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ARS Home » Northeast Area » Ithaca, New York » Robert W. Holley Center for Agriculture & Health » Plant, Soil and Nutrition Research » Research » Publications at this Location » Publication #320551
Title: Extrinsic labelling of staple food crops with isotopic iron does not consistently result in full equilibration: Revisiting the methodology

Author
item Glahn, Raymond
item CHENG, ZHIQIANG - Cornell University
item GIRI, SHREE - Cornell University

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/19/2015
Publication Date: 10/11/2015
Citation: Glahn, R.P., Cheng, Z., Giri, S. 2015. Extrinsic labelling of staple food crops with isotopic iron does not consistently result in full equilibration: Revisiting the methodology. Journal of Agricultural and Food Chemistry. 63(43):9621-9628.

Interpretive Summary: Intestinal absorption of iron (Fe), an essential nutrient, is often studied by adding a rare form of iron (i.e. an isotope) to the food of study, and then measuring the concentration of that Fe in the blood. This method is known as “extrinsic isotopic labeling” and has been used for over 50 years to measure Fe absorption. This method is based on the assumption that the added or “extrinsic” Fe mixes completely with the Fe already in the meal (i.e. the “intrinsic Fe”). Thus, if the mixing is complete, and by knowing the total amount of Fe in the food, then absorption of the extrinsic Fe can be used to accurately measure the total Fe absorbed from the meal or food to which it was added. The present study tested this assumption via use of a simulated digestion method. Samples of maize, white beans, black beans, red beans and lentils were evaluated. In the beans, the results clearly showed that the extrinsic Fe did not completely mix with the intrinsic Fe of the food. The effect varied depending upon the seed coat color, the harvest (i.e. sample), and the concentration of the extrinsic Fe. For lentils and maize the extrinsic Fe also did not exchange well with the intrinsic Fe and the effect varied depending on variety and harvest. Overall, there was no crop in this study that consistently demonstrated complete mixing of the extrinsic Fe with the intrinsic Fe. These observations call into question the accuracy of Fe absorption studies where isotopic extrinsic Fe was used to evaluate Fe absorption. The results further suggest that at the very least, the primary assumption of the extrinsic labeling method should be tested under the conditions specific to a given study, and if possible, depending on the results, calculations and presentation of the data should be adjusted to account for potential error in measurement due to incomplete mixing of the extrinsic Fe.

Technical Abstract: Extrinsic isotopic labeling of food Fe has been used for over 50 years to measure Fe absorption. This method is based on the assumption that complete equilibration occurs between the extrinsic and the intrinsic Fe prior to intestinal absorption. The present study tested this assumption via use of in vitro digestion of varieties of maize, white beans, black beans, red beans and lentils. Prior to digestion, foods were extrinsically labeled with 58Fe at concentrations of 1, 10, 50 and 100% of the intrinsic 56Fe. Following an established gastric and intestinal in vitro digestion protocol, the in vitro digest was centrifuged and the Fe solubility of the extrinsic 58Fe and the intrinsic 56Fe were compared as a measure of extrinsic/intrinsic equilibration. In the beans, significantly more of the extrinsic Fe (up to 2-3 times) partitioned into the supernatant. The effect varied depending upon the seed coat color, the harvest (i.e. sample), and the concentration of the extrinsic Fe. For lentils and maize, the extrinsic Fe tended to partition into the insoluble fraction and also varied depending on variety and harvest. There was no crop in this study that consistently demonstrated full equilibration of the extrinsic Fe with the intrinsic Fe. These observations call into question the accuracy of Fe absorption studies where isotopic extrinsic Fe was used to evaluate Fe absorption and bioavailability. The results further suggest that at the very least, the primary assumption of the extrinsic labeling method should be tested under the conditions specific to a given study, and if possible, depending on the results, calculations and presentation of the data should be adjusted to account for potential error in measurement due to incomplete equilibration of the extrinsic Fe.