Vitamin D – are you getting enough – Why is it so important?

Why is Vitamin D so important?


The link to vitamin D deficiency and the development of rickets in children is well known.  Bones are living things – they need nutrition to keep them healthy. Rickets is a condition in infants and children of soft bones and skeletal deformities caused by failure of bone tissue to harden. The role of vitamin D in bone development is to help the body to absorb and retain calcium and phosphorus. Both of these are needed to help the body to build bone.


Vitamin D does not only help bone health. It can reduce cancer cell growth, help to control infections and reduce inflammation.


Vitamin D is both a nutrient that we eat and a hormone our bodies make. The trouble is that very few foods naturally contain Vitamin D and therefore we need to take a supplement. Furthermore, if we live in UK we often do not get enough vitamin D because we need sunshine and there is often not enough of that here!

That, coupled with the fact that few foods naturally contain vitamin D, means that for most of us we need to take a supplement of vitamin D.

Where do we get our vitamin D from?

Vitamin D supplements are available in two forms: vitamin D2 (“ergocalciferol” or pre-vitamin D) and vitamin D3 (“cholecalciferol”).

Both are also naturally occurring forms that are produced in the presence of the sun’s ultraviolet-B (UVB) rays, hence its nickname, “the sunshine vitamin,” but D2 is produced in plants and fungi and D3 in animals, including humans. Vitamin D production in the skin is the primary natural source of vitamin D hence the problem for a lot of people who live in places where sunlight is limited in winter, or because they have limited sun exposure due to being inside much of the time. Also, people with darker skin tend to have lower blood levels of vitamin D because the pigment (melanin) acts like a shade, reducing production of vitamin D (and also reducing damaging effects of sunlight on skin, including skin cancer).

Recommended Amounts

The Recommended Dietary Allowance (RDA) for vitamin D provides the daily amount needed to maintain healthy bones and normal calcium metabolism in healthy people. It assumes minimal sun exposure.

RDA: The Recommended Dietary Allowance for adults 19 years and older is 600 IU daily for men and women, and for adults >70 years it is 800 IU daily.

UL: The Tolerable Upper Intake Level is the maximum daily intake unlikely to cause harmful effects on health. The UL for vitamin D for adults and children ages 9+ is 4,000 IU.

What happens if we take too much without realising?

The use of vitamin D supplementation has increased substantially. Growing awareness of vitamin D in the general population, and over-the-counter vitamin D with often very high doses, include the risk for uncontrolled use and us imbibing too much vitamin D. This can result in hypercalciuria and finally hypercalcemia (excess calcium in our urine and in our blood).

Reports of vitamin D overdose are rare in the literature, however, there are also causes of hypervitaminosis D by those with lymphomas which are cancers that target our lymphatic system.

Vitamin D accumulation due to excessive intake lasts up to 18 months and may cause chronic toxic effects such as nephrocalcinosis (too much calcium in the kidneys) following hypercalcemia and hypercalciuria.

Since the 1930s, public health officials in the United States and the United Kingdom have recommended routine fortification of foods like milk to prevent vitamin D deficiency and low vitamin D status. However, there was then an increased incidence of hypercalcemia due to massive intakes of vitamin D from various food fortifications. In some cases, hypercalcemia was associated with drinking vitamin D-fortified milk, revealing a fortification of up to 232,565 IU instead of standard 400 IU/quart, and consequently, prohibition of milk fortification.

However, current evidence suggests that vitamin D fortification prevents deficiency safely and effectively. Feeding animals might represent an additional source of vitamin D without compromising product quality. For example, consumption of vitamin D-enriched eggs from hens.

Too much or too little – how do we know?

Worldwide, an estimated 1 billion people have inadequate levels of vitamin D in their blood, and deficiencies can be found in all ethnicities and age groups. Thus too little is a much bigger problem than too much.

In industrialized countries, doctors are seeing the resurgence of rickets, the bone-weakening disease that had been largely eradicated through vitamin D fortification.  There is, however, much scientific debate about how much vitamin D people need each day and what the optimal serum levels should be to prevent disease.

