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Fat-busting bacteria not a hard act to swallow

How much energy you get from the food you eat may be determined by the bacteria in your stomach. Dr Karl chews on some tasty findings to reveal what decides your final body weight.

Big jeans on small person

(Source: iStockphoto)

Last time, I talked about weight control and how, if your energy in equals your energy out, your weight should be stable. And then I discussed the case of a young woman who ate exactly what the dietitian had calculated would keep her weight stable. Even so, she gained about half a kilogram each day. What was going on?

I should remind you that there are ten-times as many microbial cells in your gut as there are cells in your body. You might also remember that these microbes help the normal growth and development of both your gut and your immune system. And they extract a lot more energy out of what we eat than we can with our normal gut activities.

In humans, the microbes in our gut increase the amount of energy available to us by about 10–30 per cent. Polysaccharides are long chains of repeating identical sugars. They are the most common biological polymers on Earth. But without gut microbes, we cannot extract the energy they contain.

The microbes in our gut are pretty clever. They somehow know how much energy we eat and adjust their population numbers to suit.

A neat little experiment back in 2006 proved this. The scientists, doctors Ley and Gordon and colleagues, found 12 obese volunteers and five lean volunteers.

They compared the gut bacteria in their volunteers. The obese people, as compared to the lean people, had 20 per cent more firmicutes and 90 per cent fewer bacteriodetes. (The way I remember this, is 'F' for fat equals 'F' for firmicutes.)

Then the scientists put the obese volunteers on a low-energy diet (either low-fat or low-carbohydrate). After one year, the previously obese people lost as much as one-quarter of their body weight. At the same time, the ratio of firmicutes to bacteroidetes changed. There were fewer firmicutes and more bacteroidetes.

This showed that if you were lean, your microbe populations were different to when you were fat. In other words, your level of body fat can change the microbe populations in your tube of life (your gut).

But Dr Turnbaugh and colleagues showed the opposite: that changing microbe populations can make mice fat.

First, they transplanted microbes from a human gut into very lean microbe-free mice. These microbes grew and flourished in these so-called 'humanised' mice and the mice got fatter.

Second, they fed half of the humanised mice a low-fat diet and fed the other half a high-fat diet. As you would expect, the mice fed a high-fat diet became much fatter than the mice fed on a low-fat diet. No surprises.

And, just like the doctors Ley and Gordon study, the fatter mice had higher proportions of firmicutes bacteria in their gut microbiome.

Amazingly, the proportions changed in the very short time of just one day!

Third, they did the clever bit. They kept the diets the same but transplanted the microbes between the lean and the fat mice.

Lean mice, now carrying the gut microbiome from the high-fat-fed humanised mice, quickly became obese. In other words, the fat-adapted gut microbes can, if transplanted, make you fat. And so yes, being fat might be catching.

Perhaps this is how Janet S, Dr. Atkinson's 25-year-old, 158 kilogram patient, was able to gain weight when she ate a supposedly neutral-weight diet.

Now you can easily see how microbes in your gut are related to how much energy your gut extracts from the food you eat and, as a result, your final body weight. But what about a microbe in your nose?

A study by Dr Goodson compared 313 overweight women (body mass index of 27–32) with 232 normal-weight women. It looked for differences in the microbes in the mouth.

Over 98 per cent of the overweight women had a single specific species of bacteria (Selenomonas noxia) making up over one per cent of the total number of species in the saliva.

If you swallow a litre of saliva per day, that works out to one gram of bacteria in total — and about a billion cells of Selenomonas noxia — sloshing down into the gut.

The big unanswered question is this: did the bacteria cause the overweight or vice versa? Or are they nothing to do with being overweight?, and are they just an accidental association?

Well, it's early days in our scientific research into the tube of life, so tune in next time for how catching a certain virus can make you fat.

Tags: obesity, biology, stomach-and-digestion

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Published 17 May 2011

© 2017 Karl S. Kruszelnicki Pty Ltd