New Research Shows Vitamin D Supplements Not Useful for Preventing Cancer, Cardiovascular Disease or Diabetes

Vitamin D plays a key role in bone health. It maintains normal blood levels of calcium and phosphate, minerals necessary for bone growth and strength, and helps bones absorb calcium. Vitamin D also promotes muscle health and strength.[1] For many years, vitamin D supplements have been prescribed to improve bone health.

In recent years, some studies have shown that low blood vitamin D levels are associated with increased risks of cancer, cardiovascular disease (including heart attacks and strokes) and type 2 diabetes.[2],[3] These findings have prompted proposals to use vitamin D supplements to decrease the risks of these diseases.

However, the results of two large, well-designed, randomized clinical trials that were recently published in the New England Journal of Medicine (NEJM) demonstrated that high-dose vitamin D supplementation was not useful for preventing cancer, cardiovascular disease or diabetes. These important trials were funded by the National Institutes of Health.

Vitamin D sources

Vitamin D is available in several foods. It is found naturally in some fatty fishes, fish-liver oils, egg yolks and chicken livers. Food producers also fortify many foods with vitamin D, including milk, orange juice and breakfast cereals. In addition, vitamin D is produced naturally in the skin during exposure to sunlight.

Several forms of vitamin D supplements are available for people who clearly have inadequate vitamin D dietary intake and skin production from sun exposure. The most commonly used forms are cholecalciferol, also known as vitamin D3, and ergocalciferol, or vitamin D2.[4]

Official U.S. nutritional guidelines recommend that healthy adults up to age 70 consume 600 international units (IU) of vitamin D daily.[5] After age 70, the recommended daily allowance increases to 800 IU. Daily vitamin D intake for healthy adults should not exceed 4,000 IU to avoid toxicity.

Cancer, cardiovascular disease prevention trial[6]

In the first trial, which was published in the NEJM on Jan. 3, 2019, the researchers enrolled 25,871 healthy adults throughout the U.S. from November 2011 to March 2014. To be eligible for the trial, men had to be aged 50 or older and women aged 55 or older. At the time of enrollment, the subjects had no known history of cancer (except nonmelanoma skin cancer) or cardiovascular disease (such as a prior heart attack or stroke) at the time of enrollment.

The subjects were randomly assigned to receive either oral vitamin D at a high dose of 2,000 IUs or a placebo once daily. Subjects were required to limit their intake of vitamin D from other supplemental sources to 800 IU daily. The subjects were followed for four to six years (median five years).

The researchers found no statistically significant differences between the vitamin D and placebo groups in the rates of invasive cancer of any type or of major cardiovascular events (heart attack, stroke and death from cardiovascular causes combined), which were the two primary outcomes of the trial. There also were no significant differences between the two groups in the rates of breast cancer, prostate cancer, colorectal cancer, death from cancer and death from any cause.

Finally, there were no significant differences between the groups with respect to adverse events that might be linked to vitamin D treatment, such as high blood calcium levels and kidney stones.

In conclusion, the trial demonstrated that daily supplementation with high-dose vitamin D for five years in healthy adults does not reduce the risk of cancer or major cardiovascular events.

Diabetes prevention trial[7]

In the second trial, which was published in the NEJM on Aug. 8, 2019, researchers enrolled 2,243 adults in the U.S. from October 2013 to February 2017. The subjects were at high risk of type 2 diabetes based on being overweight or obese and having blood sugar tests showing prediabetes (a condition in which blood sugar levels are higher than normal, but below levels seen with diabetes). To be eligible for the trial, subjects also had to be aged 30 or older (or aged 25 or older for American Indians, Alaska Natives or natives of the Pacific Islands, including Hawaii; such individuals have a greater risk of diabetes).

The subjects were randomly assigned to receive either oral vitamin D at a high dose of 4,000 IUs or a placebo once daily. The subjects were followed for a median of 2.5 years.

At the end of the trial, there was no statistically significant difference in the rate of new-onset diabetes between the vitamin D and placebo groups. Also, like in the first trial, the rate of adverse events was similar for both groups.

In conclusion, the trial showed that daily supplementation with high-dose vitamin D for more than two and a half years did not decrease the risk of type 2 diabetes in patients with prediabetes, a group at high risk of developing the disease.

What You Can Do

You should not take high-dose vitamin D supplements in an effort to reduce your risks of cancer, cardiovascular disease or type 2 diabetes. If you clearly have inadequate vitamin D dietary intake and skin production from sun exposure, you should discuss with your doctor the use of vitamin D supplements to improve your bone health.
 

References

[1] Bischoff-Ferrari HA. Relevance of vitamin D in muscle health. Rev Endocr Metab Disord. 2012;13(1):71-77.

[2] Manson JE, Cook NR, Lee IM, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med. 2019;380(1):33-44.

[3] Pittas AG, Dawson-Hughes B, Sheehan P, et al. Vitamin D supplementation of prevention of type 2 diabetes. N Engl J Med. 2019;381(6):520-530.

[4] Pazirandeh S, Burns DL. Overview of vitamin D. UpToDate. Last updated January 15, 2016

[5] Institute of Medicine. 2011. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: The National Academies Press.

[6] Manson JE, Cook NR, Lee IM, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med. 2019;380(1):33-44.

[7] Pittas AG, Dawson-Hughes B, Sheehan P, et al. Vitamin D supplementation of prevention of type 2 diabetes. N Engl J Med. 2019;381(6):520-530.