Biomarkers

GLP-1 Medications and Nutrient Deficiencies: What to Check

New research links starting a GLP-1 medication with new nutrient deficiency diagnoses within a year, most often vitamin D. Here is what the studies found, what they cannot show, and which blood tests to discuss: vitamin D, vitamin B12 and ferritin.

Published

October 6, 2026

Read time

7 min read

Medically reviewed by

Rhonda Collins, FNP-C

Share

Copy link

If you take a GLP-1 medication such as Ozempic or Wegovy, you probably track your weight and blood sugar. Nutrient levels are worth a look too. Two large claims studies found that about 1 in 6 children and teens, and more than 1 in 5 adults, most of them with type 2 diabetes, received a new nutrient deficiency diagnosis within a year of starting a GLP-1. Vitamin D deficiency was the most common in both. The researchers could not show that the drugs caused these deficiencies. Still, checking vitamin D, vitamin B12 and ferritin before or early in treatment is a simple way to see where you stand.

This matters to a lot of people. In a KFF poll fielded in late October and early November 2025, about 1 in 8 US adults (12%) said they were currently taking a GLP-1 drug.

What the new pediatric study found

Researchers from Abbott and Ann & Robert H. Lurie Children’s Hospital of Chicago found that 16.9% of young people starting a GLP-1 were diagnosed with at least one nutritional deficiency or deficiency-related complication within a year. The paper by Kerr, Ryder and colleagues appeared online in Childhood Obesity on September 17, 2026.

The team analyzed US insurance claims from 2017 to 2022 and identified 2,031 GLP-1 users aged 10 to 17 with no earlier deficiency diagnosis. Key findings from the published abstract:

• Vitamin D deficiency was the most frequent diagnosis, at 12.4% within one year.
• Nutritional anemia (1.55%) and iron-deficiency anemia (1.44%) came next.
• Nutrition counseling was uncommon. Only 23.3% of patients had a nutrition therapy or counseling visit within 180 days of starting, and the average wait for a first visit was 149 days.
• Liraglutide made up 78.6% of prescriptions, followed by dulaglutide (10.4%) and semaglutide (9.1%).

The authors describe nutritional deficiencies as an under-recognized risk in this age group and call for nutrition care to be built into pediatric GLP-1 treatment. The testing guidance later in this article is written for adults.

What the adult data show

A 2025 study in Obesity Pillars by Butsch and colleagues, with researchers from Cleveland Clinic, Abbott and Pennington Biomedical Research Center, found similar patterns in adults. The team followed 461,382 adults who newly started a GLP-1 between mid-2017 and the end of 2021 and had no recent deficiency diagnosis. Most (80.5%) had type 2 diabetes. The rest had type 1 diabetes, prediabetes, or no recorded diabetes diagnosis.

New diagnoses after starting a GLP-1, from the full text:

The researchers also compared 4,505 adults with type 2 diabetes taking a GLP-1 plus metformin with 4,505 closely matched adults taking metformin alone. At 12 months, 18.6% of the GLP-1 group had a new deficiency diagnosis or related complication, compared with 16.5% of the metformin-only group. That 2.1-point difference was statistically significant, and it is far smaller than the headline 22.4% suggests. Many people with type 2 diabetes would receive some of these diagnoses whether or not they took a GLP-1.

Why eating less can mean fewer nutrients

GLP-1 medications reduce appetite and slow digestion, so many people eat noticeably less. Fewer and smaller meals usually mean less vitamin D, iron and vitamin B12 from food as well as fewer calories. The Butsch study authors point to this reduced intake as the reason to look, and note that people with obesity already have a higher risk of some micronutrient deficiencies. Vitamin D is a particular focus, because it is stored in fat tissue and many people get limited sun exposure.

For vitamin B12, metformin is a separate consideration. According to its FDA prescribing label, about 7% of patients in controlled trials lasting 29 weeks developed subnormal vitamin B12 levels, and the label recommends periodic vitamin B12 measurement. Many people with type 2 diabetes take metformin and a GLP-1 together.

What these numbers can and cannot tell us

In both studies, researchers counted diagnosis codes on insurance claims rather than measured lab values, could not show that GLP-1 medications caused any deficiency, and drew the adult numbers from a group made up mostly of people with type 2 diabetes. A deficiency only appears in claims data if someone was tested and coded, so true rates could be higher or lower. In the pediatric study, patients who saw a nutrition professional had more recorded deficiencies (23.2% vs 14.8%). That may simply mean more of them were checked, and there is no way to tell from claims data alone. Researchers employed by Abbott, which makes nutrition products, co-authored both papers, and Abbott funded the adult study.

