Have you seen articles on the web about the alkaline diet? The idea is that foods leave alkaline or acidic byproducts after digestion, and that alkaline foods help maintain calcium in your bones. I found an interesting study by Susan A. Lanham-New showing a positive effect of eating potassium-rich, bicarbonate-rich foods (i.e., fruits and vegetables) on bone density. It's published in the Journal of Nutrition.
There's a table in the article that shows the values of different foods on the acid/alkaline scale, with negative values being more alkaline. For example, spinach is -14 and cheddar cheese is +26. Surprisingly, citrus fruits such as lemons are alkaline (they leave alkaline "ash" in the body). So in addition to eating lots of fruits and vegetables (the study recommends 9 servings a day), sipping lemon water all day can help tip the balance towards alkalinity. Researchers have not yet shown exactly HOW this helps bones, but the trend seems clear that eating more than 5 fruits and veggies per day is good for your bones as well as for many other aspects of your health.