Vitamin D is important for good overall health and strong and healthy bones. It’s also an important factor in making sure your muscles, heart, lungs, and brain work well and that your body can fight infection. … It also may hold the key to longer breast cancer survival, according to an analysis published in JAMA Oncology.
Your body can make its own Vitamin D from sunlight. You can also get Vitamin D from supplements and a very small amount comes from a few foods you eat. Vitamin D deficiency affects an estimated 1 billion people worldwide or 90 percent of the world's population. 
Severe Vitamin D deficiency can cause rickets in children and osteomalacia in adults. Both of these conditions cause soft, thin, and brittle bones. A lack of Vitamin D has also been linked to cancer, asthma, type-II diabetes, high blood pressure, depression, Alzheimer’s and autoimmune diseases like multiple sclerosis, Crohn’s and type-I diabetes.
“This large prospective observational study provides compelling evidence that higher levels of Vitamin D at the time of breast cancer diagnosis can reduce the risk of breast cancer progression and death.”
Song Yao, Ph.D.
Roswell Park Cancer Institute
Some of the functions of the body that Vitamin D helps with include:
- Immune system, which helps you to fight infection
- Muscle function
- Cardiovascular function, for a healthy heart and circulation
- Respiratory system, for healthy lungs and airways
- Brain development
- Anti-cancer effects
Song Yao, an associate professor at the Roswell Park Cancer Institute in Buffalo, N.Y., led a study to determine Vitamin D levels at the time of diagnosis with breast cancer survival. The researchers used data from 1,666 Kaiser Permanente patients, testing their Vitamin D blood levels. Yao's team followed the women's health for seven years. 
The data analysis was performed from Jan. 5, 2014, to March 15, 2015.
Compared with women whose Vitamin D levels were under 17 nanograms per milliliter, women with levels higher than 25 had a 28 percent higher likelihood of surviving during the study, even after adjusting for tumor stage, grade, and type.
The effect was stronger for premenopausal women. Those with the highest Vitamin D levels were 55 percent more likely to survive. Also, they were 42 percent more likely to survive free of invasive disease and 63 percent less likely to die of breast cancer.
How Vitamin D works
Vitamin D travels through the bloodstream to the liver, where it's turned into 25-hydroxycholecalciferol (25(OH)D or calcidiol). This is a prohormone or precursor for the Vitamin D hormone.
The Vitamin D prohormone travels through the bloodstream to the kidneys, where it's turned into the active form, 1,25-dihydroxycholecalciferol (1,25(OH)2 D3 or calcitriol). 1,25(OH)2 D3 is the active Vitamin D hormone. It is released back into the bloodstream where it then regulates how your body uses calcium and phosphorus.
Because the liver and the kidneys are involved in the production of calcitriol, diseases of these organs may affect your ability to make this hormone.
Active Vitamin D works by entering cells and attaching to a protein called the Vitamin D receptor, located in the nucleus of cells, where the genetic material is located. This combination of calcitriol and its receptor stimulates the cell to make proteins that regulate the way the body works.
Vitamin D receptors also are present in most other tissues, including the brain, heart, skin, ovary and testicle, prostate gland, and breast, as well as the cells of the immune system, including white blood cells and other key immune cells. 
Did You Know?
Vitamin D Deficiency Signs
• Aches and pains
• Low energy
• Brain fog
• Mood fluctuations
• Sleep disturbances
Breast cancer and bacteria
One in eight women in the U.S. are diagnosed with breast cancer during their lifetimes, but its origins remain unknown in most cases. Age, genetic predisposition, and environmental changes often are implicated — and according to a growing body of research, bacteria may be one of those environmental factors. 
Researchers from Western University in Ontario, Canada, discovered in 2014 that a variety of bacteria were detected in breast tissue regardless of the location samples — tissue from close to the nipple to as far back as the chest wall.
The scientists discovered that in the women with breast cancer, there were significantly higher levels of Enterobacteriaceae, Staphylococcus, and Bacillus bacteria.
The participants without breast cancer, on the other hand, had higher incidences of Lactococcus and Streptococcus bacteria, which are thought to have strong anticarcinogenic properties.
Could there be a link between bacteria and a lack of Vitamin D in breast cancer patients, in light of the fact a Vitamin D deficiency hampers the immune system?
“This large prospective observational study provides compelling evidence that higher levels of Vitamin D at the time of breast cancer diagnosis can reduce the risk of breast cancer progression and death,” Yao said.
“The reduced risk was more pronounced in younger women, specifically those diagnosed with breast cancer before menopause. Our study suggests that Vitamin D may extend survival in women diagnosed with breast cancer.”