What vitamins are you deficient in on GLP-1 medications?

TL;DR: GLP-1 medications dramatically reduce how much you eat - and that smaller plate means smaller nutrient intake too. Clinical reviews flag vitamin B12, vitamin D, iron, magnesium, and zinc as the nutrients most at risk. Most prescribers don't routinely test for these. Knowing which gaps to watch for is the first step to protecting your energy, bone density, and long-term health while on treatment.

If you're taking semaglutide (Ozempic, Wegovy) or tirzepatide (Mounjaro, Zepbound), your appetite has probably dropped dramatically. That's the whole point. But here's what most people don't realize: when you eat 30–50% less food than you used to, you're also taking in 30–50% fewer vitamins and minerals. Your body doesn't get a pass on needing those nutrients just because you're losing weight.

A 2026 review published in PLOS Medicine by researchers at Johns Hopkins, the University of Colorado, and the University of Pennsylvania flagged micronutrient deficiencies as an explicit concern with long-term GLP-1 use - noting they "remain a concern and may necessitate monitoring and supplementation." Yet most GLP-1 prescriptions are written without a single baseline nutrient panel.

Why GLP-1 medications create nutrient gaps

GLP-1 medications work by slowing gastric emptying and signaling fullness to your brain. The result: you feel satisfied on far less food. In the STEP-1 trial, patients on semaglutide 2.4 mg lost an average of 14.9% of body weight over 68 weeks - a meaningful result, but one that also reflects months of substantially reduced caloric intake.

The math is simple. If you were eating 2,000 calories a day and now eat 1,200, you've cut nutrient intake by roughly 40%. Most people on GLP-1 medications don't radically change the quality of what they eat - they just eat less of it. If your diet was already light on leafy greens and rich in processed food before starting, the gap between what you're getting and what you need widens fast.

There's also a secondary mechanism: GLP-1 side effects like nausea and vomiting - reported in up to half of patients during dose escalation - make eating certain foods actively unappealing. Foods rich in B12 (red meat, shellfish), iron (organ meats, legumes), and magnesium (nuts, seeds, dark chocolate) are often the first to disappear from the plate when nausea hits.

The nutrients most at risk

Vitamin B12

B12 deficiency is the most consistently flagged nutrient risk in GLP-1 patients. It's found almost exclusively in animal products, and it requires adequate stomach acid and a protein called intrinsic factor for absorption. GLP-1 medications slow gastric transit, which may impair this absorption process. B12 deficiency develops slowly - it can take 2–3 years to become symptomatic - which means you might be depleting your stores for a long time before noticing fatigue, tingling in your hands and feet, or cognitive fog.

The risk is compounded if you're taking metformin for type 2 diabetes alongside your GLP-1 medication. Metformin is a well-documented B12 depleter, with studies showing it reduces B12 levels in up to 30% of long-term patients. If you're on both, get your B12 tested now if you haven't already.

Vitamin D

Vitamin D is fat-soluble, meaning it's absorbed alongside dietary fat. When you eat less overall - and especially when nausea pushes you toward low-fat, easily tolerated foods - vitamin D absorption takes a hit. Most people are already suboptimal in D before starting GLP-1 medications. Combine pre-existing inadequacy with reduced fat intake and less sun exposure (a common pattern in people with obesity), and deficiency becomes the rule rather than the exception.

Vitamin D does more than support bone health. Early findings suggest it plays a role in muscle function, immune regulation, and insulin sensitivity - all relevant to your GLP-1 treatment goals. Researchers are still investigating the full interaction between vitamin D status and GLP-1 response.

Iron

Iron deficiency is the world's most common nutrient deficiency, and GLP-1 patients face particular risk for two reasons. First, reduced food intake means less dietary iron. Second, the iron-richest foods - red meat, organ meats, shellfish - are often the hardest to tolerate when nausea is present. Heme iron from animal sources has roughly 15–35% absorption rate, while non-heme iron from plants is closer to 2–20%. If nausea is pushing you toward plant-based, easily tolerated foods, your iron absorption may be lower than the food labels suggest.

Watch for fatigue that feels disproportionate to how well you're sleeping, shortness of breath, pale skin, or unusual cold sensitivity. These are early signals worth checking against a serum ferritin test, not just a standard hemoglobin count.

Magnesium

Magnesium participates in over 300 enzymatic reactions in your body, including muscle contraction, nerve signaling, blood sugar regulation, and protein synthesis. It's also one of the nutrients most commonly lost when GLP-1 side effects like diarrhea occur. Diarrhea - reported in a significant portion of semaglutide and tirzepatide patients - actively flushes magnesium out before your gut can absorb it.

Low magnesium often manifests as muscle cramps, poor sleep, headaches, and anxiety - symptoms that are easy to attribute to "adjusting to the medication" when they may actually be pointing to a fixable deficiency.

Zinc

Zinc is involved in immune function, wound healing, and the sense of taste and smell. Reduced zinc intake can actually dull your ability to taste food - which creates a feedback loop: food becomes less appealing, you eat even less, you take in even less zinc. Zinc is found in highest concentrations in oysters, red meat, and pumpkin seeds - foods not well-represented in a typical GLP-1-era plate focused on tolerable, soft, low-fat options.

