A: This common belief probably originated with studies that linked soft drinks to osteoporosis, like the 2006 Framingham Osteoporosis Study that compared bone density in 2,538 men and women with their self-reported intake of soda. Drinking more than three colas a week—but not other carbonated beverages—was associated with lower bone density in women, though not in men.
The scientists could not explain the difference between the sexes, but at first they suspected that the phosphoric acid in cola might be interfering with bone metabolism. After all, our bones are constantly being dissolved and reformed in a complex dance that involves calcium, phosphorus, vitamin D, and parathyroid hormone. But the phosphorus in cola is only a tiny fraction of what we get from the rest of our food; a cup of skim-milk yogurt has more than seven times the amount in a can of soda. So then the experts looked at the subjects' calcium intake. And they found that the women who consumed the most cola also consumed the least calcium.
Soft drinks tend to displace healthier foods from the diet. So it's not the soda itself but the diet as a whole that matters. An unhealthy diet with too much soda and not enough sources of calcium (like dairy, fish, and leafy greens) can weaken bones over time.
Harriet Hall, MD, is an editor of ScienceBasedMedicine.org.
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