This review summarises published evidence on effects of GLP-1 and dual GIP/GLP-1 medicines beyond nausea: food intake, protein, lean mass, fibre and constipation, vitamins and minerals, and weight after treatment stops. It includes data from Asian populations where available.

Summary of findings

• In a controlled study, people on a GLP-1 medicine ate 35% less at a free-choice meal than people on placebo.[1]

• In the two largest obesity trials, lean mass, which includes muscle, made up between about a quarter and just under half of the weight lost.[4]

• In a large US cohort, about 1 in 5 people had a diagnosed nutritional deficiency within a year of starting treatment. Vitamin D was the most common.[11]

• One year after treatment and lifestyle support stopped, trial participants had regained about two-thirds of the weight they had lost.[19]

• A 2025 joint advisory recommends adequate protein, resistance exercise and monitoring of nutrient levels during treatment.[7]

Food intake and stomach symptoms

These medicines act on the hormone GLP-1, which signals fullness and slows how fast the stomach empties.[7] In a 20-week controlled study, people on a GLP-1 medicine ate 35% less energy at a free-choice lunch than people on placebo.[1]

In the main 68-week obesity trial, 44% of people on the medicine reported nausea and 23% constipation, compared with 17% and 10% on placebo. Most cases were mild to moderate.[2]

Protein intake

The 2025 joint advisory from four US nutrition and obesity societies suggests 1.2 to 1.6 g of protein per kg of body weight per day during active weight loss. Where body composition is known, it gives an alternative of about 1.5 g per kg of lean body mass. It advises avoiding intakes below 0.4 to 0.5 g/kg and prolonged intakes above 2 g/kg, and individualising targets for kidney function.[7]

Applied to a range of adult body weights, the advisory's range gives:[7]

Lean mass and muscle

Lean mass is all body tissue that is not fat. It includes muscle, but also water, organs and bone, so it is an imperfect measure of muscle.[4]

In the two largest obesity trials, body composition was measured by DXA scan:

• With a GLP-1 medicine, fat mass fell by 19% and lean mass by 10%.[2]

• With a dual GIP/GLP-1 medicine, fat mass fell by 34% and lean mass by 11%.[3]

Converted to kilograms, lean mass made up about 45% and 26% of the weight lost in these trials.[4] Lean mass is also lost with diet and with weight-loss surgery.[4]

Authors writing in JAMA noted that fat-free mass falls proportionally less than fat mass, and concluded that concern about frailty from this weight loss is not supported by data.[5] In a one-year trial, people who combined a GLP-1 medicine with structured exercise maintained their lean mass while losing fat.[6] The 2025 advisory recommends resistance exercise at least three times a week and 150 minutes of moderate aerobic activity a week.[7]

Asian reference values. The Asian Working Group for Sarcopenia defines low muscle strength as a handgrip below 28 kg in men or 18 kg in women, or taking 12 seconds or more to rise from a chair five times.[8]

Sarcopenic obesity. A 2022 consensus statement from two European societies defines sarcopenic obesity as the co-existence of excess body fat and low muscle mass or function. It describes diagnosis in two steps: muscle function is assessed first, and body composition is then measured to confirm excess fat together with low muscle mass.[17]

Body size and body composition in Asian adults

A 2004 World Health Organization expert consultation reviewed evidence that Asian populations have different relationships between body mass index (BMI), body fat percentage and health risk than European populations. It concluded that a substantial proportion of Asian people have a high risk of type 2 diabetes and cardiovascular disease at BMIs below the international overweight cut-off of 25 kg/m². It kept the international cut-offs and added public health action points of 23.0, 27.5, 32.5 and 37.5 kg/m².[15]

Singapore's Ministry of Health clinical practice guidelines use 23 kg/m² for overweight and 27.5 kg/m² for obesity, because Singaporeans have a higher prevalence of type 2 diabetes and cardiovascular risk factors at BMIs below 25.[16]

The same Singapore study notes that BMI is not a good indicator of body fat on its own, because body weight includes both fat and fat-free mass.[16]

Fibre and constipation

Singapore's Health Promotion Board recommends 20 g of dietary fibre a day for women and 26 g for men.[9] The 2025 advisory recommends increasing fibre from food gradually, and notes that fibre supplements may help with constipation.[7]

A meta-analysis of 16 randomised trials in adults with chronic constipation found:[10]

• 66% responded to fibre supplements, compared with 41% on control

• Psyllium showed the clearest benefit

• Doses above 10 g a day and treatment for at least 4 weeks were associated with better results

• Highly fermentable fibres such as inulin increased wind

These trials were in people with chronic constipation in general, not specifically people taking GLP-1 medicines.[10]

