People prescribed the new generation of weight loss drugs may not receive sufficient nutritional guidance to support safe and sustainable weight loss, leaving them vulnerable to nutritional deficiencies and muscle loss, say experts at UCL and the University of Cambridge.

In research published today in Obesity Reviews, the team, led by Dr Marie Spreckley from the MRC Epidemiology Unit, found a lack of robust evidence surrounding nutritional advice and support and the impact this has on factors such as calorie intake, body composition, protein adequacy, and patient experiences.

Weight loss drugs such as semaglutide and tirzepatide – available under brand names including Ozempic, Wegovy and Mounjaro – mimic the naturally-occurring hormone glucagon like peptide-1 (GLP-1), released into the blood in response to eating. The drugs suppress appetite, increase a feeling of being full, and reduce food cravings.

Addressing the unintended consequences of weight loss drug use

Weight loss drugs can reduce calorie intake by between 16-39%, making them a powerful tool to help people living with obesity and overweight. However, there has been little research to examine their impact on diet quality, protein intake, or adequacy of micronutrients (vitamins and minerals). Evidence suggests that lean body mass – including muscle – can constitute up to 40% of total weight lost during treatment.

Dr Adrian Brown, an NIHR Advanced Fellow from the Centre of Obesity Research at UCL, the study’s corresponding author, said:

Obesity management medications work by suppressing appetite, increasing feelings of fullness, and altering eating behaviours, which often leads people to eat significantly less. This can be highly beneficial for individuals living with obesity, as it supports substantial weight loss and improves health outcomes.

“However, without appropriate nutritional guidance and support from healthcare professionals, there is a real risk that reduced food intake could compromise dietary quality, meaning people may not get enough protein, fibre, vitamins, and minerals essential for maintaining overall health.”

Guidelines from the National Institute for Health and Care Excellence (NICE) recommend semaglutide as an option for weight management for individuals who meet specific criteria, such as a body mass index (BMI) of at least 35.0 kg/m² and comorbidity (that is, they also have other conditions such as type 2 diabetes, cardiovascular disease, etc). But the guidelines stress that when administered through the NHS, treatment is part of a programme alongside a reduced-calorie diet and increased physical activity in adults.

Most people, however, access the drugs privately. There are currently around 1.5 million people in the UK using GLP-1 drugs, of whom the vast majority – 95% – are thought to access the medication privately, where they are not always provided with the additional nutritional advice and support.

Dr Spreckley said:

Use of GLP-1 receptor agonist therapies has increased rapidly in a very short period of time, but the nutritional support available to people using these medications has not kept pace. Many people receive little or no structured guidance on diet quality, protein intake, or micronutrient adequacy while experiencing marked appetite suppression.

“If nutritional care is not integrated alongside treatment, there’s a risk of replacing one set of health problems with another, through preventable nutritional deficiencies and largely avoidable loss of muscle mass. This represents a missed opportunity to support long-term health alongside weight loss.”

Inadequate micronutrient intake can put people at risk of a range of conditions, from fatigue and an impaired immune response to hair loss and osteoporosis. Loss of lean mass – in most cases, muscle – increases the risk of weakness, injury and falls.

Missing evidence on weight loss drugs and nutrition

The researchers identified just 12 studies that looked at nutrition and diet alongside treatment with semaglutide or tirzepatide. Even then, as the trials varied widely in their approaches to dietary advice and nutritional assessment and lacked standardised protocols and reporting, it was difficult to reach robust conclusions to inform the optimal support for people taking weight loss drugs.

Given the widespread – and increasing – use of these medications and the urgency of providing advice to individuals using the drugs, as well as calling for more research the researchers argue that we can turn to what has been learnt from the guidance and support offered alongside weight loss surgery, such as gastric bands, which leads to similar outcomes to GLP-1 drugs.

Dr Cara Ruggiero, co-author from the MRC Epidemiology Unit, said:

While GLP-1 receptor agonists are increasingly used, there remains a clear gap in structured nutritional guidance. In the interim, we can draw on well-established post-bariatric nutrition principles. Our previous work highlights the importance of prioritising nutrient-dense foods including high-quality protein intake, ideally distributed evenly across meals, to help preserve lean mass during periods of reduced appetite and rapid weight loss.”

While there was insufficient evidence from the studies to recommend strict low-fat diets to complement the weight loss drugs, some observational studies found that people on the treatments often consumed excessive levels of total and saturated fat. This suggests that there might be a need for individualised fat intake guidance, aligned with broad national recommendations, the team says.

Similarly, although meal frequency was not explicitly tested in most trials, they say that strategies such as small, frequent meals may help reduce common side effects such as nausea and improve tolerability, especially in the early stages of treatment.

Understanding lived experiences to improve support

In calling for more research in this area, the team says it is important that people who use the treatments are consulted to help identify the types of information and wrap-around support that are most meaningful and needed in real-world care. They have launched AMPLIFY (Amplifying Meaningful Perspectives and Lived experiences of Incretin therapy use From diverse communitY voices) to understand how people experience next-generation weight loss therapies when used for weight management.

Dr Spreckley, who is leading AMPLIFY, said:

These medications are transforming obesity care, but we know very little about how they shape people’s daily lives, including changes in appetite, eating patterns, wellbeing, and quality of life. That’s what we’ll explore, working in particular with people from communities historically under-represented in obesity research, to help shape the future of obesity treatment.”

The research was funded by the National Institute for Health and Care Research (NIHR), with additional support from the Medical Research Council and the NIHR UCLH Biomedical Research Centre.

Reference

• Spreckly, M et al. Nutrition Strategies for Next-Generation Incretin Therapies: A Systematic Scoping Review of the Current Evidence. Obesity Reviews; 8 Jan 2026; DOI: 10.1111/obr.70079