In recent years there has been a tremendous increase in the awareness and use of vitamins and minerals.

Although there have been many new discoveries on how our body metabolizes and deals with these micronutrients, we are far from knowing it all where these substances are concerned.

It is thus very important for all of us to stay as current as possible on what new discoveries are being made, where our health is concerned, as what may have been unnecessary in the past today is necessary or vice-versa.

In today’s tutorial let us cover one of the most talked about vitamins today – vitamin D.

Vitamin D is a fat soluble vitamin. It can be found in various forms in the body such as cholecalciferol, calcidiol and calcitriol.

Sources of Vitamin D

Due to its regulatory properties and the tissues it targets, vitamin D in its active form is regarded like a steroid hormone. It can be obtained from 3 major sources:

  • Sunlight
  • Supplements
  • Dietary food products

Sunlight is the major source of vitamin D, since UV rays are needed for the reaction that converts 7-dehydrocholesterol to cholecalciferol (vitamin D3) and this prompts further conversion to the most active form calcitriol.

We only need to spend an average of 15 – 30 minutes in the sun, depending on our skin color, to get the minimum recommended intake. This is still by far the most efficient and easiest way to obtain vitamin D.

Supplements are available of pure vitamin D or in combination with other vitamins (ie. multivitamin mixes). Usual strength found of supplemental vitamin D alone in today’s pharmacies is 400 or 800IU, with 1000IU and even 2000IU becoming more popular. In multivitamin packages such as Centrum the dose is also 400IU, however others contain lower amounts varying to even below 100IU.

Although supplements are the preferred method of choice for taking in vitamin D, they are the least efficient where our body is concerned as it is not the most natural way for our body to obtain and metabolize this vitamin.

The third source of vitamin D can be obtained from foods like fish, eggs, some organ meats, as well as fortified milk and dairy products. However, none of these are good choices for optimal health given that the risks and drawbacks of eating them greatly outweigh any benefit they can offer. For example, fish and seafood are some of the riskiest foods we can possibly eat.

Dietary vitamin D is a good choice for most and much more efficient in terms of actual absorption of the vitamin compared to supplements but its drawback is that it is not feasible for those who choose to avoid animal products for other health reasons.

Vitamin D Metabolism

So based on the above 3 sources, as we already mentioned vitamin D from sunlight is the easiest form to obtain, most efficient for our body and plentiful upon adequate exposure.

The problem with dietary vitamin D is that it is not fully active and must be converted in the body by the liver to be used.

Metabolism of vitamin D from sunlight begins with cholesterol being converted to 7-dehydrocholesterol which is secreted onto the skin’s surface. Upon UV exposure, 7-dehydrocholesterol absorbs specific wavelengths and is converted to vitamin D3 in the skin. Other byproducts are formed, but these are usually lost with skin turnover.

Vitamin D3 then enters the blood stream, being carried by a vitamin D binding protein and is transported to the liver where it is converted to calcidiol (25-OH-Vit D3).

Dietary vitamin D follows somewhat a similar path except that it is packaged into special units with the help of bile salts and absorbed into the intestinal cells. It is carried through by the lymphatic system into the blood and following to the liver where it encounters the same fate of conversion to calcidiol.

From the liver calcidiol is transported to the kidney via the blood. There a reaction occurs producing the active form of the vitamin – calcitriol (1,25-(OH)2Vit D3).

Calcitriol is then the form of the vitamin used by bodily tissues and processes.

Very little vitamin D is excreted, however that which is lost is through the bile.

From this one can see that lacking exposure to the sun, such as during the winter months or absence of equal dietary amounts will have an adverse effect on all those functions that depend on its presence.

Functions of Vitamin D In Our Body

Vitamin D is an essential nutrient for bone and tooth development and maintenance.

Vitamin D is also crucial for calcium absorption since it increases it by 30-80%.

Calcitriol mainly functions alongside the parathyroid hormone (PTH) in maintaining steady calcium concentrations in the blood. When these decline PTH secretion increases and this signals calcitriol to take action.

As a hormone, vitamin D targets 3 main tissues: the intestine, kidney and bone.

In the intestine, calcitriol works to increase the absorption of calcium and phosphorus. There it interacts with genes that code for proteins involved in calcium transport. The newly synthesized proteins act in the intestine to enhance calcium absorption.

Calcitriol also promotes this process by helping to open up voltage gated calcium channels for rapid calcium absorption.

In the elderly, the relevance of the action of vitamin D in the intestine is that with age intestinal calcium absorption decreases and this can be linked with a diminished capacity to catalyze vitamin D from the sun. This therefore results in a decreased capacity to convert vitamin D to the required metabolites.

In the kidney, calcitriol along with the parathyroid hormone is involved in the reabsorption of calcium and phosphorus from the distal renal tubule.

Again with age as kidney functions decrease, these processes are less efficient.

In bone, calcitriol works with osteocalcin. Here its main function is bone mineralization, bone modeling and remodeling.

With regard to the metabolism and function of vitamin D in humans, it is therefore very important to get an adequate intake to prevent deficiencies and possible future conditions such as osteoporosis.

Old nutrient recommendations suggested 200IU for children, 400IU for adults and 800IU for the elderly. Modern recommendations range from 1,000 to 5,000 on average, as experts claim that over 75% of people are vitamin D deficient. One of the greatest causes for this is the lack of time spent in the sun, or time spent covered in sunscreen spent in the sun. For the proper reaction to take place in our skin, we need proper sun exposure.

The minimum about of time we should each aim for is spending about 15 min/day in the sun.

Higher doses are required in elderly individuals since with age there can be increased bone loss. In the case of post menopausal women the decline of estrogen levels causes increased bone loss as well.

In general only individuals at risk for osteoporosis or other related conditions would require higher intakes of vitamin D. This includes: smokers, heavy drinkers, people lacking dairy products or ones not eating a balanced diet, as well as individuals with a genetic predisposition to the disease.

Also individuals on chronic steroids need to consume higher intakes, as well as have an increased calcium consumption (1500mg/day). This is due to the fact that steroid therapy results in bone loss which is actually most rapid during the first 6 months.


Therefore we can see that not only is vitamin D critical for many chemical reactions in our body, but it itself has many biochemical reactions it has to undergo in order to be used appropriately.

Today vitamin D is suspected to be one of the leading vitamins deficient in the majority of the population in developed countries. It has also been linked to the prevention of cancers such as breast cancer as well as other conditions, including those tied to the immune system. Hence it is critical to make sure we all get enough of this very important vitamin.

Thus, as noted supplements may be necessary for some people. However, for the majority of the population daily sun exposure is not only sufficient but also the most efficient method for obtaining this vitamin and having the body use it. For safe sun exposure see Part 5 of our Sun Smart Series.