Well, life is full of things that lend themselves to completely obvious, incorrect initial conclusions. For most Americans, and certainly those who eat dairy, their weak bones are most likely not the result of inadequate calcium intake. There are factors causing calcium to be liberated from bone, and processes preventing calcium for being stored in bone. Although calcium is stored in the bones, calcium has many other vital functions including stimulating muscle contraction, buffering the body's pH against acidic foods (that's why soda is bad for your bones), and playing a role in the clotting cascade among many others. The important point is that calcium is stored in the bones, but the body takes calcium from the bones as needed for other functions more important than skeletal strength.
So, if the problem isn't inadequate calcium intake, what is it? For many, including about 85% of Massachusetts residents this time of year, Vitamin D deficiency is a big one. Our skin produces Vitamin D when exposed to UVB light, found in sunshine, and think how much less our skin is exposed to sunlight than nature anticipated. Dietary sources of Vitamin D aren't even close to adequate because our bodies were designed to get Vitamin D from sunlight on a large surface area of the body. Calcium absorption depends upon Vitamin D status. Their relationship goes well beyond just absorption; it seems that Vitamin D is required to deposit calcium where it's supposed to go. This makes giving extra calcium to a Vitamin D deficient patient not only foolish, but dangerous.
A large meta-analysis (a study that looks at a bunch of studies and groups data together to paint a bigger picture) of over 20,000 participants published in the July, 2010 British Medical Journal found that calcium supplements, at a dose of > 500 mg/day, given without Vitamin D, were associated with a statistically-significant 31% increased risk of heart attack. Use of calcium supplements was also correlated with an increased risk of stroke and death, but the association was just a smidge too weak to prove significant. The article is called "Effect of Calcium Supplements on Risk of Myocardial Infarction and Cardiovascular Events: Meta-analysis" and you can access the full text of the study here.
Now, that study only looked at calcium supplements without Vitamin D, but the amounts of Vitamin D in most supplements is simply too low. They give lots of calcium (often 1200 MG), and then a sprinkling of Vitamin D (200 IU or 400 IU) in a dry tablet form, which is poorly absorbed. Supplementing with that much calcium isn't just missing the point for most patients; it's actively harmful. There is emerging research to suggest that calcium plays a causative role in the development of plaque in the arteries, and it seems logical to me that Vitamin D deficiency leads to abnormal calcium deposition in the arteries. As such, given the fact that calcium supplements at best only prevent further bone loss, I believe the risk of calcium supplements as a means of bone protection outweigh the potential benefits for most everyone.
In general, if you're Vitamin D deficient, the calcium isn't going to help you, and it probably hurts you. And if your Vitamin D status is adequate then there is no need for supplemental calcium because you'll properly absorb calcium from dairy, nuts, seeds, green leafy vegetables, and more. Giving supplemental calcium to someone with an optimal Vitamin D level can actually lead to dangerously high levels of calcium in the blood. Humans evolved to run on much lower intakes of calcium because nature didn't expect us to use another animal's milk as a major source of nutrition in adulthood. I believe that the minimum calcium requirement has been greatly exaggerated because the people developing the standards don't understand that the majority of Americans are Vitamin D deficient.
The dopes at the Institute of Medicine will tell you that blood levels of 20 ng/ml are adequate. That's idiocy. There is crystal clear data to show that blood levels >30 ng/ml are needed simply to prevent the body from breaking down bone for calcium. As far as I'm concerned, anyone who tells you that blood levels less than 30 ng/ml are adequate doesn't know what they are talking about, and you shouldn't entrust your health to them if you wish to live.
30 ng/ml has been clearly shown to be the level simply needed to prevent obvious deficiency, and I'm in the camp that believes there are significant benefits to running higher than that, but those benefits have yet to be captured in placebo-controlled trials because everyone is so behind the times. As such, I am forced to guess as to what is the best blood level to aim for. I believe that blood levels of at least 40 ng/ml are needed for optimal health. That number is likely too low, and please know that I believe that setting 40 ng/ml as the minimum level is actually a conservative recommendation even though it is double that of the Institute of Medicine.
My read of the data is to steer clear of calcium supplements, know your 25-hydroxy Vitamin D level, supplement with Vitamin D3 (cholecalciferol) in an oil-based gelcap to achieve blood levels of at least 40 ng/ml (I suspect the optimal level is between 50 and 60 for most people, but it's tricky because of individual and ethnic variation). If you're an adult and you don't know your Vitamin D level, you may wish to consider taking 2000-2500 IU of Vitamin D daily until you can find out your 25-hydroxy Vitamin D level with the goal to to supplement to at least 40 ng/ml while keeping it under 70 ng/ml.
Vitamin D is safe when taken in physiologic doses, but as with anything, it is not without side effects. Those who suffer from primary hyperparathyroidism should avoid Vitamin D supplements, and those with sarcoidosis, granulomatous tuberculosis, and granulomatous cancers should only take only take Vitamin D under the supervision of a competent physician (which isn't easy to find these days) because these patients may produce high levels of the active form of Vitamin D in the blood. Otherwise healthy patients may rarely develop elevated blood calcium levels with Vit D supplementation. There is a risk-to-benefit ratio that must be calculated for anything. Just as you wouldn't worry about causing obesity by feeding a starving patient, or water toxicity in someone with extreme dehydration, concerns about Vitamin D toxicity are overblown to the point of absurdity. An exhaustive review has found no documented Vitamin D toxicity with doses of less than 10,000 IU per day. Even the conservative (conservative is a nice way of saying incompetent) Institute of Medicine just doubled the Tolerable Upper Intake Level to 4,000 IU per day, meaning even they believe 4,000 IU per day is totally safe.
No one really knows the perfect dose, and there is no magic number for everyone, so my best guess at this point is to aim for about 50 ng/ml. For more information about Vitamin D deficiency, I recommend you check out the Vitamin D council (http://www.vitamindcouncil.org/) run by a very smart guy named Dr. John Cannell. For more information on heart disease, the benefits of a low carb diet, Vitamin D deficiency, and just about everything, I advise you to check out the website of Dr. William Davis, the most knowledgeable cardiologist I've ever come across, http://www.heartscanblog.com/. And if your primary-care doctor advises you to take calcium supplements for your bones, at least ask for an explanation, and see how it rates next to mine.

