Evidence Based Nutrition

Vitamin D may be labelled as a vitamin however technically it is more like a steroid hormone that regulates various tissue processes (5). Vitamin D is produced mostly in the skin and production depends on skin pigmentation either natural or caused by sunburn, which is then metabolised into more active compounds (21). The discovery that most tissues in the body have a vitamin D receptor (VDR) and several tissues possess the ability to convert the circulating vitamin D into its active form has provided new insights into this vitamin and the interest has grown to discover the roles it can play in decreasing the risks of chronic illnesses (14). There are two forms of vitamin D, vitamin D₃ and vitamin D₂. D₃ is synthesised in the skin and D₂ is the form converted from our diet, the differences are very minor, and both join to all VDR therefore it does not matter which form of vitamin D we are receiving (8). 

Vitamin D’s main and most well-known source is the sun, exposure to sunlight and UVB radiation on our skin converts 7-dehydrocholesterol to pre-vitamin D₃ which is then converted to vitamin D₃ (14). Overexposure to sunlight does not cause vitamin D intoxication as excess pre-vitamin D₃ is destroyed by sunlight, also the pigmentation of the skin changes when sunburnt this reduces the amount of vitamin D absorbed creating a negative feedback loop (14, 21). Vitamin D can also be obtained through the diet although most modern diets contain inadequate vitamin D (5). Non the less, food sources include fatty oily fish such as salmon and mackerel, mushrooms, liver and egg yolk (15). A lot of foods can be fortified with vitamin D now such as milk, orange juice, bread, cereals, yoghurts, and margarine however, there is only around 100IU of vitamin D in an 8-oz of milk or orange juice serving (15). Whereas vitamin D found naturally in food can vary for example wild-caught salmon contains approximately 500 – 1000IU per 3.5oz, compare that to farmed salmon in the US containing approximately 100 – 250IU per 3.5oz (15). There are also over the counter supplements available providing a wide range of dosages using both vitamin D₂ and D₃. Both forms of vitamin D whether ingested or synthesised through the skin goes to the liver where it undergoes a process that eventually turns it into the active form of vitamin D, Calcitriol (7). Calcitriol can then bind to VDR all over the body such as intestines, bones, kidneys and muscles (7). 

Most European countries including the UK currently recommend a minimum intake of 10 – 15 mg (400 – 600IU) per day of vitamin D (12). However, most experts agree that without adequate sun exposure children and adults require 800 – 1000IU per day and from the evidence I have found I agree that the recommended daily allowance (RDA) should be increased. The elderly are at higher risk of falling and if deficient or low in vitamin D levels then these falls are more likely to cause serious injuries. Vitamin D enhances the absorption of calcium, so these two micronutrients work together in maintaining bone strength and health amongst other things. A study found vitamin D alone without calcium had no significant effect on the risk of falling however, 8 weeks of 800IU of vitamin D₃ combined with 1200 mg of calcium reported reduced secondary hyperparathyroidism, body sway and the number of falls after 1 year in elderly women (24). Other studies have found similar results in a meta-analysis of 5 randomised clinical trials that found vitamin D intake reduced the risk of falls by 22% they also discovered that 800IU of vitamin D per day plus calcium reduced the risk of falls whereas 400IU did not (25). Sufficient serum blood levels of vitamin D are defined as >30 ng/mL and it is thought that 1000 IU is required to maintain good serum levels however may not be enough to help increase levels enough if a person is insufficient or deficient (13). The Canadian Cancer Society recommends all Canadian adults ingest 2000IU of vitamin D a day to decrease the risk of cancer, (15) lactating women given 4000IU a day saw an increase in serum levels to over 30ng per mL and was also able to transfer enough vitamin D into their milk to satisfy an infant’s requirements (11). 4000IU is considered to be the highest recommended tolerable amount of vitamin D for everyone without causing toxic effects. Toxic effects of vitamin D are very rare in fact 10,000 IU of vitamin D₃ for 5 months does not cause toxicity, however, if this is ingested for more than 6 months risk of intoxication increases (14A consistently high dosage a day will cause intoxication, 50,000 IU can raise serum levels to 150ng per mL and is associated with hypercalcemia (which can cause kidney stones and weakens bones) and hyperphosphatemia (which can cause chronic kidney disease amongst other conditions) again this is very rare and getting that amount consistently could be difficult (14). Therefore, it is not advised to take more than 4000IU a day just to be on the safe side as each person is different and this is considered a safe level for everyone. 

