What happens to your gut when fibre goes to zero

Fibre is the part of plant food that survives the small bowel. Take plants out of the diet and the intake goes to approximately zero. Something changes in the gut when that happens, and it is worth knowing what before you are three weeks in and wondering why nothing is moving.

The UK target and how far most people miss it

The recommendation for adults in the UK is 30g of fibre a day, set by the Scientific Advisory Committee on Nutrition in 2015. The National Diet and Nutrition Survey has consistently found adult intake well below that, in the region of 18 to 20g, so most people arriving at a meat-only diet are already below the line rather than crossing it.

That does not make 30g a magic number. It is a population target built from evidence on bowel habit, lipid markers and long-term cohort outcomes, and it is a long way from a measured requirement for an individual.

What fibre was doing

Three jobs, roughly.

It holds water and adds bulk, which gives the colon something to move and keeps transit regular.

It is fermented by bacteria in the large bowel into short chain fatty acids, mainly acetate, propionate and butyrate. Butyrate is the preferred fuel of the cells lining the colon, which is why the fermentation story gets so much attention.

It binds bile acids and, in some forms, slows the absorption of glucose. Viscous fibres such as beta glucan from oats and psyllium lower LDL cholesterol modestly, which is the basis for several authorised claims in the health claims register.

What happens when the intake is zero

Stool volume falls first. Stool is mostly water, bacteria and undigested residue, and the residue disappears when the plants do. Stools get smaller and firmer within about two weeks, and often score 1 or 2 on the Bristol scale.

Transit becomes variable. Some people pass a stool every day; some go every second or third day. Both can be entirely normal for that person, provided there is no pain, no straining, and no distension. Frequency alone is a poor measure of whether something is wrong.

Constipation is the real risk, and it is more likely if you are also drinking less than you did, which many people are, because carbohydrate holds water and plant food carries a lot of it.

What the colon ferments instead

When the fibre goes, protein and amino acids reach the colon in greater quantities and get fermented. The products are different: branched chain fatty acids, ammonia, amines, phenols and sulphides rather than the short chain fatty acids that dominated before.

If you read that and feel alarmed, keep the size of the evidence in view. The shift in fermentation products is measurable and reasonably well described. What it means for someone's health over ten or twenty years has not been measured at all, because nobody has followed a large group of people eating this way for that long. That is a gap in the evidence, not a finding of harm, and it should be stated as a gap.

Managing it without guessing

Water first, and more than feels natural: 2 to 2.5 litres a day in a temperate climate, more in summer or if you are training.

Salt matters here too, because the kidney sheds more sodium on a low carbohydrate intake, and low sodium with low water is a reliable route to hard stools.

Fat at meals helps. The gastrocolic reflex is triggered by food arriving in the stomach, and a meal with a decent amount of fat produces a stronger signal than a dry one.

Movement after eating, ten minutes of walking, does more for transit than most supplements.

Magnesium is the thing people reach for, and it works for some. A pharmacist can advise on a form and dose; too much produces loose stools, which tells you the ceiling quickly.

When to stop managing it yourself

Constipation that lasts beyond about three weeks, or comes with abdominal pain, vomiting, blood in the stool, or weight you did not mean to lose, is a GP appointment.

The argument that you do not need fibre

You will read that fibre is unnecessary, that traditional peoples ate none, and that the 30g figure was invented. The strongest version of the argument is that the evidence base for the target is largely observational and was generated in populations eating plant foods, and that an individual can be perfectly regular without it.

The weakest version is the claim that it makes no difference at all. Zero fibre populations have not been studied long term, and the case reports of people who did fine are not data.

My own position after two years is unglamorous: fibre is not optional for everyone, it is optional for some people, and there is currently no way to know in advance which group you are in. So decide in advance what you will do if week three brings constipation, and write it down, because the decision is easier to make now than it is at 11pm.