Walk into any pharmacy or health food store and you will see shelves filled with vitamin D supplements. Then stroll over to a bookstore and you will find all sort of books touting the wonders of the vitamin. Next, search PubMed for articles about vitamin D. There will be thousands, with different conclusions about what constitutes adequate blood levels and how these are to be achieved. Where will that leave you? Confused, I suspect.
Let’s start with the facts. There is no question that vitamin D is essential for the absorption of calcium and phosphorus from food. Since these in turn are essential for bone formation, it is obvious that insufficient levels of vitamin D will lead to weakened bones. Of course, the question is what levels are deemed to be insufficient and what is to be done about them.
Since sunlight can convert 7-dehydrocholesterol that is present in the skin into vitamin D, lack of sun exposure may be a cause of low levels. The classic example is the epidemic of skeletal deformation known as rickets that was observed in children during the Industrial Revolution in the U.K. when clouds of dense smoke spewed out by factories obscured the sun. So, there is no doubt that sun exposure is important, but we do not have to roast ourselves on a beach to get a good dose of vitamin D. Fifteen to thirty minutes walking around outdoors gets the job done. Our body also stores vitamin D in the liver, so even being mired in darkness for a few days is not a problem. Still, it is possible that in the winter in northern climates with our prolonged indoor entrapment, blood levels could drop.
How would we know if we are vitamin D deficient? Well, there is a blood test. But there is a question of how to interpret it. Back in 2010, the American “Institute of Medicine (IOM),” a group of experts tasked with advising the public on matters of health, examined available data on blood levels and bone strength. Their conclusion was that 20 nanograms of vitamin D per milliliter of blood offers adequate protection. (20 ng/mL is equivalent to 50 nmol/L, the other unit that is also used.) This was commonly, and erroneously, interpreted to mean that 20 ng/mL was the minimum amount that was needed for protection and that more was better. In fact, the IOM analysis actually stated that 12-15 ng/mL was enough, but suggested 20 ng/mL, “just to be sure.”
A year later, the Endocrine Society came to a different conclusion. Its minimum level was 30 ng/mL, and it recommended 40-60 ng/mL to be sure of “sufficiency.” The rationale was that the Society took into account possible benefits of vitamin D, other than bone health. For example, if calcium levels are low because of insufficient vitamin D, the parathyroid glands can become overactive and churn out a hormone to normalize blood calcium. Such “hyperparathyroidism” can cause fatigue, muscle weakness, cramps and even depression. The recommendation to strive for a blood level of 40-60 ng/mL was not without controversy, with some experts claiming that the chair of the Endocrine Society committee had a conflict of interest because of ties to a company that sold vitamin D supplements.
When you peruse the books about vitamin D you pull off the shelf in the bookstore, you’ll learn that you should indeed be loading up on this vitamin, not only to have super bones, but also to protect you from heart disease, cancer and diabetes. And you will be inundated with references. For example, you may learn that the Framingham Heart Study that followed some 1700 people with no prior cardiovascular disease for five years found an association between low blood levels of vitamin D and increased risk of developing heart disease. Or you may come across a Norwegian study that correlates higher blood levels of vitamin D with a lower incidence of type 2 diabetes. Keep in mind, though, that such observational studies have to be taken with a grain of salt. It may be that people with low levels of vitamin D have poor diets, and it may be some other aspect of their diet that is responsible for the increase in heart disease. “Correlation is not causation” is the mantra that cannot be repeated too often!
Nevertheless, these observational studies created much interest and stimulated randomized, placebo-controlled trials in which thousands of subjects were given as much as 4000 IU (100 micrograms) of vitamin D daily and were followed for years. The results were surprising. The supplements offered no protection from heart disease, diabetes, cancer, atrial fibrillation, risk of falling or even fractures. That, though, may be because the subjects in these studies were not representative of the general population. About 85% of already had blood levels above 20 ng/mL so they weren’t deficient. But there were some positive findings as well with autoimmune diseases such as rheumatoid arthritis, Crohn’s disease and psoriasis. Symptoms were reduced after taking a daily supplement of 2000 IU for two years.
When it comes to the general population, several studies have documented deficiencies. Both the National Health and Nutrition Examination Survey (NHANES) in the U.S. and the Biobank study in the U.K. found that roughly 30% of the population have blood levels below 20 ng/mL.
Where does this leave us? Most health agencies around the world agree that blood levels of 20 to 30 ng/mL mean that intake is adequate and that this can be met with a daily intake of roughly 400-600 IU of vitamin D. However, that intake is not so easily met since vitamin D doesn’t occur widely in the diet. Oily fish, mushrooms, egg yolk, fortified dairy products and cereal are the best sources, but are unlikely to meet the body’s needs unless coupled with sun exposure. This is especially problematic for people with kidney, liver, Crohn’s or celiac disease. These conditions either interfere with the absorption of the vitamin or its conversion into the active form, 1,25-dihydroxyvitamin D. In this case supplements are definitely indicated.
But what about healthy people taking a 600-1000 IU dose, the safety of which is well established, “just for insurance?” The fact is that for many people, both dietary intake and sun exposure is uncertain. Then there is the emerging research about the role of Vitamin D in immune function, including a reduction in the severity of a COVID-19 infection. After having digested all the information, I come to the conclusion that popping a 1000 IU pill a day, “just for insurance,” is not a bad idea.
