Could gut bacteria microbes make you fat? That is the question raised by the article on the BBC website.

It begins by noting that our guts contains trillions of microbes that are collectively know as gut microbiota. No two persons' microbiomes are the same. They are the products of what we inherit from out mothers, our diets, environment and lifestyle.

According to the BBC, researchers have known for some time that the gut plays an important part in numerous systems of our body, including digestion and hunger. Now they are discovering the connection between gut microbiomes and weight.

A varied biome of gut bacteria is thought to have several benefits, even so far as improving our moods. New evidence suggests it could also have a physical effect – making you skinnier, or even heavier.

According to Nature.com, studies on mice in the mid-2000s showed a correlation between gut microbiota and weight gain.

In one study, the researchers observed that germ-free mice were leaner and had lower body fat than mice colonized with a conventional microbiota, despite the fact that the latter ate less. Notably, when the germ-free mice were colonized with the conventional gut microbiota, they gained weight and showed increased levels (over 50%) of body fat.

But the mice were not eating more — in fact, they were eating less — so the presence, or absence, of a gut microbiota must have at least contributed to weight gain. When probed further, the gut microbiota was found to have a crucial role in helping us extract nutrients and energy from food. The processed nutrients are delivered to the liver, and are ultimately converted to fat and deposited in the body.

In another study, the researchers sought to determine what the differences there were in the gut microbiota of obese and lean mice. They did find some.

Specifically, obese mice were found to have far fewer (50% less) Bacteriodetes bacteria in the distal part of the gut than lean mice, and instead showed an increase in the proportion of Firmicutes.

By now, researchers had concluded that the gut microbiota contributes to energy extraction and fat deposition, and that obesity drives changes in the gut microbiota.

Then another group of researchers wanted to test a hypothesis–whether obesity is, at least in part, driven by more efficient extraction of energy from food by the gut microbiota. To test this hypothesis, the researchers analyzed the microbiomes of obese and lean mice.

They found that the microbiomes from obese mice are enriched in enzymes that break down polysaccharides that are indigestible to humans, in this way ‘helping’ the mice get more out of their food. Not only that, but obese mice had far less energy remaining in their faeces than lean mice — they were just better at getting energy out of food.

In a Harvard article, Jemila C. Kester, PhD discusses the topic “Can your microbiome be the key to long-lasting weight loss?” She points out that two people can eat the same portions of the same food and have vastly different results when it comes to weight loss. She notes that our traditional understanding of digestion is that food gets broken down and absorbed in our intestines and either burned for energy or stored as fat. However, she says that there is more to it than that.

Food is mechanically and chemically broken down in the stomach. Next, it travels to the small intestine where enzymes break down the food even more into parts small enough to be absorbed into the body’s circulation and used for fuel.
 
The nuance arises from what happens to the food we can’t process. As humans, we don’t make the enzymes required to break down 45-65% of what our species has historically eaten, namely indigestible carbohydrates, or fiber. This indigestible portion of what we eat travels to our large intestine (also called the colon), where the resident microbes break it down for us, releasing molecules we are able to absorb and use.

Most importantly, Kester says that “microbiomes isolated from obese individuals have been shown to harvest more energy from the same amount of food.”

Kester next discusses fad diets and says,

Many popular diets call for the reduction in—or even exclusion of—a macronutrient: carbohydrates, proteins or fats, primarily. While cutting out a macronutrient can lead to fast weight loss in up to 15% of dieters, it fails to maintain weight loss, with 95-99% regaining the weight with a year.

While Kester's article is thought provoking, she doesn't give offer much practical advise to the dieter. She concludes by saying that future research will continue to expand our understanding of the microbiome and suggest ways to manipulate it to achieve our weight-loss goals. She does suggest that avoiding fad diets is the best practice for weight loss. She recommends for now to eat a complex diet including all macronutrients.

Sources:

www.bbc.com/future/article/20190212-could-gut-bacteria-microbes-make-you-fat

www.nature.com/articles/d42859-019-00003-5

sitn.hms.harvard.edu/flash/2018/eating-for-a-trillion/