Vitamin D - The Sunshine Vitamin
Vitamin D is a fat-soluble vitamin with hormone-like effects. It can be obtained through the diet or synthesized by the body from exposure to sunlight (UVB, wavelength 290 to 315 nm) 1. More than 90% of circulating vitamin D in the body comes from exposure to sunlight on the skin 2. Vitamin D is stored in the liver, as well as in fat and muscle tissue when there is an excess. Vitamin D in foods is light and oxygen sensitive, and found in small amounts.
Forms of Vitamin D
In nature, vitamin D comes in two forms: vitamin D2 and vitamin D3:
- Vitamin D2 (ergocalciferol) is produced by plants.
- Vitamin D3 (cholecalciferol) is produced by animals.
Vitamin D2 is just as effective as vitamin D3 in maintaining the 25-hydroxyvitamin D (abbreviated: 25(OH)D) status 3.
Absorption and Conversion
Vitamin D2 and D3 are precursors of the biologically active vitamin D. Therefore, they must first be converted into the active form of vitamin D (calcitriol). Together with fat, vitamin D2 and D3 are absorbed through the last part of the small intestine and then converted into 25-hydroxyvitamin D (calcidiol) in the liver. Calcidiol then travels through the blood to the kidneys where it is modified into 1,25-dihydroxyvitamin D (calcitriol) 4. Calcitriol is a steroid hormone and the active form of vitamin D. It binds to the vitamin D receptors (VDR) of the cells and regulates a variety of functions such as the use of calcium in the body.
About 50% of the vitamin D contained in food is absorbed by the body 5. The remaining 50% is lost during digestion.
If the body is deficient in magnesium, it can lead to a disturbance in the absorption and metabolism of vitamin D 6. Magnesium also helps in the conversion of vitamin D to its active form, calcitriol, in the body, and is required as a cofactor for the binding of vitamin D to its transport protein (vitamin D-binding protein; abbreviated VDBP), which enables the transport of vitamin D through the blood 78. Magnesium-rich foods can be found here (magnesium table).
Functions and effects of vitamin D
Vitamin D has numerous important functions in the body, which arise due to the hormone-like and biochemical effects (endocrine and autocrine) of vitamin D. The endocrine mechanism regulates calcium absorption and the autocrine mechanism regulates gene expression, which enables, for example, the formation of proteins 9.
The most important functions of Vitamin D summarized:
- Vitamin D is needed for the absorption of calcium and phosphorus from food 10. These two minerals are building blocks for bones and teeth.
- Vitamin D, in its active form, is an influential immune modulator and thus involved in the regulation of the immune system 11. In particular, for innate immune disorders, the immune system can be strengthened with the help of Vitamin D 12 13.
- Vitamin D increases the release of the enzyme tyrosine hydroxylase 14. Tyrosine hydroxylase is involved in the conversion of the amino acid tyrosine into the extremely important neurotransmitter dopamine ("happy hormone").
- Along with omega-3 fatty acids, Vitamin D also controls the synthesis and functions of the "feel-good hormone" serotonin 15.
- Vitamin D stimulates muscle and fat cells to create new insulin receptors, which can improve insulin sensitivity 16 17.
Additionally, Vitamin D appears to regulate the development and homeostasis of the nervous system and skeletal muscles 18.
Preventive effects of Vitamin D
In addition, vitamin D has numerous disease-preventive effects. According to a large summary of studies from 2014, there is promising evidence of a connection between higher vitamin D concentrations in plasma and a reduced risk of: colon cancer, bone fractures, cardiovascular disease, high blood pressure, stroke, ischemic stroke, cognitive disorders, depression (especially in older people), higher body mass index, metabolic syndrome, type 2 diabetes, pregnancy disorders, gestational diabetes 19 20.
Overview of the preventive effects of vitamin D:
- According to recent study results, Vitamin D has a cancer-preventive effect against colon, breast, and prostate cancer, and possibly lung and skin cancer 21 22 23 24.
- Vitamin D is also essential for the cardiovascular system 25. It promotes cardiovascular health and reduces the risk of cardiovascular diseases with adequate Vitamin D supply.
- Vitamin D, together with calcium, plays a major role in the prevention and treatment of osteoporosis 26.
- Periodontal diseases such as periodontitis can be prevented, as Vitamin D stimulates the synthesis of defensins and cathelicidins, which target microorganisms and protect the oral cavity from harmful bacteria 27 28.
- Vitamin D helps with psoriasis caused by a disruption in the production of Cathelicidin 29 30 31. Vitamin D indirectly stimulates the formation of Cathelicidin.
- Vitamin D reduces the risk of developing dementia, Alzheimer's disease, type 1 diabetes, and autoimmune diseases such as multiple sclerosis 32 33 34. In 2014, scientists also found a link between vitamin D and Parkinson's disease 35.
