Butter is a rich dietary source of conjugated linoleic acid (CLA) and is also a good source of vitamin A (through its beta-carotene content). However, butter is also a rich source of saturated fat and cholesterol, both of which have been linked to increased breast cancer risk. "Butter" refers to butter made from cow's milk in this website.
Butter consumption has been found to be associated with increased risk of leukemia, as well as oral, esophageal, thyroid, stomach, pancreatic, endometrial, colorectal, prostate and testicular cancer. In addition, butter consumption has been linked to higher rates of cardiovascular disease and heart attacks in women as well as in men.

Breast cancer-related effects of consuming butter

As noted above, butter incorporates CLA, which has been reported to have anti-breast cancer properties. However, the fat profile of butter is cancer-promoting and epidemiological evidence suggests that this factor overpowers the potentially positive effects of the CLA content.

Conjugated linoleic acid (CLA) and breast cancer

CLA has been found to inhibit breast cancer in the laboratory at concentrations equivalent to human consumption levels. Several population studies have been performed specifically to evaluate the possible association between consumption of CLA in butter and other dairy products and the risk of breast cancer. The study results have been contradictory.
For example, a Finnish study of women who already had breast cancer found that dietary CLA and levels of CLA in the blood were significantly lower in postmenopausal breast cancer cases than in postmenopausal controls without cancer. However, a large prospective Netherlands population study found a weak positive association between CLA intake and the risk breast cancer (i.e., the association was in the opposite direction expected if CLA was protective) and concluded that the apparent anticarcinogenic properties of CLA in animal and tissue culture models had not been confirmed in humans.

Butter has an unfavorable fat profile

Butter is a source of several types of saturated fat, as well as cholesterol, all of which have been found to be associated with increased breast cancer risk. For example, butter is a rich source of myristic acid, a saturated fat that has been linked to greater breast cancer risk in epidemiological studies. Intake of animal fats has been found to be associated in several studies to increased breast density, a risk factor for breast cancer and its recurrence. In addition, high cholesterol has been shown to promote metastasis in an animal model of breast cancer.

Human studies

Women in the Nurses' Health Study who consumed more butter during high school were found to have a higher risk of breast cancer in adulthood. Another study found that butter consumption was associated with increased risk of recurrence of early stage breast cancer, especially among premenopausal breast cancer cases. Other U.S, Canadian and European studies have found that the use of butter in soups or sauces (i.e., boiled butter), butter used for frying food, and butter at the dinner table were associated with higher risks of breast cancer. A 2021 meta-analysis of data from previous studies with a total of 1,019,232 participants reported that high-fat dairy was associated with increased breast cancer risk in both premenopausal and postmenopausal women.
High intake of animal fats has been linked in several studies to increased breast density, a risk factor for breast cancer and its recurrence. A 2021 U.S. prospective study reported that high circulating myristic acid was associated with increased breast cancer risk.
In addition, high consumption of butter by rat mothers during pregnancy has been reported to heighten the mammary tumor-promoting effects of gestational exposure to bisphenol A (BPA) in their offspring.

Varieties of butter and margarine

While grass-fed butter has a higher beta-carotene content than conventionally-produced butter (as evidenced by it's richer yellow color) and it's fat profile may also be somewhat less harmful, these advantages are not enough to make it safe for breast cancer patients and survivors. While we do not recommend such butter, modest amounts consumed infrequently might not be harmful.
Another alternative is carefully-selected margarine. Most margarine is more detrimental than butter since it is a highly processed food that does not incorporate CLA or vitamin A and is made with trans fat as well as saturated fat. High quality yogurt-based margarine can be an acceptable choice.

Additional comments

Ghee is also not recommended

Ghee is a type of clarified butter used in South Asian, middle Eastern and North African cuisines. Ghee has a far longer shelf life than butter. To make ghee, the water and milk solids are removed from butter or cream by heating it (possibly after fermentation) to eliminate the water, and straining. Ghee normally is composed of at least 99% milk fat. However, part of the nonfat milk solids remain in ghee, giving it a distinctive flavor. The flavor is also determined by any fermentation and the heating process. Ghee is used in cooking and for frying, as well as in dressings or toppings for various foods.
Ghee is considered to have medicinal properties in Ayurveda medicine. For example, ghee is thought to help to "increase carcinogen detoxification" and to reduce the risk of cancer. However, consumption of ghee has been linked cardiovascular disease in India and elsewhere. Ghee has a higher saturated fat and cholesterol content than butter (approximately 58 g saturated fat and 300 mg cholesterol per 100 g ghee compared to 51 g and 215 mg for butter). The predominant saturated fatty acids in ghee are palmitic, stearic, and myristic acid.
Ghee has been reported to have a protective effect against carcinogen-induced mammary tumors in rats compared to soybean oil, in part because ghee increases carcinogen detoxification activities in mammary tissues. (However, note that soybean oil is also not recommended for breast cancer.) Ghee contains high levels of CLA, and is a source of beta-carotene and vitamin E. One study reported that the CLA content of ghee was up to 600 mg per 100g, whereas it was 300 mg in butter. CLA-enriched ghee has been shown to inhibit the development of mammary tumors in a rat model of breast cancer.
However, the breast cancer-promoting effects of the high saturated fat and cholesterol content of ghee appear to outweigh the chemopreventive properties of the CLA, although the evidence is sparse. Several Indian and Middle Eastern studies have reported that high fat diets including ghee are associated with increased risk of breast cancer. One Iranian study reported that women with the highest intake of ghee had double the risk of breast cancer as those with the lowest level of consumption.
Banaspati ghee may be particularly harmful
One Pakistani study reported that women who routinely used banaspati ghee in cooking had a sharply higher risk of breast cancer than those using regular oils in cooking. Banaspati ghee is a vegetable ghee typically made from hydrogenated palm oil (instead of butter) and is high in trans fats. Some "vegan ghee" sold in the U.S. in made with coconut oil. In either case, banaspati ghee should probably be avoided.

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 butter and its components. For a more complete list of studies, please click on butter.