What is breast cancer?
Tumors are made when there is a buildup of unneeded, old or damaged cells that form a mass. Breast cancer is a malignant tumor, which is a group of cancerous cells that can grow and spread to surrounding tissues. If the cancer spreads away from the breast cells to other areas of the body, this is called metastasizing.
Breast tumors can either be benign or malignant. Benign tumors are usually not harmful, whereas malignant tumors are and can spread to other parts of the body. When breast cancer cells spread, they usually first spread to the lymph nodes, which are located at the underarms and above the collarbone.
The stage of breast cancer depends on the size of the tumor, and whether or not it has metastasized. A cancer that is Stage I is early-stage breast cancer, whereas a cancer that is Stage IV is advanced cancer that has spread to other parts of the body.
How common is breast cancer?
Breast cancer is the second most common cancer among women in the United States. About 1 in 8 women will develop breast cancer in their lifetime. Breast cancer occurs almost entirely in women, but men are able to get it, too. White women have the highest chances of getting breast cancer compared to blacks and Hispanics , but blacks have the lowest survival rates .
There are many factors that can increase your likelihood for developing breast cancer. Non-modifiable risk factors include:
- Gender: Being female is the main factor that increases your chances of developing breast cancer. Men can also develop breast cancer.
- Age: The risk of developing breast cancer increases as you get older.
- Genetics: Some breast cancers are related to having unusual forms of some genes such as BRCA1 or BRCA2. 
- Family history of breast cancer: If you have someone in your extended family that has breast cancer, you are more likely to get it.
- Race and ethnicity: White women are more likely to develop breast cancer.
- Women who began menstruation early (before age 12) are at higher risk for breast cancer.
- Pregnancy and breastfeeding: Women who do not have children or those who do not breastfeed their children are at a greater risk. Pregnancy and breastfeeding reduce the number of menstrual cycles in a woman’s life, which may decrease the risk of breast cancer.
- Long-term birth control: Women who use oral birth control pills for most of their life have a slightly higher chance of developing breast cancer.
- Hormone therapy: Using combined hormone therapy with both estrogen and progesterone for over 2 years increases the risk of developing breast cancer.
- Alcohol consumption: Chronic alcohol consumption may also increase the likelihood of breast cancer.
- Overweight or obesity: Those who are overweight or obese have greater chances of developing breast cancer.
- Inadequate exercise: Those who live a sedentary lifestyle are at an increased risk of breast cancer.
- Cigarette smoke: Long-term smoking or exposure to secondhand smoke will increase the risk of developing breast cancer.
What causes breast cancer?
While it is known that there are many factors that can increase an individual’s risk for developing breast cancer, researchers still don’t know exactly what causes breast cancer and causes cells to become cancerous. Hormones are believed to play a large role in breast cancer; though, it is not fully understood how or why.
Researchers do know that cancers come from abnormal changes in cells and genes. For example, genes called tumor suppressor genes can slow cells from growing into cancerous cells. When people have altered versions of these genes, they are unable to slow the cancer cell growth, so breast cancer is more likely to develop. Some individuals may have changes to their oncogenes, genes associated with cancer, usually responsible for increased cell division and tumor production. When these genes are overexpressed, it increases one’s risk for developing breast cancer.
Sometimes individuals can develop these DNA changes over time, while others inherit them at birth. Researchers believe that being exposed to radiation or chemicals may cause DNA changes over time and result in cancer.
It’s clear that a combination of both genetics and environmental factors contribute to the development of breast cancer.
What is the link between breast cancer and vitamin D?
Many studies have shown that there is a link between vitamin D and breast cancer. Women who have breast cancer tend to have low levels of vitamin D.
Researchers have found a potential mechanism to explain the relationship between vitamin D and breast cancer. Vitamin D receptors are found on the surface of a cell where they receive chemical signals. By attaching themselves to a receptor, these chemical signals direct a cell to do something, for example to act in a certain way, divide or die.
There are vitamin D receptors in breast tissue, and vitamin D can bind to these receptors. This can cause cells like oncogenes to die or stop growing, and can stop the cancer cells from spreading to other parts of the body. Therefore, it is thought that vitamin D may help in protecting against breast cancer, by making cells in the breast smarter.
What does the research say in general about vitamin D and breast cancer?
Preventing breast cancer
Some studies have been conducted which have found that women with low levels of vitamin D are more likely to develop breast cancer. A recent review of many studies found that post-menopausal women with low levels of vitamin D had a higher risk of developing breast cancer compared to post-menopausal women with high levels of vitamin D. 
Other studies have found what is called a dose-response relationship, where for each increase in vitamin D levels in the body, there is a decrease in breast cancer risk 
Treating breast cancer
Some studies have shown that there is a link between vitamin D levels and recurrence of breast cancer, tumor size and death from breast cancer. This means that having enough vitamin D may be able to help keep a cancer from getting worse. In fact a recent review concluded, “high vitamin D status is weakly associated with low breast cancer risk but strongly associated with better breast cancer survival.”
