Our bodies require a whole alphabet of vitamins and minerals to function optimally. From vitamin A to zinc, our bodies need to replenish a variety of different elements every day to stay healthy. Most of these vitamins and minerals come from the foods we eat, but many of us don't get enough of certain ones, and that could lead to problems and potentially cancer in some cases, recent research has suggested.
A study published in 2016 in the journal Endocrinology used mice to explore the relationship between vitamin D and breast cancer. The study notes that "patients with breast cancer frequently have preexisting vitamin D deficiency when their cancer develops," and the study looked at whether this was coincidental or if low levels of vitamin D contributed to the development of cancer. The results of this study indicate there could be a causal connection, but some other studies have offered conflicting assessments or inconclusive evidence of an association between vitamin D deficiency and breast cancer.
Hollie Zammit, an outpatient oncology dietitian at UF Health Cancer Center, Orlando Health, says that vitamin D status is a problem in the general population, not just among cancer patients, but she cautions that we shouldn't jump to too many conclusions just yet about the potential connection between breast cancer and vitamin D levels. "There's a lot of studies emerging on that, but in many of these studies, the results have been mixed or inconsistent, and further research is needed to determine where the connection is," she says.
As with many other aspects of nutritional research, it's difficult to say for certain whether a low level of vitamin D in the body is a risk factor for developing breast cancer, but one thing seems certain: Most Americans are vitamin D deficient. A 2009 study published in the Journal of the American Medical Association found that 75 percent of American teenagers and adults had suboptimal levels of vitamin D in their blood.
Vitamin D is important for helping the body absorb calcium. In children, vitamin D deficiency can cause the disease rickets, which makes the bones soft and weak. In adults, vitamin D deficiency has been linked with cardiovascular disease, diabetes, depression, cognitive decline and cancer, among other diseases. JAMA study co-author Adit Gide, an assistant professor at the University of Colorado Denver School of Medicine, told Scientific American, "we're just starting to scratch the surface of what the health effects of vitamin D are."
Some of the widespread vitamin D deficiency in modern America is likely the result of decades of promotion of sun safety to reduce the incidence of skin cancer. Vitamin D is the only vitamin humans can manufacture, and this process happens in the skin when it's exposed to sunlight. Sunscreen, sunblock and clothing all interfere with the production of vitamin D in the skin, with sunscreen blocking upwards of 90 percent of vitamin D production in the skin, Zammit says.
Simultaneously, as technology increases, more jobs are moving indoors and out of the sun. A review study published June 21 in the journal BMC Public Health found that occupation is a major factor in vitamin D deficiency and insufficiency, noting that a lack of vitamin D is highest among shift workers and those who work indoors. The study found that 77 percent of indoor workers were vitamin D deficient, and 91 percent of indoor workers had insufficient levels of vitamin D, meaning their levels of vitamin D weren't as low as the deficient workers', but still lower than recommended for optimal health.
This migration away from the sun means that we need to find ways to boost vitamin D in the diet, but "getting it from your diet alone is really hard," Zammit says. The Food and Nutrition Board of the Institute of Medicine sets the vitamin D recommended dietary allowance for adults through age 70 at 600 international units per day. For adults aged 71 and above it's 800 IUs per day. "A cup of milk is only 100 IUs, so you'd have to drink 6 cups of milk per day," Zammit says, and that's a lot of calories and fluid to take in just to reach your daily vitamin D allowance. "Salmon is also a natural source of vitamin D, which provides anywhere from 300 to 400 IUs per 3 to 4 ounces, but you'd still need to eat 6 ounces of salmon every day," to hit the 600 IU RDA. Some food items, such as orange juice and packaged cereals, are fortified with vitamin D, "but even then, it's going to have low amounts of D in it," she says. Dairy products, eggs, tofu and cod liver oil are also good sources of vitamin D.
Because of the challenges we face in getting enough vitamin D from food sources, Zammit says that a lot of people do take vitamin D supplements. But because vitamin D is a fat-soluble vitamin, there's an outside chance that if you're taking extremely high doses of vitamin D, it can become toxic. The Mayo Clinic reports that taking 50,000 IUs daily for several months could lead to toxicity. Therefore, Zammit recommends that you speak with your doctor about any supplements you're taking and have your vitamin D levels tested to see if you're deficient and to determine if supplementation is a good option for you. "Discuss with your primary care physician and oncologist if you haven't already had your serum concentration of 25-hydroxyvitamin D levels checked. It's a lab test that measures the serum [blood] concentrations of the vitamin, and it's the only way to know what your vitamin D status is," she says.
If supplementation isn't for you, Zammit says you can also carefully increase your exposure to the sun by getting about 10 minutes of sun (without sunblock on) three times a week between the hours of 10 a.m. and 3 p.m. How much vitamin D you'll make in the skin with this level of sun exposure likely varies from person to person, Zammit says, but it's typically not enough to result in a sunburn or a big increase in risk for skin cancer. Taking a brisk walk around the block may even be enough exposure to help boost your vitamin D levels, and Zammit says the exercise will do you good, too. "For breast cancer patients specifically, we want them to be as physically active as they can be. We advise them to strive for 30 minutes per day."
Lastly, whether you have breast cancer or are concerned you could develop it, be sure to eat a healthy diet and control your weight. Although vitamin D levels have not been conclusively tied to risk of developing breast cancer, obesity has been. Zammit says you should aim to "be as lean as possible without being underweight and limit your alcohol consumption," which is another risk factor for developing breast cancer. "Focus on eating a plant-based diet with lots of fruit, vegetables and moderate amounts of animal proteins. That's what can help reduce your risk of developing cancer, reduce your risk of recurrence and reduce your risk of developing secondary cancers."
Elaine K. Howley is a freelance Health reporter at U.S. News. An award-winning writer specializing in health, fitness, sports and history, her work has appeared in numerous print and online publications, including AARP.org, espnW, SWIMMER magazine and Atlas Obscura. She's also a world-record holding marathon swimmer with a passion for animals and beer. Contact her via her website: elainekhowley.com.