A couple of months ago, I addressed the question of whether eating meat affects your hormone levels. Since then, several of you have written to ask the same question about cow’s milk.

All milk (whether from cows, goats, humans, or porpoises) naturally contains small amounts of various hormones, including estrogen and progesterone. Because hormones like estrogen are fat-soluble, the level of hormones is higher in whole milk than in skim milk. Organic milk, however, contains about the same amount of hormones as conventionally produced milk.

DOES DAIRY PROMOTE BREAST CANCER?

Some worry that the hormones in cow’s milk could cause health problems in humans. People have suggested that estrogen in cow’s milk might promote the growth of hormone-sensitive cancers such as breast or ovarian cancer, for example. Others worry that hormones in milk might cause early puberty in children.

Before I go any further, let me say once more that I fully support anyone who decides that they would rather not consume dairy. It’s not essential for a healthy diet. And there are any number of reasons you might decide that you don’t want to consume dairy products. You might feel, for example, that it’s an exploitation of animals. Or you might not feel comfortable with the conditions that dairy cows are raised in.

But if your primary reason for avoiding dairy is concern over hormones, let me share what I’ve uncovered on this front.

The amount of hormones in milk is tiny compared to what’s already in your body. Guys, your bodies produce about 6,000 times as much estrogen every day as you’d get in a glass of whole milk. Ladies, your bodies are producing 28,000 times that much. Compared to what your own body pumps out, the amount of estrogen you’re exposed to from dairy products is far too little to have any physiological effect.

If drinking milk could raise your estrogen levels, we could expect it to be useful in treating symptoms of menopause. Ironically, soymilk may be more effective than cow’s milk in beating hot flashes.

There’s no clear link between intake of dairy products and early puberty. It’s true that kids are reaching puberty earlier these days but I don’t think dairy is the cause. For one thing, kids are drinking less milk than they used to. If the hormones in dairy caused early puberty, we’d expect to see the average age of puberty go up as dairy consumption went down, but the opposite is true. I think the increase in childhood obesity is a much more likely explanation for early puberty because circulating estrogen levels increase with body fat.

There’s no clear link between intake of dairy products and increased risk of breast or ovarian cancer. Some people have pointed out that breast cancer rates are lower in cultures or countries where dairy is not commonly consumed. But there are so many other differences between these populations that it’s impossible to say that the difference in cancer risk—or any other difference, for that matter—is due to the difference in dairy intake.

When we compare people from the same country and culture, those who drink the most milk don’t have a higher incidence of breast cancer than those who drink the least. In fact, several studies have found a modest protective effect.

There may be a link between dairy intake and prostate cancer. Prostate cancer is also a hormone related cancer, of course, and studies have found that men who consume more than four servings of dairy a day have a slightly elevated risk of this common disease. It’s not clear whether this is due to a hormonal effect or something else. But the fact that dairy doesn’t seem to affect breast cancer risk or puberty rates suggests that the hormones in milk may not be the most likely explanation. It’s even possible that the link between dairy intake and prostate cancer is purely coincidental.

How Much Dairy Should You Eat?

If you choose to consume dairy products, I think it makes sense to aim for no more than 3 servings a day

If you choose to consume dairy products, I think it makes sense to aim for no more than 3 servings a day—especially if you choose whole milk and other full-fat dairy products. Three servings of dairy a day is enough to meet your daily requirement for calcium (without overdoing it) and it keeps you well within the recommended amount of saturated fat.