Races react differently to dietary salt

African-American and Caucasian adolescent girls handle sodium and calcium differently, which may help explain why the races have different rates of hypertension and osteoporosis, according to research at Purdue University.

In a study published in the April edition of the Journal of Clinical Endocrinology and Metabolism, nutrition researchers discovered Caucasian girls lose more calcium in their urine than African-American girls, but both races lose calcium at an accelerated rate when they consume a high-salt diet.

"While we found a racial difference in calcium retention in adolescents, we also confirmed that blacks retain more sodium on a high-salt diet than whites," said Connie Weaver, distinguished professor and head of Purdue's Department of Foods and Nutrition. "This proves that salt is processed differently in the races, but too much salt in the diet reduces bone density in both races."

One out of four Caucasians will be diagnosed with osteoporosis, a bone-loss disease that costs Americans $14 billion a year in health care. The disease strikes one out of 10 African-Americans, but studies show they are more susceptible to hypertension, Weaver said.

"Sodium causes water retention, which leads to high blood pressure, and that could be related to the high prevalence of hypertension in adult blacks," Weaver said. "So even though salt intake is less critical to blacks with respect to building bones, we still have to be concerned about how sodium affects heart health."

The research results were based on figures from Purdue's Camp Calcium. The summer camp, funded by the National Institutes of Health since 1990, is designed to investigate various aspects of calcium metabolism in adolescent girls and boys.

Thirty-five campers were selected to participate in two, 20-day summer camps separated by two weeks. While housed on campus, the girls ate a controlled diet that provided certain amounts of calcium and other nutrients under 24-hour supervision.

The camp participants included 22 African-American girls and 13 Caucasian girls between the ages of 10 and 15. Berdine Martin, research associate and Camp Calcium project director, said the age range was important because, as an earlier Camp Calcium study found, calcium absorption is highest just after a girl's first menstrual cycle.

"Hormones affect the way the body retains calcium," Martin said. "Almost 40 percent of adult peak bone mass is acquired during adolescence, so we can directly affect the way a body ages by regulating calcium at this age."

During the first session, half of the girls received a low-sodium diet and half received a high-sodium diet. The diet was reversed during the second session.

Researchers found:

• Adolescent girls need to have the equivalent of four cups of milk a day to take full advantage of the time when their bodies can develop peak bone mass.
• Adolescent girls keep 25 percent of the net calcium they consume, but by the time they are young women, it drops to 5 percent.
• Caucasian adolescent girls lose more calcium through their urine than African-American girls, resulting in lower bone density.

"Salt intake affects how the body uses calcium at a critical time of bone development in young girls, but in whites more than in blacks," Weaver said. "This is something that should be easy to monitor in order not only to ensure healthy bones in adults, but also to reduce health-care costs of our aging populations."

Weaver says the strength of this research is the fact that the diets were strictly controlled and monitored in a clinical setting. While a longer study would reveal even more information, it would be difficult to control a diet as tightly as is possible at Camp Calcium, she said.