Health benefits of Vitamin D

Vitamin D is vital for the healthy functions of the entire body and mind – not just the bones.   Vitamin D supports the immune system which may explain the link between darker skinned people and worse outcomes after covid-19.  This page explains some of the scientific facts and evidence for the benefits of vitamin D and provides practical tips how to enhance levels safely.

What is Vitamin D

vitamin D covid-19, immunityVitamin D is an oil-soluble essential vitamin. Ingestible forms include vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). These are converted to 25-hydroxyvitamin D in the liver and then to bioactive calciferol in the kidneys. Calciferol regulates the absorption of calcium, magnesium and phosphate from the gut; modulates the healthy growth and remodelling of bone; and has effects on cell growth in other organs. The serum reference range of 25-hydroxyvitamin D is 75-200 nmol/l (30-80 micro g/l). Because of the importnce of vitamin D, ideally it would be a good idea to check your blood levels from time to time.

Recommended Daily Amount: Oral requirements are greater in the winter than in the summer. The 2010 RDA is 600 IU for those 1-70 years of age, and 800 IU (40 Microgram) for those over 71. Overweight individuals require more vitamin D as it can be broken down in the peripheral fat. Dark skin people would need high oral intake esepcially if they avoid sun exposure. It is important to consider other essential mineral levels as a number of minerals, namely magnesium, are involved in the bioactivity of vitamin D, meaning a deficiency can impact on vitamin D function. Vitamin D has a half-life of 6 weeks, meaning that, without high dietary intakes, three months of no sunlight exposure will lead to blood levels dropping to one-quarter of what they should be. This explains the findings published at the National Cancer Conference in 2013 that over 70% of people from the UK had suboptimal levels of vitamin D after being tested, with amounts of the vitamin in the body noticeably lower between December and March. Although Vitamin D2 is commonly used in oral supplements, a randomised trial published in 2017 in the American Society of Nutrition Journal found that D3 was better at increasing serum 25-hydroxyvitamin D than D2, especially in Asian women.

Vitamin D and health

Viral infections (Covid-19): Studies are emerging that low vitamin D increases the risk of respiratory tract infections. This is thought to be via several mechanisms involving immune defence and regulation of inflammation. Vitamin D enhances production of proteins called cathelicidins and defensins that can lower viral replication rates and reduce concentrations of pro-inflammatory cytokines. The virus attacks the lining of the lungs, leading to pneumonia. In addition  dramatic increases increasing  of anti-inflammatory cytokines leads to excess inflammation, called the cytokine storm, that causes the severe respiratory distress and even death. It is interesting that case-fatality rates increase with age, obesity  and with chronic disease comorbidity, both of which are associated with lower 25(OH)D concentration… read review article

Rickets and osteoporosis: Vitamin D deficiency in children leads to severe bone damage (rickets) and osteomalacia in adults. These conditions were common among Asian communities in the early seventies, who came to the UK from sunnier climates and struggled to adjust until supplementation and education programmes were implemented. Unfortunately, although rickets is rare, suboptimal vitamin D levels are still prevalent in Asian communities. Vitamin D deficiency, particularly if combined with a poor calcium diet and lack of weight-bearing exercise, is a risk factor for osteoporosis

Chronic disease: Epidemiological studies have shown that 25OHD deficiency is closely associated with a number of common chronic diseases such as cardiovascular disease, dementia, Parkinson’s disease, other neuropsychiatric disorders and autoimmune diseases. A recent study has also confirmed that individuals within populations with the lowest serum 25(OH)D concentrations had the highest all-cause mortality rates. This increased chance of death took into account other factors such as obesity and smoking.

Arthritis and vitamin D: Research has found that vitamin D may play a significant role in joint health and that low levels may increase the risk of arthritis.  Several studies have found low blood levels of vitamin D in patients with arthritis of the hip and knee. In another study of more than 2,000 people, researchers found that deficiency was strongly associated with disabling symptoms among those with rheumatoid arthritis.

