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Should you drink soy or cow's milk? Here's what the evidence says.

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Welcome to Dear Julia, a weekly column where readers can submit everyday health questions. Which over-the-counter painkillers work best? Is it better to run or walk for exercise? Julia Belluz will sift through the research and consult with experts in the field to figure out how science can help us live happier and healthier lives.

Dear Julia: Does drinking soy milk really cause man boobs?

This question comes up surprisingly often. There seems to be a widespread belief out there that drinking soy milk or eating tofu will lead to gynecomastia, the medical term for "man boobs."

There's actually some research on this, but we should back up for a second and think about the question rationally. Countries like Japan and Korea consume a lot of soy in their diets. But we never hear of rampant man-boob problems over there. And that's because there's no good medical evidence that consuming soy will cause men's breast tissue to grow.

Soy is rich in isoflavones, which are plant-based molecules with estrogen-like activity. Estrogen is a female hormone. That, in turn, has raised the fear that eating soy could bring out feminine traits in men, such as breast enlargement.

But whenever researchers have studied the link, they haven't found any association. In a 2010 review of the medical evidence, researchers wrote that "isoflavones do not exert feminizing effects on men," at least not when consumed at levels typical of many soy-heavy Asian diets. Another 2004 study, comparing babies who were fed soy milk with a control group, found no "estrogen-like" hormonal effects in the soy drinkers.

Now, these studies are not exhaustive. And soy now comes in many forms, so it's possible that different types of soy could have different effects on the body when consumed in large quantities.

For now, Christopher Gardner, a professor of medicine at Stanford who has studied soy milk, said there's no need to worry about this one. "I've consumed soy for 30 years, every day, and have never had to buy a bra," he quipped.

Is soy milk any healthier than dairy milk?

Another question that often comes up about soy milk: Is it any better for you than regular dairy milk? And the short answer is that there's really not a huge difference between soy milk and cow milk, nutrition-wise — though it depends on what type of milk you’re drinking.

If it's calcium you need, you're fine with either type of milk. "Dairy has calcium, but all of the non-dairy beverages add an amount of calcium that would parallel dairy milk," says Gardner.

Or you can skip milk altogether and get calcium from many other foods, including kale, bok choy, broccoli, and even dried, ground basil. There's nothing special about milk in this regard.

If you're concerned about consuming too much saturated fat, which can drive up the level of bad cholesterol in your blood, you may want to opt for soy milk. Soy milk contains unsaturated fat, which actually lowers the levels of bad cholesterol in the body (though it’s not clear what that means for long-term heart health). But then again, you can also just drink skim or low-fat dairy milk, which has hardly any saturated fat.

As for protein, cow's milk contains a little more of it than soy milk, but the difference is usually marginal, a gram or two per serving. (Almond milk, by contrast, hardly has any protein.)

And when it comes to vitamins like A and D, most types of milk, dairy or otherwise, are now fortified with these nutrients, so there's usually no trade-off here, either.

Overall, the nutritional differences between milk and its alternatives aren’t huge. The one exception here is flavored milk, which contains a lot of added sugars.

As soon as you add flavors to any type of milk, the sugar content goes up. "Chocolate dairy milk and chocolate soy milk are basically added sugar delivery systems," said Gardner.

A cup of regular 1 percent cow's milk has about 100 calories and 13 grams of sugar. By contrast, a cup of chocolate cow's milk has 150 calories and 25 grams of sugar. And a cup of chocolate-flavored soy milk has 120 calories and 20 grams of sugar. Those flavored milks are giving you about one-tenth of your recommended daily sugar intake in a single serving.

So if you think you're doing yourself a favor by drinking vanilla soy milk over plain old dairy milk, you're actually just adding more sugar to your diet. Otherwise, just drink what you like.

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