A salmon that grows to market size twice as fast as normal. Dairy cows that produce 15 percent more milk. Beef cows that grow 20 percent faster.

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 these hormones dangerous to the humans who eat the food or drink the milk? The food industry says no -- and the Food and Drug Administration (FDA) agrees, at least when it comes to cows.

The FDA, which regulates the use of hormones in livestock, hasn't yet decided whether it will approve the sale of a genetically engineered salmon patented by the biotech company AquaBounty. If the salmon -- which is wired to produce growth hormone year-round, instead of just in the spring and summer -- gets an OK from the agency, it will be the first genetically engineered animal to wind up on your dinner plate. (Genetically engineered fruits and vegetables have been around for years.)

The FDA's stamp of approval isn't likely to 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.

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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.

Higher blood levels of IGF (regardless of what causes them) have been associated with an increased risk of breast, prostate, and other cancers in humans. In a 2004 study, patients with above-average IGF levels had nearly a 50 percent higher risk of prostate cancer and a 65 percent 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, says Walter Willett, M.D., chairman of the department of nutrition at the Harvard School of Public Health, in Boston.

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," says Terry Etherton, Ph.D., 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 says.

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 chicken 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, Ph.D., a researcher in the Department of Dairy and Animal Science at Pennsylvania State University, says 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 miniscule amounts of estrogen could affect prepubescent girls and boys, says 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," says Thomas Remer, Ph.D., 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, says Marcia Herman-Giddens, an adjunct professor at the University of North Carolina School of Public Health, in Chapel Hill, 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, she says, 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 Not?

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, says 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 says, 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 recommends cutting back on dairy altogether, despite USDA recommendations that call for three servings a day of dairy.

Bruce Chassy, Ph.D., 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 argues 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 percent 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 says.

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