Vitamins and minerals

In a US study of more than 460,000 adults starting GLP-1 medicines, most of whom had type 2 diabetes, 12.7% had a diagnosed nutritional deficiency at 6 months and 22.4% at 12 months. Vitamin D deficiency was the most common, at 13.6% by 12 months.[11] The study used insurance records, so it measured diagnosed deficiencies rather than every deficiency.[11]

The 2025 advisory lists the nutrients most at risk during treatment: iron, calcium, magnesium, zinc, and vitamins A, D, E, K, B1, B12 and C. Risk is higher when intake falls below about 1,200 kcal a day in women or 1,800 kcal a day in men.[7] It recommends a nutritional assessment before treatment and monitoring of nutrient levels during it, and says supplements can be considered for nutrients at risk.[7]

Gallbladder and acute illness

A meta-analysis of 76 randomised trials found a 37% higher relative risk of gallbladder or bile duct problems with GLP-1 medicines, with higher risk at weight-loss doses and longer treatment.[12]

UK primary-care guidance advises withholding these medicines during vomiting, diarrhoea or any illness where a person cannot eat and drink normally, and restarting once eating and drinking return to normal.[13]

Weight and health after treatment stops

The main 68-week GLP-1 obesity trial had a one-year off-treatment extension in a representative subset of 327 participants. Average weight loss during treatment was 17.3%. One year after treatment and lifestyle support stopped, participants had regained about two-thirds of the weight they had lost, equal to 11.6% of their starting weight, and cardiometabolic risk factors showed similar changes.[19] The authors concluded that the findings confirm obesity is a chronic condition and suggest that ongoing treatment is needed to maintain improvements in weight and health.[19]

The extension analyses were exploratory, and participants stopped both the medicine and the lifestyle programme, so the study cannot separate the effect of each.[19]

Benefits and treatment options

In a 68-week trial in East Asian adults, people on a GLP-1 medicine lost 13.2% of body weight and 40% of visceral (abdominal) fat.[14] The 2026 American Diabetes Association Standards of Care list lifestyle therapy, obesity medicines and metabolic surgery as treatment options for obesity.[18]

Frequently asked questions

➡️How much protein is recommended during GLP-1 treatment?

A 2025 joint advisory suggests 1.2 to 1.6 g per kg of body weight a day during active weight loss, individualised for kidney function.[7]

➡️How much of the weight lost is muscle?

In the two largest trials, lean mass made up about 26% to 45% of the weight lost. Lean mass includes muscle, water, organs and bone.[4]

➡️How much fibre is recommended in Singapore?

20 g a day for women and 26 g for men.[9]

➡️Which nutrient deficiency is most common?

Vitamin D, diagnosed in 13.6% of people within 12 months in a large US study.[11]

What happens to weight after treatment stops?

➡️In a one-year follow-up of a 68-week trial, participants regained about two-thirds of the weight they had lost.[19]

➡️Which BMI cut-offs are used in Singapore?

Singapore's clinical practice guidelines use 23 kg/m² for overweight and 27.5 kg/m² for obesity.[16]

References

1. Friedrichsen M, et al. Diabetes Obes Metab. 2021;23(3):754–762.

2. Wilding JPH, et al. N Engl J Med. 2021;384:989–1002.

3. Jastreboff AM, et al. N Engl J Med. 2022;387:205–216.

4. Neeland IJ, Linge J, Birkenfeld AL. Diabetes Obes Metab. 2024.

5. Conte C, Hall KD, Klein S. JAMA. 2024;332(1):9–10.

6. Lundgren JR, et al. N Engl J Med. 2021;384:1719–1730.

7. Mozaffarian D, et al. Obesity. 2025;33(8):1475–1503.

8. Chen LK, et al. Asian Working Group for Sarcopenia: 2019 consensus update. J Am Med Dir Assoc. 2020;21(3):300–307.

9. Health Promotion Board, Singapore. Dietary fibre recommendations. HealthHub.

10. van der Schoot A, et al. Am J Clin Nutr. 2022;116:953–969.

11. Butsch WS, et al. Obesity Pillars. 2025;15:100186.

12. He L, et al. JAMA Intern Med. 2022;182(5):513–519.

13. NHS Scotland. GLP-1 receptor agonist initiation guidance for primary care. Right Decisions.

14. Kadowaki T, et al. Lancet Diabetes Endocrinol. 2022;10(3):193–206.

15. WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet. 2004;363:157–163.

16. Body mass index and percentage body fat in Singaporean adults. BMC Public Health. 2021.

17. Donini LM, et al. Definition and diagnostic criteria for sarcopenic obesity: ESPEN and EASO consensus statement. Obes Facts. 2022;15(3):321–335.

18. American Diabetes Association. Standards of Care in Diabetes—2026, Section 8: Obesity and weight management. Diabetes Care. 2026;49(Suppl 1).

19. Wilding JPH, et al. Weight regain and cardiometabolic effects after treatment withdrawal: the 68-week trial extension. Diabetes Obes Metab. 2022;24(8):1553–1564.