If these drugs have a fattening effect on livestock animals, it's logical that they would do the same for humans.
Farmers give antibiotics to animals not only to ward off disease, but also to promote growth. The drugs stimulate the appetite and the animals gain weight rapidly, reducing the number of days needed to reach maturity. The practice began in the 1950s, when ranchers discovered that cattle treated for infections gained weight, and it became common practice by the 1970s to add antibiotics to livestock feed.
While this comes with a slew of ethical issues, there are two main health concerns that arise from this practice. One is the increase in antibiotic resistance. The second concern is scientists' growing suspicions that the trace antibiotics in meat might be doing the same thing to humans who eat it as they did to the animals -- making them fatter.
This disturbing hypothesis suggests that antibiotics in food kill gut flora and throw off the balance of microorganism species in the intestines. This imbalance changes the way food is digested. From Berkeley Wellness, of the University of California:
"Many types of bacteria in your gut help your body absorb calories from food. If you have an imbalance of bacteria — too much of the type that breaks food down into energy — you may be absorbing more calories from the same amount of food you eat than you would otherwise."
Researchers are only just beginning to understand the effect of microflora on the human body. One fascinating case of fecal transplant resulted in a previously thin individual becoming obese after treatment, attributed to the fact that the newly introduced microflora came from an obese donor. A controlled trial of the reverse scenario gave fecal transplants from lean donors to overweight people with metabolic syndrome, and the recipients showed improvement in their degrees of insulin resistance.
The idea that weight gain is tied to antibiotics stems from the fact that the obesity epidemic ramped up in the last 20 years, just as livestock production intensified. Of course, there are other factors that could drive weight gain, such as increased access to junk food and a more sedentary lifestyle, but as James Hamblin wrote in The Atlantic, the diet wasn't all that great in the mid-twentieth century, either: "Even while the food Americans ate in the 1950s was more reasonable than it is now, it was not idyllic. White bread and American cheese and hot dogs and Spam were staples then, too." And now at least the average American has a gym membership of some kind.
Dr. Lee Riley, who has examined the link between antibiotics and obesity in depth, pointed out the same strange spike in obesity to Berkeley Wellness:
"Our diet and lifestyle is certainly a factor in obesity, but can't explain it completely. It's true that we eat more calories today than in the past. The US food supply provides about 3,900 calories per person a day today, compared to 3,400 calories in the early 1900s. Yet obesity increased very slowly until the middle of the 1970s, then exploded. What changed? The level of antibiotics in our food and water. It makes sense. We know that antibiotics promote weight gain in livestock animals. We're animals, so why should we be different?"
We should be deeply concerned, then, that meat contains as many antibiotics as it does. In 2015 the U.S. Food and Drug Administration said that American antibiotic use in animals had increased to nearly 34 million pounds in a single year; and this amount increased 20 percent between 2009 and 2013, following a global trend. The U.S. is not alone in its excessive use; a graph in the "Review on Antimicrobial Resistance" (p. 16) reveals that Portugal, Hungary, Italy, Spain and Cyprus use even more antibiotics than the U.S., based on the number of milligrams purchased per kilogram of livestock raised.
What is the solution?
The FDA has asked the meat industry to phase out the use of antibiotics voluntarily, whereas the Canadian government has taken a more drastic approach by requiring veterinary prescriptions by the end of this year. Some people think that legislation could help -- if it could get past two of the most powerful lobby groups in the U.S., the meat and pharmaceutical industries -- but that's a tough thing to regulate for a global market.
What is most likely to effect change is consumer pressure. Once shoppers refuse to buy meat laced with antibiotics, many large-scale operations will be forced to change their production standards. It is possible to find antibiotic-free meat, but it usually costs more -- not a bad thing, considering that we should all be eating far less meat for environmental reasons. If you do eat conventional meat, then choose carefully; pork has the highest levels of antibiotics, followed by chicken, then beef.
Going vegan could help reduce exposure significantly, although antibiotics have been detected in a number of fruits and vegetables, due to the contaminated manure used as fertilizer.
It is a messy, complicated situation, and an unfortunate one to find ourselves in for so many reasons; but never underestimate the power of being a consumer. For the sake of your health, any weight loss goals you may have, and the wellbeing of future generations, make buying antibiotic-free meat a priority, adopt a reducetarian approach to animal products, or eliminate meat and dairy completely from your diet.