Key takeaways:
You can safely take calcium and iron together, but calcium may affect how much iron your body absorbs.
For most healthy adults, taking calcium and iron together is unlikely to lower iron levels by a meaningful amount.
If you have iron deficiency, it’s best to take calcium and iron supplements at least 2 hours apart.
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If you take supplements as part of your daily routine, there’s a good chance you take them all together to keep things easy. And in general, there’s nothing wrong with that. But if calcium or iron are one of your more important supplements, you may want to rethink when you take them.
Can you take calcium and iron together?
Yes, you can take calcium and iron together. But calcium can block iron absorption. So in some cases, it may be better to take them separately.
Before you switch up your routine, let’s go through some of the details. First, we need to look at the different kinds of calcium and iron.
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Calcium supplements come in eight different forms. But the two most common are:
Calcium carbonate, which is better absorbed when taken with food
Calcium citrate
There are four different forms of iron supplements:
Ferrous sulfate
Ferrous gluconate
Ferric citrate
Ferric sulfate
But in the research studies, the results were similar no matter which type of iron supplement people took.
And in food, there are two other forms of iron:
Heme iron is found in animal foods and is easily absorbed by your body.
Non-heme iron is found in plant foods and is more difficult for your body to absorb.
With this in mind, let’s walk through what the research shows when people take calcium and iron together.
One analysis looked at 30 studies and found that:
Calcium can reduce iron absorption, especially when you take 2,000 mg or more of calcium at one time.
Calcium is more likely to impact iron absorption when it comes from a supplement rather than calcium-rich foods.
The effect is short-lasting. The authors concluded that, over the long-term, calcium doesn’t have a meaningful effect on iron levels in your body.
An older review paper found similar results:
Calcium chloride and calcium phosphate supplements lowered iron absorption more than other forms of calcium supplements. It didn’t matter whether the iron came from food or a supplement.
Larger doses of calcium blocked more iron absorption. In particular, 300 mg of calcium was the magic number. Higher doses, like 2,000 mg of calcium, didn’t inhibit iron absorption any more than the 300 mg.
Milk and cheese reduced iron absorption from meals, but less than calcium supplements did. Milk interfered most with the absorption of non-heme iron from food.
All these details might feel a bit confusing. But the key takeaway from both of these large reviews is that calcium intake didn’t cause as big of a change in a person's iron levels.
This means that most people can take calcium and iron together without worrying it’ll cause a problem. That being some, people being treated for low calcium levels or low iron levels may want to take the extra precaution of taking them separately.
This may also be more important if you’re at high risk of calcium deficiency or iron deficiency. This includes:
Older women
Pregnant women
Women with heavy periods
People who don’t consume dairy products
People with heart failure
People with cancer
People with gastrointestinal disorders, like Crohn’s disease or ulcerative colitis
People who donate blood frequently
Why does calcium affect the absorption of iron?
Experts aren’t 100% sure why calcium affects iron absorption.
It’s thought that calcium may interfere with iron transportation in your blood and block its absorption. But again, this effect seems to be a short-term effect.
In the lab, researchers found that calcium interfered with how iron moved through cells and into the bloodstream. But after just 4 hours, the effects disappeared. This may explain why calcium intake doesn’t seem to have a long-term effect on iron levels.
What is a good time gap between iron and calcium?
Since calcium can block iron absorption, it’s recommended to take calcium and iron at least 2 hours apart.
So, if you’re enjoying breakfast with a glass of milk, maybe wait 2 hours before taking your iron supplement. Or, if you take both calcium and iron supplements, take one with breakfast and the other with lunch or dinner.
When is the best time to take calcium?
The best time to take calcium depends on the form of supplement you’re taking:
Calcium carbonate is best taken with meals for two reasons. It can cause stomach upset, so taking it with food can help. Food also slows down how quickly calcium moves through your gut, giving your body more time to absorb it.
Calcium citrate can be taken at any time.
Is it better to take iron in the morning or at night?
It’s okay to take iron in the morning or at night. Here are two things to consider:
Taking iron on an empty stomach can help your body absorb it. You may find that taking it when you wake up, about 30 minutes before breakfast, is easy to remember and convenient.
Taking iron with vitamin C may also help your body absorb it. If you take a vitamin C supplement, you can take it at the same time as your iron. Or, you might prefer taking iron with breakfast if you usually drink a beverage rich in vitamin C, like orange juice.
Frequently asked questions
No, if you’re taking iron supplements, you don’t need to avoid calcium-rich foods. Any effect on iron absorption is small and temporary.
Similarly, you don’t need to avoid iron-rich foods if you’re taking calcium supplements. Iron-rich foods are unlikely to significantly interfere with calcium absorption.
If you’re taking iron supplements, you don’t need to avoid calcium. But it may make sense to take your iron 2 hours later if you eat a meal that’s heavy in dairy or drink a glass of milk.
No, it’s safe to take calcium and iron together. There isn’t any known harm in taking them at the same time.
No, if you’re taking iron supplements, you don’t need to avoid calcium-rich foods. Any effect on iron absorption is small and temporary.
Similarly, you don’t need to avoid iron-rich foods if you’re taking calcium supplements. Iron-rich foods are unlikely to significantly interfere with calcium absorption.
If you’re taking iron supplements, you don’t need to avoid calcium. But it may make sense to take your iron 2 hours later if you eat a meal that’s heavy in dairy or drink a glass of milk.
No, it’s safe to take calcium and iron together. There isn’t any known harm in taking them at the same time.
The bottom line
Yes, you can safely take calcium and iron together. Calcium may slightly reduce your body’s ability to absorb iron, especially when taken as a supplement at a dose of 300 mg or more daily. But this effect is short-lasting and it won’t affect your body’s iron levels over the long term. If you have iron or calcium deficiency, separating your supplements by 2 hours may help your body absorb both nutrients as much as possible. Otherwise, don’t stress too much about perfect timing. The best supplement schedule is one you can stick with.
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References
Abioye, A. I., et al. (2021). Calcium intake and iron status in human studies: A systematic review and dose-response meta-analysis of randomized trials and crossover studies. The Journal of Nutrition.
Deng, J., et al. (2024). Efficacy of vitamin C with Fe supplementation in patients with iron deficiency anemia: A systematic review and meta-analysis. Blood Vessels, Thrombosis & Hemostasis.
Harris, S. S., et al. (2008). The effect of calcium consumption on iron absorption and iron status. Nutrition in Clinical Care.
Lönnerdal, B. (2013). Calcium and iron absorption - mechanisms and public health relevance. International Journal for Vitamin and Nutrition Research.
Lynch, S. R., et al. (2001). The effect of calcium on iron absorption. Nutrition Research Reviews.
National Institutes of Health, Office of Dietary Supplement. (2023). Calcium, fact sheet for consumers.
National Institutes of Health, Office of Dietary Supplement. (2023). Iron, fact sheet for consumers.
New York State Department of Health. (2015). All about calcium supplements.
Piskin, E., et al. (2022). Iron absorption: Factors, limitations, and improvement methods. ACS Omega.
Skibsted, L. H. (2016). Mineral nutrient interaction: Improving bioavailability of calcium and iron. Food Science and Biotechnology.