What the research says about monitoring

The 2026 PLOS Medicine review by Chao, Gilden, and Wadden - drawing on data from over 700 studies and noting a 700% increase in GLP-1 prescriptions without diabetes from 2019 to 2023 - explicitly flagged nutrient monitoring as a clinical gap. Yet standard GLP-1 prescribing protocols from most primary care practices don't include routine micronutrient panels.

This is a structural problem. Bariatric surgery patients, who experience similar dramatic reductions in food intake, receive mandatory pre- and post-operative nutrient testing as a matter of protocol. GLP-1 patients, who may lose equivalent amounts of weight over similar timeframes, often receive none. The clinical literature is beginning to push back on this inconsistency.

Current evidence-based suggestions from the research community include:

• Baseline bloodwork before starting: B12, vitamin D (25-OH), ferritin, magnesium, zinc
• Re-testing at 6 months and annually thereafter
• Special attention for people on metformin (B12), postmenopausal women (iron, D), older adults (B12, D, magnesium), and those with GI side effects (all of the above)
• Dietary assessment to identify food groups being avoided due to nausea or texture aversion

How to protect yourself

Prioritize nutrient-dense foods first

When your appetite is suppressed, every bite counts more than it ever did before. You can't afford to fill your small plate with low-nutrient foods. Prioritize protein from eggs, fish, and Greek yogurt. Add leafy greens even in small amounts - spinach, kale, and chard deliver iron, magnesium, and B vitamins in compact portions. Aim for variety across food groups rather than defaulting to the three things your stomach currently tolerates.

Don't skip meals entirely

Some GLP-1 patients report going 12–16 hours without eating because they simply don't feel hungry. This is where nutrient intake collapses fastest. Even if you can only manage a small amount, eating something three times a day keeps your micronutrient intake more consistent than two large meals with long gaps between.

Consider targeted supplementation

Food first, always. But the gap between what your reduced intake delivers and what your body actually needs is often real enough to warrant supplementation - especially for B12, D, and magnesium. This is where a product specifically designed for GLP-1 patients becomes relevant. Sign me up was built exactly for this scenario: to restore the nutrients that a smaller plate stops delivering, without the guesswork of assembling five separate supplements yourself.

Talk to your prescriber

Ask specifically for a nutrient panel. Many prescribers don't order one unless you ask. A simple B12, vitamin D, ferritin, magnesium, and zinc test costs very little and can reveal deficiencies you wouldn't otherwise find for years. Don't wait for symptoms - by the time B12 deficiency is symptomatic, it's been building for months.

The bigger picture

GLP-1 medications like Ozempic, Wegovy, Mounjaro, and Zepbound are genuinely effective tools for weight management. The weight loss data from clinical trials is real and meaningful. But the mechanism that makes them work - appetite suppression - is also the mechanism that creates nutrient gaps. These aren't rare edge cases. They're predictable, physiologically logical consequences of eating substantially less food for months or years.

The good news is that these gaps are entirely manageable once you know about them. Regular testing, thoughtful food choices, and targeted supplementation can keep your nutrient status healthy throughout your GLP-1 journey. The problem is just that most prescribers aren't telling patients about this, and most patients aren't asking.

You're asking now. That already puts you ahead.

Frequently asked questions

Do all GLP-1 medications cause the same nutrient deficiencies?

The core mechanism - reduced food intake - is similar across semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound). The specific nutrients affected depend largely on how much your eating patterns change and which foods you stop tolerating due to nausea or appetite suppression. People on higher doses tend to eat less, so the gap can be more pronounced. The risk profile is broadly similar, not drug-specific.

What vitamins should I take on Ozempic or Wegovy?

Based on current clinical evidence, B12, vitamin D, magnesium, and iron are the most commonly flagged deficiency risks. However, what you need specifically depends on your baseline bloodwork, your diet, and whether you have other conditions like type 2 diabetes (which adds metformin-related B12 risk). Get tested first, then supplement based on results - don't guess.

How quickly do nutrient deficiencies develop on GLP-1 medications?

It varies by nutrient. Magnesium can drop relatively quickly if you have ongoing diarrhea as a side effect. Iron stores (measured by ferritin) typically take several months to significantly deplete. B12 takes the longest - often 1–3 years before deficiency becomes symptomatic - but the depletion is silent the entire time. This is why baseline and follow-up testing matters even when you feel fine.

Can I just take a regular multivitamin?

A multivitamin covers the basics, but most standard formulations contain inadequate amounts of B12 (often as cyanocobalamin, the less bioavailable form), low-dose vitamin D, and minimal magnesium. A formula designed specifically for GLP-1 patients, with the right forms and doses of the most at-risk nutrients, is likely to be more effective than a generic multi. Check the label: you want methylcobalamin for B12, D3 for vitamin D, and glycinate or malate forms of magnesium for better absorption.