Weight loss medications such as semaglutide and tirzepatide have transformed the treatment of obesity. They can reduce appetite, increase feelings of fullness and make it significantly easier for some people to eat less.
But if you are taking a GLP-1 based medication, you may also notice that your digestive system behaves differently.
Nausea, constipation, diarrhoea, bloating, reflux and burping are all commonly reported gastrointestinal side effects. For many people these symptoms are mild and improve as their body adjusts, particularly after the first few weeks at a new dose. For others, digestive symptoms can make eating well surprisingly difficult.
From a dietitian's perspective, the aim is not simply to "eat less" while taking weight loss medication. It is to find a way of eating that allows you to meet your nutritional needs while keeping digestive symptoms as manageable as possible.
Why do weight loss medications affect digestion?
GLP-1 based medications work in several ways. As well as influencing appetite signals in the brain, they affect the gastrointestinal system and can slow the rate at which food leaves the stomach.
This contributes to feeling fuller for longer, which can be helpful for weight management. However, it can also contribute to symptoms such as nausea, fullness, bloating, reflux and belching.
Gastrointestinal symptoms are particularly common when starting treatment or increasing the dose. They often improve with time, but this doesn't mean that significant symptoms should simply be ignored.
What and how you eat can make a considerable difference.
Nausea: when the thought of food is unappealing
Nausea is one of the most frequently reported side effects of GLP-1 based weight loss medication.
The temptation when you feel nauseous is to skip food altogether. Occasionally eating very little for a short period may be unavoidable, but repeatedly skipping meals can make it difficult to consume enough protein, vitamins and minerals.
Instead, think small, simple and frequent.
Try smaller meals rather than attempting to eat your previous portion sizes. Eat slowly and stop as soon as you start to feel comfortably full.
Cold foods can sometimes be easier to tolerate because they have less aroma. Foods such as yoghurt, fruit, overnight oats, a small sandwich or a nourishing smoothie may feel more manageable than a large cooked meal.
Very rich, fatty or heavily spiced meals may make nausea worse, particularly when gastric emptying is already slower.
Ginger tea or foods containing ginger may also be worth trying.
Most importantly, don't force yourself to finish a meal simply because that is the portion you would normally eat. Learning to recognise your new fullness signals is an important part of eating well on these medications.
Constipation: don't just add lots of fibre
Constipation can be a frustrating side effect.
There may be several contributing factors. You may be eating less food overall, drinking less because you don't feel as hungry or thirsty, and consuming less fibre. Changes in gut motility may also play a role.
Increasing fibre can help, but there is an important caveat: more fibre without enough fluid can make constipation worse.
Build fibre gradually and drink regularly throughout the day.
Useful high fibre foods include:
- oats
- fruit and vegetables
- beans, lentils and chickpeas
- wholegrain breads and cereals
- chia and flaxseeds
- nuts and seeds
Kiwifruit and flaxseeds can be particularly useful additions for some people with constipation.
If your appetite is very small, adding fibre strategically can be easier than simply trying to eat larger quantities of vegetables. Adding chia or ground flaxseed to yoghurt or porridge, for example, can increase fibre without dramatically increasing meal volume.
Movement matters too. Even a regular walk can support normal bowel function.
If constipation becomes persistent, painful or doesn't respond to dietary changes, speak to your healthcare professional. You don't need to simply put up with it because you are taking weight loss medication.
Diarrhoea: temporarily change your priorities
At the other end of the spectrum, some people can experience loose stools or diarrhoea.
When this happens, hydration becomes particularly important because diarrhoea can increase fluid and electrolyte losses.
For a short period, plain, lower-fat foods may be better tolerated. Rice, potatoes, toast, oats, bananas, yoghurt if tolerated, eggs, chicken or fish can all be useful options.
You may also need to temporarily reduce foods that can stimulate the bowel or worsen symptoms, including:
- very fatty meals
- large amounts of fibre
- alcohol
- caffeine
- very spicy foods
- large quantities of sugar alcohols such as sorbitol, mannitol or xylitol
This does not mean you should follow a permanently low-fibre diet.
Once symptoms settle, gradually bring vegetables, fruit, wholegrains, nuts, seeds and pulses back into your diet. Fibre remains important for gut health, cardiovascular health and overall dietary quality.
