GLP-1 medication and fibre: smaller meals, same need

Why fibre matters when you're eating less

Fibre is the part of plant food that your body can't digest on its own. Rather than being absorbed in the small intestine, it travels on to your large intestine, where trillions of gut bacteria are waiting for it. They ferment it, and in doing so produce short-chain fatty acids such as butyrate, the main source of energy for the cells lining your colon. Fibre also adds bulk and holds on to water, which is part of how it keeps digestion moving.

When your appetite shrinks, there is simply less food each day to deliver everything your body needs, and fibre is one of the easiest things to lose along the way. That's where nutrient density comes in: making each mouthful count by building smaller meals around vegetables, pulses, whole grains, fruit, nuts and seeds, alongside a good source of protein.

What the research shows

Most of us were short on fibre long before any change in appetite. The UK recommends that adults eat 30g of fibre a day^[2], yet the latest National Diet and Nutrition Survey puts the average adult intake at around 16g, with only 4% of adults reaching the full 30g^[1].

Eating less can widen that gap. One study of 69 adults taking GLP-1 medication in the US found they were eating an average of 14.5g of fibre a day, less than half the UK recommendation^[3]. The same study found that fewer than half of participants were eating enough protein for their body weight, and that intakes of calcium, iron, magnesium and vitamin D were also below recommended levels. Because it looked at people's diets at a single point in time, the study can't tell us whether the medication caused these shortfalls. But it fits with what dietitians are seeing in practice.

The British Dietetic Association has called for people using these medicines to be supported to get enough protein, fibre, fluids, vitamins and minerals, and to keep eating and drinking regularly even when portions are smaller^[4]. The British Nutrition Foundation makes the same point: most adults already fall short of 30g a day, and eating less may widen that gap^[5].

Adding fibre without upsetting your stomach

GLP-1 medicines work partly by reducing appetite and slowing how quickly the stomach empties. Side effects such as nausea, bloating, constipation and diarrhoea are common, particularly in the first weeks of treatment and after a dose increase^[6]. Because your gut bacteria produce gas as they ferment fibre, a sudden jump in how much you eat can add to that feeling of fullness and wind. Expert guidance on nutrition during GLP-1 treatment recommends increasing fibre gradually, alongside plenty of fluids^[7]. In practice, that means:

• Going slowly. Add fibre a little at a time over a couple of weeks, so your gut bacteria have time to adjust, rather than making a big change all at once.
• Drinking enough. Fibre works by holding on to water, so it needs fluid to do its job^[8]. Drink regularly through the day, and stir any fibre powder into a full glass of water or another drink.
• Starting with food. We always recommend whole foods come first. Pulses, oats, vegetables, fruit, nuts and seeds bring fibre along with the vitamins, minerals and protein that smaller meals still need to provide. A fibre powder can then help top up what you're already eating.

If you take any medication or have a medical condition, speak to your GP, prescriber or pharmacist before adding a new supplement to your routine.