Best Food to Eat on Retatrutide: Trial Info

Retatrutide is an investigational triple hormone receptor agonist developed by Eli Lilly, with no UK marketing authorisation. It is available only within authorised clinical trials. Questions about the best food to eat on retatrutide are relevant to trial participants, as the medicine's mechanism affects gastric emptying, appetite and gastrointestinal tolerance. This article outlines what clinical trials have reported about eating patterns, what nutrition experts broadly recommend during GLP-1 based therapies, which foods may worsen side effects, and when participants should seek medical help. Dietary advice within any trial should always come from the trial team.

What is retatrutide and where does it stand with UK regulators?

Retatrutide has no UK marketing authorisation and has not been evaluated by the MHRA for routine clinical use. It is accessible only through an authorised clinical trial, and no timeline for UK authorisation can be given.

Retatrutide holds no marketing authorisation in the UK or anywhere else, and it can be prescribed or supplied only within an authorised clinical trial.^[1][2] It is an investigational medicine developed by Eli Lilly that acts on three hormone receptors simultaneously: GIP (glucose-dependent insulinotropic polypeptide), GLP-1 (glucagon-like peptide-1) and glucagon receptors. This triple agonist mechanism distinguishes it pharmacologically from currently authorised medicines such as tirzepatide (Mounjaro) and semaglutide (Wegovy), though no head-to-head trials have been conducted and no direct comparisons can be drawn.

Because retatrutide has no UK marketing authorisation, the MHRA has not evaluated it for routine clinical use, and no Integrated Care Board (ICB) has commissioned it. NICE has not issued guidance on it. Anyone who encounters a website offering retatrutide outside a clinical trial should be aware that such a product would not be a licensed medicine and could be falsified or unsafe.

If you are interested in licensed weight-management treatments, a GP can discuss options that are currently authorised and available. For retatrutide specifically, participation is possible only through an authorised clinical trial, and no timeline for UK authorisation can be given.

What did clinical trials report about appetite and eating patterns?

Phase 2 trial data published in the New England Journal of Medicine reported reduced appetite, earlier satiety and decreased interest in high-calorie foods among participants receiving retatrutide. These findings apply only to defined trial populations under controlled conditions.

Phase 2 trial data published in the New England Journal of Medicine (2023) reported that participants receiving retatrutide experienced substantial reductions in body weight over 48 weeks, alongside marked reductions in appetite and food intake.^[3][4] The TRIUMPH programme (TRIUMPH-1 to TRIUMPH-4) and the TRANSCEND-T2D-1 trial, published in The Lancet, have since extended the evidence base into Phase 3 populations, including people with type 2 diabetes.^[5][6]

Across these trials, participants reported earlier satiety, reduced hunger between meals and a decreased interest in high-calorie foods. These effects are consistent with the GLP-1 receptor agonist component of retatrutide's mechanism, which slows gastric emptying and modulates appetite signalling in the brain.^[14][15] The glucagon receptor activity may additionally influence energy expenditure, though the precise contribution of each receptor pathway in humans continues to be studied.^[7]

It is important to note that these are findings from defined trial populations under controlled conditions, with specific inclusion and exclusion criteria. They describe what was observed in those participants during those trials, and they do not predict what any individual outside a trial would experience. Nutritional guidance within trials is typically provided by the trial team and is tailored to the study protocol.

What do nutrition experts generally recommend during GLP-1 based trials?

Nutrition experts recommend prioritising protein-rich, nutrient-dense foods, non-starchy vegetables, healthy fats and small frequent meals when appetite is reduced. Trial participants should follow the specific dietary advice provided by their trial team.

Nutrition experts generally recommend prioritising protein-rich, nutrient-dense foods when appetite is significantly reduced, because eating less overall makes the quality of each meal more important. This principle applies broadly to GLP-1 receptor agonist-based therapies and is relevant to the dietary context of retatrutide trials, though specific dietary protocols vary between studies and are set by the trial team.

The following principles are widely supported by dietitians working in the field of obesity medicine:

• Adequate protein intake: Lean meats, fish, eggs, legumes, low-fat dairy and tofu help preserve muscle mass during periods of caloric reduction.^[11][12]

• Vegetables and fibre: Non-starchy vegetables, wholegrains, lentils and beans support gut health and help maintain satiety between meals.

• Healthy fats in moderation: Oily fish, avocado, nuts and olive oil provide essential fatty acids without excessive caloric density.

• Small, frequent meals: Because gastric emptying is slowed, smaller portions eaten more regularly are often better tolerated than large meals.

• Adequate hydration: Drinking sufficient water throughout the day supports digestion and helps distinguish thirst from hunger.

These recommendations align with general NHS healthy eating guidance and are not specific to any investigational medicine. Anyone participating in a clinical trial should follow the dietary advice provided by their trial team rather than general online guidance, as trial protocols may include specific nutritional requirements.

Are there foods or drinks that could worsen side effects?

High-fat foods, very spicy foods, carbonated drinks, alcohol, high-sugar foods and very large portions are commonly associated with worsened nausea, bloating or discomfort during GLP-1 based therapies. Participants should report dietary difficulties to their trial clinician.

