Achieving True Vitamin D Sufficiency Cuts Breast Cancer Risk In Half

It's concerning enough that more than 40% of American adults1 meet the criteria for a clinical insufficiency in vitamin D (and a whopping 29%2 are straight-up deficient in the essential vitamin). But, disturbingly, a 2017 Breast Cancer review shows that up to 96%3 of people diagnosed with breast cancer are also deficient in vitamin D. 

Some studies show a specific association between circulating vitamin D—i.e., 25(OH)D serum vitamin D levels—and breast cancer risk. For example, a 2015 study published in Breast Cancer Research and Treatment found that women who had circulating vitamin D levels below 20 ng/ml had a 27% higher risk4 of breast cancer than those with sufficient vitamin D levels. (Worth noting: Clinical vitamin D deficiency is defined as levels below 20 ng/ml, while vitamin D sufficiency is defined as levels above 30 ng/ml5.)

A meta-analysis published in Anticancer Research found that a circulating vitamin D level of 47 ng/ml lowered breast cancer risk by 50%6, suggesting that levels even above 30 ng/ml are the goal. 

Recommendations around vitamin D intake are a little confusing. While the recommended dietary allowance (RDA) from the National Academies for vitamin D is 600 IU for most healthy adults, some research7 suggests that you need a minimum of 3,000 IU of vitamin D3 a day to get a total serum 25(OH)D (the clinical biomarker of your body's vitamin D status) of 30 ng/ml, aka clinical sufficiency. Some leading wellness experts suggest that 30 ng/ml is still well below truly optimal vitamin D levels (i.e., 50 ng/ml).

"There's a lot of information suggesting that 600 IU—the current RDA for vitamin D—isn't enough to optimize your vitamin D status," says Samantha Cassetty, M.S., R.D., a nutrition and wellness expert and co-author of Sugar Shock. It's possible to get vitamin D from salmon, egg yolks, certain mushrooms, and fortified milk, but many plant-based milks aren't fortified to the same degree as cow's milk. As a result, "vegans and vegetarians are at higher risk for vitamin D insufficiency," Cassetty says. Even if you eat animal products, it's hard to get enough vitamin D from food and exposure to sunlight (another vitamin D source) alone.

Ultimately, "most people tend to need a supplement for vitamin D," says Jessica Cording, M.S., R.D., CDN, registered dietitian and author of The Little Book of Game-Changers. When selecting a vitamin D supplement, Cording recommends looking for one with vitamin D3. "It's the most absorbable form," she says, adding that she especially likes formulations that include plant-based fats, like olive oil, avocado oil, or flax oil.

There is an association between vitamin D status and breast cancer, but it's not a definitive link at this point.