Hormones in Food: Should You Worry?

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Salmon that matures to market size twice as fast as normal, dairy cows that are producing 15% more milk than average, and beef cattle that grow at a 20% faster rate. What do these hyper-productive animals have in common? Thanks to injections and implants—in the case of cows—or genetic engineering—in the case of salmon—they contain artificially high levels of sex or growth hormones.

Are Hormone Enhanced Foods Safe?

You might question if these hormones are dangerous to the humans who consume the food from these enhanced animals. The food industry says no—and the Food and Drug Administration (FDA) agrees, at least when it comes to cows and fish.

In 2015 the FDA, which regulates the use of hormones in livestock, approved the sale of a genetically-engineered salmon called AquaAdvantage patented by the biotech company AquaBounty.

The salmon—which is wired to produce growth hormone year-round, instead of just in the spring and summer—was the first genetically engineered animal to wind up on dinner plates. (Genetically engineered fruits and vegetables have been around for years.)

It's required that all genetically-engineered salmon sold in the U.S. must list on the label that it's bioengineered.

The FDA's stamp of approval may not reassure those who worry that excess hormones in the food supply are contributing to cancer, early puberty in girls, and other health problems in humans.

For years, consumer advocates and public health experts have fought to limit the use of hormones in cows, and some support a ban on the practice similar to the one in place in Europe, where food regulations are generally more stringent than in the U.S.

But it's not clear if such hormones truly are bad for our health. Surprisingly little research has been done on the health effects of these hormones in humans, in part because it's difficult to separate the effects of added hormones from the mixture of natural hormones, proteins, and other components found in milk and meat.

Buying organic may reassure shoppers, but there's little proof these products are indeed safer.

Growth Hormones

In 1993, the FDA approved recombinant bovine growth hormone (rBGH), a synthetic cow hormone that spurs milk production when injected into dairy cows, and consumer groups have been concerned about it ever since. The manipulation of growth hormone in the AquaBounty salmon has sparked similar concerns.

By itself, rBGH has no discernible effect in humans and is of little concern to your health, and the growth hormone in AquaBounty's salmon is expected to be inconsequential to your health as well.

The actual fear is that manipulating growth hormones in cows—or salmon—may increase another hormone, insulin-like growth factor (IGF), which could mimic the effects of human growth hormone in harmful ways. In fact, research has found that milk from rBGH-treated cows contains up to 10 times more IGF than other milk.

A study published in 2020 in the journal Cancer found an association between higher blood levels of IGF (regardless of what causes them) with an increased risk of breast, prostate, colorectal, and thyroid cancers in humans.

In an older 2004 study, patients with above-average IGF levels had nearly a 50% higher risk of prostate cancer and a 65% higher risk of hormone-dependent premenopausal breast cancer than people with below-average levels.

Many factors—including genes, smoking, and fat intake—contribute to these cancers, but "it's very likely that at least part of that [risk] is related to IGF levels," especially where prostate cancer is concerned, said Walter Willett, MD, professor of epidemiology and mutrition at Harvard T.H. Chan School of Public Health.

While consuming lots of milk and other dairy has been shown to raise blood levels of human IGF, the increase is probably not a direct effect of the animal's IGF level or the IGF found in these foods. That's because the amount of IGF in dairy products—whether or not it's from rBGH-treated cows—pales in comparison to what is naturally in your body.

"Just [to get] the amount of IGF secreted in your saliva and digestive tract in a day, you'd have to drink about 95 quarts of milk," said Terry Etherton, PhD, a professor of dairy and animal science at Pennsylvania State University and the author of a blog about food biotechnology.

And you'd have to eat at least 170 three-ounce servings of genetically modified salmon. (The IGF levels in the AquaBounty salmon and regular salmon are comparable, although consumer advocates say the studies that determined this are too small to be reliable.)

So if the amount of IGF in milk is negligible, how does milk consumption increase our IGF levels? Milk in general—and the proteins, sugar, minerals, and non-IGF hormones it contains—may somehow cause the human body to make more of its own IGF, Dr. Willett said.

Sex Hormones and Early Puberty

IGF isn't the only hormone found in the food supply. Ranchers have been fattening up cattle with sex hormones—most notably estrogen—since the 1950s.

