Researchers trying to figure out if microbes living in your body might be a factor in weight gain say a breath test could show if you’re loaded with greedy germs that pull every last calorie out of food.

Study after study is showing that people are covered in bacteria, fungi and other microorganisms that help digest food, that can keep teeth healthy and even that cause dandruff. And they’re finding that the types of microbes living in the colon and intestines may play a major role in just how much nutrition the body gets out of food.

“Normally, the collection of microorganisms living in the digestive tract is balanced and benefits humans by helping them convert food into energy,” says Dr. Ruchi Mathur, an endocrinologist at at Cedars-Sinai Medical Center in Los Angeles.

Mathur and colleagues were looking at a species of bacteria called Methanobrevibacter smithii – M. smithii for short. As its name indicates, it makes a lot of methane – the odorless gas responsible for burps and other inconvenient emissions.

People who produced the most methane and another gas, hydrogen, in their breath weighed more and had more body fat than people who produced the lowest amounts, they reported in the Journal of Clinical Endocrinology & Metabolism.

“This is the first large-scale human study to connect the dots and show an association between gas production and body weight,” Mathur said in a statement.

The team tested 792 volunteers, dividing them into four groups – those with “normal” levels of gases in their breath, those who had more methane than average, those who breathed out more hydrogen than average and those who produced extra amounts of both methane and hydrogen.

Those in the last group, exuding the highest concentrations of both hydrogen and methane, also had higher body mass indexes or BMI, the standard measure of height to weight that doctors use to determine obesity. They also had more body fat than the others.

This fits in with other work Mathur’s team has done on the role of this particular bug in obesity, they noted. For one, obese people with more methane detectable in their breath weighed nearly 15 pounds more than other obese people who didn’t produce as much methane.

M. smithii needs hydrogen, and it gets it from other bacteria living in the gut, which produce hydrogen gas as a byproduct of metabolizing food.  The researchers are not entirely sure how a methane-producing bug might make people fatter, but said it’s possible methane gas slows the passage of food through the intestines and colon, allowing more calories to be extracted.

Diet could affect this, and the researchers didn’t ask their volunteers for details about what they ate. “However, given the large sample size, these individual variations may be mitigated between groups,” they wrote.

Researchers are trying to figure out if it’s possible to kill off the guilty germ and help people lose weight. But they know better than to just kill gut bacteria willy-nilly – studies have shown that taking antibiotics can alter the balance of microbes in a bad way, causing stomach upset, allowing deadly infections such as C. difficile to take hold and, perhaps, even allowing a takeover by the obesity-generating germs.

Related:

• Are your gut bacteria in charge?
• Overweight? Blame the bacteria in your gut
• Antibiotics may raise obesity risk

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AP

A clump of Staphylococcus epidermidis bacteria (green) in the extracellular matrix, which connects cells and tissue, taken with a scanning electron microscope. At right, the bacterium Enterococcus faecalis, which lives in the human gut, is just one type of microbe that live on your skin, up your nose, in your gut; enough bacteria, fungi and other microbes that collected together could weigh a few pounds. (AP Photo/National Institute of Allergy and Infectious Diseases (NIAID, Agriculture Department)

Could antibiotics make you fat?

Two studies this week suggest that using antibiotics may save people’s lives, but could also change their metabolisms. Put together, the studies suggest that taking antibiotics might alter digestion to help people absorb calories from food they normally would be unable to digest.

Every human carries pounds of microorganisms that we couldn’t live without. They break down food and extract nutrients like Vitamin K for us. Antibiotics will kill some of these beneficial organisms, which is why so many doctors now tell patients to eat yogurt after taking a course of the drugs, to replace some of the good guys.

“There is emerging evidence suggesting the importance of the microbes in our intestines and their role in absorbing food,” said Dr. Leonardo Trasande of New York University, who led one of the studies.

The two studies look at different sides of the coin, and help answer two questions -- whether antibiotics really do affect how we absorb nutrients, and how they might do so. Together, they support the idea that the drugs kill off some populations of bacteria and allow microbes to flourish that are very good at getting calories out of hard-to-digest plant foods.

