One of the biggest nutritional concerns of vegans is getting enough iron.
Most iron in plant sources is non-heme iron, which has a low absorption rate (the average is usually between 8-17% depending on the exact source and circumstances ).
Anecdotally, my iron levels went down for the first few times I donated blood after going vegan.
Naturally, I did some research online and learned that vitamin C increases non-heme iron absorption  significantly, up to 3-6 times the normal absorption rate .
It’s so effective that it’s even included in most of the best vegan iron supplements.
But I couldn’t find a clear answer to exactly how much vitamin C I needed, and when I needed to have the vitamin C.
So I was left to dig into studies and eventually found my answers, which I’m going to share with you here.
There are 2 main questions I’m going to answer:
- When do you need to consume vitamin C? (While eating the iron? Within 30 minutes? A day?)
- How much vitamin C do you need to have for iron absorption to be effective.
Before we get started, here’s a quick post on the top plant sources of iron if you need a quick refresher.
1. How soon after eating non-heme iron do you need to consume Vitamin C?
I’ve read a few times that it doesn’t matter when you have the vitamin C, as long as it’s within the same day.
All the studies I’ve read say this is wrong. You need to eat the vitamin C at the same time (or very shortly after) you eat the iron.
In one such study, subjects were split into 3 groups: self-selected diet, high vitamin C diet, and low vitamin C diet .
All 3 of these types of diets only considered vitamin C throughout the day, but not at specific meals.
They found that the high vitamin C has a very small effect on iron absorption.
As you can see, there’s no real correlation between the iron absorption and vitamin C intake.
Compare that to other studies that find the iron absorption goes up by up to 4 times when pairing vitamin C with iron intake at specific meals .
In short: When you’re eating high non-heme iron meals, try to include as much vitamin C as possible. More usually results in a higher rate of absorption.
2. How much vitamin C do you need to consume?
I’ll give you the short and simple answer first:
You should have at least 14 mg of vitamin C per mg of iron. It doesn’t matter whether that vitamin C comes from food or supplements.
Let me go over where I found that number in research, so you know that I didn’t just pull it out of nowhere.
There were 2 studies in particular that both found that this minimum ratio is highly effective [5,6]
Both studies found that a molar ratio of 4:1 of ascorbic acid to iron was the most effective.
For reference, the molecular weight of ascorbic acid (vitamin C) is 176 grams per mole, and the molecular weight of Fe is 55.84 grams per mole.
I’ve done the math and double checked with the numbers in each study, and a 4:1 molar ratio works out to about 70 mg of vitamin C per 5 mg of iron (or 14:1 on a mg basis).
This “magic” ratio resulted in a 2.9x increase in iron absorption.
That seems to be about as good as you’ll get from what I’ve read in studies. It also doesn’t matter if the vitamin C comes from food or supplement form.
What does this look like in real food situations?
Referring back to that list of the top vegan food sources of iron, oats are right near the top.
They contain 7.4 mg of iron per cup, meaning you’d want around 103.6 mg of vitamin C. Naturally, it makes sense to pair oatmeal with fruit.
Here’s a list of the best vegan foods with vitamin C.
Contrary to popular believe, apples and blueberries both suck when it comes to vitamin C. Your only real option here is strawberries; 7 strawberries have about 112 mg of vitamin C.
So even though oats are a good source of plant iron, it’s hard to get enough vitamin C with them unless you love strawberries, or want to use vitamin C supplements.
Let’s look at legumes on the other hand, all of which have lots of iron.
For example, 1 cup of lentils have 6.6 mg of iron. There’s a lot of vegetables high in vitamin C, so you have a lot of options to reach the 14:1 ratio (92.4 mg vitamin C):
- About 1/2 red bell pepper
- 1 bunch of spinach
- 1 cup of broccoli
- 2 cups of peas
Long story short, vitamin C is probably the best way to increase the amount of iron you actually absorb, which is particularly important for vegans.
The research shows 3 main things:
- You should eat vitamin C alongside iron (or very shortly after).
- You want to aim for about a 14:1 ratio of vitamin C to iron (on a mg basis). It results in about a 2.9 times increase in absorption.
- It doesn’t matter if vitamin C is from food or supplements.
Finally, I highly recommend focusing on getting most of your iron from legumes from a practical standpoint. In my opinion, legumes pair with foods that are high in vitamin C much easier that oats, seeds, or nuts.
If you have any questions, just leave a comment below.