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.

Can I have both at once?

Nothing prevents it, and the two would produce overlapping symptoms if you did. The awkward part is that one test cannot show you both, because the sugar you are given decides which question is being asked. Establishing both means two separate tests asking two separate questions, which is worth knowing before assuming one result covered the ground.

If a lactose test is positive, do I still need the other one?

That is a clinical decision, and a positive on one does not answer the other. A lactose result tells you about lactose. It does not tell you whether there are too many bacteria in the small intestine, which is a different question asked with a different sugar. People do carry both findings at the same time.

Is it SIBO or a food intolerance?

By BeatSIBO editorial team · Updated August 6, 2026

At a glance

The question
SIBO
Where bacteria are
Food intolerance
What your body can process
Mechanism
SIBO
Fermentation too high in the gut
Food intolerance
A sugar unabsorbed or an enzyme missing
Tested with
SIBO
Lactulose or glucose challenge
Food intolerance
Lactose or fructose challenge
Same machine
SIBO
Yes
Food intolerance
Yes. The sugar decides the question

SIBO compared with Food intolerance, on the dimensions that separate them
Dimension	SIBO	Food intolerance
The question	Where bacteria are	What your body can process
Mechanism	Fermentation too high in the gut	A sugar unabsorbed or an enzyme missing
Tested with	Lactulose or glucose challenge	Lactose or fructose challenge
Same machine	Yes	Yes. The sugar decides the question

The one that matters: Which question your breath test answered was decided entirely by which sugar you were given.

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

What actually separates them

The short answer

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

The short answer

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 study

And where patients took both overgrowth tests, under one in ten diagnoses were made by both. 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

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.
Can I have both at once?: Nothing prevents it, and the two would produce overlapping symptoms if you did. The awkward part is that one test cannot show you both, because the sugar you are given decides which question is being asked. Establishing both means two separate tests asking two separate questions, which is worth knowing before assuming one result covered the ground.
If a lactose test is positive, do I still need the other one?: That is a clinical decision, and a positive on one does not answer the other. A lactose result tells you about lactose. It does not tell you whether there are too many bacteria in the small intestine, which is a different question asked with a different sugar. People do carry both findings at the same time.