16 February 2021
By Jack Coxall

What is Vitamin D?

Vitamin D is a fat soluble vitamin (a vitamin that is absorbed along with fats in the diet and can be stored in the body’s fatty tissue) and is one of the 24 essential micronutrients needed to keep the human body functioning properly. It is one of the few vitamins that humans can produce, something dependent on exposure to sunlight. Vitamin D helps keep bones, teeth and muscles healthy by regulating the amount of calcium and phosphate in the body. It also plays a role in proper immune function, preventing certain diseases, promoting good cardiovascular and reproductive health and numerous other bodily functions. Vitamin D deficiencies are common due to a lack of exposure to direct sunlight and the difficulty in consuming adequate amounts through dietary sources.

What does it do?

Bone and teeth health

Vitamin D aids with the absorption of calcium, which is critical for teeth and bone health. Together, these micronutrients improve bone mineralisation and correct secondary hyperparathyroidism (a medical condition characterised by excessive secretion of parathyroid hormone) which can cause bone disease and calcium build up in the tissues of the cardiovascular system. Low levels of Vitamin D are linked with osteomalacia, osteoporosis (both conditions that cause the weakening of bones) and bone fractures.

Muscle strength and power

Skeletal muscle is a target organ for Vitamin D, meaning deficiencies lead to weakness. A lack of Vitamin D leads to abnormalities in muscle contraction and relaxation which affects muscle force production. A positive correlation between adequate levels of Vitamin D and a reduction in the degradation of protein has also been found.

Furthermore, a 2008 study found that optimal Vitamin D levels improved the production of muscle power development and resulted in a greater jump height.

Lean body mass

Vitamin D plays a significant role in maintaining muscle and avoiding the development of fat in muscle, thereby contributing to greater lean body mass. There has been evidence to show body fat mass is higher in those with a Vitamin D deficiency, something which correlates with elevated levels of parathyroid hormone and intracellular calcium both of which are considered major factors in determining obesity. Increased calcium levels promote an accumulation of fat tissue while suppressing fat burning.

Treatment of skin disorders

Vitamin D lessens inflammation which means it can play a role in treating skin disorders such as psoriasis. Studies have shown a correlation between patients suffering from inflammatory skin conditions including psoriasis, dermatitis, dandruff, eczema, rosacea, and severe acne and Vitamin D deficiency.

Blood sugar regulation

Adequate levels of Vitamin D play a role in increasing insulin sensitivity and decreasing insulin resistance, suggesting it may be effective in countering the symptoms of diabetes. Low Vitamin D decreases pancreatic cell function, impacting insulin sensitivity due to the presence of Vitamin D receptors on insulin cells.

A 2010 study of women with insulin sensitivity (who were pre-diabetic) found that daily vitamin D supplementation resulted in significant decreases in insulin sensitivity. This followed a 2009 study that found higher vitamin D levels lowered diabetes risk.

Disease prevention

Studies have shown a positive correlation between Vitamin D supplementation and a lowered risk of Multiple Sclerosis. One study of over 190,000 women found that those who supplemented had a 40% lower risk of MS than those that did not. Low Vitamin D levels have been identified as a major risk factor in MS susceptibility and severity.

Vitamin D has also been linked with fighting a number of cancers including breast, colon and prostate. There is evidence to suggest that supplementation may combat elevated levels of an enzyme that is associated with the development of aggressive lung cancer tumors. A 2011 study displayed a positive correlation between the 5 year survival rate of patients with a high level of Vitamin D (81% compared to 41% of those with low levels.)

There is also some evidence of a relationship between low Vitamin D levels and early onset Parkinson’s disease.

Testosterone levels and reproductive health

Low testosterone and poor fertility is often associated with low Vitamin D. A deficiency in men often correlates with lower levels of free testosterone and higher levels of estrogen. Vitamin D supports testosterone production because there are  Vitamin D receptors in the cells and glands that release testosterone. Vitamin D is also though to inhibit aromatization, wherein testosterone is changed into estrogen.

A study from the University of Copenhagen found that normal sperm count and motility (the efficiency at which sperm moves) is associated with adequate Vitamin D levels. Those with a deficiency had a lower proportion of mobile sperm than those with a high Vitamin D level. In terms of female fertility, Vitamin D has been shown to have a positive effect on rates of fetal implantation in the uterus. Furthermore, Vitamin D has also been shown to play a role in proper fetal brain development.

