🩸 Can low B12 or folate be why iron isn't fixing your anemia?

Working through iron keeps teaching me the same thing, and keeps surprising me. To understand how iron works, I have to take it one piece at a time — copper one week, vitamin A the next, stomach acid after that. One nutrient, one mechanism. It’s the only way I know how to get my head around something this complicated.

But our bodies don’t run in pieces. It runs as a whole complex system, and every one of those pieces has to be working for your iron to work too.

Vitamin B12 and folate are a good example of this. B12 comes up more frequently than I’d expect among my friends, especially after having kids.

And folate, we hear about all the time. It’s important enough that governments add it to our food. We know they’re important. What do they mean for iron?

That’s what this issue is about: the two vitamins that decide whether the iron in your body ever gets used.

The main reason you have iron is to build red blood cells. About two-thirds of your body’s iron sits in them, as the hemoglobin that carries oxygen. But iron is only half of what a red cell needs.

Building a red blood cell requires iron — its cargo. But first, the marrow has to make the cell: divide it, over and over, and that division needs vitamin B12 and folate — the vehicle carrying the cargo — to build the DNA for each new cell.

So if vitamin B12 or folate is missing, the marrow can’t push out new cells and your iron effectively has nowhere to go.

What’s crazy is that absorption, transport, and storage can all be working properly. The developing cells still enlarge, load up on hemoglobin…and then, die in the marrow before they mature. The result is anemia, even though your iron levels are objectively fine.

Here’s what causes diagnosis problems for even good workups. When your doctor does a blood work up looking at anemia, one of the first numbers they read is your MCV — mean corpuscular volume, the average size of your red blood cells.

MCV comes standard on a blood count and its used to shortcut to the cause: Small cells (low MCV) point to iron deficiency. Large cells (high MCV) point to B12 or folate deficiency.

Having both at once means you get two populations: some too small and some too large. And since MCV is their average, you can land squarely in the normal range while nothing about your cell size is really normal.

But your panel isn’t actually blind to this. RDW — red blood cell distribution width — is a measure of how much your cells vary in size and also comes standard. Research shows it’s widened in nearly every case of these double deficiencies.

RDW can be the clue you need to measure your B12 and folate directly.

Only if you’re short. Adding vitamin B12 or folate helps only when you’re actually deficient.

There’s no evidence extra B12 or folate does anything more for iron or energy in someone who already has enough.

A note. Vitamin B12 tends to be the more likely hidden deficiency, since grains are often fortified with folate.

🔎 Check the MCV and RDW of your last blood panel. A wide RDW beside a normal average cell size could be your cue to get B12 and folate tested.

💊 Are you on a PPI (omeprazole, lansoprazole, esomeprazole) or metformin? Both lower B12 over time. Worth noting so that your B12 actually gets checked.

🥩 Vitamin B12 comes almost entirely from animal foods: meat, fish, shellfish, eggs, and dairy. Plants don’t supply usable B12, so if you eat little or none, you’ll need fortified foods or a supplement. If you already eat these and are still low, absorption might be the larger issue for you.

🥬 Folate is in leafy greens, beans, lentils, and (in the US and UK) fortified grains. Glance at your week. A few servings and you're almost certainly covered.