Jarrow Formulas CalMag contains vitamin D2, calcium and magnesium which together contribute to normal muscle function, blood clotting, production of digestive enzymes, neurotransmitter function and also the maintenance of normal bones and teeth. Magnesium’s role in the body is broad and is possibly the most important mineral for stress management as it is often depleted quickly when we are experiencing high stress. 
Vitamin D is our sunshine vitamin and is a key nutrient that the government guidelines recommend we supplement during the winter months to avoid deficiency as an estimated 50% of the global population are deficient in vitamin D. This vitamin is produced in our skin when we are exposed to sunlight, but while the high deficiency rate is largely a result of lack of sun exposure there are other factors influence this including:
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Using protective sunscreen. Studies show that wearing factor 30 reduces vitamin D by 95%.
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Our skin tone. Individuals with darker skin tones have increased amounts of melanin, a pigment which protects skin from the harmful effects of UV radiation and therefore reduces vitamin D production.
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Aging. Vitamin D requires activation by the liver and kidneys once in the body, however as we age, our kidney function naturally declines, therefore reducing the ability to activate vitamin D.
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Magnesium deficiency. Magnesium has a regulatory role of vitamin D and a deficiency can reduce vitamin D production.
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Consuming vitamin D alongside calcium supports calcium absorption from the intestine, making the most of dietary and supplemental intake.
Sources
Calcium