New weight loss medications such as semaglutide and tirzepatide have changed how obesity and overweight are treated, helping many people lose substantial amounts of weight by suppressing appetite and increasing feelings of fullness. But a new review suggests that without adequate nutritional support, people using these drugs may be at risk for nutrient deficiencies and unintended muscle loss.
The review, published in Obesity Reviews, examined the existing evidence on nutrition strategies used alongside next-generation incretin-based therapies, including GLP-1 receptor agonists. Researchers found that while these medications are effective at reducing calorie intake, there is surprisingly little high-quality evidence guiding how people should eat while taking them.
Weight loss drugs can reduce daily calorie intake by roughly 16% to 39%, according to previous trials. While this reduction drives weight loss, it can also make it harder to consume enough protein, fiber, vitamins and minerals. Evidence from clinical studies suggests that up to 40% of the weight lost during treatment may come from lean body mass, including muscle.
“Obesity management medications work by suppressing appetite, increasing feelings of fullness, and altering eating behaviours, which often leads people to eat significantly less,” said Dr. Adrian Brown of University College London, a corresponding author of the review. “However, without appropriate nutritional guidance and support from healthcare professionals, there is a real risk that reduced food intake could compromise dietary quality.”
In the United Kingdom, national guidelines recommend that weight loss medications be prescribed alongside a reduced-calorie diet and increased physical activity. In practice, however, most people using GLP-1 drugs obtain them through private providers rather than the National Health Service, where structured dietary support is not always included.
The researchers identified only 12 studies that directly examined nutrition, diet quality or nutrient intake in people taking semaglutide or tirzepatide. Many of these studies varied widely in how dietary advice was delivered and how nutrition outcomes were measured, making it difficult to draw firm conclusions.
The review highlights several potential concerns. Inadequate protein intake may accelerate muscle loss during rapid weight reduction, increasing the risk of weakness and injury. Insufficient intake of micronutrients such as iron, calcium and B vitamins may contribute to fatigue, impaired immune function and bone health problems over time.
“If nutritional care is not integrated alongside treatment, there’s a risk of replacing one set of health problems with another,” said Dr. Marie Spreckley of the University of Cambridge, who led the study.
Until more research is available, the authors suggest that clinicians may be able to draw on established nutrition principles used after bariatric surgery, which also produces rapid weight loss and appetite changes. These approaches typically emphasize nutrient-dense foods, adequate protein intake spread across meals, and monitoring for vitamin and mineral deficiencies.
Some observational studies included in the review also found that people taking GLP-1 drugs consumed higher-than-recommended amounts of total and saturated fat, suggesting a need for individualized dietary guidance rather than blanket advice.
The researchers stress that medications alone are unlikely to address long-term health if diet quality is overlooked. They also note the importance of incorporating patient perspectives when designing nutrition support, as appetite changes, side effects and daily eating patterns can vary widely.
As use of weight loss drugs continues to expand rapidly, the authors argue that nutrition support should not be treated as optional but as a core part of safe and effective care.
The research was supported by public funding from the U.K.’s National Institute for Health and Care Research and the Medical Research Council.