The microbes in our gut don’t provide us with a great immune system and various metabolic functions for free. To maintain a healthy symbiotic relationship we have to feed the metabolic powerhouse in our guts with fermentable food ingredients – prebiotics – that these microbes are able to utilize. The fact is that the human host only produces the needed enzymes to digest starch, monosaccharides, and some disaccharides, and that the rest of the carbohydrate we eat are either passed through undigested or broken down by bacteria in the colon.
One of the problems with the typical western diet – with plenty of refined food high in fat and/or sugar – is that it’s primarily absorbed in the small intestine and contains little prebiotic fiber for our old friends in the colon. We’re virtually starving 90% of the cells in our body, and on top of that we’re probably changing the balance of bacteria in the upper gastrointestinal tract by eating massive amounts of refined carbohydrates and fats (10).
There are now plenty of studies showing that the typical western diet – that is certainly without evolutionary precedent – alters the balance of microbes in the gut (11,12,13,14). It’s also been shown that a single high-fat meal, low in dietary fiber, can initiate endotoxemia in just a few hours, and that levels of C-reactive protein shoot up quickly after consuming a typical McDonald’s breakfast (15,16). When we lived as hunter-gatherers in the wild, this response to a high-fat meal would have benefitted us in the sense that low-grade inflammation initiated increased fat storage and therefore better survival when food was scarce. However, in the industrialized world we have virtually unlimited access to calories, and refined junk foods are especially cheap. This daily influx of hyper-palatable food certainly drives weight gain through several mechanisms, and changes to the gut microbiota and increased translocation of bacterial toxins seem to be very important in that regard.
So, diet can change the composition of bacteria in our bodies. What else? We know that antibiotics dramatically alters the microbiome and that antibiotic exposure in early life is associated with consistent increases in body mass during the next years of life (17,18). Other factors that seem to play an important role are the increased rates of c-sections, bottle-feeding in infancy, reduced exposure to soil microbes, modern hygiene, and limited consumption of fermented food.
The gut microbiota and regulation of body fat
Microorganisms are found in most parts of the human body, and each location has a unique composition of bacteria that are able to live in symbiotically with the human host in that specific environment. The large intestine is where most of the microbes live, and this collection of gut microbes – gut microbiota – is also the part of the microbiome that is most important in terms of weight regulation.
In 2004, researchers at Washington University in St. Louis were some of the first to document a link between the gut microbiome and host energy homeostasis. They noticed that mice raised without a microbiome ñ germ-free mice ñ had 40% less total body fat than conventionally raised mice, while at the same time eating 29% more energy. Even more surprisingly; when these germ-free mice were colonized with normal microbiota, they experienced a 60% increase in body fat content within 14 days (3).
It’s important to note that we can’t live without the microbiome and that the complete absence of microbes – as seen in this trial – is not something to aim for.
In the decade that has passed since this first glimpse into the connection between the gut microbiota and weight regulation, the research in this field has exploded, and these are some of the things we now know:
Both studies in animals and humans show that obesity is characterized by an “obese microbiota” that is quite different than the microbiota of a lean person (19,20,21).
Germ-free animals who are inoculated with microbiota from an obese person gain more weight than animals who receive microbiota from a lean person (22).
Changes in the gut microbiota contribute to reduced host weight after gastric bypass surgery (23,24).
Changes in the bacterial communities can cause weight gain (25,26,27).
Transfer of intestinal microbiota from lean donors increases insulin sensitivity in individuals with metabolic syndrome (28).
How can microbes affect whether we gain or lose weight?
While most of the studies have focused on the gut microbiota in obesity, it’s apparent that the critters within play an essential role in weight regulation for everyone. But how is it that microorganisms are able to influence our body weight?
Since we know that the microbiome constitutes most of our cells and DNA, and that the gut microbiota provides so many important metabolic functions in our body, it’s no surprise that disruption of healthy gut microbiota can promote weight gain through several different mechanisms. While a lot of the early animal studies focused on the role microbes play in energy harvesting, it’s now becoming clear that this enhanced energy extraction from food only is a small part of a much bigger picture.