Vitamin D is actually a hormone (steroid) and not a vitamin, as our bodies can synthesize vitamin D from UVB rays from the sun and convert it into its active form which then acts as a hormone in the body. It’s very difficult to get all our vitamin D from food sources alone, and we should only rely on food sources/supplementation during a vitamin D winter, when getting vitamin D from the sun is not possible, such as during cloudy days, winter and indoor jobs, as sunlight is our best and most potent source of this hormone vitamin.

Healthy levels of vitamin D should range from 35ng/ml and higher. As the sun rays hit us, 7-dehydrocholesterol in our skin is converted into vitamin D3. It then enters the circulation of the blood stream and is transported to the liver where it is hydroxylated to form 25-hydroxyvitamin D3 (calcidiol a.k.a. 25(OH)D, the inactive form). 25(OH)D then requires a further hydroxylation in the kidneys by the enzyme 25-hydroxyvitamin D-1α-hydroxylase (CYP27B1) to be converted into its active form, namely, 1,25-dihydroxyvitamin D (calcitriol a.k.a. 1,25(OH)D). Calcitriol is the most potent form of vitamin D and performs most of it’s physiological actions in this form. The production of calcitriol in the kidneys is regulated by a few factors, namely, serum phosphorus, calcium, parathyroid hormone (PTH), fibroblast growth factor 23 (FGF-23), and calcitriol itself (negative feedback loop).

Few other tissue other than the kidneys also contain the enzyme, 1-α-hydroxylase, that converts 25(OH)D to 1.25(OH)D, such as a few glands, skin, bones and immune cells. 25(OH)D levels in the body are regulated by enzymes that increase or decrease its concentration, and it would be impossible to get vitamin D toxicity from sunlight alone. Toxicity is possible however with supplementation.

Vitamin D binds to its receptor (VDR) and dimerizes (combine with a similar molecule to form a dimer) with the retinoid X receptor (RXR), which is a retinoic acid (vitamin A) receptor. Vitamin A and D work in synergy throughout the body. Without sufficient vitamin A, vitamin D is unable to properly fulfill its functions. More about vitamin A here

Other factors that increase vitamin D receptors are vitamin D itself, retinoic acid (vitamin A) and zinc.

20-30 minutes of whole-body exposure to sunlight can produce between 10,000 IU and 20,000 IU of Vitamin D depending on season, sunscreen, time of day and race (the color of your skin). Wearing a synthetic sunscreen reduces vitamin D synthesis in the skin by more than 99%. (R) The perfect time to get sun exposure for vitamin D would be in the middle of the day, between 10 am and 3 pm. I advise putting on coconut oil before going out into the sun as this will serve as a natural sunscreen. Another natural alternative would be cold pressed olive oil as both these oils will not prevent the vitamin D formation and absorption by the skin.

Vitamin D is probably best known for bone metabolism and calcium absorption. In a vitamin D deficient state, only 10–15% of dietary calcium and about 60% of phosphorus are absorbed. This will lead to low calcium and phosphorous levels, which will increase parathyroid hormone (PTH) to increase calcium levels. A minimum of 32ng/ml of vitamin D levels are needed in order to optimize calcium absorption, as chronic elevated PTH can be real detrimental to your health.

Vitamin D promotes muscle growth and exercise performance and lowers fat mass

1) A significant correlation between serum  IGF-1 and vitamin D have been found. An increase as much as 31.3ng/ml in IGF-1 where seen after supplementing with 7 000IU vitamin D/day for 12 weeks. (13) Also, as 25(OH)D increases, so does IGFBP-3 – which is the transport protein for IGF-1 – leading to more IGF-1 transport to muscle for anabolic activity. (14)

2) Vitamin D is inversely correlated with adiposity. It’s able to decrease leptin, resistin and inflammation (inhibiting NF-κB signaling and cytokines such as IL-1, IL-6, IL-8, IL-12 and TNFα), it also increases adiponectin (which increase fat oxidation, insulin sensitivity and reduce inflammation), boosts thyroid, metabolism and uncoupling protein (UCP).

1,25(OH)D controls the expression of UCP (which promotes thermogenesis) and is also able to increase thyroid hormones. As vitamin D deficiency is associated with hypothyroidism, metabolism will be slower and UCP will be less due to low thyroid hormones (R). UCP ‘leak’ protons, which enables the mitochondria to burn energy faster to make up for the ‘loss’ of energy, thus increasing fat/energy burned, hence speeding up your metabolism.

More vitamin D = more thyroid hormones + UCP = faster metabolism = more fat burned.

3) Vitamin D lowers cortisol. A 40% decrease in cortisol is seen when supplementing with 2000IU of vitamin D a day from just 2 weeks (20). Cortisol is catabolic, so keeping is low should aid in muscle growth.

4) Vitamin D receptors are found in human skeletal muscles as well as satellite cells (which regulate muscle growth). Vitamin D is directly involved in muscle protein synthesis (by increasing gene transcription of mRNA), ATP production, cross bridge formation, strength, exercise capacity, and physical performance. A deficiency is known to cause muscle weakness and atrophy specific in type II muscle fibers. Vitamin D increases serum calcium which is important for muscle contraction and also muscle protein signalling. (27)

5) It would also appear that VO2max increases as vitamin D levels increase and is speculated that it might be due to:

“the CYP enzymes that activate vitamin D3 into 1,25-dihydroxyvitamin D3 have heme-containing proteins and could potentially affect the binding affinity of oxygen to hemoglobin” (28)

Vitamin D speeds up recovery and muscle growth by:

  1. attenuating inflammation directly post-workout
  2. improving mitochondrial function (29)
  3. increasing the differentiation and proliferation (rapid multiplication) in muscles
  4. inhibiting myostatin (an inhibitor of muscle growth) and its mRNA gene expression (3031)
  5. promoting myogenic differentiation by increasing IGF-I and follistatin expression (follistatin inhibits myostatin) (32)

Magnesium is an extremely important component for vitamin D function. Magnesium is a cofactor for vitamin D synthesis (to its active form), transport, and activation at receptors. More on magnesium here

Zinc is another important mineral, not only involved in forming and giving structure to vitamin D receptors, but it also increases the rate at which vitamin D activates the expression of genes. More on zinc here

1000IU+ a day of vitamin D3 might be a good place to start, depending on your current vitamin D levels, so check your levels first before determining your vitamin D dose.

Good supplements include:

Vitamin D cofactors:

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Published by Hans Amato

Hi, I'm Hans and I'm a physique, strength and nutrition specialist/coach. My passion is building the best physique naturally (both functional and super strong) and enabling others to achieve their dream physique and health goals as well. Nothing is impossible if you set your mind and heart to it - not even you're "genetic limit" will be able to stop you then.

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