The most important and thoroughly researched function of vitamin D is maintaining the right balance of calcium in our body, essential for the health of our bones. Vitamin D does this by facilitating the absorption of calcium in the digestive system and influencing the genetic expression of other calcium channels.
In addition to its role in assisting the absorption of calcium, vitamin D also protects the heart and brain.
Current research efforts are testing the ability of vitamin D to prevent a range of diseases, including diabetes, cancer, heart disease, stroke, Multiple Sclerosis, and chronic inflammatory bowel diseases such as Crohn’s disease and Colitis.
Another role of vitamin D, which arouses special interest during the COVID-19 pandemic, is protecting the respiratory system. A literature review has found that administering a vitamin D supplement reduces the risk of developing pneumonia. It must be emphasized, however, that this has not yet been tested on COVID-19 patients.
Vitamin D and exposure to sunlight
The main source of vitamin D is exposure to sunlight. UVB rays hit cholesterol compounds in the skin, causing a biochemical chain reaction in the cells, passing through the liver and kidneys, and ultimately producing active vitamin D called 1,25-dihydroxy-vitamin D, or calciferol.
Our modern lifestyle constantly diminishes our sunlight exposure, causing a significant lack of vitamin D in some parts of the population. To address this problem experts have recommended controlled exposure of limited skin areas (such as the cleavage or forearms) for about 15 minutes per day, depending on the amount of radiation. In Israel the climate allows for good sunlight exposure during most seasons, but on cloudy days radiation diminishes significantly, necessitating longer exposure.
The concentration of melanin (a brown or black pigment that gives skin its color) in the skin also affects the quantity of vitamin D produced, because it acts as a radiation screen preventing UV rays from penetrating through the outer layers of skin. People with dark skin, which contains large quantities of melanin, require longer periods of sunlight exposure than light-skinned individuals. Yet it must be noted that sunlight exposure is a risk factor for developing skin cancers.
Nutritional sources of vitamin D
People who are not sufficiently exposed to sunlight should consume a vitamin-D-rich diet or take dietary supplements.
The main nutritional sources of vitamin D are animal products including fatty fish (mackerel, salmon, sardines, tuna and herring), egg yolks, butter and liver. Today vitamin D is also added to milk and other dairy products, as well as cereals. Some vegetable groups also contain a very small amount of vitamin D. Relatively higher concentrations are found in mushrooms and green-leaf vegetables.
Food and sunlight are the preferable ways for obtaining vitamin D because the body knows how to dispose of excess quantities coming from these sources.
For people who do not eat animal foods and are rarely exposed to sunlight – especially now, with the coronavirus pandemic forcing us to stay at home much of the time – dietary supplements can be important. You must consult with a dietician before taking a supplement, to make sure it is the right one for you. There is a thin line between deficiency and excess which can lead to toxicity: too much calcium and phosphorus carried in the blood, and deposited in soft tissues, can cause headaches, nausea, over-calcification of bones and kidney stones.
Because the calcium and vitamin D in our bodies are strongly related, some supplements contain both, but this is not imperative. The need for each should be assessed independently, and supplements taken accordingly. Note that they do not need to be taken together for vitamin D to favorably impact calcium absorption.
The information presented in this article is general. It does not constitute medical advice or replace consultation with a physician. It should not be regarded as a recommendation or an alternative for medical treatment.
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