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When it comes to the amount of steroid hormones we are exposed to in the food supply, milk products, dairy products, supply about 60 to 80 percent of ingested female sex steroids. I’ve talked about the effects of these estrogens and progesterone in men and prepubescent children: how milk intake can spike estrogen levels within hours of consumption. But in terms of effects on women, I talked about the increased endometrial cancer risk in postmenopausal women, but what about reproductive-age women? Might dairy hormones affect reproduction?

We’ve known dairy food intake has sometimes been associated with infertility; however, little is known with regard to associations with reproductive hormones or anovulation, meaning how might dairy do it, by affecting how the uterus prepares, or affecting the ovary itself. The researchers found that women who ate yogurt or cream had about twice the risk of sporadic anovulation, meaning failure of ovulation, so some months there was no egg to fertilize at all. Now we know most yogurt these days is packed with sugar. Even plain Greek yogurt can have more sugar than a double chocolate glazed cake Dunkin’ donut. But they controlled for that, and the results remained after adjusting for the sugar content, which suggests that the risk of anovulation was independent of the sugar content included in many yogurts. Now we don’t know if this was just a fluke, or exactly what the mechanism might be, but if women skip ovulations here and there throughout their life, might they end up with a larger ovarian reserve of eggs?

Women are starting to have their first baby later and later; there’s been a rise in women having babies in their late 30s, and 40s. And we used to think that women’s ovarian reserve of eggs stayed relatively stable until a rapid decline at about age 37, but now we know it appears to be more of a gradual loss of eggs over time. Here’s what the graph looks like, where there’s a steady loss starting at peak fertility in one’s 20s. This is measuring “antral follicle count,” which is an ultrasound test where you can just count the number of like next-batter-up eggs in the ovaries. It’s probably the best reflection of true reproductive age. It’s a measure of ovarian reserve—how many eggs a woman has left.

Okay, so what does this have to do with diet? Researchers at Harvard looked at the association of various protein intakes with ovarian antral follicle counts among women having trouble getting pregnant. Even though diminished ovarian reserve is one of the major causes of female infertility, the process leading to reproductive deterioration with age is still poorly understood. In light of women delaying pregnancy until older age, the identification of reversible factors that may affect the individual rates of reproductive decline might be of significant clinical value.

They did ultrasounds on all the women and studied their diets, and concluded that higher dairy protein intake was associated with lower antral follicle counts, in other words: accelerated ovarian aging.

Here’s what the graph looked like in nonsmokers. Significantly lower ovarian reserve at the highest dairy intake, which would be like three ounces of cheese a day, compared to the lowest dairy intake. Okay, but what do these numbers mean in terms of biological age? Is 16.9 down to 12.7 really that much of a difference? If you look at women with really robust ovaries, a follicle count of 16.9 would be like what you might see in a 36- or 37-year-old. Whereas 12.7, what you can see in women eating the most dairy, is like what you might see in a really fertile 50-year-old. So, we’re talking years’ worth of ovarian aging, between the highest and lowest dairy consumers.

While it wasn’t possible for the researchers to identify the underlying mechanism linking higher dairy protein intake to lower antral follicle count, they had several educated guesses. One: it could be the steroid hormones and growth factors; two: the contamination of milk products by pesticides and endocrine-disrupting chemicals that may negatively affect the development of these ovarian follicles and egg competence.

Regarding the hormones, studies suggest that commercial milk (derived from both pregnant and non-pregnant animals) contains large amounts of estrogens, progesterone, and other placental hormones that are eventually released into the human food chain, with dairy intake accounting for 60-80 percent of the estrogens consumed, as I mentioned before. Dairy estrogens survive processing, appear both in raw and commercial milk products, are found in substantially higher concentrations with increasing amounts of milk fat, with no apparent difference between organic and conventional dairy products—that’s important to realize. It’s not just cows that have been injected with growth hormones. Their hormones are just in their bodies naturally. and once inside the human body, these bovine hormones get converted to estrone and estradiol, the main active human estrogens. And following absorption, bovine steroids may then affect reproductive outcomes.

It is imperative that further studies are designed to clarify the biology underlying the observed associations. This might be crucial, given that consumption of another species’ milk by humans is an evolutionary novel dietary behavior that has the potential to alter reproductive parameters and may have long-term adverse health effects.

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