Nutrient deficiencies on GLP-1 medications: what the research shows

How common are nutrient deficiencies on GLP-1 medications?

In a claims study of 461,382 adults, 12.7% were diagnosed with a nutritional deficiency or related complication within 6 months of starting a GLP-1 medication, and 22.4% within 12 months ^[2]. Most had type 2 diabetes and started dulaglutide, semaglutide or liraglutide between 2017 and 2021, so results for newer drugs such as tirzepatide may differ.

The study also matched 4,505 adults with type 2 diabetes who added a GLP-1 to metformin against 4,505 similar adults on metformin alone. At 12 months, deficiency diagnoses were more common with the GLP-1 (18.6% vs 16.5%), with significant differences for vitamin D, thiamine and other B vitamins. At 6 months, only other B-vitamin deficiencies differed significantly ^[2].

One finding cuts the other way. Deficiencies were recorded more often in people who saw a dietitian within 6 months of starting (29.8% vs 21.8% at 12 months), which the authors say may reflect more testing and higher-risk patients being referred, rather than harm from dietitian care ^[2].

Why do nutrient shortfalls happen on a GLP-1?

Nutrient shortfalls on a GLP-1 mostly come from eating less, and less variety. A 2025 perspective in the International Journal of Obesity links early fullness, nausea and changing food preferences to lower intake of iron, vitamin B12, vitamin D, calcium and thiamine ^[1]. In a food-record study of 69 GLP-1 users, average intake from food fell below the Daily Value for vitamin D, iron, calcium and magnesium ^[3].

Feeding-lab studies show how steep the drop can be. At week 12, adults with obesity on weekly semaglutide injections ate 24% fewer calories than on placebo, and adults with type 2 diabetes on oral semaglutide ate 38.9% fewer ^{[4, 5]}.

In the food-record study, whose participants were mostly women averaging 50 years old and mostly on semaglutide or tirzepatide, 98.6% took in less than the Daily Value of vitamin D from food, 89.9% less magnesium and 88.4% less iron. Average vitamin D intake was 4 mcg a day against a 20 mcg Daily Value, while B vitamins, zinc and selenium were adequate ^[3].

Keep two limits in mind: that study compared intake with food-label Daily Values rather than age- and sex-specific needs, and it counted food only, not supplements. It was funded by GNC, a supplement retailer ^[3].

Is vitamin D deficiency common on GLP-1 medications like Ozempic?

Vitamin D deficiency is the most frequently diagnosed deficiency after starting a GLP-1 medication. In the claims study, 7.5% of adults had a vitamin D deficiency diagnosis within 6 months and 13.6% within 12 months ^[2]. In a separate food-record study, GLP-1 users averaged 4 mcg of vitamin D a day from food, a fifth of the 20 mcg Daily Value ^[3].

Vitamin D deficiency is common in people with obesity, partly because vitamin D is held in fat tissue and partly because of limited sun exposure, and it may worsen during weight loss without enough intake ^[2].

The RDA for adults is 15 mcg (600 IU) a day through age 70 and 20 mcg (800 IU) after 70 ^[6].

What about vitamin B12 on a GLP-1?

Vitamin B12 intake from food was adequate in the food-record study, where GLP-1 users averaged 4.9 mcg a day against an adult RDA of 2.4 mcg ^{[3, 7]}. The B12 issues worth raising involve other medications and testing. Metformin, which many people with type 2 diabetes take, might reduce B12 absorption ^[7], and semaglutide may skew B12 lab results toward falsely low levels in people also taking metformin ^[2].

Diagnoses of other B-vitamin deficiencies did rise in the claims study, from 1.3% at 6 months to 2.6% at 12 months, and were more common than in matched adults on metformin alone (2.3% vs 1.4%) ^[2].

Does iron deficiency happen on GLP-1 medications?

Iron deficiency anemia was diagnosed in 3.2% of adults within 12 months of starting a GLP-1 medication ^[2]. In the matched comparison, it was no more common than in adults taking metformin alone (2.8% vs 3.0%) ^[2]. Intake may still run low: GLP-1 users in a food-record study averaged 12.1 mg of iron a day from food, and 88.4% fell below the 18 mg Daily Value ^[3].

That 18 mg Daily Value is more than many adults need. The RDA is 18 mg a day for women 19 to 50 and 8 mg for men and for women 51 and older ^[8]. The 2025 perspective names people who menstruate, along with older adults, as groups for whom a baseline nutrition assessment matters most ^[1].

More iron isn't automatically better, so ask before adding a supplement that contains it.

Are thiamine and protein a concern too?

Thiamine deficiency was rare in the claims study, diagnosed in fewer than 1 in 1,000 adults within 12 months, though more often than in matched adults on metformin alone ^[2]. Protein intake often falls short: in a food-record study, only 43% of GLP-1 users reached 1.2 g of protein per kilogram of adjusted body weight, the low end of the range the authors cite for preserving muscle during weight loss ^[3].

Muscle loss was diagnosed in 3.0% of adults within 12 months in the claims study ^[2]. The 2025 perspective recommends prioritizing high-quality protein, ideally alongside resistance training ^[1]. For targets, food strategies and how to protect muscle, see our guide to protein on a GLP-1 medication.

Which lab tests should I ask about on a GLP-1?

No universally accepted screening protocol exists yet, so testing is a decision to make with your prescriber ^[2]. The claims study authors list vitamin B12, 25-hydroxy vitamin D, iron and folate as tests that can screen for shortfalls ^[2], and a 2025 perspective recommends a baseline nutrition assessment, especially for older adults and people who menstruate ^[1].

Useful things to bring to that appointment:

• A list of every medication and supplement you take. Metformin matters here because it might lower B12 levels ^[7].
• Any new symptoms, including ones that seem unrelated.
• What you eat on a typical day now, not before you started.

Is there an official nutrition guideline for GLP-1 medications?

Not one that is universally accepted for nutrition screening and monitoring ^[2]. Four medical and nutrition societies published a joint advisory on nutrition priorities for GLP-1 therapy in 2025 ^[9], and a 2025 perspective calls for international GLP-1-specific nutrition guidelines, pointing out that bariatric surgery care already includes routine micronutrient screening ^[1].

A 2024 review in the journal Obesity also covered nutritional considerations with anti-obesity medications ^[10].

Should I take a supplement while on a GLP-1?

Whether you need a supplement depends on your diet, lab results and other medications, so check with your prescriber first. In the absence of a screening standard, a 2025 International Journal of Obesity perspective suggests a once-daily complete multivitamin and mineral with iron, zinc, copper, B12 and folic acid as a pragmatic interim approach ^[1]. Food still matters: people in the food-record study fell short on fruit, vegetables and dairy ^[3].

Check the iron dose. For men and for women over 50, the RDA for iron is 8 mg a day, so a supplement with 18 mg may give you more than you need ^[8].

What these studies can't tell you

• The 461,382-person study used insurance claims, so it counts only deficiencies that were tested for and coded, and it has no data on supplement use. Most participants had type 2 diabetes, prescriptions started between 2017 and 2021, and the study was funded by Abbott ^[2].
• The food-record study had 69 participants recruited online, relied on self-reported food records, compared intake with label Daily Values, and was funded by GNC ^[3].
• The International Journal of Obesity article is an expert perspective, not a trial or guideline, and one author reports honoraria from Novo Nordisk and Eli Lilly ^[1].
• No study here tested whether a supplement prevents deficiencies in people on GLP-1 medications.