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U.S. Plant, Soil, and Nutrition Laboratory USDA/ARS, Cornell University Ithaca, New York, 14853
* To whom correspondence should be addressed. E-mail: rpg3{at}cornell.edu.
Dear Editor,
In the May issue of The Journal of Nutrition, Storcksdieck genannt Bonsmann et al. (1) reported that a previous study by Huh et al. (2) prompted their study of the effect of glycosaminoglycan (GAG)1 compounds, namely sodium hyaluronate (NaH) and chondroitin sulfate (CS), on nonheme iron absorption of young women from a semisynthetic meal prepared from hydrolyzed corn starch, chicken egg albumin, and corn oil. One of their main conclusions was that GAG are not a major component of the "meat factor" and thus they are in disagreement with the in vitro results of Huh et al. (2).
In the study by Huh et al. (2), the "meat factors" were extracted from fish tissue incubated for 1 h at pH 2, and the combined information from a suite of analyses showed that low molecular weight carbohydrates (<5000 Da), namely glycosaminoglycans, were predominant in the isolated fractions with enhanced iron availability. It is important to note that Storcksdieck genannt Bonsmann et al. (1) used purified GAG at much higher molecular weights (NaH at MW of 1000–1200 kDa and CS at MW of 20 kDa, respectively). Thus, the experimental design in the Storcksdieck genannt Bonsmann et al. study was very different from the Huh et al. study. Huh et al. clearly pointed out that meat did not have an enhancing effect on nonheme iron uptake when it was incubated at pH 7 or in water extract, and the acid digestion procedure appeared necessary to generate the enhancing effect of meat. Furthermore, Dr. Storcksdieck genannt Bonsmann and colleagues also sent to our laboratory the same meals for iron bioavailability analysis via the in vitro digestion/Caco-2 cell culture model. As in the human study, we observed no enhancing effect of NaH and CS on iron uptake in these meals. These results were not mentioned in the Storcksdieck genannt Bonsmann et al. article and are important, as in combination with the other observations mentioned above, suggesting that an interaction between GAG and some other components of meat are involved in the enhancing effect of meat on Fe uptake. We feel it is important to mention these observations here, as it could be inferred from the Storcksdieck genannt Bonsmann article that the human studies differed from the in vitro model, when in fact they did not. This observation may be key to defining the compounds associated with the meat factor.
Although the Huh et al. work identified glycosaminoglycan components in an extract of meat, they were unable to isolate a specific compound from the extract. This is not surprising, as glycosaminoglycans are structurally diverse with regard to the number of sulfate groups, and to the biological form in which they exist, and this characteristic makes them difficult to isolate and purify (3). Continued research on the meat factor is warranted to determine whether this class of compounds is involved in the "meat factor" effect. A thorough screening of low molecular GAG for their promoting effect on Fe absorption would probably shed new light on this subject. In addition, purification and characterization of the acid extracts from Huh et al. study could be very useful in advancing knowledge about this factor. However, the challenge there will be to obtain a high level of extraction of the active factor for both in vitro and in vivo studies. It appears that the "meat factor" remains elusive.
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FOOTNOTES |
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Manuscript received 6 July 2007.
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LITERATURE CITED |
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 TOP LITERATURE CITED |
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1. Storcksdieck genannt Bonsmann S, Walczyk T, Renggli S, Hurrell RF. Nonheme iron absorption in young women is not influenced by purified sulfated and unsulfated glycosaminoglycans. J Nutr. 2007;137:1161–4.
2. Huh EC, Hotchkiss A, Brouillette J, Glahn RP. Carbohydrate fractions from cooked fish promote iron uptake by Caco-2 cells. J Nutr. 2004;134:1681–9.
3. Sasisekharan R, Myette JR. The sweet science of glycobiology. Am. Sci. 2003;91:432–41.
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