What is vitamin D good for: Bone, brain, and immune system health

Vitamin D helps the body absorb calcium, build a strong immune system, and prevent neurodegenerative conditions.Vitamin D deficiency can increase the risk of osteoporosis, especially for the elderly.Too much vitamin D causes nausea, vomiting, weakness, and kidney damage, but vitamin D toxicity is rare and usually only happens from taking too many supplements.This article was medically…
  • Vitamin D helps the body absorb calcium, build a strong immune system, and prevent neurodegenerative conditions.
  • Vitamin D deficiency can increase the risk of osteoporosis, especially for the elderly.
  • Too much vitamin D causes nausea, vomiting, weakness, and kidney damage, but vitamin D toxicity is rare and usually only happens from taking too many supplements.
  • This article was medically reviewed by Melissa Rifkin, MS, RD, CDN, owner of Melissa Rifkin Nutrition LLC. 
  • Visit Insider’s homepage for more stories.

Vitamin D’s main function is to promote bone health, but it helps with other necessary bodily functions, as well. That’s why it’s important to make sure you are getting a proper amount of vitamin D, as too little or too much vitamin D can be harmful. Here’s what you need to know.

What does Vitamin D do in the body 

Maintains strong bones: Vitamin D promotes bone health by helping the body absorb calcium, which is a mineral crucial to keeping bones strong. “Calcium is not very well absorbed by the human intestines. Calcium in your bloodstream interacts with the vitamin D in your blood that goes and activates cells in your bone to help you make stronger bones,” says David Cutler, MD, family medicine physician at Providence Saint John’s Health Center. 

Promotes muscle strength: Vitamin D can aid in keeping muscles strong. A study published in Plos One in 2017 showed a positive correlation between muscle strength and vitamin D intake.

Boosts immune system: Vitamin D is crucial for a healthy immune system and good health. Immune system cells such as B cells (cells that produce antibodies) and T cells (cells that are critical to the immune response) have receptors for vitamin D. Essentially, the vitamin helps keep the immune system balanced. 

Aids in brain function: Researchers believe there is a link between vitamin D and neurological function. Vitamin D can promote brain development and prevent neurodegenerative conditions. Additionally, preliminary research suggests there may be a link between low vitamin D levels and depression.

Health effects of Vitamin D deficiency 

Vitamin D deficiency can lead to several negative outcomes. Certain groups of people, such as people with dark skin, obese people, and elderly people, should be especially careful in making sure they get enough of the vitamin since they’re more predisposed to Vitamin D deficiency. 

According to Cutler, lack of exposure to the sun is one of the main causes of Vitamin D deficiency. A study published in the International Journal of Circumpolar Health in 2008, showed that Vitamin D supplementation is necessary to maintain good health when living in a northern latitude that has very short daylight hours in the winter. 

Additionally, having dark skin can be a risk factor. “If you have very dark skin, you’ll tend not to absorb as much sunlight, which is what converts vitamin D from its inactive to its active form,” says Cutler. Other risk factors are old age, obesity, and people with Crohn’s disease or celiac disease, according to the National Institute of Health.

Vitamin D deficiency most commonly affects your bones. In children, vitamin D deficiency can lead to rickets, a condition that results in brittle bones that don’t develop properly, causing bowed legs. According to Cutler, this was more common in the 19th and early 20th century, and has become rarer today, thanks to milk and other foods being fortified with vitamin D.

In older adults who are vitamin D deficient, they may have trouble absorbing calcium, which can cause their bones to get weak and brittle. This can contribute to bone disease osteoporosis, says Cutler. 

Health effects of too much Vitamin D 

As good as Vitamin D is for you, it is possible to have too much of it. For example, you can experience Vitamin D toxicity if you take too much through supplements. “Vitamin D is in a group of vitamins, which are fat-soluble, unlike the more common vitamins like C and B, which are water-soluble,” says Cutler. 

If you take too many water-soluble vitamins, they’ll go through your kidneys and be flushed out through urination. However, fat-soluble vitamins like vitamin D can get absorbed into your body’s fatty tissue, liver, and even the brain, according to Cutler. So, your body has a harder time removing excess amounts. That’s why it’s important not to overdose on any fat-soluble vitamins, including vitamin D.

Signs of vitamin D toxicity include nausea, vomiting, weakness, confusion, and even kidney damage. Cutler says that this really only can happen from taking too many supplements. According to Mayo Clinic, the daily recommended amount of vitamin D for children and adults is 600 IU. A toxic level is around 4,000 IU. Your body will not produce toxic amounts of vitamin D from sun exposure. 

Additionally, while some people believe vitamin D has anti-cancer properties, the opposite can be true. A study, published in the International Journal of Cancer in 2019, found a link between high levels of vitamin D with higher incidences of skin, prostate, and blood cancers, but a lower incidence of lung cancer.

If you’re concerned that you’re getting too little or too much vitamin D, speak to your doctor. Blood tests can be ordered to check the levels of the vitamin in your blood, and then you can determine what changes, if any, need to be made. 

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