 My last in-depth video series on vitamin D was done more than four years ago, as in 15,950 papers ago. I had a lot of catching up to do. Review articles like this continue to be published touting vitamin D as a veritable cure-all. The vitamin D receptor is found in most tissues in the body, including the brain. Upwards of 2,000 genes may be regulated by vitamin D. Within 24 hours of vitamin D exposure, you can change the expression of hundreds of genes. The term vitamin, though, is actually a misnomer. Vitamins, by definition, cannot be synthesized within our body, but we can make all the D we need with sufficient sun exposure. D is not a vitamin but a hormone produced by our skin in response to sunlight exposure. And it's not just a hormone of calcium regulation and bone health, but a hormone of fertility and immunity and brain function. But is it a panacea or a false prophet? Remember when vitamin E was the vitamin du jour touted as curative for many clinical disorders? Vitamin E, the radical protector with supplement sales in general, forming a billion-dollar business to capitalize on the public's fears. After all, those with low levels of vitamin E in their blood had 50% higher cancer risk. Hey, and what about vitamin A or beta-carotene? After all, people eat lots of greens and sweet potatoes. Now, the beta-carotene-rich foods have lower risk of cancer. So, hey, let's give people some beta-carotene pills. But when they were put to the test, beta-carotene pills increased cancer rates. Beta-carotene, vitamin A and vitamin E supplements may increase mortality, and in effect would be potentially paying to shorten our lifespan. So you can understand the skepticism in the medical community regarding claims about vitamin D, which is now enjoying its moment in the sun. Having a half-billion-dollar vitamin D supplement industry doesn't help matters in terms of getting at the truth, not to mention the highly lucrative vitamin D testing industry that loves to talk about the studies suggesting having higher vitamin D levels may reduce the risk of heart disease and cancer and diabetes, autoimmune diseases, and infections. But most of this research stems from observational studies, meaning studies that correlate higher D levels in the blood with lower disease risk. But that doesn't mean vitamin D is the cause. It's like the early beta-carotene data, right? Higher levels in the blood may have just been a marker of healthy eating. Who has high beta-carotene levels? Those who eat lots of greens and sweet potatoes. Just like vitamin D levels may also just be a marker for healthy behaviors. Who has high vitamin D levels? Those that run around outside. And those who run around outside run around outside. Higher vitamin D levels may just be a sign of higher physical activity, for example. So when she studies like this, showing significantly lower diabetes rates among those with higher vitamin D levels, it doesn't mean giving people vitamin D will necessary help. You have to put it to the test. And when you do, vitamin D supplements fall flat on their face, no benefit for preventing or treating type 2 diabetes. So when supplement companies wave around studies like this, suggesting vitamin D deficiency plays a role in obesity, because most population studies show lower vitamin D levels in the blood of those who are obese, is that just because they're exercising less or simply because it's a fat soluble vitamin, so it's just lodged in all that fat? One might expect obese sunbathers would make more vitamin Ds since they have more skin surface area, but the same exposure leads to less than half the D because it gets sucked away in their fat. That's why obese persons may require 2 to 3 times the dose of vitamin D, though they may get it back when they lose weight and release it back into their circulation. So that would explain this population data. And indeed, when you put vitamin D to the test, trivitamin D is a treatment for obesity. It doesn't work at all. Similar story with artery health. Those with low vitamin D levels have worse coronary blood flow, more atherosclerosis, and worse artery function. But if you actually put it to the test and randomized controlled trials, the results are disappointing. Also ineffective in bringing down blood pressures, so this all adds to this growing body of science casting doubt on the ability of vitamin D supplementation to improve anything beyond just falls, fractures, the common cold, and all-cause mortality. Wait, what? Vitamin D supplements can make you live longer? That's kind of important. I'll explore that next.