Even if we have plenty of vitamin D, we still aren’t very good at absorbing calcium, and we get worse and worse at it as we age. While infants can absorb a respectable 60 percent of the calcium they consume, adults can hope to absorb only around 20 percent, and by retirement age, that drops to 10 percent or even lower. Our intestines are so bad at extracting calcium from food that our bodies are forced to extract it from our bones instead—a strategy with devastating consequences. Without constant calcium and vitamin D supplementation, most people would develop the brittle bones of osteoporosis
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