Why there's no official upper limit
NHMRC's Nutrient Reference Values document states plainly: "There is no UL set for dietary fibre." NHMRC reviewed the potential downsides of high fibre intake and concluded they weren't well-supported enough, or serious enough, to justify a formal ceiling.
What was actually considered
The main historical concern was that fibre might bind to minerals like iron, zinc, and calcium and reduce how much the body absorbs. NHMRC's review found this largely didn't hold up: it cites an earlier literature review that found no convincing evidence of any adverse effect on mineral absorption or nutrition in humans, even at intakes as high as 50g of dietary fibre a day.
NHMRC also noted a small number of other theoretical considerations at very high intakes: a higher incidental intake of naturally occurring plant compounds and environmental residues that concentrate in bran and skins, and a general note that a diet dominated by one narrow source of fibre carries more of these theoretical downsides than one drawing on a variety of sources. NHMRC's conclusion was that none of these are likely to cause problems at realistic intake levels, provided fibre comes from a mix of sources rather than one alone.
The evidence NHMRC reviewed doesn't point to a specific gram figure where fibre stops being fine. It points to variety of source mattering more than a hard ceiling on total amount.
No upper limit isn't the same as no discomfort
A missing formal ceiling is a regulatory judgement about long-term nutritional safety, not a guarantee of comfort at any intake level. Very high fibre intakes, particularly introduced quickly, commonly produce gas, bloating, and changes in bowel habits as the gut microbiome processes the extra substrate. That's a tolerance question, separate from the safety question NHMRC was assessing.
When to speak to your GP
If you're eating a very high or rapidly increasing amount of fibre and experience persistent or severe digestive symptoms, or any sudden and unexplained change in your usual bowel habits, that's worth raising with your GP. This is general information, not a diagnosis, and it doesn't replace a conversation with someone who can examine your specific situation.
Sources
Related reading
- Why does fibre make me bloated?
- How to increase fibre without bloating
- The Fibre Gap: How Far Most Australians Fall Short