The sweet spot for vitamin D is still debated, and there are many who are confused by the current brouhaha about this vitamin.
It's almost impossible for a day to go by without noticing a mention of vitamin D in the news. This is because an increasing number of studies have revealed the importance of this vitamin in a multitude of biochemical pathways, making its deficiency a focal point in the precipitation of some diseases.
Adolf Windaus who received the Nobel Prize in chemistry in 1928 for his work on sterols and vitamins, was responsible for showing the relationship between cholesterol and vitamin D. All the cells in your body synthesize cholesterol over a series of more than 20 exquisitely regulated steps, with the liver doing most of the heavy lifting. The result of one of these steps is a molecule called 7-dehydrocholesterol, which when exposed to sunlight is eventually transformed to vitamin D3 -- one of the two major forms of vitamin D; the other is vitamin D2 (found in plants).
A few steps in the biosynthesis of cholesterol and vitamin D3. Note how acetyl CoA is the starting point -- the reader is referred to a separate article on vitamin B12, that also references acetyl CoA.
Without feeling overwhelmed by chemical structures, it's easy to notice the similarities between the molecules in this figure
and it becomes clear (even if you didn't know that a precursor to cholesterol is also a precursor to vitamin D3) that cholesterol and D3 are related.
When your metabolic pathways are shipshape, and you spend responsible amounts of time in the sun, your body has the capacity to make adequate amounts of D3. Dietary intake of foods rich in vitamin D as well as supplements can compensate for a deficiency of this vitamin. None of these forms, not even the one your body makes when exposed to sun, are the active forms of the vitamin. The liver and kidney do the hard work of converting the vitamin to the active, and useful form. And when your levels of vitamin D are being checked,it's really only the form transformed in the liver which circulates in the blood that is measured. It's not the fully active form -- the fully active form is produced on an as-needed basis.
The marvellous thing about the active form is that it acts like a hormone, directing calcium traffic in your body. When there is inadequate calcium circulating in the blood, the active form of vitamin D3 jumps into action and helps to increase the absorption of calcium from the food you eat by activating the necessary pathways. Which is why chronic vitamin D deficiency is associated with a propensity for bone fractures and muscle weakness, among other calcium-dependent conditions.
But wait, there's more!
Unlike deficiencies of other vitamins, vitamin D deficiencies result in subtle symptoms in adults; in children they are more obvious and result in significantly poor bone development resulting in a disease called rickets. Unfortunately, there isn't a global consensus on what is an adequate level of vitamin D in your blood, although the acceptable ranges are relatively close; nor is there a consensus in the units of reporting -- in the US the levels are reported in ng/ml, elsewhere they are reported in nmol/L. And often doses are prescribed in IUs (International Units). All this leaves most people quite confused particularly when different organizations recommend different normal levels of vitamin D. So if you are comparing your levels with those of someone geographically removed, make sure you check the units before you draw conclusions!
Table from the Vitamin D Council
Recent studies have shown that adequate levels of vitamin D might be necessary to reduce your susceptibility to a variety of diseases including some cancers, cardiovascular diseases, inflammatory diseases, and even in maintaining a robust nervous system. The breadth and pace of research in studying vitamin D's roles in these settings has exploded in the last decade. And this is accompanied by some physicians recommending regular evaluations of the level of vitamin D in your blood, and when necessary, prescribing appropriate doses of the vitamin.
So what's to be done?
While correcting low levels of vitamin D with steady doses of the vitamin may address the subtle symptoms of vitamin D deficiency, it's important to keep in mind that the vitamin is often a mediator rather than a direct effector in a biochemical pathway, as seen in its role to increase the absorption of dietary calcium. In fact, this is not uncommon when it comes to the role of vitamins -- they are the quiet worker bees, keeping the hive of life going! Equally important is to note that excessive dosing with vitamin D has its own problems, primarily because of an excess of calcium in the blood. A (very) broad but useful way to think about any supplements is that if they are fat soluble -- as is vitamin D -- they stay in your body much longer than water soluble ones, because they can "hide" in fatty tissues; and by staying longer, their effects are also more long-lasting.
The 2015-2020 Dietary Guidelines for Americans notes that "Nutritional needs should be met primarily from foods. Foods in nutrient-dense forms contain essential vitamins and minerals and also dietary fiber and other naturally occurring substances that may have positive health effects. In some cases, fortified foods and dietary supplements may be useful in providing one or more nutrients that otherwise may be consumed in less-than-recommended amounts." Decades old and even more recent research has demonstrated that exercise maintains calcium and vitamin D at normal levels, so with sensible nourishment and exercise it might be possible to significantly reduce expenses associated with vitamin D testing and dosing.