The Institute of Medicine recommendations increasing the daily vitamin D intake for children and adults in the U.S. and Canada, to 600 IU per day.  The report also discussed whether the upper limit from 2,000 to 4,000 IU per day.

New evidence has supported other benefits of consuming an adequate amount of vitamin D, although there is still not consensus on the amount considered to be adequate. Although observational studies see a strong connection with lower rates of certain diseases in populations that live in sunnier climates or have higher serum levels of vitamin D, clinical trials that give people vitamin D supplements to affect a particular disease are still inconclusive. This may be due to different study designs, differences in the absorption rates of vitamin D in different populations, and different dosages given to participants.

Specific Health conditions and diseases related to vitamin D deficiency


The effect of vitamin D on the lungs has a strong link particularly in acute respiratory failure.

Vitamin D supplementation reveals direct anti-inflammatory properties in the lungs and therefore those with respiratory disease should take it. Significant benefits have already been shown in adults and children with asthma, and for the prevention of respiratory tract infections, particularly in severe vitamin D deficiency.


Sepsis, a complication of severe infection, is characterized by signs of systemic inflammation expressed with failure of organs often remote from the site of the initial infection. Septic patients have high mortality and lower circulating levels of vitamin D.

Organ transplantation recipients

The recipients of solid organ transplantation are, by definition, vitamin D insufficient for many reasons, including limited sunlight exposure, limited physical activity, reduced dietary intake of vitamin D in food, as well as liver and kidney dysfunction according to their main disease. As an example, in liver transplantation recipients (a group of patients with very low vitamin D levels), osteoporosis has a high prevalence, with a large decline in bone mineral density in the first year after transplantation.


Several studies in recent years have highlighted that women are at high risk for vitamin D deficiency, and this is associated with adverse pregnancy outcomes, including preeclampsia and gestational diabetes.

Vitamin D supplementation is able to reduce adverse pregnancy outcomes when a higher level is achieved. Three major adverse pregnancy outcomes appear to improve with vitamin D supplementation: a 60% reduction in preeclampsia, a 50% reduction in gestational diabetes, and a 40% reduction in preterm delivery. The FDA has recently approved the statement “Pregnant women who have higher serum vitamin D levels have a decreased risk of preterm birth.”

Taking into account the recent literature, vitamin D deficiency is associated with worse outcomes during pregnancy, and at least 400–600 IU of daily vitamin D supplementation is reasonable for women with a vitamin D level <40 ng/mL, with higher required doses in more severe deficiency.


Vitamin D supplementation as a strategy for preventing cancer was considered, as results from several observational studies suggested an association between vitamin D deficiency and risk for several types of cancer. It was already assumed in 1980 that calcitriol could inhibit the growth of malignant melanoma cells. Studies revealed a decreased cancer mortality in areas with greater sun exposure but the cancer-protective effect of vitamin D remains unclear.

An ongoing long-term random controlled trial that investigated vitamin D supplementation and the incidence of cancer and precancerous lesions in a high-risk population (overweight adults with prediabetes), will provide further and important data on the causality.


Several studies demonstrated a link between a higher frequency of vitamin D deficiency in patients with type 1 diabetes mellitus compared with healthy individuals.

There is a clear effect of vitamin D in young adults, as low vitamin D levels were significantly associated with developing diabetes type 1. However, according to the available literature, the cause-and-effect relationship is inconclusive. In a recent study by Pittas et al., vitamin D did not significantly reduce new onset of diabetes, however, analysis of individuals with a 25(OH)D level below 12 ng/ml (30 nmol/l) revealed a significantly reduced risk of developing type 2 diabetes.

Musculoskeletal effects of vitamin D

The detrimental effects of vitamin D deficiency on the musculoskeletal system were the first visible mode of action that was attributed to vitamin D (i.e., rickets in children).

Food Sources

Few foods are naturally rich in vitamin D3. The best sources are the flesh of fatty fish and fish liver oils. Smaller amounts are found in egg yolks, cheese, and beef liver. Certain mushrooms contain some vitamin D2; in addition some commercially sold mushrooms contain higher amounts of D2 due to intentionally being exposed to high amounts of ultraviolet light. Many foods and supplements are fortified with vitamin D like dairy products and cereals.

