Generally speaking, vitamins are organic compounds that cannot be made by the body, but are necessary for proper functioning.

Obtained through the diet or supplementation, vitamins are vital to growth, development, and metabolic reactions. Hormones on the other hand, are synthesized in the body from simple precursors and go to distant tissues where they have an intended effect and make multiple metabolic improvements. In the case of the manufacture of vitamin D which requires the help of an outside source to trigger a sequence of events, the precursor of a cholesterol-like molecule found in the skin cell (7-dehydrocholesterol; provitamin D3) starts the process by absorbing just the ultraviolet B portion of sunlight to create what’s called previtamin  D3.


Previtamin D3 quickly rearranges itself with the help of the body’s heat to give birth to vitamin D, which immediately exits the skin cell for the bloodstream. The fact that vitamin D is made in living skin cells explains why it is not possible to wash off vitamin D when you bathe after being exposed to the sun. Before vitamin D can act as a hormone, however, it must go through two steps of activation – one in your liver and another in your kidneys. Most humans obtain from sun exposure their vitamin D requirement between the hours of about 10:00 A.M. and 3.00 P.M. and mainly in the late spring, summer, and early fall. Because vitamin D in fat soluble, it’s stored in body fat and released throughout the winter months, allowing you to be vitamin D sufficient throughout the year.


Vitamin D Deficiency

Nutrient deficiencies are usually the result of dietary inadequacy, impaired absorption and use, increased requirement, or increased excretion. A vitamin D deficiency can occur when usual intake is lower than recommended levels over time, exposure to sunlight is limited, the kidneys cannot convert 25(OH)D to its active form, or absorption of vitamin D from the digestive tract is inadequate. Rickets and osteomalacia are the classical vitamin D deficiency diseases. In children, vitamin D deficiency causes rickets, a disease characterized by a failure of bone tissue to properly mineralize, resulting in soft bones and skeletal deformities. Prolonged exclusive breastfeeding without the AAP-recommended vitamin D supplementation is a significant cause of rickets. Additional causes of rickets include extensive use of sunscreens and placement of children in daycare programs, where they often have less outdoor activity and sun exposure. In adults, vitamin D deficiency can lead to osteomalacia, resulting in weak bones [1,5]. Symptoms of bone pain and muscle weakness can indicate inadequate vitamin D levels, but such symptoms can be subtle and go undetected in the initial stages.



-Bone health: prevents osteopenia, osteoporosis, osteomalacia, rickets, and fractures.

-Cellular health: prevents certain cancers, such as prostate, pancreatic, breast, ovarian, and colon; prevents infectious diseases and upper-respiratory-tract infections, asthma and wheezing disorders.

​-Organ health: prevents heart disease and stroke, prevents type 2 diabetes, periodontitis and tooth loss, and other inflammatory diseases.

​-Muscular health: supports muscle strength.

​-Autoimmune health: prevents multiple sclerosis, type 1 diabetes mellitus, Crohn’s disease, and rheumatoid arthritis.

​-Brain health: prevents depression, schizophrenia, Alzheimer’s disease, and dementia.

Mood-related health: prevents seasonal affective disorder, premenstrual syndrome (PMS, also known as premenstrual tension) and sleeping disorders, elevates the sense of well-being.​



Skin color has a tremendous impact, which is evident in studies done to identify patterns of deficiency. The darker your skin, the harder it is to make vitamin D, because melanin, your skin’s pigment, that gives it color, acts as a natural sunscreen. The older you are, the harder it becomes to synthesize enough vitaminD. A seventy-year-old has only quarter of the vitamin D-making capacity that a twenty-year-old has. Because vitamin D stored in fat cells, you’d think that people with excess fat would have plenty of extra vitamin D on hand to make up any shortage. As it turns out, that thinking is wrong! The fatter you are, the higher your risk for deficiency. Why? The vitamin D essentially gets locked inside the fat cells, unavailable for use. People assume that if you have a well-balanced diet, you are getting all the nutrients you need. There is very little vitamin D from dietary sources.

It’s principally found in oily fish, mushrooms, or sun-dried mushrooms, and in fortified foods like (plant)milk, orange juice, (plant-based)yogurt, some cheeses, and some cereals. (Note: Mushrooms are the only source of natural vitamin D in the produce section. Similar to the way that humans absorb sunlight and convert it to vitamin D, mushrooms contain a plant-sterol  -ergosterol- that converts to vitamin D when exposed to light. An increasing number of mushroom-growers are now exposing their product to ultraviolet light that produces even more natural vitamin D.)


What about fish? 

A ‘serving’ (3,5 ounces - 100 g) of wild salmon can impact 600 to 1000 IU, but few people eat wild salmon most days a week. 

The same amount of cod liver oil can provide 400 IU per serving, but even that’s too low, and a very few people eat cod liver oil every day. It’s simply not practical way to get your vitamin D, and you can get too much vitamin A. Because wild salmon get vitamin D from the food chain in nature, where there is plenty of vitamin D because phytoplankton and zooplankton photosynthesize it, wild salmon contain a high level of vitamin D. Farmed salmon, on the other hand, are fed pelleted food that has very little basic nutritional value. There is essentially no vitamin D in it. Farmed salmon has 10 to 25 percent of the vitamin D content of wild-caught salmon. To eat a sufficient amount of vitamin D (100 to 2000 IU), you’d have to consume three cans of sardines, drink ten to twenty glasses of fortified (plant)milk, gulp down ten to twenty bowls of cereals, snack on fifty to a hundred egg yolks, or eat seven ounces (200 g) of wild salmon for dinner every night. The inescapable fact is that humans have evolved in such a way as to be dependent on sunshine for life and health. 

Sunlight is the fuel that enables your body to manufacture vitamin D. When your body is unable to obtain sufficient sunlight, it can’t make vitamin D on its own.

D3 vs. D2

The chain reaction starts when UVB rays hit the skin’s surface, where a precursor to cholesterol called 7-dehydrocholesterol (also called provitamin D3 or 7-DHC) gets converted to previtamin D3. This compound then rapidly changes to vitamin D3 that can be released  from the skin cells and enter the bloodstream. This form is also called cholecalciferol, or vitamin D3. Vitamin D3 is still biologically inactive until the liver takes it and creates the major circulating form, 25-vitamin D (alternatively called calcidiol). Vitamin D is fat soluble, so that’s where it can be stored during winter months and released as needed. The vitamin D that is made in your skin is vitamin D3. Most supplemental vitamin D3 comes from an animal source (lanolin – from sheep). Vitamin D2 made from yeast and used for food fortification and supplements for more than forty years. Studies confirmed that vitamin D2 is just as effective as vitamin D3.


Overdose is extremely rare

Sunlight destroys any excess vitamin D that your body makes, so you could never become vitamin D intoxicated from sun exposure. You would need to ingest more than 10,000 IU of vitamin D a day for at least half a year to begin to worry about potential toxicity from supplements. Symptoms of toxicity include nausea, vomiting, loss of appetite, constipation, increased frequency of urination, increased thirst, disorientation, and weight loss.



Dr.M.Holick - The Vitamin D Solution