Here's an excerpt on this topic:
BIHW: Does caffeine increase risk for osteoporosis?
Dr. Heaney: A moderate amount of caffeine isn’t harmful to bone health as long as people also consume enough calcium.
Controlled clinical studies show that although caffeine ingestion results in a small, temporary increase in calcium excretion, it has no effect on 24-hour urinary calcium loss. Studies have also shown that although caffeine can cause the intestine to absorb slightly less calcium when calcium intake is also low, this effect can be fully offset by as little as one to two tablespoons of milk. This helps explain why the epidemiological studies regarding caffeine and bone health are mixed. All the observations in studies linking caffeine intake to osteoporosis were made in populations with calcium intakes far below recommended levels. In studies where dietary calcium intakes were higher, at least 800 milligrams per day, caffeine intake has an almost negligible effect.
BIHW: Some people believe that sparkling soft drinks, particularly colas, adversely affect bone health. Is this true?
Dr. Heaney: No. A few observational studies have found an association between high carbonated soft drink consumption and either increased fracture risk or decreased bone mineral density, and the usual explanation given has been that one or more constituents in these beverages, such as caffeine or phosphoric acid, increases urinary calcium losses. But, this theory didn’t hold up under experimental studies done in my lab using carefully controlled calcium-metabolic methods. We found that the net effect of carbonated soft drinks, including colas, on calcium retention was negligible. As a result, it seems likely that colas’ prominence in observational studies is due to their prominence in the marketplace. For example, 27 out of 30 subjects in our study reported being cola drinkers.
The real issue is that people who drink a lot of soft drinks also tend to have an overall diet that is low in calcium and other important nutrients bones need. And, it’s the poor diet and low calcium intake impacting skeletal fragility that need to be addressed – not whether or not they drink soft drinks per se.
BIHW: Could you explain your study on the impact of carbonated soft drinks on calcium in more detail?
Dr. Heaney: In 2001, we published a calcium metabolic study in the American Journal of Clinical Nutrition that compared the impact of four different soft drinks, plus water and milk, on urinary calcium in adult women who normally consume soft drinks. Because that study evaluated a caffeine-free and a regular cola, both of which contained phosphoric acid, and a caffeine-free and a caffeinated citrus-flavored soft drink, neither of which contained phosphoric acid, we were able to evaluate the impact of soft drinks overall, as well as the individual and combined impact of these ingredients, on calcium balance. Our results showed that the effect of soft drinks on calcium losses, including those with caffeine and colas with phosphoric acid, is negligible.
Specifically, we found that phosphoric acid had no impact at all: The caffeine-free cola did not increase urinary calcium losses. In addition, although both the caffeinated cola and caffeinated citrus soft drink caused a small increase in urinary calcium loss, it was about equal to that previously found for caffeine alone.
However, since the body can compensate for the small impact of caffeine by reducing calcium losses later in the day, we determined that soft drinks, including colas with phosphoric acid and those with caffeine, have essentially no impact on calcium balance.
BIHW: Why do concerns over phosphoric acid in colas seem to linger? Is there something unique about phosphoric acid or phosphorus that negatively affects bone health?
Dr. Heaney: No. The common myth that the phosphoric acid in colas draws calcium out of the bones is likely linked to a theory that an acidic diet causes minerals to be drawn from the bones to neutralize the impact of the acid on blood pH. But, as I explained earlier, our calcium-metabolic study found that the phosphoric acid in cola had no net impact on urinary calcium losses. Now, I must admit that this finding was initially unexpected, but perhaps it should not have been, considering the body normally produces 50 to 100 mEq of acid a day during the metabolism of food. The acid load imposed by a 20-ounce cola is only about 4.5 to 5.0 mEq, or substantially less than the amount produced by eating even a moderate protein breakfast.
There is also no plausible evidence that phosphorus in the diet is harmful to bone health in humans. This theory began with studies showing a harmful effect in animals and the findings were extrapolated to humans. But the animal diets contained amounts of phosphorus up to five times the amount in a typical human diet, so the findings aren’t applicable to humans. And, although phosphorus is widely believed to form insoluble complexes with calcium and is often listed as a potential anti-absorber on web sites, studies conducted more than 25 years ago in my lab and others showed that varying phosphorus intakes have little or no effect on overall calcium balance.
Phosphorus is actually quite plentiful in the food supply. For example, depending on the brand, 8 ounces of cola contains 25-40 milligrams of phosphorus, in the form of phosphoric acid, which is used as the acidulant. The same amount of orange juice has 27 mg of phosphorus and milk has 232 milligrams in 8 ounces. To put these amounts into perspective, national dietary surveys in the U.S. show that the highest level of phosphorus intake among individuals who don’t take supplements is about 2500 mg per day, which is well within the safe limits established by the Institute of Medicine. The safe limits for phosphorus established by the Institute of Medicine are 4,000 mg (4 g) per day for individuals between the ages of 9 and 70 and 3,000 mg (3 g) per day for those over 70 and between the ages of 1 and 8.
Phosphorus Content of Common Foods and Beverages
Cola (8 oz): 25 - 40 mg
Orange Juice (8 oz), unfortified: 27 mg
Peanuts (1 oz), shelled: 113 mg
Chicken breast, roasted: 124 mg
Cheddar cheese (1 oz): 145 mg
Milk (8 oz), 1%: 232 mg
BIHW: Does the carbonation in soft drinks harm bones?
Dr. Heaney: No. In fact, research has shown that carbonated waters rich in calcium and other minerals can actually improve measures of skeletal metabolism in postmenopausal women with low calcium intakes. And, as for carbonation in soft drinks, our calcium-metabolic soft drink study also exonerated carbonation when we determined that caffeine-free soft drinks had no net effect on urinary calcium loss. This isn’t surprising since the amount of carbon dioxide absorbed from a soft drink is relatively very small compared to the amount our cells continuously produce as a byproduct of energy production.