Millions of people struggle with extra weight, and despite their best efforts to eat right and exercise, the pounds stay put.
Recently scientists have asked whether the bacteria that live in us and on us — known as the microbiome — may play a role in how easily we gain and lose weight. And if so, could it be a reason so many people get stuck in a bigger size, even when they work really hard to slim down?
Early studies on that question have been intriguing.
Take two mice specially bred to be germ-free. Give one mouse gut bacteria from a thin person and one gut bacteria from an obese person. Feed them the same amount of chow and something striking will happen: The mouse given bacteria from a lean person will stay lean. The mouse given bacteria from an obese person will gain weight.
Studies that have analyzed the gut bacteria of lean and obese twins have found that lean people have many more different kinds of bacteria than obese people do.
And in one widely discussed case, a woman who had never experienced weight problems gained 30 pounds after getting a fecal transplant from her overweight teenage daughter.
Now researchers are just starting to tackle the question: will changing a person’s gut bacteria change their weight?
Elaine W. Yu, MD, an endocrinologist at Massachusetts General Hospital in Boston, is launching a study examining whether giving obese people capsules of freeze-dried stool bacteria from lean donors will help them lose weight. We asked her why she’s trying that approach and what she hopes to learn from the study.
WebMD: Have any studies suggested that altering a person’s gut bacteria might help them lose weight?
Yu: There are not a lot of studies in humans. But there have been a couple.
There was a study published a couple of years ago that looked at fecal microbiota transplants (FMT). They took 18 patients and randomly assigned them to receiving active FMT or a placebo (their own bacteria that was given back to them). They looked at changes in insulin sensitivity. They found that those patients who received FMT from lean donors had improvements in insulin sensitivity.
There have been other studies that have looked at what happens to metabolism when you give short courses of antibiotics and have found some effects. That’s been seen in humans and animals.
But in terms of randomized, controlled trials in humans looking at weight as an outcome, there aren’t a lot of those. That’s why we’re doing our study.
WebMD: What made you think it could work?
Yu: There’s a lot of indirect evidence about the connections between gut bacteria and body weight. For example, what we already know is that obese people and obese animals have different intestinal bacterial communities as compared to lean people or lean animals.
Another piece of evidence is that in an experimental setting, you can take germ-free mice and feed them large amounts of food, and they’re resistant to diet-induced obesity, despite eating more food than their control litter mates. That’s because they have an increased metabolism. Whether that’s due to a lack of microbes in their gut, we don’t know. That’s certainly a very clear hypothesis.
Then there’s also some evidence that exposure to antibiotics early in life, whether in humans or in animals, is associated with becoming overweight later in life.
But in science, to establish a clear and causative relationship between microbes and disease, you have to transfer a microbe from one organism and put it into another organism and see if it causes disease, or in this case, the opposite of disease. That’s what we’re trying to do.
WebMD: How will your study work?
Yu: We’re taking individuals who have obesity and some sort of metabolic disarray [like insulin resistance] and we’re giving them FMT samples from lean, healthy donors and we’re going to be monitoring their body weight and metabolism afterwards.
WebMD: How do you know if your donors have bacteria that will help weight loss? Are you checking them for specific species?
Yu: Unfortunately, I don’t think we’re at the stage where we know which species are the beneficial ones. We’re still at a very rudimentary stage where we just take the entire community of gut bacteria from a healthy individual and transfer that into individuals who have certain disorders like C. diff, inflammatory bowel disease, or obesity. We’re not yet at the stage of identifying, ‘OK, It’s these five species which are the good ones that we need to introduce.’ We’re not there yet.
Once we finish the study and if there are improvements in metabolism, that’s where the real nitty gritty work will occur where we will try to drill down and figure out which species, which communities were the ones that caused these improvements.
WebMD: Will you have one donor for all the study participants?
Yu: No. There will be multiple donors. And it is possible that there may be differences amongst the donors. We do have extraordinarily strict criteria for who can be a donor. We had to get a special permit from the FDA — called an IND, for Investigational New Drug application — to do this.
There are potential risks for doing these fecal transplants. It’s a little bit nerve wracking to see that there are websites and patients and people out there who are promoting doing this on your own because you are taking material from one person and transferring it to someone else. There are infectious risks associated with doing this. These microbes have an effect on a whole host of things. It’s one of those things that should be done carefully and with much thought.
WebMD: In terms of the things that cause us to gain or lose weight — calories, exercise, etc. — where does gut bacteria fit into the equation? Any guesses as to how big or small its role may be?
Yu: I can’t even try to give you a number. We just don’t know enough about it. What makes it even harder is that the systems are so highly interconnected.
It’s very obvious that diet affects weight. And of course, it’s our hypothesis that the gut microbes can also affect weight. But diet can also affect the gut microbes, so it’s not like it’s a discreet system. And that makes it extremely complicated to figure out the various contributions of these different components.
Read more in WebMD’s special report, “Mysteries of Weight Loss.”