Recently the FDA issued a somber report: Sales of antibiotics to use in pigs, chicken, cows and seafood rose 20 percent between 2009 and 2013. About 32.6 million pounds of antibiotics were going into the animals we eat, and most of those drugs were given in low doses to promote faster growth or prevent disease in crowded livestock pens.
The FDA and the Centers for Disease Control and Prevention (CDC) are concerned this antibiotic use could contribute to the rise of resistant "superbugs" that increasingly threaten our health. But there's another concern that scientists are beginning to study. Just as antibiotics promote weight gain in food animals, are they also contributing to obesity in humans?
We spoke with Lee Riley, MD, chair of the Division of Infectious Diseases and Vaccinology at the University of California, Berkeley, who has reviewed more than 100 studies looking at the link between antibiotics and obesity. Here, he explains the research and his hypothesis that antibiotics are fueling obesity worldwide.
How do antibiotics change a person's microbiome?
Our intestinal microbiome is all the microorganisms in our gut, and you have to look at it like a human organ, just as important as your heart, brain and lungs. Our gut is teaming with over 100 trillion bacterial cells of many types. Usually these species stay in equilibrium with each other; they help our bodies digest food and function well. Antibiotics change this gut flora, killing off bacteria and changing the balance of species in our intestines.
How does this affect weight?
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.
Research shows that obese people have a different mix of bacteria in their gut than lean people. In studies with mice, scientists took fecal matter from the obese mice and transferred it into the gut of thin mice—and the thin mice got fat. Researchers have also looked at people. In identical twins, the obese twin had a very different type of bacterial population than the thin twin. In one study of twins in Malawi, they took fecal matter from the children who developed a severe malnutrition called Kwashiorkor and put it in the guts of mice. Those mice started losing weight and became malnourished.
One of the most interesting case studies involved a 32-year-old woman in Rhode Island who had Clostridium difficile colitis (C. diff), which can be a very serious disease that's difficult to cure. Her disease didn't respond to antibiotics at all, so they decided to try a fecal implant, which is actually a treatment for C. diff. She received this transplant from her daughter, who was overweight. And it worked. Her Clostridium difficile went away, but over the next six months she gained weight and became overweight for the first time in her life. She weighed 136 pounds for most of her adult life. Three years after her transplant she weighed 177 pounds despite being on a medically supervised liquid diet and exercise program.
What's the evidence that antibiotics are at the root of this difference?
Some research has found that babies who received antibiotics in the first six months of life—when their microbiome is evolving—became fatter. But I'm not convinced that therapeutic antibiotics we take for infection could cause long-term weight gain in most people. My hypothesis is that chronic low-dose exposure to antibiotics contributes to weight gain—and we get that exposure in our food and water.
The food industry discovered in the 1940s that if they gave low-dose antibiotics to animals, the animals gained weight faster. But widespread use of antibiotics to promote growth in food animals really took off over the past 20 years or so with industrial intensive farming. And that coincides with our obesity epidemic.
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 antbiotics 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?
Which foods have the most antibiotics in them?
Pigs have the most antibiotics in their meat, then chicken, then beef, according to research in the Proceedings of the National Academy of Sciences. Also, farmed shrimp and fish, even farmed salmon, have high levels of antibiotics because the drugs are needed to prevent disease in the farming pens. Even organic vegetables have antibiotics in them because about 75 percent of antibiotics fed to livestock are excreted out, and in the manure used to fertilize fields.
We also consume antibiotics indirectly through our environment. A Colorado study traced levels of antibiotics in the Cache la Poudre river from its pristine source in the mountains all the way down to the sea. There were no antibiotics in the water at its source, but the levels increased as the river passed cities, ranches and farms.
This problem is only going to increase globally. It's estimated that the use of antibiotics in food animals will increase 67 percent by the year 2030 as the demand for protein grows worldwide. In China, India, Brazil, and Russia, antibiotic use is expected increase 99 percent—far more than those countries' population growth. A lot of that food will be exported.
What's being done about this problem?
The only way this can change is if the multinational food industry stops using antibiotics for growth purposes. Right now in the US, the FDA has asked the food industry to voluntarily phase out the use of antibiotics to promote growth in animals. Some people are calling for strong legislation, but I personally don't think legislation can work—at least not globally.
Consumers are in the position to influence this change worldwide. If American consumers demand food raised without antibiotics, suppliers around the world will pay attention.
What can individuals do?
Look at food labels to see if it says No Antibiotics or Raised Without Antibiotics. Go to restaurants that have pledged not to use meat or seafood with antibiotics (Chipotle, Panera Bread, Applegate and others). Try to shop at supermarkets that don't sell meat with antibiotics, such as Whole Foods. Your choices are growing. Costco has pledged to try to phase out meat with antibiotics over the next several years.
I struggle myself with what I can do. I don't buy farmed shrimp or seafood. I rarely eat pork. And if I eat chicken, I try to make sure I look at the label to see if it says No Antibiotics. If we as consumers demand change, it will happen.