Vitamin Deficiencies on GLP-1: Which Nutrients Drop and What to Do

Yes, and the numbers are larger than most people expect. A narrative review published in Clinical Obesity in 2026 pooled six studies covering 480,825 adults with diabetes, obesity, or both who were taking GLP-1 receptor agonists. The consistent finding across those studies was that GLP-1 users are more likely than non-users to develop gaps in vitamin D, iron, and B vitamins.

The largest single dataset inside that review followed 461,382 GLP-1 users. Within six months of starting the medication, 12.7% had a newly diagnosed nutritional deficiency. That is roughly one in eight people, and most of them had no idea it was happening, because early nutrient deficiency feels like ordinary tiredness.

Here is the important nuance. A GLP-1 receptor agonist (a medication that mimics the gut hormone glucagon-like peptide-1 to slow stomach emptying and reduce appetite) does not usually block your body from absorbing nutrients. The deficiency comes from the front end, not the back end. When your appetite drops by 30-40% and you eat two small meals instead of three full ones, you take in proportionally less of everything: less protein, less iron, less magnesium, less vitamin D.

That distinction matters because it means the problem is solvable. You are not fighting your medication. You are fighting a smaller plate. The work is making that smaller plate denser in nutrients rather than just smaller in calories.

Vitamin D leads the list. In the pooled data, vitamin D deficiency was found in 7.5% of GLP-1 users at 6 months and 13.6% at 12 months. The rate roughly doubles over the first year, which tells you this is a slow drift rather than a sudden crash. Vitamin D matters here for a specific reason: it works alongside calcium to maintain bone, and rapid weight loss already puts bone density under pressure.

Iron is second and arguably the most interesting. Beyond simply eating less red meat and leafy greens, there is early evidence of an absorption effect. A prospective pilot study of 51 adults with type 2 diabetes found markedly reduced intestinal iron absorption after 10 weeks of semaglutide therapy. Separately, a register-based cohort of 439 patients with type 2 diabetes and hereditary hemochromatosis found GLP-1 users had 26-30% lower ferritin levels than people taking SGLT2 inhibitors. Ferritin is your stored iron, and it falls before your hemoglobin does.

B vitamins, especially B12 and folate, round out the top three. B12 comes almost entirely from animal foods, and many people on GLP-1s find meat unappealing during dose escalation. There is also a testing quirk worth knowing: semaglutide may interfere with some B12 assays, which can produce a falsely low reading, particularly if you also take metformin. If your B12 comes back low, ask your clinician about confirming with a methylmalonic acid test.

Other nutrients worth watching include magnesium, zinc, and calcium, though the evidence base for these on GLP-1s specifically is thinner.

The signs are frustratingly vague, which is exactly why deficiencies get missed on GLP-1s. Almost every early symptom overlaps with something you would expect from losing weight quickly.

Fatigue that does not match your sleep is the most common flag. If you slept seven hours and still feel wrung out by 2pm, low iron or low B12 is worth ruling out before you blame the medication itself.

Hair shedding three to six months in is often chalked up to rapid weight loss, and much of the time that is correct. But low iron and low ferritin are independent triggers for telogen effluvium, the type of diffuse shedding most GLP-1 users experience. If you are already dealing with [GLP-1 hair loss](/blog/glp1-hair-loss-why-it-happens-and-how-to-prevent-it), a ferritin check is one of the highest-value labs you can request.

Brittle nails, mouth corner cracks, and a smooth or sore tongue point toward B vitamins and iron. Pins and needles in hands or feet is a B12 signal and deserves prompt attention, because prolonged B12 deficiency can cause nerve damage that does not fully reverse.

Bone or muscle aching, and frequent illness, lean toward vitamin D. And muscle cramps or heart flutters may be magnesium or potassium, which overlaps with the [electrolyte issues common on GLP-1s](/blog/electrolytes-on-glp1-why-they-matter-and-how-to-get-them).

None of these symptoms proves a deficiency on its own. All of them justify a blood test rather than a guess.

The right answer is a baseline panel before you start, then a repeat at 6 and 12 months. Most people skip the baseline, which makes later results hard to interpret, so if you are still in your first weeks, ask now.

A reasonable panel to discuss with your prescriber includes:

• •Complete blood count (CBC) to catch anemia
• •Ferritin plus iron studies, not just hemoglobin. Ferritin drops first and is the early warning
• •Vitamin D (25-hydroxyvitamin D), the standard measure of vitamin D status
• •Vitamin B12, ideally with methylmalonic acid if the result is borderline or you take metformin
• •Folate
• •Magnesium, and a comprehensive metabolic panel for potassium, sodium, and kidney function
• •Albumin and prealbumin if your protein intake has been low, as markers of protein status

A practical note on timing: run labs before a dose escalation rather than during the worst of your nausea week, so the result reflects your typical intake rather than a bad few days.

