Fibre, the ageing gut, and why constipation is not inevitable

Fibre intake in most Western countries sits around 15 to 20 grams a day against recommendations of 25 to 30. That gap is not new and not specific to older adults. What changes with age is how much the shortfall costs you.

Transit time slows. Physical activity, which stimulates colonic motility, tends to fall. Thirst perception blunts, so fluid intake drops. Dentition problems push people away from raw vegetables, nuts, wholegrains and anything requiring chewing. And the medication list grows, bringing with it a long roster of drugs that slow the gut.

The two kinds of fibre, and why the distinction matters practically

Soluble fibre dissolves into a gel — oats, barley, psyllium, apples, citrus, beans. It slows gastric emptying, moderates the glucose response to a meal, binds bile acids and modestly lowers LDL cholesterol. It softens stool by holding water.

Insoluble fibre does not dissolve — wheat bran, wholegrains, vegetable skins, nuts. It adds bulk and speeds transit.

Most foods contain both, and the practical relevance is mostly this: in someone with a slow, hard, difficult stool, adding a large amount of insoluble bran can make things worse rather than better, producing bloating and discomfort without resolving anything. Soluble fibre, particularly psyllium, has the better trial record for chronic constipation and is the more sensible first move.

The medication problem

Before rearranging anyone's diet, it is worth looking at what they swallow. Constipation is a side effect of a remarkable number of commonly prescribed drugs in older adults.

• Opioids, including low-dose codeine in combination painkillers, which many people do not think of as opioids at all.
• Anticholinergics — older antihistamines, tricyclic antidepressants, some bladder antimuscarinics.
• Calcium channel blockers, particularly verapamil.
• Iron supplements, reliably.
• Calcium supplements, and some antacids.
• Diuretics, indirectly, through fluid loss.

Fixing a drug-induced problem with dietary fibre is possible but inefficient, and in opioid-induced constipation specifically, fibre is largely ineffective and can worsen symptoms — that situation needs an osmotic laxative and, sometimes, a targeted agent.

The microbiome part, kept honest

The gut microbiota does change with age — generally toward reduced diversity, fewer of the short-chain-fatty-acid producers like Faecalibacterium prausnitzii, and greater instability. The ELDERMET study found that in older adults in long-term residential care, this shift tracked closely with dietary diversity and with frailty measures.

That is a genuinely interesting finding and it is frequently over-extrapolated. What we can say with reasonable confidence: dietary fibre is the principal substrate for the colonic bacteria that produce butyrate, which is the main energy source for colonocytes and has anti-inflammatory effects locally. A varied, plant-heavy diet supports a more diverse microbiota.

What we cannot yet say: that any particular probiotic product will meaningfully alter an older person's microbiome, or that doing so would improve a specific health outcome. The probiotic evidence is strongest for narrow indications — antibiotic-associated diarrhoea, some IBS symptoms — and weak to absent for the general claims on the packaging.

Getting to 30 grams without misery

The arithmetic is more achievable than it sounds once you see where fibre actually concentrates.

• A cup of cooked lentils — about 15 g. Beans and pulses are far and away the most efficient source, and the most neglected.
• A pear or apple with skin — 4 to 5 g.
• Two slices of proper wholemeal bread — 5 to 6 g.
• A bowl of porridge — 4 g.
• A tablespoon of ground flaxseed — 3 g, and easy to add to anything.
• A handful of almonds — 3.5 g.
• A portion of broccoli or Brussels sprouts — 3 to 4 g.

Pulses once a day gets most people over the line on their own. If chewing is the limitation, cooked and blended vegetable soups, stewed fruit, ground seeds and well-cooked pulses deliver fibre without requiring good teeth.

When it is not just constipation

Some things should not be managed with diet. A change in bowel habit that persists for more than a few weeks in someone over fifty warrants investigation, as does any rectal bleeding, unexplained weight loss, iron-deficiency anaemia, or a family history of bowel cancer. New constipation with abdominal pain and vomiting is an obstruction until proven otherwise and needs urgent assessment.

Fibre is a good answer to a common problem. It is a bad answer to an uncommon one, and telling them apart is what the appointment is for.