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Matthew J. Edlund M.D.

Can Bacteria Make You Fat?

The transplant made me do it.

What Your Gut Does to You

Forget the screaming headlines and the infomercials: Dieting, weight gain, and weight loss are not simple matters. Did the fecal transplant received from her daughter make a Rhode Island woman ravenous, rapidly fattening despite liquid diets, exercise, and strict medical supervision? Are the supplements sold everywhere as probiotics the answer to irritable bowel syndrome and inflammatory bowel disease, or are they themselves potential causes of illness? Do we really know how the gut and brain interact, or is our present semi-hierarchical version of different organ systems (brain, GI tract, muscle) making us miss how information flows in the body?

Let’s take a look.

Keeping Things in the Family

In Open Forum Infectious Disease, Neha Alang and Coleen Kelly of Brown University describe an unusual patient. She was first treated with antibiotics for a vaginal infection. She did not feel better. Not at all. Instead she developed clostridium difficile infection, or CDI – also diagnosed in a family member. Though provoked by antibiotics killing lots of gut bacteria, CDI is itself scarily antibiotic resistant. It kills thousands of people in the U.S. every year.

Next our patient went on “standard” treatment for CDI: metronidazole. That failed. This was followed by vancomycin, the “gold standard” of treatment. Two weeks later, she had added a new infection of helicobacter pylori, the bacterium that causes ulcers and stomach cancer. Next up: a further 12 weeks of vancomycin. Two weeks after finishing, diarrhea recurred. Now she’s given rifaximin with saccharomyces boulardii bacteria. The H. pylori infection remains. She's still infected.

Her doctors then tried the most effective treatment for clostridium difficile. They gave her a fecal transplant – an infusion of stool to replace her own. But from whom do you get the right stool? Just as you might with a kidney transplant, the patient sought a source she knew and trusted: her daughter.

She got the fecal translplant – after another course of antibiotics to kill off the H. pylori.

Finally the treatment worked. But the side effects were unexpected.

The Weight of Your Gut

Prior to her diarrheal disaster, the woman had a stable weight of around 134 pounds – which, amazingly, stayed the same throughout antibiotic treatment. After the fecal transplant, however, she felt ravenous. A little over a year later, she had gained 36 pounds, going from a body mass index of 26 to 33. Three years after the ordeal, she weighted 177 pounds, and was chronically constipated.

Her daughter? At 140 pounds prior to providing her stool to Mom, she subsequently gained 30 pounds herself. Mom remained adamant that her desire to eat rocketed following the fecal transplant.

Why Did She Gain Weight?

Changes in bacterial populations certainly had something to do with it.

As Alang and Kelly reported, treating H. pylori is known to cause weight gain. That curious bacterium can change the levels of ghrelin, one of the prominent hormones controlling satiety. But neither mother nor daughter had been obese prior to the fecal transplant. Both, now with presumably similar bacterial populations, quickly got fat.

Is there evidence to support that bacteria change people’s weight? Sure. It works that way in animals. Obese humans have notably different bacterial populations than slim ones. But what’s doing what?

Brain and Bacteria

Consider this:

  1. Changing bacterial populations are related to people’s moods.
  2. Viral infections – as in adenovirus – can cause weight gain.
  3. Women who harbor more toxoplasma parasities in their blood – usually obtained from local cats – are more prone to suicide.
  4. The gut, also known as the “second brain,” has a lot more serotonin than what's in your head — one possible reason many antidepressants provoke weight gain.
  5. People with irritable bowel and esophageal dysmotility have high rates of depression and anxiety problems.

So pointing a finger at the fecal transplant as the cause of this woman’s rapid weight gain wouldn’t win in a court of law. But it sure is suggestive.

GI problems are on the rise — not just celiac disease, but irritable bowel syndrome appears more common. Plus we know next to nothing about how gut bacteria change the brain, or vice versa.

We will, in time, once we view the body as a living, breathing information system whose complexity makes the flow of information on the Internet look like a game of checkers.

In the meantime, people need to remain careful and skeptical. The fecal transplant centers recently set up throughout the world will have to prove that they effectively treat ulcerative colitis and Crohns, which they’re getting paid to do. People who get fecal transplants will have to be followed for decades to find out what actually happens to them.

More significantly, people will need to see themselves as vast ecosystems whose non-human inhabitants influence very personal decisions, like what they want to eat and when. Then, if we’re lucky, humans might notice that we’re part of a much larger ecosystem, in which our eating, socializing, driving, and entertainment habits affect millions of other species on which our survival depends.

We’re all in this together. Obesity isn’t just about diet and exercise. Thousands of species inside and outside of us have a say — and they don’t have to register to vote.