The main and most well-known function of vitamin D in the body is to help keep bones, teeth and muscles strong making sure maximum function is kept. Vitamin D increases the absorption of calcium and phosphorus, phosphorus is a component of bones, teeth, DNA and RNA, which play a role in vitamin D’s main functions (7). Vitamin D has been found to play a role in a wide variety of biological processes and has many other functions that do not include calcium metabolism or muscle health. Up-regulation of genes, downregulation of inflammatory markers and has an antiproliferative effect are other benefits of vitamin D that has been found (18). The antiproliferative effect of vitamin D may also be why vitamin D has been associated with a decreased risk of certain cancers. For example, in the American population, vitamin D deficiency or insufficiency was associated with all-cause mortality in those with colon cancer (1). Woo et al., (2005) found that men with metastatic prostate cancer who received 2000IU of vitamin D a day had 50% reduction in prostate-specific antigen levels after 21 months. Immune cells also have a VDR, therefore, indicating a role vitamin D could play in the immune system. It is only just recently we have discovered vitamin D deficiency is associated with disease progression and increased mortality (10). Vitamin D’s effects on viruses are less clear however, has been growing in interest especially infections of the respiratory tract as VDR are also found in the respiratory system. This is why the role of vitamin D on COVID-19 has been heavily researched since 2020 as COVID is known to cause respiratory issues. Randomised control trials have found vitamin D supplementation reduces respiratory diseases (23) therefore it was thought it could do the same for those with COVID. There were studies done on this and some of those studies found vitamin D supplementation to be associated with less severe COVID and improved outcomes also deficiency was associated with an increased risk of COVID infection and mortality (3, 4, 19, 22, 2). However, none of these studies are conclusive so it cannot be stated that vitamin D can be used as a cure for COVID although evidence suggests it may be an important ally for fighting against the virus as again low levels were associated with severity and mortality (9). 

Vitamin D deficiency is defined as serum levels of <20 ng/mL and insufficient levels are considered to be between 21-29 ng/mL (15). Vitamin D deficiency is a worldwide problem and has been estimated that 1 billion people are deficient or insufficient if we are going by the definition previously mentioned (14). Vitamin D deficiency causes Rickets in children or Osteomalacia in adults these affect bone density and growth due to failure to mineralise newly formed bones in children or bone demineralisation in adults (12). This leads to bowlegs, knock knees, bone pain, deformities and muscle weakness (12). Studies also indicate impaired vitamin D metabolism can lead to health problems such as cognitive decline, depression, cardiovascular disease, diabetes and cancer, deficiency can also affect immune regulation, genome stability and neurogenesis (7). Those at higher risk of vitamin D deficiency are those with darker pigmented skin, those with malabsorption, obesity and have low sun exposure, especially those living in countries in the northern hemisphere such as the UK (18). 

Vitamin D has a direct impact on skeletal muscle function, due to muscles having VDR it has been found that these receptors can promote protein synthesis directed through vitamin D, vitamin D also stimulates muscle cell uptake which is important for muscle contraction through the production of ATP and creatine phosphate (21). Vitamin D may also protect against overuse injuries such as stress fractures and poor vitamin D status affects muscle strength therefore, adequate vitamin D levels may well be just as important or even more so for athletes (20). It was found that athletes are no different to non-athletes when it comes to vitamin D intake or serum levels (17), indoor athletes are particularly at risk, but also outdoor athletes are deficient. Willis et al., (2008) found 77% of German gymnasts had levels below 35ng/mL and 37% had below 10ng/mL. Testing on the New York Giants football team (American football) found 81% were vitamin D deficient however, this was done during spring (26). Additionally, elite female football (soccer) players showed they had an average intake of 163IU per day which would be low however, the same athletes showed serum levels to be 30ng/mL possibly due to sun exposure (26). Would improving vitamin D levels increase performance in athletes well, Bezuglov et al., (2019) observed no difference in young football players in 5, 15 and 30m sprint tests and standing long jump test in those with serum levels below 30ng/mL compared to those with serum levels between 61-130ng/mL. Kim et al., (2019) found no correlation between the sufficient and insufficient/deficient and shoulder muscle strength in male professional volleyball players. However, one study had shown that vitamin D supplementation improved muscular performance, not strength so balance and reaction time for example other studies have also found an increase in type 2 muscle fibres and improvements in muscle atrophy due to vitamin D treatment (5). Therefore, type 2 muscle fibres are sensitive to vitamin D deficiency, type 2 muscle fibres are important for those short quick bursts needed in most sports and are important for avoiding falls (26). 

Conclusion

Vitamin D is not a regular vitamin it is more like a steroid hormone, as most cells contain a VDR there are probably many more uses yet to be discovered for this precious vitamin. There are many benefits of supplementing with vitamin D healthy bones and teeth, healthy and strong muscles, possible improved muscle performance, increase in type 2 muscle fibres, lower risk of cardiovascular disease, lower cancer risk and potentially decrease in the severity of COVID. These are just what have been mentioned in this blog there are also neurological effects and effects on DNA amongst others that I have not managed to fit in therefore I advise strongly taking supplements, especially in the winter and enjoy the sunshine as much as possible when it is available.

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• Everything You Need to Know About Vitamin C
• Vitamin D and the Winter Months
• Everything You Need to Know About Vitamin K
• The Mediterranean Diet
• Everything You Need To Know About Vitamin E

References

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