Are Antibiotics Making Us Fat?

Cleanliness may be next to godliness, but it might also do us in. A growing body of data suggests that a wide range of ills, from allergies and asthma to metabolic disease and superbugs, may be the consequence of our war on germs.
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Cleanliness may be next to godliness, but it might also do us in. A growing body of data suggests that a wide range of ills, from allergies and asthma to metabolic disease and superbugs, may be the consequence of our war on germs.

The territory in need of a cleansing now ranges from our garage to our gut, from our teeth to our toilet. Whether it's the surface of our kitchen counter or the surface of our face, cleaner is not just better, according to popular culture -- it's safer.

A kind of homeland security mentality has invaded the cosmos of clean. This is a two-front war, the world outside our bodies and the biosphere inside us. The axis of evil here is bacteria and all bacteria are bad. But nothing could be further from the truth. We can't live without bacteria.

The bottom line: Bacteria are an essential part of us.

Our bowels are a perfect example. There are way more bacterial cells living in our gut than the total number of our own cells in our entire body. We are, so to speak, colonized. These gut microbes turn out to be incredibly important. Anyone who has been on antibiotics, which kill many of these bacteria, can attest to the stomach misery caused by upsetting the balance of these little lodgers. Growing evidence suggests that a reduced diversity of these bugs is associated with inflammatory bowel disease, metabolic syndrome (prediabetes), asthma and even obesity.[1] So why would you want to "cleanse" a colon?

The first explanation for the proliferation of these disorders was the "hygiene hypothesis." This theory suggested that as we moved from rural to urban living, children were exposed to fewer bacterial strains that are essential to normal immune function. This has proven insufficient to explain the whole picture.[2] Now, the dominant paradigm in making sense of the explosion of these illnesses has less to do with external environmental exposure and more to do with the loss of our ancestral internal bacterial colonies as a consequence of antibiotic exposure.[1]

In the United States, the average child has received between 10 and 20 courses of antibiotics by age 18.[3] While there is no question that antibiotics have saved millions of lives, we are just beginning to appreciate the collateral damage. Without certain bacteria that we have experienced historically and that our bodies evolved to cohabitate with, bad bacteria are able to proliferate. This loss of "good bacteria" by antibiotic exposure appears to be linked to the huge increase in metabolic (diabetes, obesity) and allergic disease.

Let's look at obesity.

In the early 1950s, shortly after the discovery of antibiotics, researchers discovered that the administration of low doses of antibiotics to livestock dramatically increased their ability to convert food into biomass.[4] In other words, they grew much faster and reached a larger size. Between 1955 and 1995 (a before and after antibiotic-augmented feed comparison) the average broiler chicken changed such that it took 35 percent less time to reach market weight and had increased that weight by 50 percent.[5]

The way antibiotics enhance growth is poorly understood, but is presumed to be a consequence of their antibacterial action (rather than a side effect) because so many different types have this capacity. So antibiotics change the bacterial populations in these animals in a way that makes them grow bigger and faster. The younger an animal begins an antibiotic-augmented feed, the greater the effect.[1]

Are we fattening ourselves the same way we did our chickens?

It appears increasingly plausible that the widespread administration of antibiotics to young children is altering the bacteria in their intestines in a way that increases their ability to convert food into biomass, just like our livestock. In this way, antibiotics seem to be directly contributing to the obesity epidemic.

Let me close with a plea on behalf of the friendly bacteria that have taken up residence inside of us early in our development as a species. Antibiotics have rendered them an endangered species whose extinction threatens our future.

The ecology of our internal environment is no less important than our external one. We must be green inside and out.

What You Can Do

  • Make sure you use antibiotics only when necessary.
  • Try to avoid eating animals that have been fed antibiotics. (The use of antibiotics in livestock has been banned by the European Union due to the spread of antibiotic-resistant bacteria.)
  • Follow the advances in the use of probiotics.

When it comes to dangerous bacteria, we must cultivate the good bacterial part of us to survive them. Diversity is a good thing in man and microbe.


1. Blaser et al. What are the consequences of the disappearing human microbiota? Nature Reviews Microbiology, 2009 Dec7(12)

2.Okada H et al. The hygiene hypothesis for autoimmune and allergic disorders: an update Clin Exp Immunol 2010 Apr:160

3.Nyquist A, Gonzales R, Steiner JF, Sande MA. Antibiotic Prescribing for Children With Colds, Upper Respiratory Tract Infections, and Bronchitis. JAMA. 1998;279(11):875-877. doi:10.1001/jama.279.11.875.

4.Boyd W. Making meat: science, technology, and American poultry production. Technol Cult. 2001;42:631-64.

5.US Government Accountability Office Report on Antibiotic Resistance, Sept. 2011

For more by Paul Spector, M.D., click here.

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