But the same pattern appeared in children and adults, vitamin D deficiency was the most common diagnosis in both, and the adult comparison with metformin alone still showed a small, statistically significant gap. That is enough to make a baseline check sensible, even before better data arrive. Researchers who measure blood levels before and during treatment could answer the question more directly.

What to check while taking a GLP-1, and when

Neither study sets a testing schedule. Based on when new diagnoses showed up in the adult data, a practical approach is to test before you start, or as soon as you can, and again a few months into treatment. With a second result, you can see whether a level is holding steady or falling as your eating changes. Bring the results to the clinician who prescribes your GLP-1 so the two of you can decide whether anything needs to change.

Vitamin D (25-hydroxyvitamin D)

Most vitamin D blood tests measure 25-hydroxyvitamin D, because it is the most accurate way to see whether you have enough vitamin D, according to MedlinePlus. Vitamin D deficiency was the most common new diagnosis in both studies. See the vitamin D biomarker guide for what the result means, and the vitamin D test cost guide for how testing options compare.

Vitamin B12

Vitamin B12 is worth checking if you eat much less meat, fish, eggs or dairy than before, or if you also take metformin. The vitamin B12 biomarker guide explains the result, and how B12 deficiency is diagnosed covers the follow-up tests clinicians use when a level is borderline.

Ferritin, for iron stores

A ferritin blood test shows how much iron you have stored in your body, according to MedlinePlus. MedlinePlus also lists checking for low iron before symptoms develop in people at higher risk, such as those who are underweight or cannot absorb food well. The ferritin biomarker guide and the ferritin level guide explain how to read the number.

Most people on a GLP-1 also track metabolic markers such as glucose, HbA1c, lipids and liver enzymes. The GLP-1 medications and monitoring guide covers those.

How to test with Mito

Mito’s marketplace sells each of these as an individual blood test, with no membership required and a draw at a partner lab near you:

• Vitamin D, 25-Hydroxy (Calcidiol) Blood Test
• Vitamin B12 (Cobalamin) Blood Test
• Ferritin Blood Test

Mito does not prescribe GLP-1 medications. Your results appear in your Mito dashboard, so you can share them with the clinician managing your treatment. With Mito Membership, you can also track them over time and compare a later result with your baseline.

Sources

• Kerr KW, Chang AT, Sulo S, et al. Nutritional Deficiencies, Complications, and Nutrition Therapy/Counseling in Pediatric Patients Using GLP-1 Receptor Agonists. Childhood Obesity, published online September 17, 2026 (PubMed abstract)
• Butsch WS, Sulo S, Chang AT, et al. Nutritional deficiencies and muscle loss in adults with type 2 diabetes using GLP-1 receptor agonists: A retrospective observational study. Obesity Pillars, 2025;15:100186
• KFF Health Tracking Poll: 1 in 8 adults say they are currently taking a GLP-1 drug, November 14, 2025
• Metformin hydrochloride tablets, prescribing information, DailyMed
• Vitamin D Test, MedlinePlus
• Ferritin Blood Test, MedlinePlus

Frequently asked questions

Can Ozempic cause low iron?

Researchers have not shown that semaglutide or any other GLP-1 medication causes low iron. In a 2025 claims study of more than 461,000 adults who started a GLP-1, 3.2% received a new iron-deficiency anemia diagnosis within 12 months. Eating less can mean taking in less iron, so a ferritin test is a reasonable way to check your iron stores during treatment.

Does Ozempic cause vitamin B12 deficiency?

There is no direct evidence that semaglutide itself lowers vitamin B12. In the same 2025 study, 2.6% of adults had a new diagnosis in the group of B vitamin deficiencies that includes B12 within a year of starting a GLP-1. Metformin, which many people with type 2 diabetes take alongside a GLP-1, has a known link to lower vitamin B12 on its prescribing label.

Do I need nutrient blood tests if I am on a GLP-1 for weight loss only?

Most of the adults in the large 2025 study had type 2 diabetes, so the numbers fit people taking a GLP-1 for weight loss alone less closely. Eating less can still mean taking in less of several nutrients, and the study authors recommend that clinicians prescribing GLP-1 medications monitor patients for nutritional deficiencies.

Which GLP-1 medications were included in these studies?

In the adult study, which covered people starting treatment from 2017 to 2021, the most prescribed drugs were dulaglutide, semaglutide and liraglutide. In the pediatric study, liraglutide made up 78.6% of prescriptions, followed by dulaglutide and semaglutide. Neither paper reports results for tirzepatide, sold as Mounjaro and Zepbound.