Bacteria in Our Gut: Essential for Staying Slim?

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By Dov Michaeli

No, I am not talking about the metaphorical bugs that bug us. I am talking about the real ones, of the bacterial persuasion, that populate our gut. So why talk about this doubly yucky subject at all? For two reasons: First, because I am fighting a cold right now and the one thing I don’t want to do is wipe out my gut bacteria. Second, because the story those bacteria in our guts is becoming truly fascinating.

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The Cold Conundrum

Let’s start with the cold. Over 99% of all colds are caused by viruses, especially Rhinoviruses. Yet, how many of us weren’t tempted to call the doctor, secretly hoping for a prescription of antibiotics? These drugs don’t affect viruses. But what’s the harm in taking a course of antibiotics? Maybe just this time… I feel crappy, there is a chance of bacterial super-infection, it is settling in my chest and I am hacking and coughing and wheezing.  Doc, can you spare a couple of pills?

The answer is an emphatic NO!

Our normal gut flora contains over 1000 species. If you weighed them they would exceed 2 pounds, about 100 trillion bacteria, give or take, easily exceeding our National Debt, dollar for bacteria. Here is another sobering thought: the total number of cells in our body is about 10 trillion. Not too shabby until you realize that we are mostly bacteria, and only about 10% human…

 What’s their function? Here there is a perfect symbiotic relationship: we provide them with foodstuffs, they help in digestion and protect us from a variety of disorders. Just one example: they form a continuous film along the intestinal wall, preventing undigested food from gaining access to the blood circulation. Incompletely digested food in the blood is one cause of food intolerance.

But wait, there is more!

Bacteria can make us fat, or lean.

In a 2004 paper in Nature, Jeffrey Gordon from Washington University in St. Louis, Missouri, and his colleagues demonstrated that intestinal bacteria could also contribute to weight gain in mice. In 2005, microbial ecologist Ruth Ley, a postdoctoral fellow working in Gordon’s lab, discovered that lean and obese mice have different microbial communities in their gut. And in 2 papers in the December 20, 2006 issue of Nature, Gordon and his colleagues have shown this difference exists in people as well, and that diets can shift the microbial balance.

Ley put a dozen obese volunteers on either a low-fat or a low-carbohydrate diet for a year. At regular intervals, she surveyed their intestinal microbial communities–which can contain hundreds of species. At the onset, Firmicutes, a type of gram-positive bacteria, represented more than 90% of the microbes present in the volunteers, while gram-negative Bacteroidetes species barely made up 3%. (By contrast, Ley found, Bacteroidetes account for some 30% of microbes in people of healthy weight.) After a year, the volunteers had lost 2% to 6% of their weight, the Firmicutes had dropped to 73%, and the Bacteroidetes increased to about 15%, irrespective of the diet followed,

A second paper suggested the microbes themselves help determine weight. Gordon’s graduate student Peter Turnbaugh analyzed the genomes of gut microbes in two pairs of mice. In each pair, one mouse was healthy, while the other, a sibling, carried two mutant copies of a gene called leptin, which made it obese. Microbes from the obese mice had more genes for processing starches and complex sugars and produced more simple sugars and fatty acids–that is, calories–for the gut to absorb. When the researchers transplanted microbes from the obese mice into germ-free mice, the recipients had a 47% increase in body fat over 2 weeks. In contrast, germ-free mice supplied with microbes from lean mice had only a 27% increase.

 Other experiments suggest that gut microbes play important roles in insulin resistance, which can lead to type 2 diabetes, and in fatty liver disease, a cause of serious liver damage. For instance, Nicholson and his colleagues ath Imperial College in London showed in 2007 ( Molecular System biology, May, 2007), that by changing the gut flora of mice into human flora, the metabolism of bile acids, which are responsible for fat absorption from the small intestine, changed for the worse: the mice absorbed more fat that was stored in the liver, and more LDL cholesterol (‘bad” cholesterol). What this means is that altering gut flora has far-reaching physiological and medical consequences.

So why not take antibiotics if not absolutely necessary?

It is obvious: antibiotics are pretty dumb; they wipe out all the bacteria, good or bad. In the case of a viral infection, there are no bad ones ,only the good ones are decimated. What happens next? A race to re-populate the gut gets under way. In most cases, the healthy balance that existed before the antibiotic catastrophe (from the bacterial point of view) is restored. But in an increasing number of cases, some bacteria that had lived in perfect harmony and balance with the thousands of other species multiply much faster and become predominant. Specifically, a species called Clostridium difficile (see picture below), which had been kept in check, now divides rapidly and takes over. The symptoms are definitely not pleasant: severe cramps, bloody diarrhea, and malabsorption of necessary nutrients .From its name you could surmise that C. difficile is difficult to eradicate. Because of indiscriminate prescriptions (and feeding poultry with antibiotics to increase their weight) this bug developed resistance to most known antibiotics. It still responds to one, vancomycin, but there are already several reports of vancomycin- resistance.

So the lesson of this sermon?  Resist temptation! Take antibiotics only when absolutely indicated. Like all other drugs –they are not benign.