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Calcium may actually inhibit the body’s ability to transport iron into cells, but this only occurs under specific conditions and to a pretty minimal extent.
For several decades now, a quieter vein of research drowned out by generic pro-milk and pro-beef awareness campaigns has been pointing out important distinctions that these highly vocal ad agencies keep glossing over: even healthy nutrients don’t always play well together.
Nutrition scientists are still working on the mechanisms driving this effect, and some argue that the relationship between calcium intake and iron absorption doesn’t exist at all.
Even if the jury weren’t split, how does the everyday person act on this information?
Are calcium and iron the only two nutrients seemingly at odds with each other?
Let’s start with a look at the evidence.
How Calcium May Block Iron Absorption
A sizable portion of studies focusing on the calcium-iron conundrum don’t dive into the specific mechanisms involved, but a few more robust studies have at least thrown out some theories.
One such theory is that calcium suppresses a protein called ferroportin (FPN).
The Ferroportin (FPN) Suppression Theory:
A study from UC Davis confirmed the author’s hypothesis that calcium intake can suppress the action of ferroportin, a protein that transports iron from the inside of a cell to the outside.
The researcher added calcium chloride to human intestinal cell cultures (cells grown in a lab environment), measuring the presence of ferroportin and other substances involved with iron uptake at one and a half and four hours after the calcium was administered.
He then discovered that, even though total iron uptake was unaffected at the 1.5-hour mark, the presence of FPN at the cell membranes was decreased.
At the four-hour mark, however, FPN levels had largely recovered, “suggesting a rebound effect.”
In other words, calcium didn’t affect the amount of iron that entered the cells, but it did temporarily affect the cells’ ability to export the iron into (what would be) human circulation.
Considering most other studies using larger windows of time argue that there is no effect of calcium on iron absorption whatsoever, this study reinforces the idea that the effect is short-lived.
Still, just because the body can recover its ability to transport iron, that doesn’t mean it recoups all the iron it didn’t absorb after rebounding.
Effects of the Calcium-Iron Ratio on Iron Absorption
Apart from any investigations into the specific mechanisms involved, several studies have managed to draw some connections between calcium-iron levels and the severity of any resulting loss in iron absorption.
For example, this finding from the Kansas University Medical Center Department of Medicine found that all three forms of calcium given to its 61 participants (calcium citrate, calcium phosphate, and calcium carbonate) suppressed iron absorption when taken with food.
As one might expect, high-iron, low-calcium meals were less effective at suppressing iron absorption (28%) than low-iron, high-calcium meals (55%).
Where most sources agree that this dip in iron absorption is negligibly small for the majority of people, those with greater needs for calcium and/or iron may fall short of their higher daily requirements if they have their supplements at mealtime.
What This Means for Iron-Deficient Populations
Disorders, pregnancy, gender, and age can all raise a person’s need for iron.
There are plenty of common and less common disorders that can affect iron uptake, but for the sake of capturing the largest chunk of the general population, here are some of the largest populations at greater risk for iron deficiencies:
- Vegetarians and vegans
- Pregnant women
- Children and infants failing to thrive
- Young and middle-aged women
- People with highly processed food diets
- Iron deficiency anemia patients
It is especially important for pregnant women, who tend to be deficient in iron as well as calcium, to optimize absorption of both of these key nutrients every day.
So, how can pregnant women and the rest of these populations accomplish this?
Considering the impact of food on iron absorption, it’s best to take iron with water alone, barring any objections from your physician.
By allowing your body to take in the iron without any competing calcium, you can maximize absorption.
For those who can’t take iron outside of mealtime for whatever reason, you can still compromise by swapping out dairy-based proteins (aka, calcium-rich foods) for non-dairy protein sources like lean meat, nuts (looking at you, vegetarians/vegans), legumes, etc.
Of course, simply upping your iron intake will help, but that’s kind of like constantly pouring more water into a leaky container instead of just patching the container.
With these two methods—taking iron supplements outside of mealtime and/or swapping some dairy-based proteins for others—you can keep your iron absorption capability on the higher end.
One Connection Among Many
We wish we could tell you that calcium is the only nutrient that can elbow others out of contention, but there are many, many other associations like the calcium-iron connection among dozens of other nutrients.
In a karmic reversal of sorts, even calcium uptake can be inhibited by foods that contain phytic and oxalic acids (spinach, beans, nuts, sweet potatoes, and others), making it tougher for people to meet their daily requirement.
It’s not all bad news, however, because we Westerners especially could do with a bit of competition on the fat absorption front.
Ultimately, it’s a matter of targeting what you’re deficient or borderline-deficient in and clearing the way for optimal absorption.