Which vitamins matter on a GLP-1

Short answer: the vitamins and minerals most likely to run low on a GLP-1 medication are the ones a 2025 joint advisory names: vitamins A, D, E, K, B1, B12 and C, plus iron, calcium, magnesium and zinc.¹ Of those, vitamin D, B12 and calcium are the three it singles out as candidates for supplementing.¹ A blood test tells you which apply to you.

This is the vitamin-by-vitamin companion to our guide to what to eat when you're eating less.

Why vitamins run low

It is arithmetic. People on semaglutide or tirzepatide eat 16% to 39% fewer calories, and vitamins and minerals arrive with food.¹ The advisory warns that shortfalls become likely below about 1,200 calories a day for women and 1,800 for men.¹ Stomach upset can reduce absorption further.¹

The ones that matter most

Vitamin D

Adult RDA: 15 mcg (600 IU), rising to 20 mcg (800 IU) after age 70. Upper limit: 100 mcg (4,000 IU).² Few foods supply much: fatty fish, fortified milk, eggs. The Dietary Guidelines list it as under-consumed by most Americans.³ A blood test (25-hydroxyvitamin D) is the way to know.

Vitamin B12

Adult RDA: 2.4 mcg. No upper limit has been set.⁴ B12 comes from animal foods, so it falls with portions of meat, fish, eggs and dairy. Metformin and acid-reducing drugs lower absorption from food, which matters because many people on a GLP-1 take one of them.⁴ Supplements often contain far more than the RDA because only a small fraction of a large oral dose is absorbed.⁴

Thiamin (B1)

Adult RDA: 1.1 to 1.2 mg. The body stores only a few weeks' worth, so it depends on steady intake. It is on the advisory's list of nutrients of concern.¹

Vitamin C

Adult RDA: 75 to 90 mg. Easy to cover with one or two servings of fruit or vegetables; easy to miss when produce drops off the plate.

Folate and B6

Found in beans, leafy greens and fortified grains. Not on the advisory's shortlist, but intake follows the same foods that supply fiber.

Vitamins A, E and K

Fat-soluble vitamins found in vegetables, nuts, seeds and oils. On the advisory's list of concern.¹ A varied diet or a standard multivitamin covers them; high-dose single supplements are not advised without a reason.

The minerals that go with them

Mineral	Adult RDA	Upper limit
Calcium	1,000–1,200 mg	2,000–2,500 mg
Iron	8 mg (men), 18 mg (women 19–50)	45 mg
Magnesium	310–420 mg	350 mg from supplements
Zinc	8–11 mg	40 mg

From the NIH Office of Dietary Supplements fact sheets.^5,6,7,8 Iron deserves care: take it only if blood work shows you need it.

Signs worth acting on

The advisory lists these as signs of a true deficiency: tiredness beyond what you would expect, heavier hair shedding, flaky or itchy skin, muscle weakness, slow wound healing and unusual bruising.¹ They overlap with many other things, so the right response is a conversation and a blood test. On tiredness specifically, see why a GLP-1 can make you tired.

What the evidence actually supports, nutrient by nutrient

The claims in this category run well ahead of the data. It is worth separating what is established from what is plausible from what is being asserted.

Nutrient	How strong is the case?	What's actually known
Vitamin D	Strong	Already widely under-consumed before any medication.3 Hard to get from food. Named by the joint advisory as one to consider proactively.1
Calcium	Strong	Also already under-consumed.3 Intake falls directly with dairy intake, which is one of the first things to go.
Iron	Moderate	Red meat is a food people commonly go off. A 2025 pilot study reported reduced intestinal iron absorption with semaglutide4 — small and preliminary, a reason to test rather than to assume.
B12	Weaker than it's presented	See below. The headline study is not what it is usually described as.
Magnesium, potassium	Moderate	Lost through vomiting and diarrhea, and intake falls with food.
Vitamins A, E, K	Plausible	Fat-soluble, so intake tracks dietary fat, which often drops. Deficiency from diet alone is uncommon.
Chromium, niacin "for GLP-1 activity"	Not supported	Several ranking pages claim these improve GLP-1 receptor activity. No credible evidence supports it.

The B12 claim, examined

Almost every article on this topic tells you GLP-1 medications lower B12. It is worth knowing how thin that is.

The study usually cited found lower B12 levels in people using semaglutide — who had previously had bariatric surgery. Bariatric surgery independently and substantially impairs B12 absorption, by removing or bypassing the part of the stomach that produces intrinsic factor. Whether semaglutide lowers B12 in people who have not had that surgery is, on current evidence, unknown.

What is well established and genuinely relevant: B12 comes almost entirely from animal foods, so intake falls when meat, fish, eggs and dairy fall. Long-term metformin lowers absorption, and so do proton-pump inhibitors.² Those are real reasons to check your level, and they have nothing to do with the GLP-1.

The practical answer does not change much — test, and supplement if low. But "the medication depletes your B12" is being stated with more confidence than anyone has earned.

Water-soluble and fat-soluble: why the distinction matters

It determines what happens when you take more than you need.

Water-soluble — the B vitamins and vitamin C. Excess is largely excreted, so the margin for error is wide. B6 is the exception worth naming: sustained high doses, generally well above 100 mg a day over long periods, can cause nerve damage. The rest are forgiving.

Fat-soluble — A, D, E and K. These accumulate in body tissue, so excess builds rather than passing through. Vitamin A is the one to be careful with, particularly in pregnancy. Vitamin D is the one people most often take at high doses for long periods without testing.

The practical rule: being approximate with a B-complex is low-risk. Being approximate with fat-soluble vitamins over months is not, and is exactly the situation where a blood test earns its cost.

Signs that point at a specific nutrient

None of these are diagnostic. They are reasons to ask for a test rather than to start taking something.

What you notice	Worth checking
Tiredness out of proportion to sleep; breathlessness on stairs; pale skin	Iron (with ferritin), B12, folate
Pins and needles, numbness in hands or feet	B12
Hair shedding more than usual	Iron, zinc, and overall protein and calorie intake
Muscle cramps, twitching	Magnesium, calcium, potassium
Bone or muscle aching; frequent infections	Vitamin D
Mouth ulcers, cracks at the corners of the mouth	B vitamins, iron
Cuts healing slowly; taste seeming duller	Zinc

Hair shedding deserves a note, since it is a common worry. It most often follows rapid weight change itself rather than any single nutrient, typically two to three months after the change, and it is usually temporary. The evidence, and the blood tests worth asking for, are in GLP-1 medications and hair shedding.

Food, then a supplement

A plate with protein, a fruit or vegetable and a dairy or fortified food at each small meal covers most of this list. The pattern is in what to eat on a GLP-1.

If food is not enough, the choice is between a standard multivitamin-mineral and targeted products. We compare the options in what supplements to take with a GLP-1.

For transparency about our own product: Zornava Replenish supplies vitamins C, D3, B1, B6, folate and B12 with zinc, alongside electrolytes and a prebiotic fiber. It does not contain iron or vitamins A, E or K, so it is not a complete multivitamin. Forms and purposes are on our ingredients page.

A caution about "GLP-1 vitamins"

Some products are sold as vitamins "for GLP-1". There is no special vitamin for the medication. The nutrients are the same ones everyone needs; the only difference is that eating less makes them harder to get. More in what a "GLP-1 supplement" is.