Two people get their gut test back with the same reassuring ratio. One has a healthy microbiome. The other has almost nothing living in there. What the Firmicutes to Bacteroidetes ratio is actually telling you — and what it quietly discards.
Two people get their gut test back. Both have a Firmicutes to Bacteroidetes ratio inside the stated reference range.
One has a gut with reasonable bacterial numbers and a sensible balance between the two dominant families.
The other has almost nothing living in there at all.
Same reassuring number. Completely different situations. And if you only read the ratio, you would miss it entirely.
A ratio is a division sum. It tells you the relationship between two numbers and deliberately discards the numbers themselves.
Which is fine when both are in a sensible range. It stops being fine the moment one of them is not.
Think of a football score. Two-one and twenty-two to eleven are both a 2:1 ratio. One is a football match and the other is something that has gone badly wrong with the rules.
The F:B ratio behaves exactly like that. It can look normal because both phyla are healthy. It can also look normal because both have collapsed together, or shifted because one is raised rather than the other being adequate. The division sum cannot tell you which.
So the ratio is a supporting observation. The absolute counts are the finding.
Firmicutes and Bacteroidetes are the two dominant bacterial phyla in the human gut — between them, usually the large majority of what is in there.
Firmicutes include most of the important butyrate producers. Butyrate is the short-chain fatty acid that the cells lining your colon use as their primary fuel — not glucose, unusually, but a by-product of bacterial fermentation. Faecalibacterium prausnitzii and Roseburia both sit here, and both appear individually on a GI-MAP.
Bacteroidetes are versatile carbohydrate degraders. They break down complex plant polysaccharides that human enzymes cannot touch, and they are central to how much you actually extract from dietary fibre.
You need both. The interesting question is never which one wins.
Ratio 0.31. Comfortably inside range. And underneath it, both Firmicutes and Bacteroidetes flagged low, with Roseburia below range and Akkermansia present only at the very bottom of its range.
This is a gut with nothing wrong in it and not much in it. No pathogens, no inflammation, no yeast. Just low overall bacterial biomass and reduced diversity — a garden with nothing poisonous growing and fewer species than it should support.
The ratio said fine. The counts said depleted.
Ratio 0.05, sitting inside a reference stated simply as “less than 1.0”. Underneath: Bacteroidetes raised well above range, both keystone species undetectable, and a substantial post-antibiotic overgrowth of opportunists.
The ratio was low because one phylum was elevated, not because the other was healthy. Read alone, it looked acceptable. Read against the counts, it described a gut stripped by repeated antibiotic exposure.
In both cases the ratio was not wrong. It was answering a different question from the one that mattered.
Bacterial biomass is not fixed. It responds, and several common things reduce it.
Antibiotics, and repeated courses compound. Recovery after a single course takes months rather than weeks, and some species do not return at all without deliberate rebuilding.
Dietary restriction, in every form. Low-carbohydrate diets, elimination diets, restrictive eating for any reason. These bacteria eat plant fibre. Remove the fibre and you remove the food supply — which is why elimination diets that resolve symptoms in month one can produce a worse gut by month six.
High training load without adequate intake. Under-recognised. Sustained heavy training with insufficient energy availability suppresses the microbiome, and it is part of the picture in relative energy deficiency in sport. Young female athletes are the classic presentation, but it applies to anyone training hard and eating too little.
Infection and travel. A gastrointestinal infection does not simply clear — it can leave the community altered long afterwards.
Chronic stress. Sustained HPA activation changes gut motility, secretions and barrier function, and the bacterial population changes with them.
Post-infectious and post-antibiotic recovery is almost never addressed. Nobody’s follow-up appointment after a course of antibiotics involves rebuilding what was cleared. The gut is assumed to sort itself out. Often it does. Sometimes it does not, and nobody checks.
If your colonic cells are short of butyrate, the effects do not stay in your gut. Butyrate fuels the cells maintaining the intestinal barrier and has substantial anti-inflammatory activity. Reduced production is associated with barrier compromise and with a low-grade inflammatory state.
Most of your immune system sits around your gut, and it is trained by the microbial diversity it encounters. Less diversity means less education. A large share of the body’s serotonin is produced in the gut, and bacterial populations also influence bile acid metabolism, which affects oestrogen recirculation.
None of which means a depleted microbiome causes any specific condition. The literature is a good deal more preliminary than the internet suggests. But it does mean this is not a bowel-habit issue, and treating it as one underestimates it.
Plant diversity is the strongest single lever. The target from the large microbiome cohort work is thirty different plant species a week — not portions, species. Vegetables, fruit, whole grains, nuts, seeds, pulses, herbs and spices all count. A tablespoon of mixed seeds is three or four.
Resistant starch specifically feeds butyrate producers. Cooked and cooled potato or rice, oats, slightly green bananas, beans and lentils. The cooking-and-cooling thing is real — it changes the starch structure so more reaches the lower gut instead of being absorbed higher up.
Then the specific fibres, added slowly: inulin from chicory, onion and Jerusalem artichoke; arabinoxylan from whole grains; partially hydrolysed guar gum where tolerance is poor.
Go slowly. A depleted gut fed aggressively produces bloating and wind, people conclude fibre does not agree with them, and they retreat to the restricted diet that caused the problem. One addition at a time, several days apart, is faster in the end.
And probiotics are not the main answer here. They are transient, strain-specific, and generally do not colonise. Useful for particular conditions. Not a substitute for feeding what should already be there.
Someone with a depleted microbiome may have entirely normal bowel habits — that was true of both examples above. There is no way to infer this from how you feel. And these changes take months, so symptom improvement is a poor proxy for whether the keystone species have returned.
This is not really about bacteria.
Ratios are everywhere in testing, and they are genuinely useful — calcium to magnesium, sodium to potassium, zinc to copper, albumin to globulin. Each tells you something a single value cannot.
But every ratio discards information by design. Two numbers go in and one comes out, and what is lost is the scale.
Read the ratio. Then read what it was calculated from. If someone shows you only the ratio, that is not a reassurance — it is an unanswered question.
A GI-MAP reports both the ratio and the absolute counts. If you have had one done and only the ratio was discussed, the numbers underneath are still on the report.
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