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Thanks for your excellent question! This is a controversial issue, and you may receive different answers from the proponents of cows milk and those of soy milk. To begin with, it's great to remember that either milk or calcium-fortified soy milk is a great bone-building alternative to calcium-free beverages such as sodas, and it's great to hear you're making the effort to stave off osteoporosis with smart choices in your diet.   When comparing the two, it's worthwhile to remember cow's milk (whether whole, low-fat or skim) generally has about 300 mg per cup. This is a good figure to compare to the amount of calcium present in whichever fortified soymilk you might choose (which could vary). However, some studies have indicated that the calcium in fortified soymilk is more difficult to digest than that in cow's milk, so you would need to drink more to get the same amount. Some soy milk brands dispute this assertion and suggest their calcium is just as absorbable -- the decision is up to you.   You may want to also consider other factors besides osteoporosis -- for example, some experts tout soy milk for vascular health, while others warn excessive soy consumption can be problematic for those with estrogen-responsive breast cancer. You may want to discuss the issue with your doctor if you have either of these conditions.   Best wishes!