How many times have you heard that fibre in the diet will keep you ‘regular’ and help avoid constipation? We’ve been told that so often it must be true, right? But is there any scientific evidence to back it up? Prof Grant Schofield and research writer George Henderson take a good look at fibre and the theories around it.


By Prof Grant Schofield and George Henderson


There are three broad theories about the value of fibre in the diet:

  • The constipation theory
  • The roughage theory
  • The biome theory


The constipation theory

This theory argues that if you have constipation, then if you were to eat bulkier food that transferred through the gut, that would help the stools – the poo – come out more easily. Imagine a blocked up freeway and to clear it we’re putting bigger cars and trucks on it. Is that going to help?


[…] 100 percent of people improved on the no-fibre diet and 100 percent of people got worse on the high-fibre diet.


We often hear about this idea that eating fibre from vegetables will keep you regular.

But how regular do you want to be? We don’t think there’s any science to back up any particular idea of regularity. You just don’t want to be so irregular that it causes you a problem.

There are some experimental evidence of people with severe constipation – they hadn’t been to the toilet in a week – who were treated with either a high-fibre, or a no-fibre diet. The results showed that 100 percent of people improved on the no-fibre diet and 100 percent of people got worse on the high-fibre diet. So why are we persisting with recommending a high-fibre diet?



If you’re constipated and haven’t gone to the toilet for seven days, then taking fibre, experimentally, will make that worse. However, it does depend on the type of fibre, type of constipation and what caused it.

We’d like to mention that there is some evidence that viscous fibres, which are things like psyllium and basil seed, can help with constipation. Also, if you have diarrhoea, these can help make things a bit more solid, so sort of a two-way benefit.

Another line of evidence is, does bowel motion frequency correlate with the risk of colorectal cancer? There’s no strong association between the two, apart from severe constipation of the sort that’ll see you hospitalised. There’s no correlation between how often you go to the loo and your risk of colorectal cancer.


The roughage theory

The roughage theory is the idea that damaging something repeatedly is going to improve its function. Imagine some sort of scrubbing brush going through the colon, with the indigestible fibre somehow cleaning it out. As far as we know, there is no evidence to support this theory.

For muscle this theory works, because that’s how muscles naturally grow. It’s the natural history of muscle but it’s not so much the natural history of the gut.

There is research that says this approach could contribute to cancer, including Prof Potter’s research (Massey University). 


The biome theory

It is a fact that microbes in the gut are able to ferment fibres from plants into short chain fatty acids, butyrate, and the like. Depending on the microbe – if you have the right microbe and the right type of fibre – you may get a bit of butyrate being formed that will be similar to the beta-hydroxybutyrate, the ketone, and stimulate AMPK pathways. That’ll help you burn a little bit of fat, and it mimics fasting. But there’s also a lot of butyrate in butter and some organ meats as well. It’s not exclusively a fibre thing.


There is plausible evidence that fibre might be useful for some things but certainly not constipation.


We have nothing against this theory, except to say that you don’t know that you have the right bacteria. You could have bacteria that do something quite different and make you feel sick and make you ill, potentially. A number of things have to line up. In the modern environment, we have exposure to antibiotics and to different types of foods that could kill the required bacteria.

So, there is plausible evidence that fibre might be useful for some things but certainly not constipation. Fibre can be helpful via your gut microbes, but if this isn’t what is happening down there, if you’re eating the usual fibrous plant foods and still suffering constipation, then restricting fibre has science on its side.



Reynolds et al, Carbohydrate quality and human health: a series of systematic reviews and meta-analyses, The Lancet, 2019

Hartley et al, Dietary fibre for the primary prevention of cardiovascular disease, Cochrane Library, 2016

M.L.Burr et al, Effects of changes in fat, fish, and fibre intakes on death and myocardial reinfarction: diet and reinfarction trial (DART)The Lancet (not publicly available)


Learn more

  • Video: Dr Zoë Harcombe, What about fibre?

Prof Grant Schofield is the lead instructor in the Certificate of Advanced Nutrition. Take your nutrition knowledge to the next level. Learn more here.