Traci Stein Ph.D., MPH

The Integrationist

What's Gut Got to Do With It? Bacteria, Weight, & Health

How to keep your microbiome balanced, your weight in check, & feel better.

Posted Jan 03, 2017

Both obesity and unbalanced gut bacteria have been linked to a variety of health issues. And a growing body of evidence has found that imbalances in gut bacteria increase the risk of obesity, depression and anxiety, stroke, dementia, and changes in immune function. Obesity has also been linked to problems with learning, memory, and executive function (e.g., planning, decision-making, attention). The link between the higher rates of cognitive and psychological issues in obese patients is possibly explained in part by inflammation caused by an unbalanced microbiome (gut bacteria and other microorganisms).

The Microbiome

The microbiome, or your collective gut bacteria, account for much of what makes you you. Although estimates were previously much higher, it is now thought that your average-sized man is comprised of about 40 trillion bacterial cells and roughly 40 trillion human cells, give or take (with the ratio of bacteria to human cells estimated to be somewhere between 1 to 1 and 3 to 1). Some estimates place this range higher, in the neighborhood of up to 100 trillion microbial cells.

These bacteria are diverse, and perform a number of specific and necessary functions. For instance, microbes in the gut break down many of the proteins, fats and carbohydrates in our diet into nutrients that we can then absorb. They also produce beneficial compounds, like vitamins and anti-inflammatory chemicals that we otherwise could not produce. In fact, serotonin, an important neurotransmitter, is primarily made in the gut.

The Gut Out of Balance

When our microbiome is in balance, inflammation is kept in check, our brains function better, our weight is more stable, and we have better resistance to illness. Oral probiotics, which repopulate the gut microbiome with beneficial bacteria, have been linked to decreased severity of symptoms in schizophrenia and decreased anxiety, including decreased social anxiety. An unbalanced microbiome has also been linked to inflammatory bowel disease, irritable bowel syndrome, and treatment-resistant infections such as Clostridium dificile. Some studies have suggested that the diversity of the bacterial species in the human gut may be an important indicator of health in general.

Changing the Microbiome: Fecal Microbiota Transplantation

Fecal microbiota transplantation (FMT) involves obtaining fecal matter from a donor and introducing it into the intestinal tract of a recipient for the purposes of improving health. FMT has been employed to treat Clostridium difficile infection that has been resistant to other methods, as well as to treat inflammatory bowel disease and other illnesses. It’s been reported that over 500 cases of C. difficile have been treated this way, and approximately 80-90% treated successfully. The success rate has been more variable for inflammatory bowel disease but the results are encouraging.

FMT & Microbiome Changes in the Obese

FMT studies have found that the gut bacteria of obese individuals differs significantly from that of thin ones. For example, normal weight, germ-free mice who received FMT from obese mice experienced subsequent weight gain and increases in body fat. There has also been anecdotal evidence in humans regarding a similar effect on weight in FMT recipients whose donors were obese. Interestingly, transfer of gut bacteria from obese humans into mice has also led to increases in body fat and weight gain in recipients. Yet, when the mice who received this same type of bacteria were housed with mice receiving bacteria from lean donors, the mice carrying the obesogenic gut bacteria remained lean. (This effect is probably due to the fact that mice eat each others feces...!) 

Additionally, a small 2012 study found that transplanting the bacteria from lean human donors into obese human recipients resulted in improvements in insulin sensitivity and microbiome diversity.

It’s unclear exactly how altered gut bacteria lead to obesity. It’s possible that this occurs due to increased extraction of energy from food, by reducing calorie expenditure (burning fewer calories), or by decreasing satiety, making those with obesogenic microbiomes require more food to feel full.

Practical Strategies for Rebalancing the Microbiome

Chronic stress, poor diet, travel, illness, being delivered via caesarian section, and overuse of antibiotics are among the factors that can unbalance the microbiome. Although the association between gut bacteria and health are intriguing, there is still much remains to be understood about this link.

Fecal microbiota transplantation has shown some promise at rebalancing gut bacteria. For most people, however, mundane changes are probably more practical, have less of an "ick factor," and should still result in some benefit over time.

Currently, the recommendations for improving the microbiome, and hopefully, both mental and physical health, include increasing consumption of probiotic rich foods, taking a high-quality probiotic supplement, practicing good stress management, and consuming a fiber-rich, low sugar diet comprised largely of unprocessed foods. For more severe imbalances, the most effective means of rebalancing the microbiome will likely require addressing the issue via food intake, supplementation, and stress reduction combined, rather than by one approach alone.

Specific foods that will help rebalance the microbiome include fermented foods, such as kimchi, sauerkraut, and high quality yogurt. Increasing dietary fiber may also help. Meditation, regular exercise, prayer, engaging in a hobby you enjoy, and other similar activities can help reduce stress and balance mood as well.

In summary, your gut health impacts your mental health, risk of physical illness, and general well being. So, it's worth making steps to take good care of your microbiome as in investment in overall health.


Gupta, S., Allen-Vercoe, & Petrof, E. O. (2016). Fecal microbiota transplantation: in perspective. Therapeutic Advances in Gastroenterology, 9(2), 229-239.

Hilimire, M. R., DeVylder, J. E., & Forestell, C. (2015). Fermented foods, neuroticism, and social anxiety: An interaction model. Psychiatry Research, 228, 203–208.

Vrieze A., Van Nood E., Holleman F., Salojärvi J., Kootte R., & Bartelsman J., et al. (2012). Transfer of intestinal microbiota from lean donors increases insulin sensitivity in individuals with metabolic syndrome. Gastroenterology 143, 913–916.