Vitamin D is a fat-soluble vitamin essential for human survival, however technically it is not a vitamin since it is not an essential dietary factor (Norman, 2008), i.e. we can produce it without getting it from the diet.
When talking about vitamin D it can be somewhat confusing since vitamin D exists in many different forms. Vitamin D only refers to a group of similar/related molecules that together can increase the amount of active vitamin D (1,25-OH2D3).
A precursor of vitamin D, 7-dehydrocholesterol, can be converted under the skin to one of the two main forms of vitamin D, Vitamin D3 (cholecalciferol), which is also an inactive precursor. The other main form being Vitamin D2 (ergocalciferol), which is substantially less effective than vitamin D3 in elevating levels of 25-OHD (the vitamin D we measure in blood) (Norman, 2008).
Vitamin D can then be converted to the active form through conversions in the liver (to 25-hydroxyvitamin D (25-OHD)) followed by the kidney to 1,25-dihydroxyvitamin D (1,25-OH2D3), which is the active form of the vitamin (Marshall Brinkley et al., 2017), and is classified as a steroid hormone (Norman, 2008).
The main functions of vitamin D are related to calcium and phosphorous metabolism, basically it´s involved in bone formation and maintenance. Vitamin D is an important factor in preventing rickets (disease characterized by a defective mineralization or calcification of bones) in children. Vitamin D increases the calcium absorption in the gut to combat low blood levels of calcium and low levels of Vitamin D can increase the risk of osteoporosis and fractures (by causing an increase in parathyroid hormone (PTH)) (Lips, 2006).
Also, vitamin D has later been shown to affect immune function, anti-oxidation, inflammation and more (Wang et al., 2017).
Production of the active form of vitamin D, 1,25-OH2D3, is stimulated by a hormone important for bone and mineral regulation, parathyroid hormone (PTH), and low calcium levels.
Vitamin D is also implicated in chronic diseases and has been associated with bone disorders, cancers, cardiovascular diseases, and diabetes. However, it´s not clear whether vitamin D is the cause or solely a consequence of disease (Wang et al., 2017).
We can synthesise (produce) vitamin D3 under the skin after sun exposure. More specifically, the ultraviolet light band B (UVB) stimulates a nonenzymatically vitamin D photo-production. Therefore, sunlight is the main source of vitamin D for most people, with around 80 % of daily vitamin D requirements expected to come from sunlight exposure (Wimalawansa, 2016).
Factors affecting vitamin D synthesis in the skin include intensity of sunlight, duration of skin exposure to UVB rays, area of skin exposed, angle of the sun (e.g. zenith or at angle), skin thickness, skin color (darker skin is a barrier to UVB penetration and therefore generate less vitamin D), use of sunscreen (or other sun-blocker, e.g. clothes) (Wimalawansa, 2016), season, and living in air polluted cities (Manios et al., 2017). Sunlight in the morning and in the afternoon, are less effective at stimulating vitamin D generation.
A latitude nearer the equator results in more UVB sunlight exposure than higher latitudes (closer to the poles) and thus more vitamin D (Manios et al., 2017).
Amount of sunlight exposure recommended differs depending on the factors mentioned above, but a dose equivalent to ~400 IU is generally obtained for most latitudes during a midday sun exposure of an hour, with amounts equal to ~1000 IU with exposure of more skin areas. Being at a latitude close to the equator where the sun is closer to zenith results in more vitamin D synthesis (Wimalawansa, 2016).
Another way to obtain a vitamin D precursor (ergosterol) is through the diet. Ergosterol is then absorbed in the upper part of the small intestine called duodenum and converted to vitamin D2. However, vitamin D and its precursors only exist in small amounts and in a limited number of foods. It is therefore impossible to reach a sufficient amount of vitamin D through a regular/normal diet only. Another aspect to consider with vitamin D in food is the variable vitamin D content, even in fortified foods (Wimalawansa, 2016). Sun exposure is, therefore, an important part of vitamin D sufficiency.
Foods containing vitamin D include egg yolk, fatty fish like salmon, mackerel, and herring. In addition, some countries sell vitamin D fortified milk.
Different organisations have different recommendations on the optimal vitamin D intake. The vitamin D intake should also be specific to different populations, e.g. dependent on age, male/female, pregnant etc. Recommendations range from around 600-2000 IU in an adult population with upper tolerable intake levels ranging from 4000-10000 IU per day (“The Biphasic Effect of Vitamin D on the Musculoskeletal and Cardiovascular System. – PubMed – NCBI,” n.d.).
Other sources recommend around 3000-5000 IU of vitamin D per day (Farrokhyar et al., 2015).
