselves to nearly that level of effort."
"This report must be kept in perspective," warned Dr. Louis W. Sullivan of Morehouse School of Medicine in Atlanta in an editorial published in the medical journal along with the study.
"There is no justification for discrimination in employment, job promotions, insurability or other aspects of daily living," Sullivan wrote. "For the overwhelming majority of these persons, all available evidence suggests that sickle-cell trait is a benign condition that, with rare exceptions in special circumstances, has no adverse effect on health."
For the moment, Kark and Sullivan said, people with sickle-cell trait who engage in strenuous physical activities, particularly at high altitude or in extreme heat, may wish to take precautions against overexertion. These include the maintenance of good hydration, avoidance of prolonged exposure to extreme heat and the gradual conditioning of muscle groups.
Sickle-cell anemia is so called because of the characteristic abnormal shape of red blood cells that resembles a sickle. It afflicts about 50,000 black Americans, who have inherited two copies of the abnormal sickle-cell gene. It also afflicts a small proportion of whites of Mediterranean descent.
The distorted red blood cells contain defective molecules of the oxygen-carrying protein hemoglobin. These red blood cells can change their shape when deprived of oxygen, clogging the circulation and cutting off normal oxygen supplies. A wide range of life-threatening complications can result, from low blood counts and recurrent pain to blood clots and infections.
The far larger number of sickle-cell carriers, who have inherited only one copy of the abnormal gene, remain largely free of complications except, for example, occasional blood in the urine.
Nevertheless, researchers have debated for years conflicting reports about the potential vulnerability of sickle-cell carriers to extreme physical conditions, such as flying in unpressurized aircraft or playing organized football.
In the new study, apparently the largest of its kind ever conducted, Army researchers reviewed the medical records for all deaths during the period 1977 to 1981 that occurred among 2.1 million enlisted personnel ages 17 to 34. Previous studies, involving 5,000 or fewer individuals, had been too small to detect a condition responsible for a death rate of about 1 in 3,000.
Around the time of their deaths, all blacks had blood tests to determine if they had sickle-cell trait; blacks in the Air Force, Marines and Navy and some in the Army had also been tested at entry into the service.
In total, 28 of the 62 natural deaths during basic training occurred suddenly and were not explained by other causes. Of these 28 deaths, 12 occurred among the estimated 37,000 black recruits with sickle-cell trait.
By comparison, there were 16 unexplained deaths, including five in blacks, in the more than 2 million other military recruits.