Tucker, also a professor at the Friedman School of Nutrition Science and Policy at Tufts, and colleagues analyzed dietary questionnaires and bone mineral density measurements at the spine and three different hip sites of more than 2,500 people in the Framingham Osteoporosis Study whose average age was just below 60. In women, cola consumption was associated with lower bone mineral density at all three hip sites, regardless of factors such as age, menopausal status, total calcium and vitamin D intake, or use of cigarettes or alcohol.
However, cola consumption was not associated with lower bone mineral density for men at the hip sites, or the spine for either men or women. The results were similar for diet cola and, although weaker, for decaffeinated cola as well.
Men reported drinking an average of six carbonated drinks a week, with five being cola, and women reported consuming an average of five carbonated drinks a week, four of which were cola. Serving size was defined as one bottle, can or glass of cola. "The more cola that women drank, the lower their bone mineral density was," says Tucker, who is corresponding author of the study. "However, we did not see an association with bone mineral density loss for women who drank carbonated beverages that were not cola."
"Carbonated soft-drink consumption increased more than three-fold" between 1960 and 1990, cite the authors. They also note that more than 70 percent of the carbonated beverages consumed by people in the study were colas, all of which contain phosphoric acid, an ingredient that is not likely to be found in non-cola carbonated beverages.
While previous studies have suggested that cola contributes to bone mineral density loss because it replaces milk in the diet, Tucker determined that women in the study who consumed higher amounts of cola did not have a lower intake of milk than women who consumed fewer colas. However, the authors did conclude that calcium intake from all sources, including non-dairy sources such as dark leafy greens or beans, was lower for women who drank the most cola. On average, women consumed 1,000 milligrams of calcium per day, and men consumed 800 milligrams per day, both lower than the daily recommended 1,200 daily milligrams for adults over age 50.
"Physiologically, a diet low in calcium and high in phosphorus may promote bone loss, tipping the balance of bone remodeling toward calcium loss from the bone. Although some studies have countered that the amount of phosphoric acid in cola is negligible compared to other dietary sources such as chicken or cheese," Tucker says, "further controlled studies should be conducted to determine whether habitual cola drinkers may be adversely affecting their bone health by regularly consuming doses of phosphoric acid that do not contain calcium or another neutralizing ingredient."
Tucker stresses that as with any epidemiological study, the results should be taken with caution. "We are not certain why women who drank more cola also had lower bone mineral density," says Tucker. Although adjustment for fruit juice intake did not change results, women in the study who drank a considerable amount of cola not only consumed less calcium, but less fruit juice as well. Previous studies have also shown that low fruit and vegetable intake may affect bone mineral density.
The message from experts is clear that overall nutritional choices can affect bone health, but "there is no concrete evidence that an occasional cola will harm the bones," says Tucker. "However, women concerned about osteoporosis may want to steer away from frequent consumption of cola until further studies are conducted."