Young woman in white sun dress and walking on sandy beach at summer sunset, absorbing vitamin D

Why Our Bodies Need Vitamin D

The importance of vitamin D

It’s the perennial summer conundrum. Though exposure to prolonged UV rays can cause skin damage, premature aging, and cancer, our bodies nevertheless need vitamin D to thrive. A vitamin D deficiency can cause significant health risks.

What is vitamin D?

Vitamin D is a prohormone, or a substance that the body converts to a hormone. In a healthy body, the kidneys produce vitamin D, which controls blood calcium concentration. Vitamin D also has an impact on the immune system. It goes by several names: calcitriol (the active form), calcifediol (25(OH)D3), ergocalciferol (vitamin D2), and cholecalciferol (vitamin D3).

The body naturally produces vitamin D in a chemical reaction when sunlight hits the skin. This reaction produces cholecalciferol, which is converted in the liver to calcifediol. The kidneys further break down calcifediol into calcitriol.

Functionally, vitamin D assists the body in absorbing calcium, balancing blood calcium levels at their optimal level. This enables the bone mineralization that’s needed to develop strong, healthy bones.

Sources and deficiencies

Food sources such as oily fish, egg yolks, and mushrooms contain small amounts of vitamin D2 and D3. However, sunlight remains the best and most direct source of vitamin D. In areas of the globe without consistent sunshine (i.e. the Arctic Circle) and limited food sources, residents may need to take vitamin D supplements to make up for the lack of sunshine.

Other factors can also change the amount of vitamin D the body absorbs. Cloud cover and air pollutants scatter and absorb the sun’s rays, reducing the UVB light that reaches the ground. People over the age of 50 produce less vitamin D than their younger counterparts. Less time spent outdoors, existing kidney problems, and fewer skin receptors account for the vitamin deficiency within this population, regardless of where they live. An estimated 40% of older people living in sunny South Florida, for example, are vitamin D deficient.

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Vitamin D deficiency

Approximately 40% of Americans present with some degree of vitamin D deficiency, with people of color at the highest risk of deficiency. Studies during the COVID-19 pandemic show that vitamin deficiency may also be associated with a higher risk of testing positive for the virus. In one review, over 80% of patients hospitalized with COVID-19 were vitamin D deficient.

Rickets is the disease most people think of when it comes to vitamin D deficiency. In cases of rickets, the bone tissue does not adequately mineralize, resulting in soft bones and deformities.

Research, however, is unveiling a multitude of additional health conditions related to vitamin D deficiency, some of which include:

  • Bone pain and muscle weakness
  • Infections and immune system disorders
  • Low bone density and osteoporosis
  • Rheumatoid arthritis
  • Multiple sclerosis
  • Thyroid disease
  • Osteomalacia

The usual (and unusual) suspects

Aside from external factors like latitude and air pollution, what are the internal culprits responsible for vitamin D deficiency? Illnesses like cystic fibrosis, Crohn’s disease, and celiac disease prevent the intestines from absorbing vitamin D. Bariatric surgeries, which reduce the stomach’s size or bypass the small intestine with the goal of helping the patient lose weight, increase the difficulty of consuming adequate quantities of specific nutrients, minerals, and vitamins.

Vitamin D deficiency is seen more often in those with a body mass index greater than 30. Fat cells keep the vitamin D isolated, preventing its release and use in other parts of the body. Obese individuals need to consume larger doses of vitamin D to maintain normal vitamin D levels. Additionally, people with problems digesting fats will not absorb vitamin D, as it is a fat-soluble vitamin.

Kidney and liver diseases can reduce the amount of the specific enzyme required to convert vitamin D to a usable form within the body. Without this enzyme, a vitamin D deficiency is likely to occur.

Additional factors

  • Lifestyle can have a critical impact on vitamin D exposure and absorption. Patients who are home-bound or on bed rest, as well as indoor employees and night shift workers, may not be able to access the outdoors and have limited sun exposure.
  • Human breast milk only contains small amounts of vitamin D, as does infant formula. Babies who are exclusively breastfed may be at risk for a vitamin D deficiency.
  • Diet and medication also have an effect on vitamin D absorption. Laxatives, steroids, cholesterol-lowering drugs, seizure-control drugs (phenobarbital, phenytoin), tuberculosis medicine (rifampin), and weight-loss drugs (orlistat) can all lower vitamin D levels.

Deficient or toxic?

A vitamin D deficiency will show up in a simple blood test. Many healthcare professionals consider blood levels of less than 30 ng/mL to be deficient.

However, there can be too much of a good thing. High doses of vitamin D3 taken for prolonged periods can result in toxicity (i.e. blood levels above 150 ng/mL). As a fat-soluble substance, excess vitamin D is stored in fat, which the body releases slowly into the bloodstream. This slow release can cause elevated blood levels even months after taking supplements.

Although rare, vitamin D toxicity can be dangerous. Also called hypercalcemia, vitamin D toxicity symptoms include nausea, abdominal pain, vomiting, dizziness, weight loss, hardening of the arteries, irregular heart beat, fatigue, confusion, excessive thirst, and frequent urination. Excessive vitamin D can also cause bone pain and calcium stones.

Fortunately, it is not possible to reach toxic levels through diet and sun exposure alone.

Striking the right balance

Sun exposure comes with high risks as well as high benefits. While high exposure can cause sunburn, cancer, wrinkles, heatstroke, dehydration, eye damage, and a lowered immune system, too little sun can can prove unhealthy as well. Finding the right balance is key.

Research shows that midday sun exposure is optimal for producing vitamin D. However, people living further from the Equator will need to spend more time in the sun to make up for the indirectness of its rays, whereas those living on the Equator will get a healthy dose in much less time.


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