Slave-ship hypothesis of hypertension.

Slave-ship hypothesis of hypertension

Conditions on slave ships traversing the Atlantic Ocean during a 350-year period beginning in the 16th century may be responsible for the increased prevalence of high blood pressure among blacks in the United States, according to a hypothesis by Clarence E. Grim of the Charles R. Drew University of Medicine and Science in Los Angeles. After studying rates of hypertension among blacks on both sides of the Atlantic, as well as historical data about the slave trade, Grim concludes that voyages on the ships could have resulted in "survival of the fittest," with those better able to retain salt more likely to live through the lack of food and water and the dehydrating seasickness and diarrhea. This more efficient use of salt, however, has subsequently caused problems among slave descendants with salt-rich diets, suggests Grim.

The unproven hypothesis depends partly on the complicated question about which factors influence a person's high blood pressure. Studies conducted by various groups on twins and adopted children indicate that blood pressure is largely set by genetics--data that Grim calls "very powerful evidence that your blood pressure is set by something other than your environment."

He says his earlier studies among blacks in Indiana also showed that blood pressure levels and the ability to excrete sodium are "strongly inherited." For example, when black patients were given salt, they had a faster rise in blood pressure at a lower level of salt than did white patients. On the basis of these results and his ongoing study of black twins in the Los Angeles area, Grim concludes that 60 to 80 percent of the variability in blood pressure seen among individuals is related to heritable factors, with the remainder due to environmental factors like stress and diet.

Grim's conclusions conflict with other theories that higher blood pressure among U.S. blacks is primarily caused by stress, or that their African ancestors retained more salt and water in order to survive hot, humid weather. But Grim says several comparisons show that blacks in the United States, Jamaica and Belize have consistently higher blood pressure than those in Africa, indicating changes occurred after slaves were removed from Africa. He also discounts the possibility that living conditions after the slaves were sold would be a major evolutionary factor, saying such conditions were too inconsistent to account for such widespread hypertension.

In order to prove his hypothesis, Grim says he will expand his studies to include African blacks matched with U.S. blacks on the basis of economics, education and other characteristics. Also included will be studies of blood pressure patterns among black families whose ancestors arrived after the slave period. Among the health benefits of confirming that high blood pressure is an inherited condition among blacks could be the identification of a genetic marker, says Grim. He suggests that such a marker should alert physicians and lead to earlier changes in diet as a preventive measure.