Certain foods and drinks are widely recognised by dietitians as more likely to worsen the gastrointestinal side effects associated with GLP-1 receptor agonist-based therapies, and this general principle is relevant to the dietary context of retatrutide trials. Because retatrutide slows gastric emptying and reduces gastrointestinal motility, foods that are already difficult to digest may be particularly poorly tolerated.

The following categories are commonly associated with worsened nausea, bloating or discomfort in this setting:

• High-fat and fried foods: Fatty meals slow gastric emptying further and are a common trigger for nausea and upper abdominal discomfort.

• Very spicy foods: These can irritate the gastrointestinal lining and exacerbate nausea or reflux symptoms.

• Carbonated drinks: Fizzy beverages introduce gas into the stomach and may worsen bloating and belching.

• Alcohol: Alcohol can irritate the stomach lining, contribute to dehydration and interact with blood glucose regulation, which is particularly relevant for participants with type 2 diabetes.^[16]

• High-sugar foods and drinks: Rapidly absorbed sugars may contribute to nausea and can cause blood glucose fluctuations.

• Very large portions: Eating beyond comfortable fullness when gastric emptying is already slowed is a common cause of nausea and vomiting.

These are general dietary considerations and are not a substitute for the personalised guidance provided by a trial team. Participants in a retatrutide trial should report any dietary difficulties to their trial clinician.

What are the known gastrointestinal side effects and when should you seek help?

Nausea, vomiting, diarrhoea and constipation were the most commonly reported side effects in retatrutide trials, generally mild to moderate and more frequent during dose escalation. Severe or persistent abdominal pain requires immediate assessment via 999 or A&E.

The most commonly reported side effects of retatrutide in clinical trials have been gastrointestinal in nature, consistent with the GLP-1 receptor agonist component of its mechanism. In the Phase 2 trial reported in the New England Journal of Medicine, nausea, vomiting, diarrhoea and constipation were among the most frequently observed adverse events, and they were generally more common at higher doses and during dose escalation periods.^[3][4]

Most gastrointestinal symptoms in trials were described as mild to moderate in severity and tended to improve over time as the body adjusted. However, some participants discontinued treatment due to these effects. The UK SmPCs for authorised GLP-1 receptor agonists also note that acute pancreatitis has been reported, including post-marketing reports of necrotising pancreatitis and reports with a fatal outcome, though no risk magnitude is established.^[8]

Anyone participating in a retatrutide clinical trial should report all side effects to their trial team under the study protocol. Suspected side effects can also be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

As a general guide for when to seek help:

• Contact your trial team if you experience persistent nausea, vomiting, diarrhoea or constipation that is affecting your ability to eat or drink normally.

• Contact NHS 111 for urgent advice if symptoms are severe but not immediately life-threatening.

• Call 999 or go to A&E if you experience severe, persistent abdominal pain, particularly if accompanied by vomiting, as this may indicate a serious condition requiring immediate assessment.

Do not attempt to manage significant symptoms with dietary changes alone without informing your trial team.

Scientific References

1. EMA Paediatric Investigation Plan Decision for Retatrutide (P-0336-2024).
2. Medicines and Healthcare products Regulatory Agency.
3. Triple-Hormone-Receptor Agonist Retatrutide for Obesity (Jastreboff AM et al.).
4. Triple-Hormone-Receptor Agonist Retatrutide for Obesity – PubMed record (Jastreboff AM et al.).
5. Efficacy and safety of retatrutide, a GIP, GLP-1 and glucagon receptor agonist, in people with type 2 diabetes (Bajaj HS et al.).
6. Retatrutide in adults with obesity and type 2 diabetes (TRANSCEND-T2D-1). The Lancet.
7. Effects of Glucagon-Like Peptide-1 Receptor Agonists on energy expenditure (Vieira FT et al.).
8. GLP-1 receptor agonists and dual GLP-1/GIP receptor agonists: strengthened warnings on acute pancreatitis, including necrotising and fatal cases.
9. The Yellow Card scheme: guidance for healthcare professionals, patients and the public.
10. Adverse reactions to drugs – BNF.
11. Risk of protein intake deficiency during treatment with GLP-1 and GIP/GLP-1 receptor agonists: considerations for secondary sarcopenia. Advances in Therapy.
12. Biological Pathways of Strength Preservation During Calorie Restriction-Induced Weight Loss Among Adults Without Obesity. Obesity (Silver Spring).
13. Dietary Fiber and GLP-1 Receptor Agonists in Obesity Management: Converging Mechanisms, Interactions, and Strategies for Durable Weight Control. Advances in Nutrition.
14. GLP-1 and Dual GIP/GLP-1 Receptor Agonists in Pediatric Obesity: From Neuroendocrine Mechanisms to Clinical Application. Int J Mol Sci.
15. Mechanisms of GLP-1 Receptor Agonist-Induced Weight Loss: A Review of Central and Peripheral Pathways in Appetite and Energy Regulation. Am J Med.
16. Hypoglycemia in diabetes. Diabetes Care (Cryer PE).

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