Persistent diarrhoea should be discussed with your healthcare team, particularly if you are struggling to maintain hydration.
Acid reflux and heartburn
Feeling fuller for longer is one of the intended effects of GLP-1 medications, but a stomach that remains full for longer can also make reflux more noticeable.
Large meals are often the biggest problem.
Smaller meals, eaten slowly, are usually better tolerated. Try to avoid lying down immediately after eating and leave some time between your evening meal and going to bed.
High-fat meals, alcohol, chocolate, caffeine, acidic foods and spicy foods can trigger reflux in some people, although triggers are very individual. Rather than unnecessarily eliminating everything on a generic "reflux foods" list, pay attention to what actually affects you.
A simple food and symptom diary can be helpful if you are struggling to identify a pattern.
What about sulphur burps?
"Sulphur burps" are an unpleasant symptom sometimes described by people taking weight loss medication. The burps can have a distinctive rotten-egg smell caused by hydrogen sulphide gas.
Slower movement of food through the digestive system may contribute to increased fermentation and gas production.
There isn't one specific food that causes sulphur burps for everyone, so cutting out every sulphur-containing food is unlikely to be helpful.
Instead, start with the basics:
Eat smaller portions, eat slowly and chew your food thoroughly. Avoid very large or particularly fatty meals and limit carbonated drinks if these make symptoms worse.
Keeping track of symptoms can help you identify whether particular foods or meal sizes are consistently associated with the problem.
If burping is severe, persistent or accompanied by significant abdominal pain, vomiting or other concerning symptoms, seek medical advice rather than continuing to restrict your diet.
Hydration matters more than you might think
One thing I regularly emphasise with people taking weight loss medication is hydration.
When appetite falls, drinking can sometimes fall alongside it.
Dehydration can make constipation worse and becomes particularly important if you are experiencing vomiting or diarrhoea.
Rather than relying entirely on thirst, drink regularly throughout the day. Water, sugar-free squash, herbal teas and other low-sugar drinks can all contribute.
If drinking a large glass alongside a meal makes you feel excessively full, try sipping fluids between meals instead.
Eating less means nutrition matters more, not less
One of my biggest concerns as a dietitian is not that people on weight loss medication are eating too much.
Sometimes it is that they are eating too little of what their body needs.
When appetite is significantly suppressed, it can become difficult to consume adequate protein, fibre, vitamins and minerals.
This is why simply pursuing the lowest possible calorie intake is not the goal.
Prioritise nutrient-dense foods and try to include a protein source whenever you eat. Depending on your preferences, this might include eggs, Greek yoghurt, cottage cheese, fish, chicken, lean meat, tofu, beans or lentils.
When appetite is particularly poor, liquid nutrition can also be useful because drinking something nourishing may feel much more manageable than sitting down to another plate of food.
This is where an EXALT protein smoothie can provide a practical option. It can help you incorporate protein and nutrition when appetite is reduced, particularly on days when solid food feels difficult. It shouldn't replace a varied diet, but it can form part of your nutritional toolkit when meeting your requirements through food alone is challenging.
Should you push through digestive side effects?
No.
Mild gastrointestinal symptoms are common, particularly when starting medication or increasing the dose, and they often settle as the body adjusts.
But severe or persistent symptoms shouldn't automatically be accepted as the price you have to pay for weight loss.
If symptoms become difficult to manage, speak to your prescribing clinician. Sometimes remaining at a dose for longer, delaying an increase or adjusting treatment may be more appropriate than escalating the dose despite significant symptoms.
Seek medical advice promptly if you experience severe or persistent abdominal pain, repeated vomiting, an inability to keep fluids down, signs of significant dehydration, or other severe or unusual symptoms.
The dietitian's bottom line
Weight loss medication can make eating less considerably easier, but eating less and eating well are not the same thing.
Your digestive system may need some time to adapt, and what worked for you before starting medication may no longer feel comfortable.
Think smaller portions, slower eating, adequate hydration and good-quality nutrition. Adjust fibre according to your symptoms rather than simply adding more, and prioritise protein and nutrient-dense foods when your appetite is limited.
Most importantly, listen to your body.
The aim isn't to eat as little as possible. It is to use the medication alongside good nutrition to support weight loss while protecting your health, strength and wellbeing along the way.
