Obesity & The Gut

Weight loss is often a lifetime battle – one in which the body constantly fights (and usually prevails) to regain lost weight. This seems to be especially true for those who are overweight or obese from a young age. But what is it that drives the body’s need to revert back to its high set point, and how can those who struggle every day to lose or maintain weight be successful?

A recent study published in the journal Obesity triggered a lot of discussion about the myths and realities of weight loss, most notably a major feature in the New York Times by science writer Gina Kolata. The study followed contestants on the NBC reality show The Biggest Loser who shed extraordinary amounts of weight (239 pounds in 7 months for the season 8 winner) only to quickly regain it. The conclusion was that the rapid return to the previous weight was due to a decrease in resting metabolism caused by the weight loss.

But metabolism isn’t the only force at play here: the trillions of organisms that call our bodies home, collectively known as the microbiome, exert tremendous influence over our weight – and may be the hidden key to permanent weight loss.

These days, we can determine whether an individual is lean or obese with 90 percent accuracy just by examining their gut bacteria. A higher ratio of Firmicutes to Bacteroidetes phyla, as well as reduced microbial diversity compared to their lean counterparts are characteristic of the “obese microbiome”. And like other conditions involving bacteria, certain forms of obesity may be communicable: researchers at Washington University in St. Louis took gut bacteria from identical twins, where one was lean and one was obese, and transplanted them into germ-free mice. Within weeks, the mice that received microbes from the obese twin became obese, and the ones who received microbes from the lean twin stayed lean – validating the concept that our microbes, not our genes, may be primarily responsible for changes in our weight.

Although we’re not sure exactly how this happens, there are a number of ways bacteria can increase or decrease the amount of calories they extract from food: by altering the transit time through the digestive tract; by influencing hormones that determine whether calories are deposited as fat versus used as energy; and by consuming the extra calories themselves.

We’re learning more about which species of bacteria are associated with leanness and which diets may cultivate those species. Researchers have identified a family of bacteria called Christensenellaceae that seem to help people stay lean. Mice transplanted with these microbes tend to gain less weight than untreated mice eating the same diet. Christensenellaceae are only one example of probably hundreds of different microbes—some previously described and others not yet discovered— whose presence may impact our weight.

Bacterial imbalance – an overgrowth of pathogenic bacteria and a decrease in beneficial species and overall bacterial diversity – can cause excessive and hard-to-control cravings for sugary, starchy foods, which can lead to weight gain. But even after eliminating those foods, following a restricted diet, and getting plenty of exercise, many people continue to have difficulty losing weight because they’re still colonized with the wrong bacteria.

It may therefore seem that a simple solution to the obesity epidemic is to replace our not-so-great microbes with better ones; but that’s easier said than done. If we swap out our microbes for ones associated with leanness, but don’t eat the right mix of foods to nourish and sustain them, they’re not going to survive or reproduce long enough to recolonize our gut. So even though we can temporarily change our microbial composition by inoculating ourselves with different bugs, we tend to revert to our “old” microbiome pretty quickly – and our old set point on the scale.

A deeper understanding of the role of gut bacteria in harvesting energy from food is critical to achieving successful weight loss. What we do know is that foods high in a kind of indigestible plant fiber known as inulin such as leeks, asparagus, artichokes, lentils, oats, garlic and onion can help us cultivate a healthy microbiome populated with species associated with leanness. Avoiding overconsumption of sugary, starchy and high-fat foods can do the same. Fecal microbiota transplants that transfer “skinny” microbes from lean donors may ultimately prove to play a role in the obesity epidemic, although for now, my best advice for sustainable weight loss (and better health) is eat more veggies.

By: Dr. Robynne Chutkan