It can, and trials probably undercount it. Across 19 randomized trials, investigators logged malnutrition in 0.12% of participants [1]. Before joint replacement surgery, 38% of GLP-1 users met a lab definition of malnutrition, against 8.8% of nonusers [2].
Those two numbers measure different things in different people. Together they say the adverse-event log is not the whole picture. The lab panel that would show more is covered in what blood tests a GLP-1 needs.
How big the eating deficit is
The meta-analysis pooled trials from the SURMOUNT, STEP, SCALE and OASIS programs [1]. Daily energy intake fell between 24.0% and 39.2%, depending on the drug.
The authors model deficits reaching 1,200 kcal a day. That figure is estimated, not measured.
A deficit that size is how these drugs work. It is also why nutrition becomes a question. Eating a third less makes it harder to keep protein at the level the body needs.
What the trials recorded
Coded malnutrition came to 0.12%. That is about one participant in eight hundred.
Lab screening in the same trials found low lymphocyte counts more often. They ran 2.90% on the drug and 1.77% on placebo. The excess is about one percentage point.
The authors conclude that standard adverse-event reporting underestimates nutritional risk. A coded side effect must be noticed and written down first. The same filter sits behind side-effect rates drawn from diagnosis codes.
Before surgery, the rate was higher
One orthopedic center checked nutrition labs before elective joint replacement [2]. Of 165 patients, 29 were taking a GLP-1.
Malnutrition ran 38% in users against 8.8% in nonusers. Severe malnutrition ran 17.2% against 2.9%. Albumin, prealbumin and total protein were all lower in users.
Two limits matter. The user group was 29 people. And one low value among seven markers, vitamin D included, counted as malnutrition.
The authors still recommend targeted nutritional screening before surgery in GLP-1 users.
Why it goes unnoticed
A 2026 perspective in the Journal of Nutrition names the gap [3]. Prescribing has outpaced nutrition support, and many patients get no dietary guidance.
The risks it lists are low protein, low fiber, lost lean mass and micronutrient shortfalls. It calls them largely preventable with proper guidance.
A 2025 BMJ editorial puts it in its title: an underestimated real-world harm [4].
Who is most at risk, and what helps
The meta-analysis singles out adults aged 65 and older [1]. Its authors propose routine albumin and lymphocyte checks, as their own proposal.
Muscle is the visible cost. Tirzepatide 15 mg took off 1.60 kg of fat-free mass, 2.80% of body weight.
That sits inside the wider muscle-loss evidence, where resistance training cut the lean share.
A dietitian and a lab panel are costs outside the drug. Ask a prescriber whether either is included before paying for a month.