Kefir is a fermented milk drink popular in the Middle East, parts of the Mediterranean and eastern Europe. Kefir grains incorporate a complex living culture of yeasts and bacteria. Kefir is prepared by inoculating milk (cow, camel, goat, sheep, or even soy or almond milk) with kefir grains, which initiates a natural fermentation process.
The mixture of microorganisms in kefir grains can vary greatly depending on geographic region and other factors. One study of 24 kefirs produced in Estonia, Latvia and Lithuania reported that only one-third of them met the minimum expected level of viable microorganisms. Kefir should be purchased organic from a reputable supplier.
Raw milk has traditionally been used to make kefir. However, commercial kefir sold in the U.S. normally is made with pasteurized milk. Kefir is one of the most important probiotic products, with stronger anti-cancer effects than other fermented milk products such as yogurt or cheese. Unless otherwise noted, the discussion below refers to kefir and similar fermented drinks made from cow's milk.

Breast cancer-related effects of consuming kefir

Human studies suggest kefir lowers BC risk

One Swedish study of the relationship between various fats in the diet and breast cancer risk for women aged at least 50 found that fat from fermented milk products was negatively associated with breast cancer risk. Another Swedish study reported that relatively high long-term consumption of fermented dairy products was associated with reduced risk of ER-/PR- breast cancer.

Cell and animal studies report kefir inhibits BC

Kefir has been shown to have antiproliferative activity against hormone receptor positive (ER+/PR+) human breast cancer cells; this anticancer effect has been observed at much lower concentrations than similar effects of yogurt. Kefir is a good source of conjugated linoleic acid (CLA), which has been shown to inhibit the growth and migration of breast cancer cells.
Several studies have demonstrated that milk fermented with L. helveticus decreases the growth rate of mammary tumors in experimental rats (including HER2+ tumors) and that this involves reduced production of pro-inflammatory cytokines, including IL-6, which is implicated in estrogen synthesis. Milk fermented with Lactobacillus casei CRL431 has been shown to delay tumor growth, inhibit angiogenesis, reduce metastasis, and increase survival in mouse models of breast cancer. Lactobacillus crispatus and Lactobacillus acidophilus have also been reported to have antiproliferative activity in breast cancer cells.

Kefir may improve gut microbial profile

Recent research suggests that microorganisms found within the gut, and gut microbial imbalances in particular, may influence breast cancer risk and development. The gut microbiome (the complete sets of genes or genetic material present in the microorganisms) incorporates more than 3.3 million genes, most of whose functions have not been determined. Gut microorganisms (including bacteria, viruses, fungi and other microbes) function as symbionts, providing beneficial functions such as nutrient and drug metabolism and protection against pathogens to their human hosts. The gut microbiome can evolve rapidly under the pressure of changing environmental factors, and is influenced by factors such as antibiotic use, obesity and diet.
Vegetarian diets and diets characterized by high consumption of animal fats or fiber each have been shown to associated with distinct patterns of gut microbiota composition. Intestinal microbes have been shown to have a role in the regulation of estrogen metabolism and links between altered gut microbiome and breast cancer have been reported. Eating fermented foods such as kefir appears to protect against breast cancer in part through interactions with the microbiome, although the mechanisms are not well understood.
In one study, mice genetically susceptible to mammary tumors were fed a Westernized diet associated with increased mammary carcinogenesis. Development of mammary tumors was shown to be inhibited by the regular addition of Lactobacillus reuteri in the animals' drinking water. This outcome was determined to be a result of microbially-triggered lymphocytes. In other words, Lactobacillus reuteri generated a host immune response that inhibited tumor progression in the mammary glands, a distant tissue.
There appear to be multiple pathways by which fermented foods such as kefir can potentially reduce breast cancer risk and progression through the gut microbiome. In addition, there is some evidence that unfavorably altered gut microbiomes may reduce the effectiveness of chemotherapy.

Kefir and cancer treatment

Chemotherapy
Kefir has been shown to reduce Adriamycin (doxorubicin)-induced multidrug resistance (which reduces the effectiveness of chemotherapy) in colorectal cancer cells. Kefir has also been reported to increase survival rates and reduce Adriamycin-induced heart damage (cardiomyopathy) in rats.
Studies on the effect of probiotics on chemotherapy-associated side effects in breast cancer patients have reported contradictory results. Kefir appears to reduce gastrointestinal side effects and reduce sleep disturbances in some patients.
Radiotherapy
Kefir has been shown to protect cells against cell death caused by radiation damage, raising the possibility that it will lessen the cytotoxic impact of radiation treatment on breast cancer cells. However, there is not enough evidence to determine whether or not it is safe to consume kefir during radiotherapy.

Sources of information provided in this webpage

The information above, which is updated continually as new research becomes available, has been developed based solely on the results of academic studies. Clicking on any of the underlined terms will take you to its tag or webpage, which contain more extensive information.
Below are links to 20 recent studies concerning this food and its components. For a more complete list of studies, please click on kefir.