 We've known for over 400 years that muscle weakness was a common presenting symptom of vitamin D deficiency. Bones aren't the only organs that respond to vitamin D, muscles do, too, but as we age our muscles lose vitamin D receptors, perhaps helping to explain the loss in muscle strength as we age. And indeed, vitamin D status does appear to predict the decline in physical performance as we age, with lower vitamin D levels linked to poorer performance. But maybe the vitamin D didn't lead to weakness, maybe the weakness led to low vitamin D. Vitamin D is the sunshine vitamin, and so if you're too weak to run around outside, that could explain the correlation with lower levels. To see if it's caused an effect, you have to put it to the test. There's been about a dozen randomized controlled trials, vitamin D supplements versus sugar pills. Put all the studies together, and older men and women do get significant protection from falls with vitamin D, especially among those who start out with relatively low levels. Leading the conservative USPSTF, the US Preventive Services Task Force, the Official Prevention Guidelines Setting Body, and the American Geriatric Society, to recommend vitamin D supplementation for those at high risk for falls. We're not quite sure of the mechanism, though. Randomized controlled trials have found that vitamin D boosts global muscle strength, particularly in the quads, which are important for fall prevention, though vitamin D supplements have also been shown to improve balance. So it may also be a neurological effect, or even a cognitive effect. We've known for about 20 years that older men and women who stop walking when a conversation starts are at particularly high risk of falling. Over a six-month timeframe, few of those who could walk and talk at the same time would go on to fall, but 80% of those who stopped when a conversation is initiated ended up falling. Other high-risk groups that should supplement include those who've already fallen once or are unsteady or on a variety of heart, brain, and blood pressure drugs that can increase fall risk. There's also a test called Get Up and Go, which anyone can do at home. You time how long it takes you to get up from an armchair, walk 10 feet, turn around, walk back, and sit down. If it takes you longer than 10 seconds, then you may be at high risk. So how much vitamin D should you take? It seems to take at least 700 to 1,000 units a day. The American Geriatric Society recommends a total of 4,000 a day, though, based on the rationale that this should get about 90% of people up to the target vitamin D blood level of 75 nanomoles per liter. 1,000 should do it for the majority of people, 51%, but they recommend 4,000 to capture 92% of the population. Then you don't have to routinely test levels, since you would get most people up there and it's considerably below the proposed upper tolerable intake of 10,000 a day. They do not recommend periodic mega doses. Because it's hard to get patients to comply with pills, why not just give people one mega dose, like 500,000 units once a year? Like when you come in for a flu shot or something. So every year you can at least guarantee everyone gets an annual spike in D levels that last a few months. It's unnatural, but certainly convenient, for the doctor at least. The problem is that it actually increases fall risk, a 30% increase in falls in those first three months of the spike. Similar results were found in other mega dose trials. It may be a matter of too much of a good thing. See, vitamin D may improve physical performance, reduce chronic pain, and improve mood so much that you start moving around more and thereby increase fall risk. You give people a whopping dose of D and you get a burst in physical, mental, and social functioning, and it may take time for your motor control to catch up with your improved muscle function. It would be like giving someone a sports car all of a sudden when they've been used to driving some beater. You gotta take it slow. It's possible, though, that such unnaturally high doses may actually damage the muscles. The evidence they cite in support is a meat industry study showing you can improve the tenderness of steaks by feeding steers of a few million units of vitamin D, so the concern is that such high doses may be over 10 derising our own muscles as well. So yeah, higher D levels are associated with a progressive drop in fracture risk, but too much vitamin D may be harmful. The bottom line is that vitamin D supplementation appears to help, but the strongest and most consistent evidence for prevention of serious falls is exercise. If you compare the two, yes, taking vitamin D may lower your fall risk compared to placebo, but strength and balance training with or without vitamin D may be even more powerful.