Can one test answer both questions?

The same breath-test technology is used for both, with different sugars. A lactulose or glucose challenge is used to look for bacterial overgrowth; a lactose or fructose challenge is used to look for malabsorption of that specific sugar. Same machine, different question.

Can bacterial overgrowth cause food intolerance?

The relationship is not established in that direction. Both involve fermentation of carbohydrate that reaches bacteria, so symptoms can look identical without one producing the other. The honest description is overlapping mechanism rather than cause.

Why does a low fermentable carbohydrate diet help both?

Because it removes the substrate in both cases. Improvement tells you fermentation was involved. It does not tell you whether the fermentation happened because there were too many bacteria in the wrong place or because a sugar was not absorbed.

Which should I investigate first?

That is a clinical decision. What is worth knowing before you pay for anything is which sugar a given test uses and therefore which question it can answer, because that is a factual matter about the test rather than about your body.

Is it SIBO or a food intolerance?

By B. Gray Randolph · Updated August 5, 2026

These two produce nearly identical experiences and are investigated with the same machine, which is a coincidence worth understanding.

What actually separates them

Bacterial overgrowth is about where bacteria are: too many in the small intestine, fermenting carbohydrate before it should be fermented. A food intolerance is about what your body can process: a sugar not absorbed, or an enzyme missing. Both end in fermentation and gas, which is why they feel the same. The difference is whether the problem is the bacteria's location or the food's handling.

The same machine answers both questions

Breath testing is used for both, with different sugars. A lactulose or glucose challenge looks for bacterial overgrowth. A lactose or fructose challenge looks for malabsorption of that specific sugar. Which question a test can answer is decided entirely by which sugar you are given, and that is worth confirming before paying.

This is the most practically useful thing on the page, and it is a fact about equipment rather than about you, which means it is answerable today.

Why timing is a weaker clue than you have been told

Both can produce symptoms within a few hours of eating, and transit speed varies enough between people that timing alone does not separate them. Record it, because a pattern over weeks is worth more than any single episode, but do not expect it to decide the question.

What is not settled here

Whether overgrowth produces food intolerances, or a food intolerance creates conditions for overgrowth, is not established in either direction. Both involve carbohydrate reaching bacteria, so identical symptoms are unsurprising without any causal relationship at all. Confident accounts of the ordering are available online and are usually attached to a protocol.

What the evidence base looks like

Pimentel M et al., American Journal of Gastroenterology 2020 Clinical guideline

No single symptom can be specifically attributed to overgrowth, and five of six guideline recommendations rest on evidence its own authors grade as very low quality.

Scalese G et al., Journal of Clinical Medicine 2025 Peer-reviewed studyWe have not yet confirmed a direct link to this document, so we are not giving you one. The citation above is enough to find it, and we would rather say this than send you to a journal front page.

And the two common overgrowth tests agreed with each other on 8.7% of cases. Before reasoning from a result, it is worth knowing how much the results disagree.

What to do next

Find out which sugar the test you are offered uses. That decides what it can tell you, and it is the question most likely to save you from paying for an answer to a question you were not asking.

Common questions

Can one test answer both questions?: The same breath-test technology is used for both, with different sugars. A lactulose or glucose challenge is used to look for bacterial overgrowth; a lactose or fructose challenge is used to look for malabsorption of that specific sugar. Same machine, different question.
Does timing after eating distinguish them?: Less than commonly claimed. Both can produce symptoms within a few hours of eating, and transit speed varies enough between people that timing alone is unreliable. It is worth recording, but it is not the discriminator it is often presented as.
Can bacterial overgrowth cause food intolerance?: The relationship is not established in that direction. Both involve fermentation of carbohydrate that reaches bacteria, so symptoms can look identical without one producing the other. The honest description is overlapping mechanism rather than cause.
Why does a low fermentable carbohydrate diet help both?: Because it removes the substrate in both cases. Improvement tells you fermentation was involved. It does not tell you whether the fermentation happened because there were too many bacteria in the wrong place or because a sugar was not absorbed.
Which should I investigate first?: That is a clinical decision. What is worth knowing before you pay for anything is which sugar a given test uses and therefore which question it can answer, because that is a factual matter about the test rather than about your body.