In probiotic therapy, beneficial strains of microorganisms are transferred to people in order to improve their health. Probiotics are not mainstream medicine treatment and are poorly understood because little is known about the interactions between human hosts and the trillions of bacteria that reside in their gastrointestinal tracts.

About 10 trillion microbial cells (mainly bacteria) live in our large intestine; they outnumber the total number of human cells in the body by 10 to 1. These microbes are essential to human biology and may play a key role in the development of obesity; it has long been known that the strains of intestinal bacteria in lean and obese individuals are different. A recent study published in Science magazine (6 September 2013) shows that when microbes from lean or obese human donors are transferred to the intestines of mice, they can confer similar patterns of obesity in the recipient mice. Moreover, obesity in mice that received microbes from obese human donors can be reversed if they are cohoused with mice who received microbes from lean human donors and also ate a healthy diet. This study presents some of the strongest evidence to date that the principle of probiotics has potential to become an important future treatment for weight loss.

Bacteria from Humans Made Mice Gain or Lose Weight

In this study, the researchers transferred microbes from the intestines of human donors who were either lean or obese into the intestines of germ- free mice. The mice took on the characteristics of their human donors: animals that received the microbes from the obese donors gained significant weight, whereas those that received the microbes from the lean donors stayed lean. All of the mice ate a low-fat, high-fiber diet without restrictions. All it took to make normal mice become obese was for them to acquire intestinal bacteria from obese human donors!

In addition, when lean and obese mice (those that received intestinal bacteria from lean and obese human donors, respectively) were housed in the same cage, the expected weight gain in the “obese”mice was prevented. Analysis of the bacteria in the “obese”mice that were cohoused with the lean mice showed that the “lean”mixture of bacteria passed from the lean mice to the obese mice, conferring protection against gaining weight and keeping these mice lean.

A Low-Fat, High-Fiber Diet Helped Invasion of “Lean”Bacteria

The researchers found that diet played an important role in allowing the successful invasion of “lean”bacteria into the intestines of mice harboring the “obese”bacteria. When the obese mice were cohoused with the lean mice and were fed a low-fat, high-fiber diet, they were “invaded”by the microbes of the lean mice and did not gain weight. In contrast, if the cohoused mice were fed a high-fat, low-fiber diet, the obese mice were not able to acquire the lean intestinal bacteria and thus remained obese.

There are three possible explanations for what appears to be a powerful effect of intestinal microbes on obesity that all involve their primary role in metabolizing fiber in the diet. The products of bacterial fiber digestion, although a source for energy in humans, also can minimize fat storage in adipose (fat) cells, preventing fat cells from enlarging (the main reason fatty areas of the body enlarge with weight gain). These fiber-derived compounds can also result in an increased basal metabolic rate (higher energy use) in humans and can even increase production of hormones that give a feeling of fullness (satiety). All of these effects, especially if combined, can result in weight loss and support the importance of a low-fat, high-fiber diet complementing the effect of beneficial microbes.

The researchers of the recent Science study point out that one of the most interesting findings of their study is that of the role of diet along with microbes in preventing obesity. In future studies of humans, it may be necessary to alter diet to facilitate the colonization of useful microbes to increase their benefit. This study elucidates the role of combining microbes and diet in reducing and preventing obesity and points to the possibility of exciting new probiotic therapies for combating obesity.

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