GLP-1 Nutrient Deficiencies: The Top 3 To Test For
A large retrospective study of more than 460,000 adults who started a GLP-1 medication found that 22.4 percent received a new nutritional deficiency diagnosis within 12 months. Most of the participants had type 2 diabetes, and the study used insurance claims data, so it shows patterns in diagnoses, not proof of cause.
The pattern matters anyway. Nutrient deficiencies are among the most preventable side effects of GLP-1 therapy, and the fix starts with a blood test.
This article covers the three deficiencies that show up most often in the research, what each one does in your body, and the single lab request to bring to your doctor this week.
Why Does This Happen?
GLP-1 medications reduce appetite, slow stomach emptying, and for many people narrow the variety of foods they eat. When you eat substantially less food, you take in fewer vitamins and minerals, even if your diet looks reasonable on paper.
Researchers believe reduced intake is the main driver, not the medication blocking absorption. That distinction matters because the solution is nutritional: eating enough of the right foods and checking your levels.
Two honest caveats apply. Most of the evidence is observational, so it shows an association, not proof that the medication causes the deficiency. And people on these medications often receive more blood work, which means deficiencies are more likely to be found. Even so, the pattern is consistent across studies, and the response is simple: check, don't guess.
Context also matters. In the same claims analysis, 18.6 percent of GLP-1 users had a deficiency diagnosis at 12 months, compared with 16.5 percent of people taking metformin. The gap is real but modest. Nutrient monitoring is a good habit for anyone losing weight, whatever the medication.
1. Vitamin D: The Most Common Deficiency
Vitamin D is the most frequently diagnosed deficiency in GLP-1 research. In the claims analysis, vitamin D deficiency appeared in 7.5 percent of users at six months and 13.6 percent at 12 months. Dietary surveys of GLP-1 users have found vitamin D intake well below recommended levels.
Why it matters: Vitamin D supports bone density and muscle function. During weight loss, you're already at risk of losing both. Low vitamin D adds to that risk. It has also been linked to fatigue and weaker immune function.
The test: A simple blood draw for 25-hydroxyvitamin D, often written as 25(OH)D.
2. Iron: The Fatigue Nutrient
Iron carries oxygen through your blood. Some research on GLP-1 users has reported lower ferritin, the blood marker for iron stores, compared with people on other diabetes medications. Dietary surveys have also found many users eating below estimated iron and calcium requirements. These specific figures come from individual studies that I was not able to confirm in the original papers, so verify them before citing exact percentages.
Why it matters: Low iron can cause fatigue, weakness, shortness of breath with exertion, feeling cold, and hair shedding. If it progresses, it can lead to anemia. Iron deficiency also makes workouts feel harder, which makes it harder to build and preserve the muscle you're working to protect.
The test: A ferritin level and an iron panel, which usually includes serum iron, transferrin saturation, and total iron-binding capacity.
Important warning: Do not start iron supplements without testing first. Too much iron is harmful, and supplementing based on symptoms alone can cause problems of its own.
3. Vitamin B12 (and Thiamine): Watch the Nerves
Vitamin B12 is one of the nutrients most often flagged in GLP-1 research, though the evidence is less consistent than for vitamin D and iron. Thiamine, or vitamin B1, also appears in large database studies.
Why it matters: Vitamin B12 supports nerve function and red blood cell production. Low levels can cause fatigue, tingling or numbness in the hands and feet, brain fog, and anemia. Your risk is higher if you take metformin or are over age 50, and your doctor may already be monitoring it for those reasons.
The test: A serum vitamin B12 level. Your doctor may also check methylmalonic acid if the result is borderline.
The One Lab Request to Make This Week
Ask your physician to check the following:
- 25-hydroxyvitamin D
- Ferritin and an iron panel
- Vitamin B12
Ask which labs are right for you. Ask before you start any supplement, not after. Reviews of this research recommend baseline screening for exactly these nutrients, and your physician can order the tests and interpret the results alongside your full history.
What a Lab Result Cannot Do
A blood test tells you what is low. It does not change what's on your plate.
GLP-1 medications are approved as an addition to a reduced-calorie diet and increased physical activity. The eating, the protein intake, the food variety, and the training are the parts that prevent deficiencies from returning. Those habits are rarely addressed in a 15-minute medical appointment.
That gap is where nutrition coaching fits. Working on protein at each meal, nutrient-dense food choices, resistance training, and habits that hold up over time is how you protect muscle and nutrient status while on medication.
Frequently Asked Questions
Do GLP-1 medications cause vitamin deficiencies? Research links GLP-1 use to a higher rate of diagnosed deficiencies, especially vitamin D. Reduced food intake is considered the main driver. Most studies are observational, so they show association rather than proven cause.
Which vitamin deficiency is most common on GLP-1s? Vitamin D is the most frequently diagnosed in the research. Iron and vitamin B12 are also commonly flagged.
What blood tests should I ask for? Ask for 25-hydroxyvitamin D, ferritin with an iron panel, and vitamin B12. Your physician can add other tests based on your history.
Should I take a multivitamin on a GLP-1? Ask your physician first. Some randomized trials of multivitamins in GLP-1 users are underway, but the right supplement depends on your test results, and iron should never be started without testing.
Can I fix this with diet alone? Sometimes, but not always. Diet is the foundation, and lab results show whether food is closing the gap. Your physician decides whether supplementation is needed.
Take the Next Step
If you're on a GLP-1 medication, start with the lab request above. Then review your nutrition and training with these free tools:
Take the Free GLP-1 Assessment: Three minutes to see where your protein, food variety, and training stand.
Download the GLP-1 Optimization Blueprint: A free framework for protecting muscle and nutrient status while on medication.
Book a Free 15-Minute Strategy Call: A no-cost conversation about whether nutrition coaching fits your goals.
Jim O'Connor is an exercise physiologist and certified nutrition coach with 37 years of experience helping professionals in Los Angeles build lasting strength and metabolic health. He is not a physician. This article is educational and does not replace medical advice. Talk with your physician before starting, stopping, or changing any medication or supplement, and before starting any iron or vitamin supplement.
Sources Referenced
- Retrospective claims analysis of 461,382 adults newly prescribed a GLP-1 receptor agonist, mostly with type 2 diabetes: Butsch, W. S., Sulo, S., Chang, A., et al. "Nutritional deficiencies and muscle loss in adults with type 2 diabetes using GLP-1 receptor agonists: A retrospective observational study."
- Cleveland Clinic study led by Steven Heymsfield, reported in coverage of the same 460,000-patient cohort.
- Separate study of about 480,000 GLP-1 users (Mexico), reported in news coverage with figures for vitamin D and iron or calcium intake. Locate the original paper before citing.
- Dietary intake study of GLP-1 users, cross-sectional: "Investigating nutrient intake during use of glucagon-like peptide-1 receptor agonist: A cross-sectional study."
- Ferritin comparison (26 to 30 percent lower in GLP-1 users vs. a comparison group)