Vitamin D and cancer: Epidemiological studies have reported men and women with jobs involving exposure to sunlight were less likely to develop several cancers including prostate, kidney and bowel cancer (Luscome, Schwartz). Calciferol has been shown in laboratory studies to slow cancer growth and spread by reducing proliferation, promoting differentiation, inhibiting invasion, preventing loss of adhesion and promoting apoptosis (Chiang). Vitamin D has also been shown to interact with the androgen-signalling pathway in animals, inhibiting the production of factors which stimulate new blood vessels growing into cancers – stopping them growing (angiogenesis). Studies have reported a strong link between low levels of vitamin D and a high chance of melanoma spreading, alongside more aggressive colorectal, breast and prostate cancers which are more likely to progress faster, respond less well to treatment, relapse after cancer treatments and be fatal (Lazzeroni, Gupta, Ng, Rose, Mondul, Giovannucci).

Vitamin D and cancer treatments: In patients with metastatic bowel cancer, low vitamin D levels were linked to a reduced response to chemotherapy. More recently, patients who used vitamin D supplements had reduced odds of developing severe diarrhoea related to colitis when taking new immunotherapies such as  PD-1 checkpoint inhibitors.

How to increase vitamin D levels

Vitamin D and sunlight:
Vitamin D levels are higher amongst those who exercise outdoors regularly as UV-B radiation interacts with the skin to make cholecalciferol from cholesterol. It is estimated that the skin produces almost 80% of the body’s required vitamin D. The most important way to increase vitamin D levels is to try and get regular but controlled sun exposure. Over the winter months, unless you can get one or two holidays in the sun, it would be sensible to take a vitamin D3 supplement at a dose of around 1000IU (50 micrograms) per day. sunlight, particularly sunburn, is the main cause of epithelial skin damage, premature ageing and skin cancers, and clearly should be avoided. On the other hand, regular, sensible sun exposure is the best way to maintain adequate serum vitamin D levels.

Vitamin D supplements:
Despite the direct causational links between sunlight, vitamin D and cancer, it has not been established that  correcting low vitamin D with supplements directly prevents cancer and improves cancer outcomes. One recent US-based study reported no reduction of cancer risk in a randomised trial involving a vitamin D supplement versus placebo. This trial, however, failed to answer the fundamental question of whether correcting low vitamin D level would be beneficial. The 2,303 postmenopausal women evaluated actually had adequate to high levels of serum vitamin D at the start of the intervention (Lappe) and, therefore, it’s logical to speculate that the results would be very different if researchers compared cancer levels among those with existing deficiencies. To support this, a study from Nebraska including 1179 women more likely to be vitamin D deficient because of their northern latitude did report a significantly lower cancer incidence in the group who had taken Vitamin D3 (Lappe).

Probiotics and gut health:
As a fat soluble vitamin, the absorption of vitamin D is strongly linked to a healthy gut. A number of studies have linked the intake of a good quality probiotic supplement together with vitamin D supplements with higher blood levels of vitamin D3 [Jones]. Other factors to improve gut health include, eating plenty of prebiotic polyphenols, reducing processed sugar intake, reducing excess meat, exercising regularly and eating bacteria rich foods more

The following foods contain some vitamin D:

  • Oily fish and fish oils
  • Fresh green leafy vegetables
  • Gains and some cereals, especially if fortified
  • Nuts
  • Yeast products
  • Sun-dried mushrooms (their skin make vitamin D, just like human skin)
  • Vitamin D is commonly fortified foods

Vitamin D excess

Vitamin D toxicity is rare and is caused by excessive supplementation rather than sunlight. A safe upper limit of supplement dose has not been established as this depends on the underlying serum levels, diet and sun exposure. High intake (>4000IU), for short periods of time, may be needed to correct a significant deficiency, but this dose is likely to create excess levels and possible toxicity if taken over several months. In general, the safe level is around 1000IU / day, but up to 4000IU is likely to be safe, albeit probably unnecessary. It is recommended that anyone taking over 1000IU a day should have their vitamin D and calcium levels checked from time to time.


Vitamin D deficiency is associated with a wide number of medical conditions, an increased risk of cancer relapse and overall lower chances of survival. Further trials are required to confirm the efficacy of correcting vitamin D levels with sunlight, diet and supplements. In the meantime, the general consensus is that preventing deficiency, particularly over the winter months, is a very sensible strategy.


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