Firmicutes, Bacteroidetes and body weight

Does your gut microbiota make you fat? What is behind the much-quoted bacterial ratio.

How solid is the research? Plausible

The mouse experiments are clear, the human studies are not: a link between the bacterial ratio and obesity could not be confirmed in a meta-analysis.

Healthy state

Most bacteria in the gut belong to two large phyla, the Firmicutes and the Bacteroidetes. They have since been officially renamed Bacillota and Bacteroidota, but studies and test reports still mostly use the old names. A phylum is a very coarse level of classification in biology, roughly as coarse as “mammals” versus “birds”. There are also smaller groups such as the Actinobacteria (which include the bifidobacteria), the Proteobacteria (which include E. coli and many enterobacteria) and the Verrucomicrobia (with Akkermansia A gut bacterium that lives on the mucus of the gut wall and stimulates its renewal. In studies, high levels often go along with good metabolic health. More in the chapter “The mucus layer and additives”).

The two main groups have different specialities. Many Bacteroidetes carry a large toolbox of enzymes for breaking down complex plant fibres, pectins and even the sugar chains in the mucus of the gut wall. This mainly produces acetate and propionate. The Firmicutes include many Butyrate A short-chain fatty acid and the main energy source for the cells of the colon wall. It strengthens the barrier and dampens inflammation. More in the chapter “Butyrate and other short-chain fatty acids” producers such as Faecalibacterium and Roseburia, but also lactic acid bacteria, spore formers and many species you have never heard of. The two groups feed each other, as described in the chapter on Short-chain fatty acids Acids such as acetate, propionate and butyrate that gut bacteria make from fibre. They nourish the gut wall and act throughout the body. More in the chapter “Butyrate and other short-chain fatty acids”.

Both groups include hundreds of very different species. A bacterium from the Firmicutes can be beneficial, like Faecalibacterium, or a pathogen, like Clostridioides difficile A bacterium that can spread after antibiotics and cause severe diarrhoea with its toxins. More in the chapter “After antibiotics: C. difficile and faecal transplants” or Staphylococcus aureus. For that reason alone, belonging to a phylum says little about the effect of a species.

The ratio of the two phyla is also not a fixed quantity. In a study of infants, adults and older people it was about 0.4, about 11 and about 0.6. So it changes strongly over a lifetime, without any disease having to be behind it. Diet shifts the proportions within a few days, and the time gut contents spend in the colon influences who thrives there: with slow transit, more protein and less carbohydrate is fermented, and the composition changes.

One more technical point: stool tests and studies mostly measure proportions, not amounts. If the share of one group rises, the share of the other has to fall, even if nothing has changed for it in absolute numbers. And a stool sample mainly shows what lives in the lumen of the colon. What sits on the gut lining or in the small intestine, it reflects only roughly.

Firmicutes and Bacteroidetes are two large groups of bacteria, each with hundreds of species.

Disrupted state: causes and effects

The idea started with mouse experiments. In obese mice, researchers found more Firmicutes and fewer Bacteroidetes. The Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity” of these animals extracted more energy from food and, when transferred to germ-free mice, made them gain more weight. From this finding a rule of thumb for humans quickly emerged: many Firmicutes, few Bacteroidetes, more weight. But laboratory mice live under strictly controlled conditions with uniform food, and humans do not.

In humans the rule of thumb does not hold. A Meta-analysis An analysis that combines the results of several studies. It is only as good as the studies it includes. of ten studies found no link between the ratio and obesity, but a weak one with lower diversity. A review calls the ratio a questionable marker because measurement methods, diet, exercise, age and medication change the values strongly. In children, by contrast, some reviews report a link more often. That, too, is no proof of cause: children with overweight often eat differently from lean ones, and research cannot separate what came first.

That does not rule out that the gut microbiota can have something to do with metabolism and weight. There are plausible routes: bacteria release short-chain fatty acids from fibre, which supply a small part of daily energy, and these substances also act as signals for satiety hormones such as GLP-1 A hormone from the gut wall that controls insulin release after eating, slows the stomach and makes you feel full. Weight-loss injections mimic it. More in the chapter “GLP-1 from the gut: what bacteria do and what “natural Ozempic” cannot do”. How much all this matters in an individual is not settled, and it probably depends more on individual species and their abilities than on belonging to a phylum.

If a commercial stool test reports an “unfavourable Firmicutes/Bacteroidetes ratio”, this is therefore not a reliable statement about your weight or your health. In addition, tests from different providers can give different results for the same sample. And the same person can score differently on two days.

In humans, lower diversity is more consistently linked to obesity, although it is open what is cause and what is effect. The environment plays a part too: people who eat a lot of processed food and little fibre often have both a higher weight and a poorer gut microbiota, without one having to cause the other. If you want to lose weight, put your energy not into a ratio but into things whose benefit has been shown in people.

In mice there was a clear difference. In humans, large studies show no clear pattern.

What often helps

Lifestyle

Diet, exercise, sleep, social contact and dealing with stress.

Focus on diversity rather than ratios A varied, plant-rich diet promotes diversity. That makes more sense than chasing a particular ratio of two groups of bacteria. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [1]
Exercise In one study, regular endurance training shifted the gut microbiota towards more butyrate producers, more strongly in lean than in overweight people. That exercise itself helps with weight is well supported independently of this. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [2]
High-fibre wholegrain foods Meta-analyses of observational and intervention studies link higher fibre intake and more whole grains with lower body weight and fewer cardiovascular diseases. Part of this effect probably works through satiety and blood sugar, and part possibly through the gut microbiota. Well supported How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [3]

Natural supplements

Supplements do not replace a medical check-up. Discuss dosage and interactions with your doctor or pharmacist.

No products against a “wrong ratio” Probiotics or powders meant to improve the ratio have no proven effect on weight. Experimental How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.

Medical treatments

Information, not a treatment recommendation. Decide together with a doctor. Approval and availability differ from country to country.

Get medical support for weight For weight loss, dietary change, exercise and, if needed, medication are well supported. The gut microbiota is not yet a target with proven benefit. Also have causes such as the thyroid or medications that affect weight checked. Well supported How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.
Read stool tests with caution An analysis of commercial microbiome tests showed that measurement quality varies between providers. Do not bring such results to the appointment as a diagnosis but as a topic for discussion, and do not let decisions depend on them. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [4]

In the mood for something completely different? Another one, at random

How we rate the evidence

Trust score 5 of 5:Established: Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies.
Trust score 4 of 5:Well supported: Several randomised studies in humans with largely consistent results.
Trust score 3 of 5:Promising: Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies.
Trust score 2 of 5:Plausible: Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms.
Trust score 1 of 5:Experimental: Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.

Sources

Your progress

Ratings by readers

No averages yet. They appear once 3 readers have answered a question.

Is something wrong here?

If you think something is wrong, outdated or misleading, report it for review. Every report is read and answered.