Nonheme iron absorption and cooking of meat. (Food Science).
Nutrition Research Newsletter, March, 2003

Iron deficiency is a major health problem in developing countries but it is also prevalent in developed countries in women of childbearing ages and children. An increased intake of animal protein could be an important dietary approacb to improve iron status. In addition to a high content' of highly absorbable heme iron, muscle protein (beef, veal, pork, lamb, chicken, and fish) enhances the absorption of nonheme and heme iron, the so-called `meat-effect'.

However, it has not been truly investigated to what extent food preparation, such as cooking, affects the iron absorption promoting effect of meat. Cysteine-containing peptides of meat have been suggested to be responsible for the meat-effect. If this is true, it could be expected to decrease at higher cooking temperatures. Alternatively, higher cooking temperatures may enhance nonheme iron absorption due to structural changes of the meat proteins, such as thermal denaturation. Therefore, a recent study in the Journal of Nutrition investigated the influence of increased cooking temperature of meat on nonheme iron absorption from a composite meal designed to provide an appreciable amount of nonheme iron, but a low availability of iron.

Twenty-one healthy non-smoking and non-pregnant women were included in this study. The subjects were given three test meals: a basic meal without meat (A) and two basic meals that also contained 75 g (raw weight) of pork cooked to different temperatures (B and C). The meat was cooked at 70, 95, or 120 degrees C and two of the three meat meals were randomly assigned to each subject. Two of the meals were served in the ABBA on four consecutive days and the remaining meal was served five weeks later on two consecutive days: CC. The meals were extrinsically labeled with [sup.55]Fe or [sup.59]Fe. Iron absorption was determined from measurements of whole-body [sup.59]Fe retention and the activity of [sup.55]Fe and [sup.59]Fe in blood samples.

The two highest cooking temperatures reduced the heine iron content of the meat to 50% that of the lowest temperature. The cysteine content decreased with increasing cooking temperature.

Nonheme iron absorptions were 0.9%, 0.7% and 2% greater when meat cooked at 70, 95 or 1200C, respectively. Increasing the cooking temperature to 95 or 120 degrees C did not impair nonheme iron absorption compared with cooking at 70 degrees C.

Heine iron is better absorbed than nonheme iron and because the heme iron content of meat diminished by 50% at the highest cooking temperature, the effect of cooking on total iron absorption from meat has to be evaluated. In addition, because the cysteine content of meat decreased with increasing cooking temperature, this argues against a specific contribution of sulfhydryl groups from cysteine residues in the promotion of nonheme iron absorption by meat proteins.

Susan B. Baech, Marianne Hansen, Klaus Bukhave, et al., Increasing the cooking temperature of meat does not affect nonheme iron absorption from a phytate-rich meal in women, J. Nutr 133:94-97 (January 2003)

Source: Nutrition Research Newsletter, March 2003