How GLP-1 Medications Can Affect Your Digestion

GLP-1 medications such as semaglutide, liraglutide and tirzepatide are increasingly being used for weight management and type 2 diabetes. They can be highly effective, but digestive side effects are common. Nausea, constipation, diarrhoea, bloating, reflux and feeling unusually full are all symptoms that some people experience when taking GLP-1 medications.

What do GLP-1 medications do?

GLP-1 (glucagon-like peptide-1) is a hormone naturally produced in the gut after eating. It helps regulate blood glucose, appetite and digestion.

GLP-1 medications mimic some of these effects by:

• Increasing feelings of fullness

• Reducing appetite

• Slowing the rate at which food leaves the stomach

• Influencing gastrointestinal motility

• Helping regulate blood glucose

These effects can be beneficial for weight management, but they can also change how your digestive system feels and functions.

Why can GLP-1 medications cause nausea and bloating?

One of the key effects of GLP-1 medications is slower gastric emptying. This means food can remain in the stomach for longer.

This contributes to feeling fuller for longer, but it may also cause:

• Nausea

• Bloating

• Belching

• Indigestion

• Feeling uncomfortably full after eating

• Reflux symptoms in some people

Research confirms that GLP-1 receptor agonists can delay gastric emptying, although the effect varies between individuals and medications. Gastrointestinal symptoms are particularly common when starting treatment or increasing the dose. For example, in the STEP 1 trial of semaglutide, gastrointestinal side effects were reported by 74.2% of participants receiving semaglutide compared with 47.9% receiving placebo. Nausea, diarrhoea, vomiting and constipation were among the most commonly reported symptoms.

Constipation and diarrhoea

It may seem surprising that GLP-1 medications can contribute to both constipation and diarrhoea, but they affect gastrointestinal motility in complex ways. Slower movement through the digestive tract may contribute to constipation. Eating considerably less can also mean you're consuming less fibre, fluid and overall food volume, which may further affect bowel regularity. On the other hand, diarrhoea is also a recognised side effect. In the STEP 1 trial, diarrhoea occurred in 31.5% of people taking semaglutide compared with 15.9% receiving placebo. If bowel changes are persistent or severe, they shouldn't simply be dismissed as a normal side effect and should be discussed with your healthcare professional.

You may need to rethink portion sizes

Because GLP-1 medications increase satiety and slow gastric emptying, large meals may become much harder to tolerate.

Some people find that smaller meals eaten slowly are more comfortable.

This is particularly important because a reduced appetite can make it difficult to consume enough:

• Protein

• Fibre

• Vitamins and minerals

• Essential fats

• Fluids

Eating less doesn't automatically mean you're meeting your nutritional needs.

How can nutrition help?

If you're experiencing digestive symptoms while taking a GLP-1 medication, nutrition can help you adapt to these changes.

Prioritise protein

When appetite is reduced, make the food you do eat count. Include a source of protein at meals, such as eggs, fish, poultry, tofu, yoghurt, beans or lentils where tolerated.

Increase fibre gradually

Fibre is important for bowel health, but suddenly increasing fibre can make bloating worse. Gradually increase fibre according to your individual tolerance and make sure you're drinking enough fluid.

Eat slowly

With increased feelings of fullness, eating quickly can make it easier to eat beyond your comfortable level. Slow down and pay attention to your body's hunger and fullness signals.

Consider meal size and timing

Large meals may be uncomfortable when gastric emptying is slower. Smaller meals may be easier to tolerate, particularly if you're experiencing nausea, reflux or prolonged fullness.

Stay hydrated

Adequate fluid intake is particularly important if you're experiencing constipation, vomiting or diarrhoea.

What about the gut microbiome?

There is growing interest in how GLP-1 medications may affect the gut microbiome. However, research is still developing and we don't currently have enough evidence to say that GLP-1 medications consistently cause an unhealthy gut microbiome. Changes in food intake, body weight, gut motility and diet may all influence the microbiome, making this an area that requires further research.

The bottom line

GLP-1 medications can have a significant effect on digestion. For many people, symptoms such as nausea, constipation, diarrhoea and bloating are mild and improve as the body adapts, particularly after dose increases. However, persistent digestive symptoms can make it difficult to eat enough and maintain a balanced diet. A personalised approach can help you work with these changes rather than simply cutting out large numbers of foods or following a generic "GLP-1 diet". If you're taking a GLP-1 medication and struggling with digestive symptoms, reduced appetite or meeting your nutritional needs, nutritional support can help you develop an eating pattern that supports your digestion and overall health. Always speak to your prescribing healthcare professional about significant or persistent symptoms, and never stop or change your medication without medical advice.

References

1. Jalleh RJ et al. Gastrointestinal effects of GLP-1 receptor agonists: mechanisms, management, and future directions. The Lancet Gastroenterology & Hepatology. 2024.

2. Hiramoto B et al. Quantified Metrics of Gastric Emptying Delay by GLP-1 Agonists: A Systematic Review and Meta-Analysis. American Journal of Gastroenterology. 2024.

3. Wilding JPH et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. New England Journal of Medicine. 2021.

4. He L et al. Association of GLP-1 Receptor Agonist Use With Risk of Gallbladder and Biliary Diseases. JAMA Internal Medicine. 2022.