The science behind vitamin D

February 26, 2024

3 min read

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Key takeaways:

  • Vitamin D supplements might be beneficial for bone health, but it’s unclear whether the effects are enough to be clinically meaningful, according to research.
  • Read more on the potential uses for the supplement.

Vitamin D supplementation may be helpful for bone health, but the research is still undecided, according to an expert.

Vitamin D is a nutrient that plays a role in immune function, neuromuscular function and other body processes, Carol J. Haggans, MS, RD, scientific and health communications consultant at the NIH’s Office of Dietary Supplements, told Healio. It also promotes calcium absorption in the gut and is necessary for bone health.

“Fatty fish, such as salmon and tuna, naturally contain vitamin D, and it is also added to fortified milk,” Haggans said. “Beef liver, egg yolks and cheese have small amounts of vitamin D.”

Enlarge  The science behind vitamin D

Haggans also noted that, for adults, the recommended dietary allowance for vitamin D is 15 to 20 µg (600 to 800 IU) per day.

“Many people in the U.S. consume less than recommended amounts of vitamin D, but people also get vitamin D from sun exposure,” she said. “The effects of vitamin D supplements on various outcomes might be affected by baseline vitamin D status.”

Summary of evidence

Haggans said that vitamin D supplements are being studied for a wide range of conditions, including bone health.

“FDA has approved a health claim for the use of supplements containing calcium and vitamin D to reduce the risk of osteoporosis, but not all research supports this claim,” she said. “The American Academy of Pediatrics recommends vitamin D supplements (10 µg [400 IU] per day) for breastfed infants.”

A 2019 systematic review and meta-analysis of 11 observational studies and six randomized trials revealed that vitamin D supplementation alone was not associated with a reduced fracture risk. However, a combination supplement with vitamin D and calcium led to a 16% reduced risk for hip fracture (RR = 0.84; 95% CI, 0.72-0.97) and a 6% reduced risk for any fracture (RR = 0.94; 95% CI, 0.89-0.99).

Additionally, in a 2023 meta-analysis of randomized controlled trials published in The American Journal of Clinical Nutrition, researchers evaluated the effects of vitamin D supplementation on bone health in adolescents and children who have a vitamin D deficiency. After analyzing results from 11 trials with 1,439 participants, the researchers wrote that what they observed in this population is likely not clinically important, regardless of baseline vitamin D status.

Vitamin D has also been investigated for its associations with mortality.

In a 2019 systematic review and meta-analysis of 50 trials that included a total of nearly 75,000 patients, researchers found that vitamin D supplementation significantly reduced the risk for cancer death — by about 15% (RR = 0.85; 95% CI, 0.74-0.97) — but not cardiovascular mortality (RR = 0.98; 95% CI, 0.88-1.08) or all-cause mortality (RR = 0.98; 95% CI, 0.95-1.02).

Also, an umbrella review of 296 meta-analyses of observational studies, 139 meta-analyses of randomized trials and 73 mendelian randomization studies revealed that vitamin D was associated with a lower risk for all-cause mortality but not type 2 diabetes, hypertension, Alzheimer’s disease or schizophrenia — indicating that the supplement “is a promising strategy with long-term preventive effects on multiple chronic diseases and thus has the potential to decrease all-cause mortality.”

Problems with exceeding recommended intake

Too much vitamin D can cause problems. Haggans said the tolerable upper intake level (UL) for vitamin D from all sources is 100 µg (4,000 IU) per day for adults.

“The UL refers to intakes from all sources — food, beverages, and dietary supplements — though it’s difficult to achieve high intakes from food and beverages alone,” she said.

Too much vitamin D can lead to “hypercalcemia, and in extreme cases, renal failure, calcification of soft tissues, cardiac arrhythmias and even death,” Haggans said. However, “health care providers sometimes recommend vitamin D doses that are above the UL for a period of time to treat a vitamin D deficiency.”

The NIH’s Dietary Supplement Label Database contains information from the labels of over 183,000 dietary supplements on the market in the U.S., and it can help primary care providers and consumers find and compare products.

The NIH’s Office of Dietary Supplements also offers other resources for health care providers and consumers. To learn more about vitamin D and other products, check out the NIH’s Office of Dietary Supplements Fact Sheets.

Editor’s Note: Healio is highlighting the clinical value of various supplements. See other installments of the series here: