Q: Is it true that caffeine interferes with calcium absorption?
A: Not enough to worry about. Caffeine can slightly reduce calcium absorption, but this is significant only if you aren’t consuming adequate calcium.
A few studies have linked caffeinated beverages to reduced bone density and increased fractures, but the effect is seen almost exclusively in people with low calcium intake. In any case, the negative effect of caffeine is small enough to be fully offset by one to two tablespoons of milk, according to Robert Heaney, M.D., a noted expert on calcium and osteoporosis at Creighton University in Omaha.
Rather than worry about calcium-caffeine interactions, just be sure to get enough calcium from food, preferably, and supplements: The recommended dietary allowance (RDA) is 1,200 milligrams a day for women over 50 and men over 70, and 1,000 milligrams for other adults. (In addition, consume enough vitamin D, which works with calcium. We advise 800 to 1,000 IU a day for most people.)
Keep in mind that many substances interfere with the absorption of nutrients, and the RDAs take most such factors into account to some extent.