Immune function and prevention of infections

The activation of immune defences is dependent on Vitamin D. Low levels inhibit the capability of the body’s T-cells to fight off infections. T-cells need Vitamin D to activate them to fight off harmful pathogens. Vitamin D also limits inflammation which can inhibit the body’s ability to heal. As well as preventing infections to begin with, Vitamin D may also be used to treat viral, bacterial and fungal infections.

Cardiovascular health

High levels of Vitamin D are associated with heart health, while deficiencies are linked to cardiovascular disease. The Framingham Heart Study indicated that maintaining optimal Vitamin D levels was a crucial factor in preventing cardiovascular disease, with supplementation considered to contribute to prevention. Furthermore, Vitamin D deficiency is strongly associated with high blood pressure, something which has been shown to be significantly reduced when Vitamin D supplementation is paired with calcium intake.

Neurological health and sleep

Vitamin D enhances the metabolic process of brain neurons that protect from degenerative processes. Additionally, it promotes nerve growth and is an essential enzyme in the production of neurotransmitters, which play a key role in mood regulation. The instance of depression and other brain disorders is much higher in those with Vitamin D deficiencies.

Vitamin D also has a positive impact on sleep. Vitamin D receptors in the brain are in a large concentration in the cells of the brainstem that allows us to sleep. If Vitamin D levels are not adequate, the sleep/wake cycle is disrupted.


As mentioned previously, a key source of Vitamin D is direct exposure to sunlight. However, this is not applicable during autumn and winter months as the sun is not strong enough for the body to produce it. Furthermore, using sunscreen during the summer months may impede the skin’s absorption of enough sunlight for Vitamin D production (SPF 15 can reduce synthesis by 98%).

While it is difficult to maintain adequate levels through diet alone, certain foods do contain Vitamin D.

  • Dietary sources of Vitamin D include:
  • Oily fish (e.g salmon, sardines, herring and mackerel)
  • Red meat
  • Liver
  • Egg yolks
  • Fortified foods (eg certain fat spreads and breakfast cereals)

There are two forms of vitamin D in the diet:

Vitamin D2 (ergocalciferol): found in some mushrooms.

Vitamin D3 (cholecalciferol): found in animal sources.

D3 is the more powerful of the two types, and raises blood levels of vitamin D almost twice as much as D2.

How can you tell if you’re deficient?

Widespread use of sunscreen, the strength of sunlight in certain months and limited dietary sources mean Vitamin D deficiencies are prevalent. Researchers at Oregon State University estimated that nearly 1 billion people worldwide are deficient. Certain populations, such as people with dark skin and the elderly, are at a higher risk.

Symptoms of Vitamin D deficiency can be subtle, but may include:

  • Fatigue or tired
  • Bone pain
  • Joint pain
  • Muscle pain
  • Sour mood
  • Low energy
  • More frequent illness
  • Anxiety
  • Irritability
  • Weight gain
  • Hair loss

The most accurate means of finding out if you’re deficient is a blood test. Due to seasonal differences in sunlight exposure, and Vitamin D levels as a result, testing every 3-6 months is recommended. Ideal blood levels of Vitamin D are between 40 and 80 ng/mL, with anything below 20 ng/mL considered deficient.

Should you supplement?

During autumn and winter months the sun is not strong enough for the body to synthesize Vitamin D. That means maintaining adequate levels is reliant solely on the diet. However, it is very difficult to get sufficient Vitamin D from the diet alone, particularly for those following a vegetarian or vegan diet due to a lack of plant based sources of D3.

In 2016, the Scientific Advisory Committee on Nutrition recommended that adults and children over the age of one should take a daily supplement containing 10mcg of vitamin D, particularly during autumn and winter. Those at a higher risk of vitamin D deficiency were advised to supplement all year round.

If your blood tests show you are deficient you should supplement. If you don’t have access to a blood test but fall into a high risk category (including not getting enough sunlight) it would be advisable to supplement due to how widespread inadequate Vitamin D levels are. The amount of Vitamin D needed to cause toxicity (around 60,000IU per day for several months) means supplementation is relatively low risk.


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