The amount of advice we find in the media, social networks and nutrition clinics it is simply overwhelming. Also overly simplistic.
One of them advise against consuming yogurt or cow’s milk when suffering from iron deficiency anemia, that is, anemia due to a low amount of iron circulating in the blood. It is based on the fact that the high concentration of calcium in these foods inhibits the absorption of iron in the intestine.
Well, this statement seems to have little scientific support. Many of the studies linking calcium and iron have been done on cells grown in the laboratory. And although in vitro tests help to understand the mechanisms involved in the transport of these molecules, they must be taken with caution.
Fundamentally because the conditions in which they are carried out do not resemble those that occur in the human intestine. But also because the cells that are used are not exactly the same as those involved in the absorption of nutrients, nor are they arranged to form an equally consistent barrier.
doors open to iron
All the nutrients that we ingest and that end up being incorporated into the bloodstream must pass, in one way or another, through the cells that line the intestine. The surface of these cells contains a series of proteins They act as gateways for the nutrients contained in the porridge that food turns into as it passes through the stomach.
In the case of minerals in the diet, these carrier proteins function as channels that open to allow them to pass into the cells and from the cells to the blood. One of these transporters, called DMT1, It is responsible for specifically incorporating iron. And interestingly, in vitro assays have shown that dietary calcium inhibits this transporter.
These interferences make sense because both compounds share transporters or they are so similar that one mineral interferes with the transport of the other. That would explain why high concentrations of iron in the diet also interfere with calcium absorption. In fact, although it is not as well known, iron supplements can cause calcium deficiency.
By the way, There is not only calcium in dairy products in the diet: it is also abundant in legumes, nuts, sardines in oil and shellfish. However, no one warns that eating meat with almonds could be a problem.
A plate of lentils and, for dessert, yogurt?
Taking into account the studies carried out in vitro, foods rich in calcium could interfere with the absorption of iron that contain meat and other foods.
But what happens when the phenomenon is studied in vivo? We assume that it is much more complicated. In general, research is carried out in which volunteers or patients take a certain amount of product and then they are monitored how and how much of that product reaches the blood. We know these procedures as bioavailability tests.
Well, the conclusions bioavailability assays contradict each other. On the one hand, certain studies carried out in fasting women show that only high concentrations of a specific calcium salt (calcium citrate) are capable of inhibiting the incorporation of iron into the blood. That would imply that the effect of calcium in dairy products would be null or, at least, biologically insignificant.
However, in another study also conducted in women, half a gram of calcium came to halve the absorption of iron. Of course, we must not lose sight of the fact that reaching 500 mg of calcium in the intestine from milk or yogurt would imply consuming a very high amount of both. Therefore, ingesting a yogurt or a glass of milk would still not be a problem.
What is true is that, When comparing human milk and cow’s milk, It has been proven that the higher calcium content in cow’s causes iron to be absorbed worse. That is why it is not recommended to incorporate cow’s milk into infant feeding until a certain age is reached.
No evidence for complex diets
In summary, we can say that human studies confirm some interference of calcium on iron absorption, but only with very high calcium concentrations.
In addition, these studies do not clarify what happens in the case of the intake of a complex diet in which a glass of milk or yogurt is one of the many nutrients and the amount of iron is not carefully controlled.
On the other hand, we also do not know what role the microbiome of each individual in the absorption of minerals, although the results obtained so far indicate that it could also be key.
In any case, the benefits of the characteristic dairy nutrients are important enough enough to recommend its consumption in those people who do not present any specific iron absorption problem.