If your insurance pushes back on the full panel, prioritize ferritin, vitamin D, and B12. Those three cover the majority of what the published data flags, and they are inexpensive.

Bring the results to a real conversation. A ferritin of 18 ng/mL is technically inside many lab reference ranges but is functionally low for someone with fatigue and hair shedding.

The strategy is nutrient density: every bite has to carry more than it used to. When you were eating 2,200 calories, you had room for filler. At 1,300 calories, you do not.

Anchor every meal to protein first. Protein is the single highest-leverage macronutrient on a GLP-1 because it protects muscle and delivers iron, B12, and zinc at the same time. A palm-sized portion of beef, chicken thigh, salmon, eggs, or Greek yogurt does triple duty. Our guide on [how much protein you really need on a GLP-1](/blog/protein-on-glp1-how-much-you-really-need) covers target amounts.

Pair iron with vitamin C and separate it from coffee. Plant iron from lentils, spinach, and beans absorbs far better alongside citrus, peppers, or strawberries. Tea and coffee within an hour of an iron-rich meal cut absorption substantially. Moving your coffee to mid-morning is a free win.

Get vitamin D from the sources that actually work. Food alone rarely covers it. Fatty fish, egg yolks, and fortified dairy help, but most people who are deficient need supplementation to correct it. Ask your clinician for a dose based on your actual lab value rather than guessing.

Do not waste appetite on low-nutrient calories. On a limited appetite, a bowl of plain pasta costs you the same fullness as a bowl of pasta with salmon and spinach, and delivers a fraction of the nutrition.

Watch fiber and hydration, which affect how comfortably you can eat at all. If constipation or bloating is limiting your intake, address that first. See our guides on [fiber on GLP-1s](/blog/fiber-on-glp1-how-much-and-which-kind) and [managing GLP-1 constipation](/blog/glp1-constipation-why-it-happens-and-how-to-relieve-it).

For many people, yes, at least as insurance. But a multivitamin is a floor, not a fix, and it will not correct a real deficiency on its own.

The logic runs like this. A standard multivitamin covers roughly 100% of the daily reference intake for most vitamins and a smaller share of minerals. If you are eating 1,200-1,400 calories of reasonably good food, a multivitamin closes the small gaps. If you have a documented deficiency, you need a targeted therapeutic dose, which is typically several times higher than what a multivitamin contains, and it should be prescribed against your lab result.

A few practical points:

Iron in multivitamins is often too low to correct a deficiency and can worsen constipation, which is already common on GLP-1s. Do not self-prescribe high-dose iron. Excess iron is genuinely harmful, and this is one supplement where more is clearly not better.

Chewables, gummies, and liquids tend to be easier to tolerate than large tablets when your stomach is emptying slowly. Gummies frequently omit iron entirely, so check the label.

Timing helps. Take supplements on your best-appetite day rather than your worst-nausea day. If you take them on an empty stomach and feel queasy, move them to your largest meal.

Calcium and iron compete, so do not take them at the same time. If you are on both, split them between morning and evening.

If you are also navigating menopause, your supplement picture is different and slightly more demanding. Our guide on [supplements on GLP-1 during menopause](/blog/supplements-on-glp1-during-menopause-what-you-actually-need) breaks down what changes.

Some situations warrant a call rather than a wait-and-see approach.

Numbness, tingling, or burning in your hands or feet should be evaluated promptly. This is the classic B12 deficiency neuropathy signal, and unlike fatigue, the damage from prolonged untreated B12 deficiency can become permanent.

Shortness of breath on stairs you used to climb easily, a racing heart at rest, or unusual paleness suggest anemia significant enough to need treatment, not just a dietary tweak.

Persistent dizziness or fainting may be dehydration and electrolytes rather than vitamins, but either way it needs assessment. See our guide on [GLP-1 dizziness](/blog/glp1-dizziness-lightheadedness-causes-and-fixes).

Losing weight faster than roughly 1-2% of your body weight per week over a sustained period is a flag. Very rapid loss increases the risk of both muscle loss and nutrient gaps, and often means your dose is ahead of what your intake can support.

Eating fewer than about 1,200 calories a day consistently makes deficiency close to inevitable regardless of how well you choose your foods. If that is where you are, your dose or your food strategy needs adjusting.

One broader point worth holding onto: none of this is an argument against GLP-1 medications. The cardiovascular, kidney, and metabolic benefits demonstrated in trials like SELECT (NEJM 2023) and FLOW (NEJM 2024) are substantial and well documented. It is an argument for monitoring. The people who do best on these medications are the ones who treat nutrition as part of the treatment rather than an afterthought.