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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