In the past 30 years, the number of obese children and adults in the U.S. has literally exploded. As scientists search for an answer to the problem, they are discovering that obesity is not a simple issue, but a complex, multi-factorial disease with a variety of causes. Some causes are obvious, such as too much food and too little exercise, but there are several other less well-known theories. In recent years, the idea that microbes might cause obesity has gained a foothold. Dr. Nikhil Dhurandhar, head of the nation’s first department of viruses and obesity, invented the term “infectobesity” to describe the emerging field. His particular interest is in the relationship between obesity and adenoviruses, which cause respiratory tract congestion and some of the “common” colds. Other scientists have been studying the role of microbes in the gut as possible causes of obesity—a theory given more credence by a new report published in the journal Science Translational Medicine.
About 100 trillion bacteria, 500 different species, live inside the human body, the greatest concentration of which live in the intestines. The majority of the bacteria fall into two major divisions, or phyla: the Firmicutes and the Bacteroidetes. One of the main functions of gut bacteria is to break down otherwise indigestible foods into a form that the body can digest. Previous research had shown that obese mice had higher levels of Firmicutes, and lean ones had more Bacteroidetes.
In the new research, Dr. Jeffrey I. Gordon, director of the Center for Genome Sciences at Washington University School of Medicine in St. Louis, and graduate student Peter Turnbaugh, along with lab members Vanessa Ridaura and Jeremiah Faith, transplanted an adult’s gut microbial community (obtained from feces) into the guts of germ-free mice. In the weeks leading up to the transplants, the mice ate low-fat, plant-rich diets. After the transplants, the mice continued to eat the same diet for one month. Stool samples were analyzed one day, one week and one month after the transplants.
A month after the transplants, half the mice were switched to a high-fat, high-sugar “western” diet. Stool samples from all the mice were analyzed 24 hours after the diet change, and then weekly for two months. Tests showed that, compared to mice on the low-fat diet, mice on the western diet had a significantly greater proportion of Firmicutes, and a reduction in Bacteroidetes—changes that Gordon’s earlier studies have linked to obesity in mice and humans. “When we switched these humanized animals (from a low-fat diet) to a junk-food diet, high in fat with lots of simple sugars, the structure of the microbial community changed dramatically and very rapidly, not only in terms of who’s there or which species but the proportional representation of species changed very abruptly within 24 hours,” explained Gordon. “These mice also became obese on Western diets.”
When the researchers took it one step further and transplanted the gut microbial communities of the humanized obese mice to germ-free mice, the recipient mice gained weight and fat, despite eating a low-fat, plant-based diet. The researchers also showed that gut microbes and their genes can be passed from generation to generation, suggesting that it is possible for mothers to pass their microbial communities to their children.
The team is already using the new mouse model to study malnutrition in children. Humanized mice are created by transplanting microbes from children living in areas of the world where malnutrition is common, and then the mice are fed diets typical of those at risk for the condition. “Pinpointing triggers of obesity or malnutrition in humans is hard because there’s a host of factors—genetic, cultural and environmental, such as diet—that are extremely difficult to control,” says Gordon. “Recreating the human gut ecosystem in mice gives us a way to control these variables.”
Gordon hopes the information from this and previous studies will allow researchers to develop hypotheses that can be tested in humans—giving new hope to those who have spent a lifetime struggling with weight gain.
by – Madeline Ellis/healthnews.com
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