Today most beef cows in the U.S.—except those labeled "organic"—receive an implant in their ear that delivers a hormone, usually a form of estrogen (estradiol) in some combination with five other hormones. (These hormones are not given to chickens and pigs because they don't have the same growth-promoting effect in these animals, although antibiotics are given to all three species for similar growth-promoting reasons.)

One concern is that such hormones may spur earlier puberty in children, who are, on average, entering puberty at a younger age than they did a generation or two ago, for reasons that are unclear.

But Ann Macrina, PhD, a researcher in the Department of Dairy and Animal Science at Pennsylvania State University, said that the amount of estrogen found in meat is vanishingly small compared to the level in our bodies.

A three-ounce serving of beef from an estrogen-treated cow contains less than a billionth of a gram of estrogen, a level around 400,000 times lower than estrogen in women and nearly 100,000 times lower than that in men.

However, even minuscule amounts of estrogen could affect prepubescent girls and boys, said Dr. Willett. "[For] a girl who's not producing hormones herself, they could be quite substantial."

A 2009 study found that children who consumed the most protein from animal sources entered puberty about seven months earlier than those who consumed the least. "It doesn't matter so much if it's milk, cheese, or meat—all these animal proteins have a clear impact on [our] IGF system," said Thomas Remer, PhD, one of the authors of the study and a professor at the Research Institute of Child Nutrition, in Germany.

Still, hormones added to the food supply are probably not the biggest culprit behind early puberty. It's more likely that meat, milk, and similar foods help trigger earlier puberty because they are rich in protein, calories, and nutrients, said Marcia Herman-Giddens, an adjunct professor at the University of North Carolina School of Public Health and the lead author of an influential 1997 study on early puberty in girls.

However, Herman-Giddens cautions that more research is needed to untangle the many factors involved. For instance, Herman-Giddens said, rising rates of overweight and obesity—and the processed foods, high-calorie drinks, and lack of exercise driving them—are "probably the biggest reason" for the trend toward earlier puberty. (Fat cells stimulate the body to produce estrogen.)

Pesticides, flame-retardants, plastics, and other chemicals in the environment that can disrupt hormones may also be partly to blame.

Organic or No?

Organic beef and dairy products certified by the U.S. Department of Agriculture (USDA) come with the guarantee that the cows were not treated with rBGH or sex hormones. They also come with a much heftier price. Is the peace of mind worth the extra cash?

Probably not, said Dr. Willett, who advocates cutting back on meat in general. Most people should eat no more than two servings of red meat per week, Dr. Willett said, and "if you're [only] having a couple of servings a week, it doesn't make much difference whether it's organic or not."

Dr. Willett offers similar advice regarding organic dairy. On the other hand, experts like Herman-Giddens urge consumers to stay away from rBGH-treated milk because of its potentially higher IGF levels, and the fact that it does not have any added health benefits over regular milk. Instead of switching to organic milk, Dr. Willett recommended cutting back on dairy altogether, despite USDA recommendations that call for three servings a day of dairy.

Bruce Chassy, PhD, a professor of food microbiology at the University of Illinois at Urbana-Champaign, says "propaganda" from organic farming groups has created misconceptions about—and resistance to—rBGH among consumers.

In fact, Chassy argued that manipulating growth hormones has benefits: rBGH-treated cows are better for the environment, not just the bottom line, since farmers can get the same amount of milk with fewer cows. Similarly, the AquaBounty salmon consumes 10% less feed during its lifecycle than a regular farmed salmon.

The most lasting effect of the fears surrounding hormones in the food supply may be the value of "organic" or "hormone-free" as selling points, Chassy said.

"I think there are a lot of farms that are not using [rBGH] because they perceive that consumers do not want [rBGH]-treated milk," Chassy said. He predicts that the AquaBounty salmon will likely inspire "marketing campaigns for 'hormone-free' fish." It's a ridiculous claim, Chassy argued, since all fish—and all meat and milk—has hormones.

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Health.com uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Food and Drug Administration. Q&A on FDA's approval of AquaAdvantage salmon.

  2. Knuppel A, Fensom GK, Watts EL, et al. Circulating insulin-like growth factor-i concentrations and risk of 30 cancers: prospective analyses in UK biobankCancer Research. 2020;80(18):4014-4021. doi:10.1158/0008-5472.CAN-20-1281

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