Your gut is home to trillions of bacteria that make up the gut microbiota. These bacteria not only help digest food, but also directly influence your body weight and metabolism.
How do gut bacteria affect your weight?
Gut bacteria influence your weight in several ways:
Extracting energy from food
Some bacteria are more efficient at extracting calories from the food you eat, especially from carbohydrates that we normally couldn't digest on our own. This means two people can eat the same thing but absorb different amounts of calories depending on their gut bacteria.
Appetite control
Bacteria produce substances that affect the hormones that regulate hunger and satiety, influencing how much you eat and when you feel full.
Fat and sugar metabolism
The microbiota takes part in how your body processes fats and sugars, affecting whether they are stored as fat or used as energy.
Inflammation
An imbalance in gut bacteria can cause low-grade inflammation in the body, which is linked to insulin resistance and fat accumulation.
Balance matters
People with obesity tend to have a different composition of gut bacteria compared to people with a healthy weight. Specifically, they usually have more Firmicutes-type bacteria and fewer Bacteroidetes-type bacteria. This imbalance, called dysbiosis, can promote weight gain.
On the other hand, beneficial bacteria such as Lactobacillus and Bifidobacterium help maintain a healthy weight by improving the intestinal barrier, reducing inflammation and better regulating metabolism.
Dysbiosis: when the imbalance affects the whole body
Dysbiosis — the alteration of the composition and diversity of the microbiota — does not stay confined to the gut. Through different communication axes, it impacts distant organs and systems, modulating immunity, metabolism, behavior and mood.
Gut-brain axis
The microbiota communicates with the central nervous system via the vagus nerve, the immune system and bacterial metabolites such as short-chain fatty acids and precursors of neurotransmitters (serotonin, GABA, dopamine). Sustained dysbiosis is associated with a higher risk of anxiety, depression, cognitive decline and neurodevelopmental disorders. In autism spectrum disorder (ASD), an altered microbiota, greater intestinal permeability and gastrointestinal symptoms have been consistently documented; studies such as fecal microbiota transplant trials (Kang et al.) showed both digestive and behavioral improvements sustained over time.
Gut-metabolic axis
Dysbiosis promotes insulin resistance, non-alcoholic fatty liver disease (NAFLD/MASLD), type 2 diabetes and metabolic syndrome, through low-grade inflammation and endotoxemia (passage of lipopolysaccharides into circulation).
Gut-immune axis
Nearly 70% of the immune system resides in the gut. Dysbiosis is implicated in inflammatory bowel diseases (Crohn's, ulcerative colitis), allergies, asthma and autoimmune diseases such as rheumatoid arthritis and multiple sclerosis.
Gut-skin axis
Acne, rosacea, atopic dermatitis and psoriasis show a relationship with alterations in the gut microbiota and improve when dysbiosis is addressed alongside dermatological treatment.
Other conditions with growing evidence
- • Parkinson's disease and Alzheimer's disease (microbiota-gut-brain axis).
- • Attention deficit hyperactivity disorder (ADHD).
- • Irritable bowel syndrome (IBS) and SIBO.
- • Infertility and pregnancy complications.
- • Certain types of cancer and response to oncologic immunotherapy.
Addressing the microbiota — diet, fiber, specific probiotics, stress management and, in select cases, targeted medical interventions — is now an evidence-backed complementary tool to improve the course of many of these conditions.
What can you do to improve your microbiota?
- • Eat more fiber: Fruits, vegetables, legumes and whole grains feed beneficial bacteria.
- • Include fermented foods: Yogurt, kefir and other fermented foods contain beneficial bacteria.
- • Limit saturated fats and ultra-processed foods: These foods can promote the growth of bacteria associated with weight gain.
- • Exercise regularly: Aerobic physical activity increases gut bacteria diversity and favors the growth of beneficial strains.
- • Avoid unnecessary antibiotic use: Antibiotics can significantly alter your gut microbiota.
The bottom line
Your gut microbiota is an important factor in weight regulation, but it is only part of the full picture. Genetics, diet, exercise and other lifestyle factors also play fundamental roles. Caring for your gut microbiota through healthy eating and appropriate lifestyle habits can be a valuable strategy for maintaining a healthy weight.
Scientific references
A selection of recent evidence supporting the role of the microbiota across the different axes.
Gut-brain axis and autism
- • Cryan JF, et al. The Microbiota-Gut-Brain Axis. Physiological Reviews. 2019;99(4):1877-2013. journals.physiology.org/doi/full/10.1152/physrev.00018.2018
- • Kang DW, et al. Long-term benefit of Microbiota Transfer Therapy on autism symptoms and gut microbiota. Scientific Reports. 2019;9:5821. nature.com/articles/s41598-019-42183-0
- • Morton JT, et al. Multi-level analysis of the gut–brain axis shows autism spectrum disorder-associated molecular and microbial profiles. Nature Neuroscience. 2023;26:1208-1217. nature.com/articles/s41593-023-01361-0
- • Valles-Colomer M, et al. The neuroactive potential of the human gut microbiota in quality of life and depression. Nature Microbiology. 2019;4:623-632. nature.com/articles/s41564-018-0337-x
Gut-metabolic axis
- • Cani PD, et al. Microbial regulation of organismal energy homeostasis. Nature Metabolism. 2019;1:34-46. nature.com/articles/s42255-018-0017-4
- • Depommier C, et al. Supplementation with Akkermansia muciniphila in overweight and obese human volunteers. Nature Medicine. 2019;25:1096-1103. nature.com/articles/s41591-019-0495-2
- • Aron-Wisnewsky J, et al. Gut microbiota and human NAFLD. Nature Reviews Gastroenterology & Hepatology. 2020;17:279-297. nature.com/articles/s41575-020-0269-9
Gut-immune axis
- • Belkaid Y, Hand TW. Role of the Microbiota in Immunity and Inflammation. Cell. 2014;157(1):121-141. cell.com/cell/fulltext/S0092-8674(14)00345-6
- • Lloyd-Price J, et al. Multi-omics of the gut microbial ecosystem in inflammatory bowel diseases. Nature. 2019;569:655-662. nature.com/articles/s41586-019-1237-9
- • Chen J, et al. Multiple sclerosis patients have a distinct gut microbiota compared to healthy controls. Scientific Reports. 2016;6:28484. nature.com/articles/srep28484
Gut-skin axis
- • Salem I, et al. The Gut Microbiome as a Major Regulator of the Gut-Skin Axis. Frontiers in Microbiology. 2018;9:1459. frontiersin.org/articles/10.3389/fmicb.2018.01459/full
- • Mahmud MR, et al. Impact of gut microbiome on skin health: gut-skin axis observed through the lens of therapeutics and skin diseases. Gut Microbes. 2022;14(1):2096995. tandfonline.com/doi/full/10.1080/19490976.2022.2096995
- • De Pessemier B, et al. Gut-Skin Axis: Current Knowledge of the Interrelationship between Microbial Dysbiosis and Skin Conditions. Microorganisms. 2021;9(2):353. mdpi.com/2076-2607/9/2/353
This selection is not exhaustive. Evidence on the microbiota is advancing rapidly; for clinical questions, consult a health professional.