The weight-loss jab. I’m sure you’ve all heard of it. Ozempic, Wegovy, Mounjaro… GLP-1 medications have become one of the biggest talking points in weight management. And quite rightly, because they do work. For a lot of people, they’ve been life changing.

However, what’s not talked about as much is the effect GLP-1 medications are having on gut health. Nausea is a common side effect, but the change in eating behaviours experienced by people taking GLP-1s also causes a steadier change to the gut microbiome.

What are GLP-1s?

GLP-1 (glucagon-like peptide-1) is a hormone produced naturally by the gut. Specialised ‘L-cells’ in the small intestine release it in response to eating, usually within 15–30 minutes of a meal entering the gut. It’s part of a group of hormones known as “incretins,” which help regulate blood sugar and appetite in response to food.

The roles of GLP-1:

Stimulate insulin release from the pancreas when blood sugar is elevated. This is why it’s such a useful target for managing type 2 diabetes as well as body weight.

Suppress glucagon, the hormone that tells your liver to release stored sugar, helping to keep blood sugar more stable after meals.

Slows gastric emptying so food stays in your stomach for longer, keeping you feel fuller.

Signals to the brain that you’re full, reducing appetite and food intake.

Medications like semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro) are GLP-1 receptor agonists - they’re designed to mimic and amplify the natural hormone, binding to the same receptors and producing a stronger and longer-lasting effect than the body’s own GLP-1 would. Tirzepatide goes a step further targeting a second gut hormone receptor (GIP), which one of the reasons why some of the newer medications produce more pronounced results.

The main effect, and the reason behind their success is appetite suppression. Taking them will make you feel fuller quicker, and for longer, which naturally leads to eating less. They make it easier to create and stick to a calorie deficit.

What’s less talked about is the indirect impact these medications have on the gut.

Less food often means less fibre.

The gut microbiome is dependent on what you eat. It’s particularly reliant on fibre from plant foods such as fruit, vegetables, wholegrains, legumes, nuts and seeds. Fibre is essentially the fuel your gut bacteria ferments to produce short-chain fatty acids (SCFAs), which support so many functions and process in your body such as the integrity of the gut lining, immune function, and metabolic health.

Here’s the problem - even in the absence of GLP-1 medications, only around 4% of people in the UK meet the recommended target of 30g fibre a day. Most of us are already under-fuelling our gut bacteria.

Now add a medication that significantly suppresses appetite. When you’re eating less overall, fibre intake is often one of the first things to drop. This is partly due to appetite suppression which tends to blunt interest in bulkier foods, and partly because, when volume is limited, energy-dense, low-effort foods are often picked first.

Over time, chronically low fibre intake can affect:

• Microbiome diversity.

• SCFA production.

• Digestive regularity.

• Overall gut health.

This isn’t me saying “you shouldn’t use GLP-1s” - it’s me saying “you should be conscious and deliberate about what you eat while taking them.”

Why do GLP-1s cause digestive side effects?

Beyond the indirect and longer-term impact of low fibre intakes on digestion, GLP-1s also have a direct mechanical effect on digestion. They slow gastric emptying, meaning food sits in the stomach for longer than usual.

This is one of the reasons you feel full, but it’s also why bloating, constipation, diarrhoea and nausea are common side effects. Your gut must work differently while you’re on the medication.

Diet quality matters, especially in the absence of quantity.

GLP-1s are a genuinely useful tool for weight loss. But if gut health isn’t factored in, you could end up losing weight while quietly under-supporting your microbiome. As a consequence, you’ll be dealing with digestive symptoms that could, at least partially, be eased through food choices.

Weight loss requires a calorie deficit – but it’s important to look beyond just ‘eating less’; what you eat needs to count! You still need dietary inclusivity to ensure you’re getting all the nutrients your body needs to be healthy.

With a supressed appetite, there’s very little room for empty calories.

• Prioritise fibre wherever you can - small, consistent amounts add up when your overall intake is lower.

• Aim for plant diversity, not just plant quantity. Different fibres feed different bacterial species.

• Choose minimally processed foods where possible, since these tend to retain more of their natural fibre and micronutrient content.

These choices won’t eliminate GLP-1 side effects entirely - some come from the mechanism of the drug itself - but they can meaningfully support gut health and reduce the risk of the microbiome declining with the number on the scale.

A side note on food and GLP-1…

There are certain foods that can stimulate your body’s own GLP-1 release, even after a meal has ended. Things like fermentable fibres, unsaturated fats, and protein-rich foods all play a role in this (PMID 25500202). This is a great example of how food and medications don’t operate silos - what you eat is already signalling to the same hormonal pathway that these drugs target.

If you’re on a GLP-1 medication and want support building a way of eating that protects your gut health alongside your weight loss goals, get in touch - this is exactly the kind of thing I work through with my one-to-one coaching clients.