Since vitamin D is a fat-soluble vitamin, intake of vitamin D supplements or other foods containing vitamin D with fat could increase the uptake.
Vitamin D3 tend to have higher absorption than the D2 form.
Deficiency is relatively uncommon but insufficiency is more common. Around between 16 and 27% were deficient in vitamin D (<25 nmol/L) when investigating a European population, with some variation between age (Manios et al., 2017).
Risk factors for vitamin D deficiency are low sun exposure, premature birth, skin pigmentation, obesity, malabsorption, and old age (Lips, 2006).
Contrary to what would be expected, Europeans in northern latitudes seem to have higher amounts of vitamin D compared to southern Europeans. One hypothesis for this is that people living closer to the equator have a more sun-avoiding behaviour, eat less fatty fish, and generally have darker skin (Wimalawansa, 2016).
We are usually measuring the 25-OHD form of vitamin D since it has a long half-life (~3 weeks), are reproducible, and reflect vitamin D exposure over weeks to months. 1,25-OH2D3 levels are more tightly regulated (Marshall Brinkley et al., 2017) but are less stable in the body (Wang et al., 2017).
25-OHD levels considered as deficient in human blood is <20 ng/L (<50 nmol/L), insufficiency 21-29 ng/ml, and sufficiency >30 ng/ml according to certain criteria (Wang et al., 2017). However, currently, no consensus exists as to what is considered deficient.
There doesn’t seem to be much evidence for any beneficial effects of vitamin D supplementation on cardiovascular disease, type 2 diabetes, body weight, or cancers (Rejnmark et al., 2017). However, vitamin D3 could possibly decrease mortality in older adults whereas vitamin D2 or 1,25-OH2D3 doesn’t seem to have the same beneficial effects, the evidence a bit unsure though. On the other hand, vitamin D3 combined with calcium could increase the risk for renal stones (Bjelakovic et al., 2014).
Vitamin D and calcium supplementation seem to prevent falls (Girgis et al., 2014) and fractures in older vitamin D deficient individuals while vitamin D supplementation only is unlikely to prevent fractures (Avenell et al., 2014).
Supplements would be best taken more often than once a month since the half-life of vitamin D is shorter than this, and the vitamin D levels risk fluctuating when consumed this seldom. A serum level of >30 ng/mL of 25-OHD seems to be protective against different diseases (Wimalawansa, 2016). Supplementing with vitamin D doesn’t necessarily lead to longevity since this issue is not well studied as of yet (Wimalawansa, 2016).
In athletes, an inadequate level of vitamin D seems to be common, especially among athletes involved in indoor sports, living at higher latitudes and during winter and early spring (Farrokhyar et al., 2015).
It has been suggested that athletes demand higher intake of vitamin D, possibly due to a higher level of physical activity (Farrokhyar et al., 2015).
Athletes with a vitamin D deficiency could potentially benefit from supplementing to reach a 25-OHD level of 50 ng/ml (125 nmol/L) (Cannell et al., 2009). However, the evidence is not completely clear as a recently published meta-analysis (a study of studies) were unable to show beneficial effects of vitamin D supplementation up to sufficient vitamin D levels (Farrokhyar et al., 2017).
A supplemented vitamin D amount of 5000 IU together with 1.5 g of calcium a day seem to be a reasonable amount since no adverse effects has been observed, including cardiovascular diseases (Wimalawansa, 2016).
Vitamin D blood levels >100 nmol/L could possibly increase the risk of cardiovascular disease. Physical activity could possibly affect the threshold of harmful effects of vitamin D (“The Biphasic Effect of Vitamin D on the Musculoskeletal and Cardiovascular System. – PubMed – NCBI,” n.d.).
Vitamin D is a fat-soluble vitamin essential for our survival.
Sun is our main source of vitamin D, but we can get some vitamin D from the diet, e.g. fatty fish, eggs or fortified milk.
Several factors affect how much vitamin D we obtain from sun exposure, for example duration of exposure, amount of skin exposed, skin color, and latitude.
To achieve sufficient levels of vitamin D it can be a good idea to spend some time during midday in the sunshine. We get around 50-90% of vitamin D through the synthesis after sun exposure.
Vitamin D is especially important for calcium metabolism and bone health. A deficiency can cause severe bone disease in children and osteoporosis in adults.
So far there doesn’t seem to exist any evidence for a causal link between vitamin D and non-skeletal diseases, e.g. cardiovascular disease.
Supplementing with vitamin D together with calcium seem to prevent fractures in older vitamin D deficient individuals.
Writen by: Fredrik Wernstål
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