  • Cod liver oil
  • Salmon
  • Swordfish
  • Tuna fish
  • Orange juice fortified with vitamin D
  • Dairy and plant milks fortified with vitamin D
  • Sardines
  • Beef liver
  • Egg yolks
  • Fortified cereals


Is There a Difference Between Vitamin D3 and Vitamin D2 Supplements?

If you purchase vitamin D supplements, you may see two different forms: vitamin D2 and vitamin D3. Vitamin D2 is made from plants and is found in fortified foods and some supplements. Vitamin D3 is naturally produced in the human body and is found in animal foods. There is ongoing debate whether vitamin D3 “cholecalciferol” is better than vitamin D2 “ergocalciferol” at increasing blood levels of the vitamin. A meta-analysis of randomized controlled trials that compared the effects of vitamin D2 and D3 supplements on blood levels found that D3 supplements tended to raise blood concentrations of the vitamin more and sustained those levels longer than D2.  Some experts cite vitamin D3 as the preferred form as it is naturally produced in the body and found in most foods that naturally contain the vitamin.

Ultraviolet Light

Vitamin D3 can be formed when a chemical reaction occurs in human skin, when a steroid called 7-dehydrocholesterol is broken down by the sun’s UVB light or so-called “tanning” rays. The amount of the vitamin absorbed can vary widely. The following are conditions that decrease exposure to UVB light and therefore lessen vitamin D absorption:

  • Use of sunscreen; correctly applied sunscreen can reduce vitamin D absorption by more than 90%.
  • Wearing full clothing that covers the skin.
  • Spending limited time outdoors.
  • Darker skin tones due to having higher amounts of the pigment melanin, which acts as a type of natural sunscreen.
  • Older ages when there is a decrease in 7-dehydrocholesterol levels and changes in skin, and a population that is likely to spend more time indoors.
  • Certain seasons and living in northern latitudes above the equator where UVB light is weaker. In the northern hemisphere, people who live in Boston (U.S.), Edmonton (Canada), and Bergen (Norway) can’t make enough vitamin D from the sun for 4, 5, and 6 months out of the year, respectively. In the southern hemisphere, residents of Buenos Aires (Argentina) and Cape Town (South Africa) make far less vitamin D from the sun during their winter months (June through August) than they can during their spring and summer months. The body stores vitamin D from summer sun exposure, but it must last for many months. By late winter, many people in these higher-latitude locales are deficient.

Are you more at risk of being Vitamin D deficient?

  • People with inflammatory bowel disease (ulcerative colitis, Crohn’s disease) or other conditions that disrupt the normal digestion of fat. Vitamin D is a fat-soluble vitamin that depends on the gut’s ability to absorb dietary fat.
  • People who are obese tend to have lower blood vitamin D levels. Vitamin D accumulates in excess fat tissues but is not easily available for use by the body when needed. Higher doses of vitamin D supplementation may be needed to achieve a desirable blood level. Conversely, blood levels of vitamin D rise when obese people lose weight.
  • People who have undergone gastric bypass surgery, which typically removes the upper part of the small intestine where vitamin D is absorbed.


 Signs and symptoms of Vitamin D Toxicity 

Vitamin D toxicity most often occurs from taking supplements. The low amounts of the vitamin found in food are unlikely to reach a toxic level, and a high amount of sun exposure does not lead to toxicity because excess heat on the skin prevents D3 from forming. It is advised to not take daily vitamin D supplements containing more than 4,000 IU unless monitored under the supervision of your doctor.

Symptoms of toxicity:

  • Anorexia
  • Weight loss
  • Irregular heart beat
  • Hardening of blood vessels and tissues due to increased blood levels of calcium, potentially leading to damage of the heart and kidneys

Don’t forget!

Catching the sun’s rays in a sunny office or driving in a car unfortunately won’t help to obtain vitamin D as window glass completely blocks UVB ultraviolet light.

Vitamin D is not a ‘cure all’, but may be an important, inexpensive, and safe additional therapy for many diseases and stages of life, including pregnancy, childhood, and old age.