This is a fascinating hormone (despite its name!), with much to know regarding evolution, geography, history, industrialisation, physics, chemistry, biology – truly a unifying wonder!
1. Considered a “hormone”, as made in skin and modified in liver and kidney, and exerts effects all through the body (classic definition of hormone is that it is made in body, and exerts actions in distant places). Main action is helping body absorb calcium from diet, as calcium is poorly absorbed otherwise, and calcium is a key nutrient for normal functioning of nerves, heart, muscles, cells. With D 70% of calcium in diet may be absorbed, without D only 20-30% absorbed. Vit D probably evolved for this reason.
2. Was classified under vitamin, as our body needs small amounts constantly, and is made by exposure of body to sunlight, or by eating other foods that contain Vit D (like classic vitamins)
3. Probably the earliest hormone in evolutionary terms, as earliest life forms that evolved in water had some primitive form of D. It is believed that this was used to maintain calcium levels in these organisms, and also for building and maintaining exo or endo skeletons.
4. Made in the skin currently of animals, and in leaves in plants (parts exposed to sunlight)
Made under influence of a particular range of UV light which acts on a type of fat or cholesterol in skin, which then makes vit D
5. More Vit D made when sun (or UV light) hits skin at 90 degrees so around noon best time.
When we just start to feel the burn in skin, Vit D is has been made. UV cannot penetrate through clothes or glass, so exposure of as much as bare skin as possible is needed.
Darker skinned people with more melanin need longer exposure to sunlight, and lighter skinned people need less to make vit D. This is one of the reasons why the further north in the world we go, people are lighter skinned, to maximise D production when sunlight is less.
6. Vit D levels may be lower in women for this reason, as we are in general more covered up. Also, as we get older, the level of the particular cholesterol in skin drops, so older people are less efficient in making Vit D on exposure to sunlight.
7. Worldwide prevalence of D deficiency increased with industrialisation and pollution. In India, worse deficiency prevalence seen in women, urban areas, children and people over 70 years, sedentary indoor workers.
8. Associations of D deficiency (< 30 ng/ml) with many diseases shown – that is people with lower levels of vit D have higher prevalence of diabetes, obesity, cancers, hypertension etc. HOWEVER, this does not imply CAUSATION as replacement studies have not shown reduced risk with replacement. It may be that the same factors that cause obesity and diabetes (being indoors with less physical activity) also causes D deficiency.
9. Best way to get vit D is by exposure to as much sunlight as possible, for urban dwellers at least 2-3 times/week for 5 minutes with as much skin exposed as socially allowed!!
Vit D is a fat soluble vitamin. This means that levels get stored in fat in the body for long periods of time. this is in contrast to water soluble vitamins (B vitamins) which get excreted easily from the body and we cannot build up toxic levels. SO there is a danger of taking too much Vit D without monitoring. We have seen people overdosed on vit D, with toxic vit D levels (which can cause headaches, vomiting, raised pressure in the brain) and high calcium causing renal failure. Supplements containing 400-1000 IU per day are probably safe, if there is no underlying high calcium problem. However levels exceeding 2000-3000 IU per day – better to monitor calcium and vit D levels. So it is NOT a vitamin to pop in as and when u feel a bit down!!! Think of it as a hormone tablet.
That being said, the tablets available do supplement the levels quite well. So supplements do work well, when needed. Rarely oral tablets do not bring up blood levels well, and one might need injections.
10. Vit D also found in egg yolks, and fish (bones and liver).
11. Some countries fortify plants and milk with D, not yet done in India.
Hope this helps. One of the most fascinating hormones, as there are geographical, biological, evolutionary, historical correlate