Low Vitamin D and Breast Cancer
Vitamin D is a staple in the wellness world, and there are good reasons for that. The nutrient helps the body absorb calcium, which is essential for good bone health. Vitamin D also helps the muscle, nervous, and immune systems function properly. Your body makes vitamin D when your skin is exposed to sunlight. Smaller amounts of vitamin D are in fortified milk and other foods, fatty fish, and eggs. You can also get it as a supplement that’s sold in many forms, including vitamin D3, vitamin D2, and calciferol.
Studies show that low vitamin D levels are linked with a higher risk of several diseases, including diabetes, osteoporosis, and breast cancer.
A simple blood test can measure your vitamin D levels. Results are reported as nanograms (ng) per milliliter (mL) of blood. Most experts say people need levels between 20 and 50 ng/mL to be as healthy as possible. People with levels below 20 ng/mL are considered vitamin D deficient.
Breast cancer and vitamin D: What the research shows
Scientists have been studying the relationship between vitamin D and cancer for decades. In the 1980s, very early research suggested that vitamin D could stop cancer from growing. Since then, researchers have looked at whether vitamin D levels affect the risk of developing breast cancer, breast cancer outcomes, and contribute to treatment-related side effects.
Vitamin D and breast cancer risk
A 2022 study found that vitamin D levels below 20 ng/mL were linked to a higher risk of breast cancer in Black and Hispanic/Latina women. A large 2023 study of more than 430,000 women (the researchers didn’t report the women’s ethnicities) found that post-menopausal women with a vitamin D deficiency had a 45% greater risk of developing breast cancer. Pre-menopausal women with vitamin D deficiency had an 18% higher risk. Vitamin D and breast cancer outcomes
A large 2026 study of more than 675,000 people diagnosed with breast cancer found that vitamin D levels below 20 ng/mL were linked to a higher risk of the breast cancer metastasizing (spreading to a part of the body away from the breast), as well as a higher risk of dying from breast cancer or another cause. These findings echoed results from a 2016 study showing that higher vitamin D levels were associated with better overall survival.
Can vitamin D make breast cancer treatment more effective and ease side effects?
Some of the most recent research on vitamin D and breast cancer is studying how the nutrient affects breast cancer treatment.
A small 2025 study done in Brazil found that women who took a vitamin D supplement got more benefits from chemotherapy before breast cancer surgery than women who didn’t take a supplement. More research is needed, but the findings suggest that doctors may want to measure vitamin D levels before women start chemo for breast cancer and prescribe a supplement for those whose levels are low.
Women with low vitamin D levels who were receiving the chemotherapy medicine Taxol (chemical name: paclitaxel) to treat breast cancer had a higher risk of neuropathy than women with sufficient vitamin D levels, according to a 2024 study.
Other small studies in people with a variety of cancers suggest that a vitamin D supplement may help ease other chemotherapy and radiation therapy side effects, including nausea, vomiting, and fatigue.
How much vitamin D do you need?
The U.S. National Institutes of Health Office of Dietary Supplements recommends that people ages 1 to 70 get 600 international units (IU) of vitamin D per day. People older than 70 need 800 IU.
Before you up your intake, though, it's important to know your vitamin D blood level. Ask your doctor to check your level the next time you’re in the office.
How to get more vitamin D
The two most reliable ways to boost your vitamin D level are to get more direct sunlight exposure and take vitamin D3 supplements. Eating foods rich in vitamin D can help, but doesn’t increase your levels as much as the other two.
Food
It’s hard to get all the vitamin D you need from food alone because few foods naturally contain the nutrient. Fatty fish, like salmon, sardines, and herring, along with cod liver oil, have the most vitamin D. Eggs also have vitamin D and milk and many cereals have vitamin D added to them.
Sunshine exposure
Even short periods of direct peak sun exposure — 15 minutes, three times a week, for example — can give many people more than the recommended daily amount of vitamin D. .
Experts recommend you continue to use sun protection when ultraviolet (UV) radiation levels are moderate or high, because sun exposure increases your risk of skin cancer, including melanoma, the most dangerous type.
There are many variables that can affect how much vitamin D you'll produce from sunlight:
People with darker skin need more sun exposure to produce the same amount of vitamin D as people with lighter skin.
The farther you live from the equator, the harder it is to produce enough vitamin D through sunlight.
Shorter days and fewer sunlight hours mean you produce less vitamin D.
As more people spend much of their time inside and away from direct sunlight, vitamin D production from sun exposure may be limited.
All these factors can make it hard to get enough vitamin D from sun exposure alone.
Supplements
Before you take any supplements, talk to your doctor about all the medicines and other supplements you’re taking because vitamin D supplements can interact with some medicines. If you have kidney stones, kidney disease, high blood calcium levels (hypercalcemia), heart disease, or liver disease, vitamin D supplements may make these conditions worse. If you’re taking diuretics or water pills — medicines that make you pee more often — a vitamin D supplement may increase the calcium level in your blood, which can create kidney stones.
Also ask your doctor about the risks and benefits of the product you want to take, as well as what a good vitamin D level is for you. If your level was low and you've been taking a supplement to get back into the normal range, have your vitamin D level checked a few months later and adjust your supplement dose accordingly. Taking too much vitamin D may cause hypercalcemia.
If you're going to take a vitamin D supplement, most experts recommend taking the D3 form of the vitamin, not D2.