Iron, calcium, and constipation, oh my!

Dear Alice,

I need to take an iron supplement and have two concerns: one, I know that many foods interfere with iron absorption. How can I maximize the absorption of any iron (and calcium) supplements? My other concern is that my GI system is very sensitive. How do I deal with constipation that often goes with iron supplements?

— Calcium and Iron Maiden

Dear Calcium and Iron Maiden, 

It seems like you're approaching a supplement regimen with a healthy consideration of various factors like absorption and effects on your system—a great idea! In terms of maximizing the absorption of iron and calcium supplements, there are a number of foods that can prevent or slow down your absorption of supplements (e.g., tea and coffee) and many foods that are rich in iron or calcium (e.g., dried fruit and leafy greens). Read on for more information. 

While there are many foods that are rich in iron, there are in fact certain foods that can inhibit iron absorption. These foods can include uncooked spinach which contains oxalic acid, dairy products and egg whites which contain phosphates, some types of beans with phytates, and tea and coffee that contain tannins. While it would take eating a lot of these foods to seriously impair your ability to absorb iron, you might consider going easy on them while trying to boost iron levels. 

Another dietary tactic to boost iron absorption is to eat vitamin C rich foods with your iron-rich foods or supplement, since vitamin C aids in iron absorption. For example, eating citrus (e.g., oranges, grapefruits, lemons) along with your steamed spinach can help unlock the iron in spinach, while also reducing the oxalic acid. Although not quite as effective as directly eating iron rich foods or taking supplements, you can also try cooking your food in cast iron pots and pans to add a little more iron to your meals. 

In terms of your sensitive gastrointestinal (GI) system, there are important things to consider with both iron and calcium supplements. Constipation is a recognized side effect of iron supplements, although some formulas like hydrolyzed protein chelate may not cause as much constipation. Iron supplements can also lead to other side effects like allergic reactions and overdoses. Calcium supplements often have few side effects, but there’s always the possibility of some tummy troubles like gas, constipation, and bloating. You might try to steer clear of calcium carbonate since this formula seems to win the award for 'Most Likely to Cause Constipation.' Generally, calcium supplements are not recommended for people with certain medical conditions, such as those with hypercalcemia. There are also some concerns about taking both iron and calcium supplements together as they can compete for absorption or can lead to more severe side effects. 

Beyond just looking for gentle and non-constipating types of supplements, you can also try to alleviate constipation by drinking plenty of water and eating fibrous foods like whole grains, fresh fruits, fresh vegetables, and other unprocessed foods (e.g., whole grain bread or pasta instead of white, brown rice instead of white). If you choose to increase your fiber intake, it’s best to do so slowly—too much too soon can cause gas and bloating. When increasing fiber intake, drinking even more water than you think you need can help you stay hydrated and keep everything moving smoothly! 

It may seem like a lot of juggling of different foods, supplements, and timing of the two, but hopefully careful consideration and knowledge will boost your iron and calcium levels to new heights! In any case, it's always a good idea to check with a health care provider first before adding a new supplement to your regime. 

Farewell, fair Calcium and Iron Maiden,

Last updated Jun 02, 2023
Originally published Apr 25, 1997

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