Risks and Controversies of Vitamin D Supplementation

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Health Benefits of Vitamin D

Hardly a month goes by without a new study suggesting that low levels of vitamin D increase the risk of cancer, heart disease or depression. Mainstream doctors and wellness gurus have endorsed the message, and vitamin D supplements sales have skyrocketed.

But only rigorous studies that randomly assign some people to take vitamins and others to do nothing can tell whether the supplements really help.

Why Are So Many People Popping Vitamin D?

The vitamin D frenzy began not in natural food stores but in medical journals. Beginning in 2000, research papers linked low blood levels of the vitamin, now considered normal, to multiple sclerosis, depression and cancer risk.

Researchers also found that people with moderate deficiency, less than 20 ng/mL, performed worse on a series of mental tests and had more than twice the risk of developing Alzheimer’s or other types of dementia.

Many doctors jumped on the bandwagon, screening healthy adults for vitamin D deficiency and prescribing supplements. In fact, more than 10 million tests for vitamin D and millions of vitamin D pills are sold annually in the US. But new studies show that these pills don’t prevent serious health problems in most people. The latest, published at the end of last month in The New England Journal of Medicine, adds to earlier trials that showed no benefit from supplementation in healthy post-menopausal women. The findings “overturn dogma and cast doubt on the value of routine screening for vitamin D deficiency and blanket recommendations for supplementation,” the authors write.

The Craze

Since 1924, when it was discovered that sunlight cures rickets, vitamin D has been something of a health fad. It’s a fat-soluble nutrient that helps absorb calcium and phosphorus to make bones strong. It’s naturally produced in the skin from ultraviolet rays and is also found in oily fish, fortified milk and some foods.

But more recent research has linked lower-than-normal blood levels of vitamin D to a host of disorders, including cancer, multiple sclerosis, depression, heart disease and weight gain. In response, doctors have urged patients to get more D through testing and supplements.

One of the leaders in the D craze is Dr. Michael Holick, an endocrinologist at Boston University. He has written book-length odes to vitamin D, and he has a following among the wellness-industrial complex: Gwyneth Paltrow’s website cites his work, and both Mehmet Oz and Oprah Winfrey have promoted his views. But an investigation by Kaiser Health News found that he has received large sums from vitamin D-related industries, including the makers of prescription drugs and indoor tanning equipment, to promote practices that may financially benefit them.

Vitamin D Deficiency

Getting enough vitamin D can help prevent rickets (softening of the bones in childhood) and osteomalacia (softening of the bones in adults). The vitamin is made by the skin when exposed to sunlight, occurs naturally in some foods — including fatty fish, such as salmon, tuna, mackerel and sardines, and fortified dairy and grain products.

The most common cause of vitamin D deficiency is not getting enough sunshine, although certain disorders also can lead to it. In extreme cases, people with low vitamin D levels experience muscle and bone weakness, a condition called rickets in infants and osteomalacia in adults.

A blood test, known as 25(OH)D, can measure vitamin D levels. A level below 30 nmol/L or 20 ng/mL is considered low and may negatively affect bone health. Kidney and liver disease can interfere with the body’s ability to turn sunlight and food into the biologically active form of vitamin D. Inflammatory bowel disease, particularly ulcerative colitis, can also lower levels of the vitamin, according to a 2013 study in Alimentary Pharmacology and Therapeutics.


Vitamin D is a group of fat-soluble hormones and prohormones (substances that have little hormonal activity by themselves but the body can turn into hormones) that helps calcium and phosphorus work together to build strong bones. It also plays other important roles in the body, including reducing inflammation and influencing immune and neuromuscular function and glucose metabolism.

People can get vitamin D from sun exposure, foods fortified with the vitamin and supplements. The form that is made in the skin is called vitamin D3, or cholecalciferol, and it is also found in fortified foods, fatty fish like salmon and tuna, liver, egg yolks and plants and fungi.

Although excess vitamin D toxicity can occur, it’s not likely from eating a healthy diet or taking supplemental amounts of the vitamin. It most commonly occurs from over-consumption and prescription errors. Vitamin D toxicity can cause high blood calcium levels, which can be harmful to the kidneys, lungs and heart.

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