If you’re living with breast cancer, you may be curious whether your vitamin D levels affect the condition’s progression. “Has anyone had issues with vitamin D?” asked one MyBCTeam member. Another said, “I am taking a booster of vitamin D.”

It’s important to understand whether there are any connections between vitamin D intake and breast cancer and if you should do anything to evaluate whether you have enough vitamin D in your diet.

What Is Vitamin D?

Vitamin D is a nutrient that your body needs to make your muscles move, help your nerves send signals, and allow your immune system to fight off bacteria and viruses that can make you sick. Vitamin D is also important so bones can absorb the calcium they need to be strong and healthy.

There are two kinds of vitamin D: vitamin D2 and vitamin D3. Vitamin D2 is mostly found in plants, mushrooms, and yeast. Vitamin D3 can be found in oily fish and is also made in the body during sun exposure. Additionally, vitamin D3 is later converted to 25-hydroxycholecalciferol, which helps turn on and off the genes that allow vitamin D to carry out its function in the body.

According to the Cleveland Clinic, foods that are good sources of vitamin D include:

• Beef liver
• Fortified cereal
• Fish (such as salmon, sardines, swordfish, and cod liver oil)
• Egg yolks
• Fortified milk and orange juice

Your body converts vitamin D into its active form, called 1,25-dihydroxyvitamin D — which is also known as calcitriol and can be found as a supplement. This active form of vitamin D can affect the cells involved in the immune system.

Is Vitamin D Deficiency Linked to Breast Cancer?

Research shows that having low levels of vitamin D may increase the risk of developing breast cancer. That’s because vitamin D may help control the normal growth of breast tissue cells.

There is more information about the link between vitamin D and breast cancer than there is for many other conditions, though vitamin D has been researched concerning several other cancers as well. Twenty different types of cancer, including colorectal, kidney, lung, and pancreatic cancer, have incidence rates that are inversely related to the concentration of serum vitamin D in one’s body.

There is research to support the idea that higher blood levels of vitamin D may be a protective factor for people undergoing breast cancer treatment. Researchers from the University of California, San Diego School of Medicine reported that “breast cancer patients with high levels of vitamin D in their blood are twice as likely to survive the disease as women with low levels of this nutrient.”

There is also evidence indicating that a lack of vitamin D may be associated with a poorer outlook for people who have already gotten a breast cancer diagnosis. A 2021 study noted that “compared to those with deficient vitamin D levels, patients with sufficient levels had significantly better survival outcomes.”

However, other studies have indicated that associations between vitamin D levels and breast cancer risk or prognosis do not prove causation. In other words, other factors besides vitamin D levels could account for the outcomes in individuals with breast cancer, and a cause-and-effect relationship between the vitamin and the condition has not been solidified.

Considering what the research indicates regarding vitamin D and breast cancer, you may be wondering if taking vitamin D supplements is right for you.

Should You Take Vitamin D Supplements?

Before starting a regimen of vitamin D supplements, check in with your doctor, who can test your levels of vitamin D serum. This can be done as a simple blood test during your regular physical. If your health care team determines that you do not have enough vitamin D, they may recommend supplements. However, keep in mind that you should only take the recommended amount because there is a risk of taking too much.

The Office of Dietary Supplements warns that too much vitamin D can cause nausea and vomiting, muscle weakness, confusion, pain, dehydration, and kidney stones, among other side effects. Vitamin D can also interact with some medications, so don’t start any supplementation plan before speaking with your physician.

Talk With People Who Understand

On MyBCTeam, the social network and online support group for people with breast cancer and their loved ones, members discuss the chronic nature of the disease. Here, more than 53,000 members from across the world come together to ask questions, offer advice and support, and share stories with others who understand life with breast cancer.

Have you investigated your vitamin D levels? Share your experience in the comments below, or start a conversation on MyBCTeam.