In an even more recent review, researchers found that low vitamin D levels were associated with a twofold-increased risk of cancer returning and mortality among women with breast cancer compared to women with breast cancer and high vitamin D levels.  The authors concluded, “maintaining an optimal 25(OH)D status at diagnosis and during the 1-year follow-up period is important for improving breast cancer patient survival.”
What does recent research say?
Preventing breast cancer
There are four types of studies that are used to see whether UVB exposure and vitamin D reduce the risk of breast cancer: geographical studies, observational studies, laboratory studies and randomized controlled trials. All four types of studies have found strong evidence that UVB and vitamin D reduce the risk of breast cancer.
In geographical studies, the geographical variation in breast cancer incidence or mortality rates is compared statistically with solar UVB amounts. Such studies in Australia, China, France, Nordic countries, Spain and the United States have found lower breast cancer rates in regions of higher solar UVB. 
Observational studies compare vitamin D levels with the incidence of breast cancer. There are two types of observational studies: those that look at vitamin D levels measured near the time of breast cancer diagnosis, called case-control studies, and those measured at the time women enroll in studies prior to diagnosis of breast cancer, called nested case-control studies. Only the case-control studies have found that lower vitamin D levels are a risk factor for breast cancer.  The reason why the nested case-control studies have not found the same result may be that breast cancer develops very rapidly, while, without supplementation, vitamin D levels tend to change little over time.
Observational studies have also found that women with higher vitamin D levels at the time of breast cancer diagnosis live longer than those with lower vitamin D levels. , In addition, black women have higher chances of mortality from breast cancer after diagnosis than white women.
Laboratory studies have identified the mechanisms of vitamin D, which contribute to the reduced risk of breast and other types of cancer. According to these studies, vitamin D is involved in the observation of cells and allows them to survive if they are the right type in the right place, or helps them commit suicide (apoptosis) if not. Vitamin D also reduces blood vessel formation around tumors and reduces the ability of tumors to metastasize. 
Finally, two randomized controlled trials found that vitamin D reduced the risk of cancer, including breast cancer. In the first trial, postmenopausal women in Nebraska received 1100 IU/d vitamin D3 and 1450 mg/d calcium, which resulted in a 77% reduction in all-cancer incidence between the first and fourth years of the trial, while those receiving only calcium had a 44% reduction.  In the second study, the Women’s Health Initiative, those women taking 400 IU/d vitamin D3 plus 1500 mg/d calcium experienced a decreased the risk of total, breast and invasive breast cancers by 14-20%. 
The role of sunlight in reducing risk of breast cancer
There is growing evidence that solar UVB reduces the risk of breast and other cancers. People who live in the more sunny regions of low to mid-latitude countries have lower breast cancer incidence and/or mortality rates than those living in the higher latitude, less sunny regions. For example, in the United States, the lowest breast cancer mortality rates are in Arizona, New Mexico and Utah, while the highest rates are in the New England states. In addition, a recent study with mice found that the progression of intestinal cancers was reduced more by UVB exposure than oral vitamin D intake, even when both treatments increased vitamin D levels about the same amount. . Individuals concerned about developing breast cancer may reduce their risk of breast cancer by spending some time in the sun daily, in a bathing suit, when their shadow is shorter than their height, which depends on the time of day and season. 
Key points from the research
- Research from many studies has shown that women with breast cancer are more likely to have low vitamin D levels.
- Some research has shown that post-menopausal women who don’t get very much vitamin D may be more likely to develop breast cancer later in life.
- Research has shown that women who have breast cancer are more likely to develop bigger tumors and are more likely to have recurrent breast cancer if they have low levels of vitamin D.
- Women with higher vitamin D levels are less likely to develop breast cancer and less likely to die from breast cancer.
- However, due to limited experimental studies, it’s not possible to say whether low vitamin D levels help cause breast cancer or lead to worse outcomes, and whether or not vitamin D can help to prevent or treat breast cancer.
- Overall, more experiments are needed to give clearer answers regarding whether taking a vitamin D supplement can prevent or treat breast cancer. These studies are underway.
What does this mean for me?
Research does show that there is a link between vitamin D and breast cancer. Studies have shown that women with low levels of vitamin D are more likely to develop breast cancer.
Studies have also shown that in women who already have breast cancer, low levels of vitamin D are linked to worse outcomes, like bigger tumors, cancer spreading to other body parts and death.
If you have breast cancer or you are trying to prevent breast cancer and want to take vitamin D, it is unlikely to make your breast cancer worse or cause you any harm, as long as you take less than 10,000 IU per day.
Also, remember that exposure to sunlight may help more than just vitamin D supplements. Our advice is to get both brief full-body sun exposure when your shadow is shorter than you are and take at least 5,000 to 10,000 IU/day. This may be especially important if you have or have had breast cancer.
If you have breast cancer, you shouldn’t take vitamin D in place of your treatment medications. Talk to your physician for more advice about taking supplements.
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This page was last revised October 12, 2015.