Of all the nutritional gaps that develop on a GLP-1, these two are the ones most likely to make you feel genuinely unwell — not just a little tired, but bone-tired, foggy, and like something is off in a way you can't quite name. That feeling gets blamed on the medication, or stress, or the pace of the weight loss. Sometimes it really is those things. Often it's B12 and iron quietly running down, month by month.

They're also the two most women don't catch until symptoms are significant — because the foods that supply them are usually the first to become unappealing.

B12 is found almost exclusively in animal products — meat, fish, dairy, eggs. These are precisely the foods that get hard to eat consistently when appetite suppression is strong. Someone who used to eat salmon twice a week may find the thought of it unappealing for months at a time.

There's also an absorption issue specific to GLP-1s. B12 uptake requires adequate stomach acid and a protein called intrinsic factor. Both are produced in smaller amounts when food intake drops and digestion slows. So even the B12 you do eat is absorbed less efficiently.

The symptom to take most seriously is tingling or numbness in your hands and feet. That means the deficiency has reached nerve tissue, and it gets harder to reverse the longer it's left. Don't wait for your next routine appointment if that's happening.

How to tell them apart

What to do about B12

Lab to request: serum B12. If it comes back borderline (roughly 200–400 pg/mL), ask for methylmalonic acid.
Best form: methylcobalamin, not cyanocobalamin. Sublingual or liquid is preferred — it bypasses the reduced stomach acid issue.
Dose: 1,000mcg daily. Water-soluble and low-risk at standard doses.

Iron is concentrated in red meat, leafy greens, legumes, and shellfish. On a GLP-1, red meat is often the first food that feels too heavy to eat, and greens get swapped for milder options. Over months, the cumulative drop is significant.

For women who are still menstruating, the risk compounds — you're losing iron monthly and replacing it at a fraction of the previous rate. That's why iron deficiency develops faster in this group, and why it can slide toward anemia without feeling dramatic until it's already well established.

Iron deficiency also feels different from B12 fatigue: more physical, more in the body. One quick check you can do at home — pull down your lower eyelid and look at the inside. It should be pink. Pale or white is worth noting.

What to do about iron

Labs to request: serum ferritin, serum iron, TIBC, and a CBC. Ferritin catches depletion before hemoglobin drops.
Best form: ferrous bisglycinate — gentler on the stomach than ferrous sulfate and better absorbed.
How to take it: with vitamin C. Not alongside coffee, calcium, or a zinc supplement.
Safe without labs? No. Excess iron causes real harm. Get ferritin checked before you supplement.

B12 and iron don't just share symptoms — they tend to develop on the same timeline, because the foods that supply both are the same foods that get hardest to eat. A woman who was borderline on either before starting is often low on both by month four or five.

When both are low at once, the fatigue compounds in a way that makes it genuinely hard to function at your previous level. That's the "something is really wrong" phase that brings women to a doctor without a clear explanation. Usually the explanation is right there in the labs.

Continue to Part 3: Vitamin D and zinc — the quiet deficiencies →