Cola may increase women's osteoporosis risk

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March 2007

News this month
Cola may increase women's osteoporosis risk

Soda consumption is at an all-time high and osteoporosis is a major public health problem, but most studies attempting to link the two trends have turned up mixed results. Recently researchers at Tufts University showed that cola, and not other carbonated soft drinks, is associated with significantly low bone mineral density (BMD) and a higher risk for osteoporosis in women.

"Women who consumed five carbonated drinks a week, including four colas, had a decreased bone mineral density at the three hip sites."

The study, published in the October 2006 issue of the American Journal of Clinical Nutrition, found a link only among women who regularly drink cola, not men.

Cola drinkers vs. non-cola drinkers
Researchers focused on 1,413 women and 1,125 men who were part of the Framingham Osteoporosis Study and who, on the average, were slightly younger than 60 years of age. The researchers gave subjects dietary questionnaires and measured their bone mineral density at the spine and three different hip locations. Then they divided the subjects into two groups: those who drank cola several days a week and those who drank it once a week or less. A serving of cola was defined as one bottle, can or glassful.

Lower BMD for cola drinkers
At the end of the study, women who consumed five carbonated drinks a week, including four colas, had a decreased BMD at the three hip sites.

The bone density among daily cola drinkers was as much as 4 percent less than women who did not consume as much cola. "This is quite significant when you are talking about the skeleton," said Katherine Tucker, director of the Dietary Assessment and Epidemiology Research Program at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. "The more cola that women drank, the lower their bone mineral density."

Doctors have assumed soft drink consumption is harmful because women who drink it may have a lower intake of milk and other calcium sources. However, the Framingham results were consistent regardless of age, menopause, total calcium and vitamin D intake, smoking or alcohol consumption. 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.

The findings were similar for regular cola and diet cola, although decaffeinated cola had less effect on BMD.

How might cola affect bone? Researchers were not sure how cola might undermine bone health, although they are suspicious about the effects of phosphoric acid, a cola ingredient that has been shown to interfere with calcium absorption.

"Physiologically, a diet low in calcium and high in phosphorus may promote bone loss," Tucker said. Some studies counter that there is more phosphorus in chicken and cheese than in cola.

However, chicken and cheese also contain calcium. "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 said.

Caffeine is also an identified risk factor for osteoporosis. Yet another possibility is that high fructose corn syrup, a sweetener in colas, may affect bone.

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Lifestyle factors are the key to prevention

As women live longer, osteoporosis can profoundly impair the quality of their lives. So it is increasingly clear that we need to refocus on exercise, nutrition and lifestyle habits that can have profound effects in the long-term on skeletal health.

The Tufts study is a large one that looks at a single lifestyle factor: drinking cola. It concludes that women who drink a lot of cola, including women who take in adequate calcium, are going to lose bone.

"So it is increasingly clear that we need to refocus on exercise, nutrition and lifestyle habits that can have profound effects in the longterm on skeletal health."

It's important to note that the Framingham group is highly respected, and that it has extensive experience with this type of research. The power of this particular study, obviously, is the sample size and the care with which researchers controlled for unknown or unaccounted for factors such as exercise, calcium consumption and other lifestyle behaviors that may influence bone mass. All of this provides some confidence that the study was well done and that it is worth careful consideration.

However, I see this is as a murky area that would benefit from further research. One problem is that there are limitations in any epidemiological study such as this.

For instance, sometimes a less obvious, unaccounted for factor is not adequately considered in the study. For example, perhaps the cola drinkers exercise less and that's why they have a lower bone density. They did mention that the subjects that had the highest cola intake also had a little bit lower calcium intake, so that may have made a difference.

Another limitation was that consumption of colas was quantified by a food frequency questionnaire, and these questionnaires are notoriously inaccurate. One thing that is especially peculiar about these findings is that when the researchers followed up on the cola drinkers, they saw changes in the three hip sites, but not in the spine. In pharmacologic studies, the spine tends to be the site that shows the most change. So I'm not sure why the spine showed the least change in this study.

In general, my feeling about epidemiological studies is that when several studies point to the same direction, they warrant serious consideration. In this case, we need another study of comparable power and size confirming the same finding. We need evidence again that this is not affecting the spine, and some plausible explanation as to why cola might cause a decrease in bone density especially in the hip.

Should women give up cola? Should women be cautious about drinking too much cola? Maybe. I would say these results add to the weight of evidence that suggest women might want to moderate their cola intake.

"I would say these results add to the weight of evidence that suggest women might want to moderate their cola intake."

More importantly, women should take their calcium. They may lose bone at an accelerated rate if they fall below their target - 1,200 milligrams a day for girls ages 9-18; 1,000 milligrams for women ages 19-50; and 1,200 milligrams for women 51 and older. In addition, women who take a calcium supplement should look for one that incudes Vitamin D to help with calcium absorption.

Weight-bearing exercise will also slow bone loss. Some of the best exercises for your bones are walking, dancing, jogging, stair-climbing, racquet sports and hiking. Yet another important strategy is to avoid smoking and excessive alcohol.

I also recommend that menopausal women have their bone density measured. These women will lose bone over the next 6-8 years, so by knowing their BMD, it can help inform their lifestyle choices.

Dr. Insogna is the director of the Yale Bone Center, an attending endocrinologist at Yale-New Haven Hospital and a professor of medicine at the Yale University School of Medicine.