by Brandon Jensen —
The prevalence of obesity in the United States has increased rapidly over the past several decades. Today, around 40% of adults in the US are considered obese. Obesity increases the risk of cardiovascular disease, diabetes mellitus, and certain cancers. It also comes with a significant financial cost – treatment of disease associated with obesity causes the average person an extra $2,000 a year over their lifetime (Kim, 2016).
Several studies have shown that only 20% of overweight individuals are successful at long-term weight loss through diet changes and exercise (Wing, 2005). This suggests that the way we understand obesity is lacking a critical element. New research suggests that the bacteria living in the gut play an enormous role on how the body processes and stores energy. Today, biologists are analyzing the complex relationship we have with these bacteria to understand how we can potentially manipulate them to better meet our metabolic needs.
A History of Coevolution
Long ago, in the days before supermarkets were available in every city, humans were required to hunt, gather, or grow their own food. This proved to be problematic especially in places with long, cold winters. The inconsistent diet at the time required the ability to absorb as many nutrients from each meal as possible and the ability to store extra energy in the form of fat. People that were able to do so were able to survive longer.
How was this achieved? Evolving additional metabolic pathways to help digest food and store energy more efficiently would be very complex and would require a long time. The bacteria in the gut, however, can digest all kinds of molecules which humans are not able to. Different kinds of bacteria have different metabolic abilities; people who had the right composition of bacteria were able to meet an equilibrium where they had sufficient stores of energy and were also able to maintain a healthier weight.
Over time, as civilization began to develop, nutrition became easier to acquire and the same bacteria that helped us survive the hunter-and-gatherer era are now preventing us from eating our favorite foods. These same bacteria could explain why many have difficulty with weight loss even after diet changes and many hours spent at the gym.
A Story of Three Mice
A study on three groups of mice revealed the drastic effect that gut bacteria can have on metabolism. One group of mice, which were born and raised as “germ-free” in a completely sterile environment grew alongside a second group of “conventional” mice, or mice that experienced a natural birth and had a normal composition of bacteria in their gut for twelve weeks. They were all fed the same diet, and their energy expenditures were tracked by measuring oxygen levels. In the tenth week, scientists transferred some of the bacteria from the conventional mice to the germ-free group, to create a third “conventionalized” group. At the end of the twelve-week experiment, both the conventional and conventionalized mice contained over 40% more total body fat than the germ-free mice, despite consuming significantly less food and exerting more energy on average. The conventional and conventionalized mice also showed signs of insulin resistance (indicating a progression toward diabetes) and increased fat content on their livers (Backhed, 2004).
Figure 1, Microbiome composition and abundance in the human GI system and factors that can affect them (Clark, 2019).
Currently, weight is thought to be influenced by a complex web of factors including age, diet, exercise, and the individual’s gut microbiome. In the future, research may be able to lead to solutions where in an individual’s gut microbiome, specific targets are identified and manipulated to control the overall metabolism within an individual, leading to healthier lives.
References
Backhed, F., Ding, H., Wang, T., Hooper, L., Koh, G., Nagy, A., 2004. The gut microbiota as an environmental factor that regulates fat storage. PNAS 101, 15718-15723.
Clark, G., Sanhu, K., Griffin, B., Hyland, N., 2019. Gut Reactions: Breaking Down Xenobiotic- Microbiome Interactions. Pharmacological Reviews 71, 198-224.
Kim, D. and Basu, A., 2016. Estimating the Medical Care Costs of Obesity in the United States: A Systemic Review, Meta-Analysis, and Empirical Analysis. Value in Health 19, 602-613.
Wing, R. and Phelan S., 2005. Long-term weight loss maintenance. Am J Clin Nutr 82, 222-225.