The concerns come from new research published recently in Obesity Reviews. Led by Dr. Marie Spreckley of the University of Cambridge, the study found limited high-quality evidence on how nutritional advice affects calorie intake, body composition, protein intake, and patient experiences while using these medications.

How GLP-1 Weight Loss Drugs Work

Drugs such as semaglutide and tirzepatide, sold under brand names including Ozempic, Wegovy and Mounjaro, work by copying the effects of glucagon like peptide-1 (GLP-1), a hormone released after eating. These medications reduce appetite, increase feelings of fullness, and help curb food cravings.

Because of these effects, calorie intake can drop by 16-39%, making the drugs highly effective for people living with obesity and overweight. However, researchers note that there has been very little study of how these medications affect overall diet quality, protein intake, or micronutrient intake (vitamins and minerals). Existing evidence suggests that lean body mass, including muscle, can make up as much as 40% of the total weight lost during treatment.

Experts Warn of Risks Without Nutrition Support

Dr. Adrian Brown, an NIHR Advanced Fellow at UCL’s Centre of Obesity Research and the study’s corresponding author, explained how the medications change eating patterns.

“Obesity management medications work by suppressing appetite, increasing feelings of fullness, and altering eating behaviors, 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, fiber, vitamins, and minerals essential for maintaining overall health.”

Public Guidelines Versus Private Use

Guidance from the National Institute for Health and Care Excellence (NICE) recommends semaglutide for weight management only for people 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). When prescribed through the NHS, the drug is intended to be part of a broader program that includes a reduced-calorie diet and increased physical activity.

In practice, most users access these medications outside the NHS. Around 1.5 million people in the UK are currently using GLP-1 drugs, and an estimated 95% obtain them privately. In these settings, additional nutrition advice and follow-up support are not always provided.

Rising Use Outpaces Nutrition Guidance

Dr. Spreckley, who works at the Medical Research Council (MRC) Epidemiology Unit at the University of Cambridge, said nutritional care has not kept up with the rapid rise in use.

“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.”

Low intake of key vitamins and minerals can increase the risk of fatigue, weakened immune function, hair loss, and osteoporosis. Loss of lean mass, most often muscle, also raises the likelihood of weakness, injuries, and falls.

Limited Research Leaves Major Questions Unanswered

The researchers identified only 12 studies that examined diet and nutrition alongside treatment with semaglutide or tirzepatide. These studies varied widely in how they delivered dietary advice and measured nutrition outcomes, and they often lacked standardized methods and reporting. As a result, the team found it difficult to draw firm conclusions about the best way to support people using weight loss drugs.

Because use of these medications continues to grow and practical guidance is urgently needed, the researchers suggest drawing lessons from nutrition care used after weight loss surgery. Procedures such as gastric bands lead to similar reductions in appetite and food intake.

Lessons From Bariatric Nutrition Care

Dr. Cara Ruggiero, a co-author from the MRC Epidemiology Unit at the University of Cambridge, said established post-surgery approaches could help fill current gaps.

“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 prioritizing 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.”

The available evidence did not support recommending strict low-fat diets alongside these medications. However, some observational studies found that people taking the drugs often consumed high levels of total and saturated fat. This points to a possible need for personalized guidance on fat intake that aligns with national dietary recommendations.

Meal timing was also rarely tested in clinical trials. Still, the researchers suggest that eating smaller meals more frequently may help ease side effects such as nausea and make the drugs easier to tolerate, especially early in treatment.

Studying Real-World Experiences

The research team emphasizes that future studies should include the perspectives of people using these medications. Understanding what information and support patients find most useful could help improve real-world care.

To address this, the researchers launched AMPLIFY (Amplifying Meaningful Perspectives and Lived experiences of Incretin therapy use From diverse communitY voices). The project aims to explore how people experience next-generation weight loss drugs in daily life.

“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, well-being, and quality of life,” Dr. Spreckley said. “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.