- Vitamin D may inhibit the formation of inducible nitric oxide synthase and increase glutathione levels, which could play a role in brain detoxification and lead to neuroprotective effects 36.
- Study results show that the risk of heart attack and heart failure could be reduced by vitamin D 37 38.
- Vitamin D reduces the risk of tuberculosis and is used for treatment 39.
- Japanese studies show that vitamin D reduces the severity of asthma attacks 40.
- Vitamin D can also prevent muscle wasting in old age. 25-hydroxyvitamin-D concentrations between 40 and 94 nmol/l in active and inactive people aged 60 years and older are associated with better musculoskeletal function in the lower extremities 41.
- Higher plasma 25-hydroxyvitamin D concentrations are associated with a reduced risk of age-related cataracts in men 42.
There is also a suspected link between vitamin D and hypertension 43. A summary of studies from 2011 associates blood 25-hydroxyvitamin D concentration inversely with hypertension 44. However, vitamin D supplementation has been shown to be ineffective in lowering blood pressure according to studies from 2015 45.
In addition, scientists also see a connection between vitamin D and neurodermatitis in children 46 47. The results are not conclusive enough to make a final judgment on whether vitamin D helps with neurodermatitis 48 4950.
Possibly, Vitamin D also plays a role in the prevention of infections 51. Furthermore, it could be used in the treatment of virus, bacterial, and fungal infections.
Recent studies also show a connection between Vitamin D and anemia 52. According to the scientists, maintaining sufficient Vitamin D levels could prevent anemia, particularly in inflammatory diseases.
Vitamin D for muscle strength, performance enhancement, and recovery
Scientific findings suggest that the Vitamin D status in the body can have significant impact on muscle performance and injury prevention and may potentially positively influence athletic performance 53 54 55. The underlying mechanisms are likely enabled through the indirect effect on calcium and phosphorus, as well as through the direct activation of Vitamin D receptors (VDRs) in muscle cells by 1,25-dihydroxyvitamin D 56. Additionally, scientists associate muscle training with an increase in 1,25-dihydroxyvitamin D levels 57.
According to studies, vitamin D plays a very important role in vegan muscle building and muscle strength 58 59. Vitamin D is associated with greater muscle strength in healthy men and women. Further study results demonstrate that vitamin D also makes performance improvements possible, especially in older adults 60. Vitamin D may also improve athletic performance in athletes with vitamin D deficiency 61. After 12 weeks of taking a high-dose vitamin supplement (20,000 and 40,000 IU), no significant improvement in physical performance was observed 62. Other studies from 2015 confirm that vitamin D supplementation does not improve performance 63.
In addition, vitamin D also contributes to the maintenance of muscle mass 64.
The muscle regeneration after an intense training is also supported by vitamin D 65. Maintaining vitamin D plasma concentrations can be advantageous for improving reparative processes after training and may contribute to facilitating subsequent hypertrophy (enlargement of tissue by increasing cell size) 66. Ultimately, vitamin D helps prevent muscle weakness and promotes improved recovery.
Scientists suspect that due to numerous positive effects on muscles, vitamin D acts on skeletal muscles through the vitamin D receptors in muscle cells. Thus, vitamin D could be important for multiple biological pathways that influence skeletal muscle function 67.
Possibly, a dietary supplement with vitamin D can increase testosterone levels in men. 68. More recent study results from 2014 with human testicular cells show that vitamin D increases testosterone synthesis and plays a crucial role in steroidogenesis 69.
Study results with adolescent boys aged 12 to 15 years underline the role of vitamin D in muscle functions 70. Boys with the highest plasma 25(OH)D concentration (> 51nmol/L) had significantly higher muscle strength compared to those with the lowest values.
Vitamin D for Weight Loss
Scientists have found that Vitamin D supports weight loss during a low-calorie diet 71. Another study also found a similar result, where people with higher levels of vitamin D in their plasma and a higher calcium intake were able to initiate greater weight loss through dieting 72.
What should vegans consider?
Vitamin D is found in very few plant-based foods like mushrooms (see foods with Vitamin D). Therefore, it is especially important for vegans to replenish their vitamin D reserves during the summer months. During the winter months, the production of vitamin D through the skin is heavily dependent on the latitude at which one is located 73. Changes in solar UVB radiation have a significant impact on cutaneous vitamin D3 synthesis, and the length of the “vitamin D winter” increases as latitude increases. During the winter months in Boston and Edmonton, sunlight exposure was not sufficient to produce previtamin D3 in human skin. This ineffective winter period lasted from November through February in Boston and from October through March in Edmonton. However, in areas located further south such as 34 degrees N and 18 degrees N, sunlight exposure effectively produced previtamin D3 even in the middle of winter 74.
Therefore, it is important to pay particular attention to a vitamin D-rich diet during these months to ensure that the vitamin D requirement is met and to prevent deficiencies.