Antibiotics and Weight Gain

by Wei Perng, PhD

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We’ve all heard why eating non-organic animal products is a bad idea: the animals may be raised in poor conditions, industrial agriculture produces large amounts of air and water pollution, it’s a leading cause of deforestation in the U.S., and the animals may be fed antibiotics.

I find the last one rather disturbing, as I recently learned that antibiotics are not only used to treat and prevent infections among animals living in close quarters, but they are also administered at low levels to promote “feed efficiency.” In other words, adding small quantities of antibiotics to animal feed makes them grow faster and gain weight more rapidly in a shorter period of time. Yum. Here’s what’s scarier: we are not so different from animals. Antibiotics may not necessarily make us grow taller faster, but they can lead to weight gain. How does this happen?

The term “antibiotics” includes a wide range of compounds that either kill or inhibit the growth of bacteria, and are prescribed liberally for a wide range of bacterial infections, including those that affect the respiratory and urinary tracts, abdominal organs, and reproductive organs. Broad-spectrum antibiotics (like amoxicillin, ciprofloxacin or “Cipro”, or tetracycline) are commonly prescribed when the bacterial strain is unknown. While these antibiotics are generally effective at getting rid of the culprit bacteria, they don’t discriminate between good and bad bacteria. It’s a bit like a non-specific but potent weed-killer that gets rid of dandelions, but could also kill your crops, your flowers, and even your grass. It might even permanently change the soil composition so that plants have a hard time thriving in the future. In similar vein, every time you take antibiotics, billions of beneficial gut bacteria known as “flora” are wiped out. This is problematic because in addition to making stools, gut bacteria synthesize essential vitamins, shield the large intestine from colon cancer, and avert yeast infections, all while stimulating lymphoid tissue to maintain the mucosal immune system. Gut bacteria also plays a key role in regulating fat storage, which might explain the link between antibiotic treatment and adiposity, as discussed in a previous blog post. Here’s what we know on the topic so far:

(1) Obese and lean individuals exhibit differences in gut flora composition. There are two main classes of bacteria that populate the human gut, known as Bacteroidetes and Firmicutes. Researchers have shown that obese people have a lower ratio of Bacteroidetes to Firmicutes in comparison to lean people. However, the relative proportion of Bacteroidetes increases with weight loss following a low-calorie diet. While such findings support a link between gut bacteria and weight, it is not possible to determine whether changes in gut flora cause weight gain, or vice versa. In other words, does weight gain change the composition of our gut bacteria, or do changes to our gut bacteria cause weight gain? Animal models to the rescue!

(2) Altering microbiota lead to changes in weight (in rodents, at least). In 2004, a team of researchers transplanted the microbiota of a group of mice prone to weight gain into another group of germ-free mice (i.e. mice with no gut bacteria of their own) and found that the germ-free animals gained excess fat, despite consuming less food. The investigators also revealed that the transplanted microbiota promoted absorption of monosaccharides, the most basic units of carbohydrates, to promote fat synthesis in the liver.  The same team later induced obesity in mice through changes in diet and noticed that Firmicute bacteria thrived in their guts. Transplanting the gut flora of obese mice into lean mice caused the lean ones to gain weight, while transplanting the microbiota of lean mice to other lean mice made no difference. Such findings provide some pretty convincing evidence of the causal role of altered gut flora in weight gain.

(3) Antibiotic treatment alters the proportion of good vs. bad bacteria, but not necessarily in the direction that we’d expect. In a recent study of 21 adults who received broad-spectrum antibiotics for a week, Panda et al. found that in addition to a 25% reduction in gut flora diversity, there was an increase in the ratio of Bacteroides (good bacteria) to Firmicutes (bad bacteria). This finding was unexpected, but it remains important for future studies to follow-up patients over time. Despite what appears to be an initial improvement in bacterial composition immediately after treatment, the “reshaping” of gut flora after antibiotic exposure also presents an opportunity for the most drug-resistant bad bacteria to thrive during the recolonization process.

We know that we should only take antibiotics when necessary (and to take the full dose to avoid antibiotic resistance!). But sometimes it’s unavoidable. So what can we do to protect ourselves from its detrimental effects on our intestinal flora? Here are some suggestions to get you started, as discussed in a recent Huffington Post article:

  · Avoid sugar and processed carbs, since Firmicutes are well-adapted to grow on simple carbohydrates. On the other hand, eating beans may promote Bacteroidete growth.

  · Adhere to a low-fat diet. Firmicutes are involved in intestinal absorption of fats, and higher-fat diets require more of them.

  · Eat and sleep according to a regular schedule. Our intestinal bacteria have a rhythm that changes throughout the day, and disturbances in our daily routine can alter gut flora.