Trasande’s team looked at the medical records of more than 11,000 newborns in Britain, who were carefully followed after they were born in the 1990s. The babies who got antibiotics before they were 6 months old were 22 percent more likely to be overweight by the time they were 3 years old, the team reported in the International Journal of Obesity. If they got antibiotics later in childhood, there wasn’t a strong effect – something that could suggest the antibiotics changed the balance of the microbes as they were just setting up shop in the infants. Babies are born with sterile digestive tracts, and they acquire bacteria, yeast and other microorganisms mostly from their mothers. The germs are collectively called “flora” by scientists.

“They play key roles in immune functions, among other things,” Trasande told NBC News. “Antibiotics disrupt the development of the healthy flora in our gut. The earlier the exposure occurs, the more disruptions occur,” Trasande says. “It seems the first few days and months are important. It is difficult to reconstitute that in later life.”

The other piece of the puzzle is whether it’s the antibiotics or something else that is doing this. Dr. Martin Blaser of New York University has been studying the effects of antibiotics on the body for years. A second team he heads has been studying what happens if you feed antibiotics to animals.

They wanted to replicate what farmers have known for decades -- that giving low doses of antibiotics to farm animals make them fatter. Many experts had thought the drugs were keeping the animals from getting infections and making them healthier, but Blaser suspected something else was going on.

When his team gave mice low doses of antibiotics long-term, the mice got fatter even though they weren’t eating any more than other mice. This, they report this week’s issue of the journal Nature, suggests the antibiotics somehow make the mice absorb more calories from their food.

“We have other work that is in process that continues to confirm and extend this,” Blaser said. “That work shows that giving antibiotics early in life, similar to what farmers do in their farm animals, is changing metabolism in mice and making them bigger and fatter.”

The gastrointestinal tract is also the center of hormone production, the researchers said. It’s possible altering the organisms in the intestines – called the microbiome -- could help people better absorb nutrients and calories from “indigestible” foods such as cellulose.

The second NYU team gave the mice varying combinations of the antibiotics penicillin, vancomycin and chlortetracycline. Mice that got the antibiotics piled on more fat than other mice, even though the fatter mice did not eat more. Also, their poop had fewer calories – suggesting they were absorbing more and eliminating less.

Other mouse studies being done by Blaser’s team show that giving antibiotics to mice every once in a while -- akin to giving antibiotics to a child to treat ear infections -- also alter the gut bacteria.

So does that explain why people are getting fatter? Does every dose of antibiotics kill off some bacteria, allowing the energy-efficient species to move in and squeeze every calorie out of an apple peel or bowl of high-fiber cereal?  

“That’s at least one of the mechanisms,” says Blaser. But he notes that studies in people suggest it’s doses very early in life that matter most, just as various colonies of bacteria are getting established in the colon and intestines. And there’s an effect on the immune system, too. Other studies show that changing the balance of bacteria effects immune cells known as T-cells – something that may someday help explain links between diet and diseases such as inflammatory bowel diseases and perhaps even colon cancer.

In other words, it is too soon to say whether a 5-day prescription of Zithromax for strep throat could make you fat.

“A lot of things are interconnected,” Blaser says. “Obesity is multifactorial. I am not saying antibiotic effects on the microbiome are everything but our work suggests it is contributory. Whether it’s 10 percent or 70 percent, we don’t know yet.”

Another big missing piece of the puzzle: Which species of bacteria are the most important? People have trillions of bacteria in and on their bodies. Microbes outnumber human cells by a factor of at least 10 to one and scientists believe at least 10,000 different species live in and on us. Healthy colonies of microbes not only process vitamins, but maintain pH balance on the skin, prevent tooth decay and even protect against infections. So which ones are killed by the antibiotics, and which do we want more of? No one knows yet.

“We are just beginning to scratch the surface,” said Dr. Ilseung Cho, who worked on the study in mice.

While it is important not to use antibiotics when they are not needed, the researchers stress that they do save lives. “I wouldn’t rush to come off any antibiotics right now,” Cho cautioned.

It’s also not clear if food like yogurt, called probiotics, help much. “There is a concept called prebiotics,” Cho said. “It is essentially introducing nutrients into your digestive tract that would select for particular bacteria. Then you might be able to alter the bacteria.”

Prebiotics are found in plain old food such as soybeans, jicama and raw oats, all of which are rich in compounds such as inulin, which people cannot digest, but which certain bacteria love.

Related links:

• Mapping one man's microbes
• Bacteria affect mood
• Ruled by your gut