Ann Prentice

"There is so much uncertainty still," said Prentice, director and head of the Nutrition and Bone Health Research Group, Elsie Widdowson Laboratory, at the Resource Centre for Human Nutrition Research in Cambridge, England. She spoke on April 7 in Snyder Auditorium.

Confusion stems from what is known as the "Calcium Paradox‚" which notes that the rate of hip fractures is highest in countries where calcium intake is also high and lowest in countries where calcium intake is low.

Prentice said the controversy over calcium intake dates back to at least 1952 when a paper that she described as seminal by Mark Hegsted, professor emeritus at HSPH, questioned minimal calcium requirements for men. Hegsted, who is a former chair of the Department of Nutrition and former chief of human nutrition at the U.S. Department of Agriculture, was in the audience for the talk. The lecture is named in honor of Hegsted and Fredrick Stare, founding chair of the Department of Nutrition.

Prentice’s own research, under way in England, The Gambia, and China, has raised more uncertainties. Mounting evidence suggests that the body, in many circumstances, adapts to the amount of calcium it receives, calling into question whether there is an optimal intake level.

One example comes from her work in The Gambia in West Africa, where calcium intake is very low, yet hip fractures are rare. Another comes from her studies of pregnant and breastfeeding women. The research shows that the women lose calcium and bone mineral density while lactating–yet their femurs compensate by expanding their circumferences where the bones meet the hips. As a result, proper distribution of body weight is maintained and risk of fracture is lessened. After lactation ends, the bones contract to their original size, and the bone density returns to pre-pregnancy levels, continuing to protect the women from fractures.

"The body appears to have a remarkable propensity to adapt to different levels of calcium intake," she said. "We adapt to what we eat."

Different recommendations for calcium intake have been established by various groups, further muddying the picture. The Food and Agriculture Organization of the United Nations/World Health Organization have recommended 1,000 mg per day for most adults and 1,300 mg per day for postmenopausal women. In the United States, the Institute of Medicine’s guidelines say that adequate calcium intake is 1,000 mg per day for adults up to age 51 and 1,200 mg per day for those older. Until definitive answers are available, Prentice said that she recommends people follow government guidelines.

Intake recommendations may prove moot, if the hypothesis that the body adapts to available levels of calcium holds true. "Does it matter?" asked Prentice. "Because there are a whole range of people who are living on low calcium intake and apparently living long and healthy lives."

She added that making blanket recommendations on calcium intake is difficult because general population-based differences exist around the world. For example, residents of The Gambia receive more sunlight and partake in different forms of exercise than people in England, possibly affecting their calcium needs and metabolism. It is unknown whether people in Western societies could thrive on the low amounts of calcium consumed in The Gambia–generally 300 to 400 mg per day.

"We can’t necessarily say that American women can live on such a low calcium intake because it may well be that other factors in the lives of people in The Gambia allow their bodies to adjust," she said. "We don’t know at this stage. We need to look at it more."

While she said there is no evidence that any one calcium source is best, Prentice said that good sources include milk, cheese, broccoli, and other green leafy vegetables. She recommends a balanced diet.

Prentice, 52, said that she personally does not worry about calcium intake. Her advice to others, particularly those going through menopause, is to continue consuming the same amount of calcium they always have eaten. "This is not the time to reduce your calcium intake," she said.

--ML