Want strong bones? Eat foods high in calcium and vitamin D, get plenty of exercise — and maybe steer clear of soda.
In recent decades, as consumption of the beverage has steadily displaced the consumption of others —particularly milk — studies have consistently linked soda consumption with weaker bones. Now scientists are trying to figure out how and why, precisely, drinking soda may affect skeletons.
One theory is that a component in cola may cause bone to deteriorate; another is that people who drink soda simply drink (and eat) fewer nutritious foods.
In the 1990s, several studies suggested soft-drink consumption might be linked to lower bone mass and reduced bone accretion — the process by which bone is built up — in children, especially teens.
In a study of 127 teens that was published in the Journal of Adolescent Health in 1994, teenage girls who drank carbonated beverages were three times as likely to suffer bone fractures compared with girls who didn't drink soda. A study by the same author published in the Archives of Pediatric Adolescent Medicine in 2000 showed the same effect — and an even stronger one for girls who drank cola beverages, who were five times as likely to suffer bone fractures.
Researchers surmised at the time that soda took its toll on bones because children who drank soda did so in place of milk. Soda drinking was also seen as a marker for a generally unhealthful diet lacking items that help foster strong bones.
It does seem to be true that soda drinkers have worse diets overall. In a study published this month in the Journal of the American Dietetic Assn., for example, among 170 girls followed from age 5 to 15, those who drank soda at age 5 were less likely to drink milk throughout childhood than 5-year-olds who did not drink soda. And they were more likely to consume diets lacking in calcium, fiber, vitamin D, protein, magnesium, phosphorus and potassium.
Such findings are significant because as much as 90% of bone mass is acquired in youth, particularly from age 16 to 25, says Dr. Jeri Nieves, director of bone density testing at Helen Hayes Hospital in West Haverstraw, N.Y.
Children who fail to get enough bone-building nutrients and bone-thickening exercise in their youth end up with increased risk of osteoporosis and fracture as they get older, adds Dr. Robert Murray, director of the Center for Healthy Weight and Nutrition at Nationwide Children's Hospital in Columbus, Ohio.
But there is also evidence that drinking sodas — specifically, colas — may take a direct toll on the skeleton, says Dr. Katherine Tucker, professor of health sciences at Northeastern University in Boston.
In a large, well-designed study published by Tucker and colleagues in the American Journal of Clinical Nutrition in 2006, women enrolled in the ongoing Framingham Osteoporosis Study who drank just three or more colas a week had a 3.7% to 5.4% lower bone mineral density in their hip bones when compared with women who didn't drink the beverage.
The study also showed what scientists call a dose response: The more soda participants drank, the lower their bone mineral density.
The effect was seen only with colas — non-cola soft drinks, such as ginger ale and orange soda, had no effect on bone density. That finding led Tucker and colleagues to suggest that the phosphoric acid in cola is behind its bone-weakening effects.
Phosphoric acid is added to colas for its tangy flavor. It's not normally found in the food chain, Tucker says. When ingested, it causes the acidity of the blood to increase; to adjust the blood's pH, the body draws calcium out of bones and into the bloodstream.
These proposed effects of phosphoric acid on bone are largely theoretical, but they are supported by animal studies and some human research. A Danish study published in the journal Osteoporosis International in 2005 measured the blood levels of bone minerals in a group of men after they consumed a low-calcium diet and 2.5 liters of soda daily for 10 days, and then again after they consumed a normal diet and 2.5 liters of skim milk for 10 days.
During the cola-drinking period, the men had higher blood levels of the bone mineral phosphate, the bone turnover protein osteocalcin and a substance called CTX — results that indicated minerals were being removed from bone, and not replaced, during the soda-drinking period.
Scientists are continuing to test the theory that phosphoric acid in soda harms bones. But even if it turns out that phosphoric acids cause only small or temporary changes in bone composition, these can add up over time, Tucker says.
In the meantime, Nieves suggests, it's probably wise to limit your intake of soda.
"It's not like alcohol, where one drink a day is OK," she says. "Because bone mass is constantly changing throughout life, soda can cause bone loss at any stage."