Both calcium and iron are inorganic minerals instrumental to the functions of the human body. Though iron can be stored internally, a certain amount of both minerals must be consumed daily in the diet. However, calcium may also have a negative effect on the absorption of iron.
Absorption
Absorption occurs after the nutrients in food have been broken down into constituent parts through the digestive process. Absorption tends to occur in the small intestine. Iron is specifically absorbed in the duodenum, the shortest part of the small intestine, by binding to the protein transferrin. Here nutrients diffuse directly into the blood so that they can be carried to the target cells and processed for their eventual functions in the body.
Calcium
The presence of vitamin D in the body is one of the few things that can improve the efficacy of calcium absorption. However, there are a number of factors that can negatively affect the rate of absorption, including the progression of age. Calcium is absorbed at a rate of 60 percent in infants and young children, who need a lot of calcium to build bones, but it decreases to 15 or 20 percent in adulthood. The efficiency of absorption can also decrease as the amount of calcium consumed during a meal increases. Other components in food such as phytic acid and oxalic acid can bind to calcium and inhibit its absorption. In addition, caffeine, protein and sodium intake increase the rate at which calcium is excreted from the body.
Iron
The body only absorbs about 10 to 15 percent of dietary iron, although the rate increases when iron stores run low. The opposite is also true: the rate of absorption decreases to guard against the toxic effects of iron overload. Heme iron -- an iron atom contained in the center of a large organic ring -- is absorbed at a rate of 15 to 35 percent. Non-heme iron, however, is only absorbed at a rate of 2 to 20 percent. Non-heme iron is the only type of iron available in plant foods such as rice, maize, soybeans and wheat. Meat protein and vitamin C can improve the absorption of non-heme iron. Polyphenols and phytates can decrease it.
Function
According to the Office of Dietary Supplements, almost two-thirds of the iron in the body is found in hemoglobin, the protein in red blood cells that carries oxygen to tissues. The rest of the iron is found in myoglobin, which is a protein that helps supply oxygen to the muscles and enzymes that facilitate biochemical reactions. Calcium, on the other hand, is an important molecule required for muscle contraction, blood vessel expansion, secretion of hormones and enzymes and the transmission of impulses through the nervous system. However, these functions constitute less than 1 percent of the calcium in the body. The remaining 99 percent is stored in bones and teeth to support their structure. Calcium is the most abundant mineral in the body.
Warning
Several studies have been conducted to discover the effect that the presence of calcium has upon iron absorption. These studies were summarized by Leif Hallberg in "The American Journal of Clinical Nutrition." He concluded that calcium inhibits the absorption of both heme and non-heme iron.
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