Micronutrients on a GLP-1: B12, vitamin D, iron and calcium, with the reference intakes

Halve your food and you halve your micronutrients, unless the half you keep is chosen. The 2025 joint advisory names the nutrients of concern on GLP-1 therapy as iron, calcium, magnesium, zinc and vitamins A, D, E, K, B1, B12 and C, and singles out vitamin D, calcium and B12 as examples warranting consideration for supplementation (PubMed 40445127). This page covers the four with the most direct relevance, with the reference intakes so you can see what enough looks like. It is about what to measure and eat, not a supplement regimen; that is a clinician's call.

Vitamin B12

Reference intake: 2.4 mcg a day for adults (NIH ODS).

Why it matters here: B12 comes almost entirely from animal foods (meat, fish, eggs, dairy) and fortified products. Intake of those often falls on a GLP-1. Separately, metformin, which many people with type 2 diabetes take alongside a GLP-1, reduces B12 absorption; the American Diabetes Association's Standards of Care recommend periodic measurement of B12 in people on metformin, especially with anaemia or peripheral neuropathy. Deficiency develops slowly, because liver stores last years, and shows up as fatigue, numbness or tingling, memory problems and a particular kind of anaemia.

What to ask about: a serum B12 (with methylmalonic acid if borderline) at baseline if you take metformin, are vegetarian or vegan, or are over 60; and again yearly. The GLP-1 Lab Guide covers the tests in more detail.

Food: 100 g of beef or salmon, or two eggs plus a serving of dairy, comfortably covers 2.4 mcg. Fortified nutritional yeast and fortified plant milks cover it for people who eat no animal foods.

Vitamin D

Reference intake: 600 IU (15 mcg) a day for adults to age 70, 800 IU (20 mcg) over 70, per the Institute of Medicine's 2011 report, with an upper level of 4,000 IU (NIH ODS).

Why it matters here: obesity is associated with lower circulating 25-hydroxyvitamin D, which the ODS fact sheet attributes partly to sequestration in fat tissue and partly to lower intake and sun exposure. Vitamin D and calcium together are what bone needs during weight loss, and rapid weight loss by any method is associated with some bone loss. The advisory lists vitamin D among nutrients to consider supplementing. Few foods contain much: oily fish, egg yolks, fortified milk and some fortified cereals.

What to ask about: a 25(OH)D level at baseline if you have never had one, and whether your clinician wants you on a supplement at the RDA. Do not take high-dose vitamin D without a measured level and a clinician's instruction.

Iron

Reference intake: 8 mg a day for men and for women over 50; 18 mg for women aged 19 to 50; 27 mg in pregnancy (NIH ODS).

Why it matters here: the most absorbable form, haem iron, is in red meat, poultry and fish, the foods a suppressed appetite most often drops. Non-haem iron in beans, lentils, tofu, fortified cereals and spinach is absorbed less well, better with vitamin C and worse with tea, coffee and calcium at the same meal. Menstruating women start with the highest requirement and the smallest margin. Iron deficiency presents as fatigue, breathlessness on exertion and poor exercise tolerance, all of which get blamed on the diet or the drug.

What to ask about: a full blood count and ferritin at baseline for menstruating women and anyone with fatigue; whether to recheck at six months. Iron supplements are constipating, which is the last thing most people on a GLP-1 need (see hydration and fibre); if one is prescribed, ask about alternate-day dosing and take it away from calcium and coffee.

Calcium

Reference intake: 1,000 mg a day for adults, 1,200 for women over 50 and everyone over 70 (NIH ODS).

Why it matters here: dairy is the main source, and dairy intake falls with total intake. Protein-first eating helps here more than anywhere: Greek yoghurt, cottage cheese, milk in a shake and hard cheese are protein and calcium at once, which is one reason they feature so heavily in the protein guide. Three servings of dairy a day covers the RDA; fortified plant milks, tofu set with calcium, canned fish with bones and leafy greens fill in for people who eat little dairy.

The others on the advisory's list

Magnesium (nuts, seeds, legumes, whole grains), zinc (meat, shellfish, legumes), thiamine (whole grains, pork, legumes), vitamin C (fruit and vegetables) and vitamins A, E and K (vegetables, oils) share a pattern: they come from the bulk of a normal diet, and the bulk is what has gone. A multivitamin and mineral at RDA doses is the usual answer clinicians reach for during rapid weight loss; the advisory frames it as something to consider, not a universal instruction. Ask.

A short protocol to raise with your prescriber

1. At baseline or your next visit: B12, 25(OH)D, ferritin and full blood count, plus whatever the practice already checks (see GLP-1 Lab Guide).
2. Eat to cover calcium and protein through dairy or fortified alternatives daily; iron and B12 through meat, fish, eggs or fortified foods several times a week.
3. Supplement only what a level or a clinician says to supplement, at RDA doses unless told otherwise.
4. Recheck anything that was low, and B12 yearly if you take metformin.

If you take a compounded GLP-1 product, it is not FDA approved and not interchangeable with brand products; the nutrient physiology is the same, but your clinician should know what you take when ordering labs.