Nutrition Facts with Dr. Greger

The Side Effects of Hormones in Dairy

February 5, 2026

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  • Drinking cow's milk, which contains actual estrogen and progesterone from pregnant cows, causes rapid, measurable increases in estrogen levels and decreases in testosterone in men within an hour, while tripling various estrogens in prepubescent children. 
  • Dairy consumption is associated with adverse reproductive outcomes, including a significantly higher risk of endometrial cancer in postmenopausal women and accelerated ovarian aging (lower antral follicle count) in women of reproductive age. 
  • Dairy products account for the majority (60-80%) of ingested female sex steroids in the diet, and these bovine hormones survive processing and can be converted into active human estrogens once consumed. 

Segments

Hormones and Vegetarian Estrogen Levels
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(00:00:22)
  • Key Takeaway: Vegetarian women exhibit 45% lower levels of potent human estrogen compared to non-vegetarians, potentially due to higher fiber intake promoting estrogen excretion.
  • Summary: Vegetarian women have significantly lower blood estrogen levels, possibly because their higher fiber intake leads to two to three times more estrogen excretion in feces. This increased excretion helps pull excess estrogen out of the system. This difference may explain the lower incidence of breast cancer among plant-based eaters.
Dairy Hormones Affecting Men
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(00:00:22)
  • Key Takeaway: Men experience a significant drop in progesterone and testosterone, alongside an increase in estrone, within one hour of consuming a quart of cow’s milk.
  • Summary: Researchers tested the immediate effects of drinking a quart of cow’s milk on men, who normally have low estrogen levels. Within 60 minutes, their estrone levels rose, while progesterone and testosterone levels dropped significantly. This effect is contrasted with men avoiding soy milk due to phytoestrogens while consuming milk containing actual estrogen.
Hormones in Children’s Milk Intake
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(00:03:09)
  • Key Takeaway: Prepubescent children experienced a more than threefold increase in multiple estrogens and a progesterone metabolite within an hour of drinking milk.
  • Summary: When prepubescent children drank milk, their circulating levels of estrone, estradiol, estriol, and a progesterone metabolite more than tripled within the hour. This occurred even when half the children failed to finish the entire quart of milk. The milk also contains 5-alpha-P, a precursor to testosterone linked to acne and prostate cancer.
Bypassing Natural Feedback Loops
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(00:03:44)
  • Key Takeaway: Ingested dairy hormones bypass the human body’s natural protective feedback system designed to regulate endogenous hormone levels.
  • Summary: The human endocrine system did not evolve to handle ingested dairy hormones and growth factors, leaving it ill-equipped to cope with this external hormonal invasion. Unlike the body’s natural dampening mechanism for its own hormones, external dairy-sourced steroids and dihydrotestosterone precursors bypass this protective feedback loop.
Dairy Exposure and Hormone Sources
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(00:05:11)
  • Key Takeaway: Dairy products contribute approximately three-quarters of the exposure to ingested female sex steroids in the diet, concentrating hormones further in higher-fat products.
  • Summary: While all animal food contains hormones, dairy is the primary source of ingested female sex steroids, accounting for about 75% of exposure. Estrogens concentrate further in cream (5x) and butter (10x) compared to milk. This exposure comes from naturally occurring hormones, not just added growth hormone injections.
Dairy Practices and Public Knowledge
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(00:06:30)
  • Key Takeaway: Many people lack basic knowledge about dairy production, such as the fact that cows only lactate after giving birth.
  • Summary: Only about half of surveyed people realize that cows must have a calf to produce milk, leading researchers to suggest public education on dairy practices. However, one response noted that informing the public about practices like separating the calf immediately might not lead to high public approval. Commercial milk contains high levels of pregnancy hormones because modern cows lactate throughout almost their entire subsequent pregnancies.
Endometrial Cancer Risk
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(00:08:28)
  • Key Takeaway: Harvard researchers found a significantly increased risk of endometrial cancer among postmenopausal women who consumed more dairy products.
  • Summary: Estrogens play a central role in the development of endometrial cancer, a cancer of the uterine lining. Milk and dairy products are sources of steroid hormones and growth factors that may contribute to this risk. Decades-long follow-up of tens of thousands of women confirmed this significantly higher risk associated with increased dairy consumption in this demographic.
Dairy and Female Infertility/Ovarian Aging
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(00:09:48)
  • Key Takeaway: Higher dairy protein intake is associated with accelerated ovarian aging, evidenced by significantly lower antral follicle counts equivalent to several years of natural decline.
  • Summary: Women who ate yogurt or cream showed about double the risk of sporadic anovulation (failure of ovulation), independent of the high sugar content in many yogurts. Furthermore, higher dairy protein intake correlated with lower antral follicle counts, a measure of ovarian reserve. For women with robust ovaries, the difference in follicle count between the highest and lowest dairy consumers represented a biological age difference equivalent to being 14 years older in terms of ovarian reserve.