BACKGROUND:

An inadequate dietary intake of calcium has been postulated to be a factor in the development of essential hypertension, and has long been considered important in the development of osteoporosis among women. Furthermore, animal experiments and limited studies in humans suggest that an increase in the intake of calcium may also have a beneficial effect on blood cholesterol levels. Despite limitations in the research data supporting its benefits, increased calcium intake is being widely advocated to the public and calcium-enriched products are being increasingly purchased and consumed by Americans. The study measured the effects of a modest increase in calcium intake, such as that currently advocated to the general public.

Previous work in the study measured the effects on blood pressures of variations in the dietary intake of sodium and linoleic acid in adolescents at the two New England boarding schools of Phillips Exeter Academy and Phillips Andover Academy. During the first year, dining room food at one school was prepared with 50 percent less sodium; total intake of sodium by students including dining hall food, outside food and snacks was reduced by approximately 35 percent. The other school served as the control. During the second year, the intervention took place at the second school. During the third and fourth years, sodium intake was kept at usual levels while changes were made in food preparation to double the polyunsaturated fatty acid intake from four percent to eight percent of dietary calories. Food diaries, duplicate food samples, and 24-hour urine collections by approximately 200 students at each school were used to monitor dietary modification and student compliance. Weekly blood pressure measurements were performed on the 200 students at each school.

DESIGN NARRATIVE:

A number of food products were modified to increase the dietary intake of calcium by an average of 500 mg/day. Calcium-enriched foods were served alternately for one school year at each of two colleges. Similar products without added calcium were served during the alternate year. It was postulated that, at the end of the school year, students at the intervention school would have lower systolic blood pressure, lower blood cholesterol, and increased bone density than students at the control school.