Races react differently to dietary salt, calcium, Purdue study finds

Main Category: Nutrition / Diet
Article Date: 11 May 2005 - 0:00 PDT

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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 American Journal of Clinical Nutrition, 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.

Research will continue to examine racial differences in how bodies handle calcium during this year's camp, when the focus will be on Asian adolescents. Girls between 12 and 14 and boys between 13 and 15 will attend the six-week camp in two three-week periods and participate in science classes, arts and crafts, sports, nutrition classes, and field trips, all while their diets are under strict supervision for calcium. For more information and an application packet, contact Berdine Martin, 700 W. State St., Purdue University, West Lafayette, Ind. 47907-2059 or call (800) 830-0175 or (765) 494-6559.

Weaver is an internationally recognized expert on calcium metabolism and bone health. She was a member of the Dietary Guidelines Advisory Committee, which gave recommendations to the U.S. Department of Agriculture to produce the most recent "Dietary Guidelines for Americans." She is director of the National Institutes of Health Botanicals Research Center for Age Related Diseases and past president of the American Society for Nutritional Sciences.

A publication-quality file photo is available at
news.uns.purdue.edu/images/+2005/campcalcium05.jpg

ABSTRACT

Racial Difference in Calcium Retention in Response to Dietary Salt in Adolescent Girls

Karin Wigertz, Cristina Palacios, Lisa A. Jackman, Berdine R. Martin, Linda Doyle McCabe, George P. McCabe, Munro Peacock, J. Howard Pratt, and Connie M. Weaver

Sodium is an important determinant of urinary calcium excretion, and race is an important determinant of calcium retention. The impact of dietary sodium on calcium retention and influence of race has not been studied in subjects in adolescence, an important life stage for accruing peak bone mass. The study reported here was undertaken to compare racial differences in calcium retention as a function of dietary salt intake. A total of 35 girls (22 black and 13 white) participated in two 20-day metabolic summer camps that were separated by two weeks and simulated a free-living environment. The effect of changes in dietary sodium on calcium retention was tested in a randomized order, crossover design of two levels of sodium, 1.30 g/day (57 mmol/day) and 3.86 g/day (168 mmol/d), and a constant calcium intake of 815 mg/day (20 mmol/day). Both race and sodium intake affected calcium retention (P < 0.01). Black girls had higher calcium retention than white girls regardless of dietary sodium level (P < 0.001). The high-sodium diet decreased calcium retention in both whites and blacks (P < 0.01), primarily through decreased net calcium absorption. Black girls excreted significantly less calcium in the urine than white girls on both the low- and the high-sodium diet (P < 0.05). Black girls have greater calcium retention than white girls and, like the white girls, retain less calcium in response to salt loading.

Contact: Maggie Morris
maggiemorris@purdue.edu
765-494-2432
Purdue University
http://www.purdue.edu