Researchers say vitamin D may make the body more resistant to breast cancer.

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Researchers say they aren’t sure if vitamin D deficiency causes breast cancer or if breast cancer brings on vitamin D deficiency. Getty Images

Research on vitamin D’s role in breast cancer hasn’t been conclusive, but one more piece to the puzzle has just been discovered.

A new study concludes that obese postmenopausal women were more likely to have vitamin D deficiency at the time of their breast cancer diagnosis than women of the same age group without cancer.

The positive relationship could suggest vitamin D deficiency is a risk factor for breast cancer, especially in women with a higher body mass index.

Dr. JoAnn Pinkerton, executive director of The North American Menopause Society and a professor of obstetrics and gynecology at the University of Virginia Health System, says this isn’t surprising.

“Many studies have shown that women whose diets are high in fat are more likely to get [breast cancer],” she explained.

They’re also more likely to be storing inactive vitamin D in their fat cells, lowering overall blood levels.

“Even if overweight women take in as much vitamin D from the sun, food, or supplements, their blood levels will tend to be lower,” Pinkerton told Healthline.

But does this mean that losing weight and taking a supplement are keys to preventing breast cancer?

The connection between obesity and breast cancer is clear.

“For cancer prevention, the American Cancer Society recommends that people achieve and maintain a healthy weight throughout life… For those who are currently overweight or obese, losing even a small amount of weight has health benefits and is a good place to start,” Marji McCullough, ScD, strategic director of nutritional epidemiology at the American Cancer Society, told Healthline.

What’s still not clear is vitamin D’s role.

“This study measured vitamin D in the blood of women already diagnosed with breast cancer,” McCullough said.

“Their findings of lower concentrations in breast cancer patients compared to controls is consistent with some earlier studies. However, since vitamin D levels were measured after breast cancer was diagnosed, it is not known whether the low vitamin D levels influenced risk or were a result of breast cancer.”

Another factor is the often overlooked fact that there are many kinds of breast cancer. How one intervention affects risk for a certain subtype may not be consistent with others.

“Breast cancer is not one disease. We know that now,” Jean Sachs, chief executive officer of Living Beyond Breast Cancer, told Healthline.

“There’s not going to be one cure. There’s going to be many cures based on your particular subtype and genomic mutation.”

Like most things medical, it depends on who you ask.

“We always want to tell women… do no harm. It’s probably not harmful to take vitamin D, so why not do that?” Sachs said.

The American Cancer Society takes into account risk factors for vitamin D deficiency, such as age and geographic location, and recommended daily allowances when suggesting supplementation:

“The current RDA for vitamin D intake is 600 IU/day for most adults and 800 IU/day for individuals over age 70. For people who do not eat foods containing vitamin D, a supplement may be necessary. The National Academy of Medicine recommends not exceeding 4,000 IU vitamin D/day because very high levels can be toxic,” McCullough said.

“The benefits of vitamin D are still not proven, but it makes sense that women of all ages need adequate vitamin D levels for their bones, their immune system, and their health,” added Pinkerton.

“However, women also need to recognize that too much vitamin D can cause abnormally high levels of calcium in blood, which can lead to problems with blood pressure, bone loss, or kidney damage.”

Sometimes, breast cancer just happens and there’s no clear understanding as to why.

“We see way too many women who are a healthy weight, have pregnancies early, took vitamin D, whatever, and they get the disease,” Sachs said.

“So we say all these things, you know, do what you can to live healthy, but that doesn’t mean you’re not going to get breast cancer because we still don’t know what causes the disease, except for that small amount of people where we know it’s hereditary.”

Still, the healthy lifestyle factor can’t be ignored. Pinkerton suggests:

  • exercising daily and incorporating a buddy system several days a week
  • choosing healthier food options and keeping healthy choices in view
  • drinking plenty of water or seltzer and avoiding sugary or diet drinks
  • if overweight, aiming to lose 10 pounds in a year through increasing movement and decreasing calories

These are things most of us already know we should be doing, so what else?

Get tested and get informed.

If you have a strong family history, insurance will cover genetic mutation testing. Speak with your doctor about seeing a genetic mutation counselor.

If you’re suspicious, Sachs says, “make sure you follow up, especially for younger women who are told, ‘Oh you don’t need a mammogram, you’re too young.’… Just really be persistent with your healthcare provider.”