High blood levels of vitamin D linked to reduced estrogen – and potentially lower breast cancer risk

Study found the hormone-lowering effect to be independent of weight loss
Image of Vitamin D3 gel capsules
New Fred Hutch research suggests that vitamin D supplementation may be a practical alternative to estrogen-lowering drugs for overweight and obese women to reduce their breast cancer risk. File photo by Moment Editorial via Getty Images

Can taking daily vitamin D supplements decrease sex-hormone levels and thereby potentially reduce the risk of breast cancer in older women?

A Fred Hutchinson Cancer Research Center study involving postmenopausal, overweight, and obese women who took 2,000 IUs of vitamin D daily for a year found that those whose vitamin D blood levels increased the most had the greatest reductions in blood estrogens, which are a known risk factor for breast cancer.

These findings, by lead author and postdoctoral research fellow Dr. Caitlin Mason, and senior and corresponding author Dr. Anne McTiernan, both investigators in the Epidemiology Program of the Public Health Sciences Division at Fred Hutch, were published earlier this month in the journal Menopause ahead of the June 2016 print issue.

The randomized, controlled, clinical trial involved 218 women with insufficient vitamin D levels at the beginning of the study. Half of the women were randomly selected to a weight loss program plus daily vitamin D, and half received a weight loss regimen plus a dummy pill, or placebo.

At the end of the year-long study, the researchers found that those in the vitamin D group whose blood levels of the nutrient rose the highest – from inadequate at baseline to normal, or replete levels – had a corresponding drop in circulating estrogens and other sex hormones that are a known risk factor for breast cancer. The reduction in sex hormones among the women with the highest levels of vitamin D was observed even after taking into account how much weight the women lost, showing that the vitamin D had an independent effect on hormone levels, regardless of pounds shed.

Scientists believe that obese and overweight people have lower levels of vitamin D because the nutrient is stored in fat deposits. A prevailing theory is that during weight loss, the vitamin D trapped in fat tissue is released into the blood and becomes available throughout the body.

Dr. Anne McTiernan
Senior and corresponding author Dr. Anne McTiernan. Photo by Robert Hood / Fred Hutch News Service

A practical alternative to estrogen-lowering drugs?

This study suggests that vitamin D supplementation may be a practical alternative to estrogen-lowering drugs for overweight and obese women to reduce their breast cancer risk, McTiernan said.

“Medications to lower blood estrogens, such as aromatase inhibitors, reduce the risk of developing breast cancer in women at increased risk, but those medications are expensive and can have adverse effects,” she said. “So it’s important to identify other ways to safely lower blood estrogens for all women.

“We’ve shown in the past that weight loss significantly lowers blood estrogens. Now we know that vitamin D can add to that, but only if a woman takes enough of it to increase her vitamin D blood levels to a normal range,” she continued.

The optimal circulating range of vitamin D is thought to be between 20 and 50 ng/ml. Blood levels lower than that may be inadequate for bone health and levels higher than that are associated with potential adverse effects, such as an increased risk of kidney stones.

There is no consensus, however, regarding the optimal blood level of vitamin D necessary for lowering sex hormones, Mason said. “We saw benefit for the women whose levels increased the most, but we don’t know whether we might have seen even more benefit had the women taken more vitamin D or had increased their blood levels to a higher threshold.”

Dr. Caitlin Mason
“Women could consult their doctor about having their blood vitamin D level checked and discuss whether they might benefit from some level of supplementation,” said lead author Dr. Caitlin Mason. Fred Hutch file

The sunshine vitamin

Vitamin D is naturally found in some foods, such as fatty fish, and it is produced within the body when skin is exposed to sunlight. According to the Institute of Medicine, just 10 minutes of sun a day is enough to trigger adequate vitamin D production. The estimated average requirement via diet or supplementation is 400 IUs per day for most adults – an amount that has been demonstrated to support bone health. The reason the participants in the current study were given a substantially higher dose is because that is the amount that was estimated to be necessary to boost their insufficient blood vitamin D levels to the desired, or replete, range, Mason said.

The nutrient plays many important roles in the body. Vitamin D promotes bone growth and bone healing and, along with calcium, helps protect older adults from osteoporosis. The vitamin also influences cell growth, neuromuscular and immune function, and it reduces inflammation.

“Vitamin D is widely available and low cost, without many side effects at moderate doses,” Mason said. “Thus, even if it delivers a small benefit in reducing breast cancer risk, that could be important at the population level.”

The bottom line, Mason said, is common-sense advice: “Maintaining a healthy weight throughout adult life – especially avoiding postmenopausal weight gain – is important for reducing breast cancer risk,” she said. “The role of vitamin D in postmenopausal breast cancer development is still unclear, but women could consult their doctor about having their blood vitamin D level checked and discuss whether they might benefit from some level of supplementation.”

Grants from the Breast Cancer Research Foundation, Susan G. Komen for the Cure and the National Cancer Institute funded the study, one of only two clinical trials known to look at the potential effects of vitamin D on sex hormones in humans.

Kristen Woodward, a science editor at Fred Hutchinson Cancer Research Center, has been in communications and media relations at Fred Hutch for more than 15 years. Before that, she was a managing editor at the University of Michigan Health System and a reporter/editor at The Holland Sentinel, a daily in western Michigan. She has received many national awards for health and science writing. She received her B.A. in journalism from Michigan State University. Reach her at kwoodwar@fredhutch.org.

Solid tumors, such as those of the breast, are the focus of Solid Tumor Translational Research, a network comprised of Fred Hutchinson Cancer Research Center, UW Medicine and Seattle Cancer Care Alliance. STTR is bridging laboratory sciences and patient care to provide the most precise treatment options for patients with solid tumor cancers.

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