![[Feedback]](/icons/banner/feedbackACT.gif){kind=icon}
![[For Subscribers]](/icons/banner/subscriberACT.gif){kind=icon}
![[Archive]](/icons/banner/archiveACT.gif){kind=icon}
![[Search]](/icons/banner/searchACT.gif){kind=icon}
![[Contents]](/icons/banner/tocACT.gif){kind=icon}
|
|
|
|
|
||
ETH Zurich, Institute of Food Science and Nutrition 8092 Zurich, Switzerland
* To whom correspondence should be addressed. E-mail: richard.hurrell{at}ilw.agrl.ethz.ch.
Dear Editor,
The comments of Jin and Glahn concerning our article (1) in the May issue of The Journal of Nutrition raise 2 issues. The first concerns the nature of the "meat factor" and the possible role of the glycosaminoglycans (GAG).1 The second issue is the usefulness of the Caco-2 cell methodology to predict changes in iron absorption in humans.
The aim of our study was to evaluate the effect of GAG on human nonheme iron absorption. We chose 2 GAG, sodium hyaluronate (NaH) and chondroitin sulfate (CS), which are representative of the sulfated and unsulfated forms (and which are present in meat in relatively high amounts). Other GAG structures, however, are also present. The NaH and CS forms we tested had no influence on human nonheme iron absorption. In contrast, Jin and Glahn, in their unpublished studies, observed a significant, 50% increase in Caco-2 cell 58Fe uptake (P 
0.05) in the presence of NaH compared with CS and control meals. Total Fe uptake was not different from control in the case of NaH, but decreased significantly in the presence of CS (P 0.05). Any activating impact of acid treatment on GAG, as emphasized by Jin and Glahn, should have become apparent during gastric digestion in our subjects, yet this was not the case. It is still possible that some other (GAG) component extracted by Huh et al. (2) from fish stimulated the Caco-2 cell system.
Human absorption studies are the gold standard for assessing dietary iron bioavailability. They obviate the need for assumptions about potentially important physiological factors when using in vitro models. The latest expert consensus (3) is that Caco-2 cell experiments are useful for predicting the correct direction of response in humans but not for predicting its magnitude. Nevertheless, Lynch (4) pointed out noteworthy discrepancies between human absorption data and Caco-2 iron uptake results. Additionally, the dose of GAG extract active in the cell culture model (2) was higher than the amounts consumed within a regular diet. We also used higher-than-normal doses of NaH and CS to avoid the risk of a nil effect due to insufficient quantities of GAG; however, the weight ratio of GAG:Fe in the Caco-2 study (2) was 
8000:1 compared with a weight ratio in our study (1) of 80:1.
Our recent publications (5,6) provided strong evidence that the "meat factor" is mainly the result of the digested peptides of meat protein. One of these (6) suggested that meat protein is different from other proteins because it is digested rapidly by pepsin in the stomach to provide a large amount of iron-binding low-molecular-weight peptides early in the digestive process. Iron binds strongly to these peptides and is kept soluble and available for absorption. The binding sites are probably cysteine, histidine, and/or aspartic and glutamic acids. We do not think that one single peptide or meal component is the "meat factor" but that the "meat factor" is the combined effect of the mass of peptides that are released upon meat digestion. It is of course possible that GAG may still play a minor role but, based on the evidence presented by Huh et al., Jin and Glahn, and ourselves, our conclusion remains that GAG at normal levels are unlikely to contribute substantially to the "meat factor" but that the weight of the evidence is tilted toward a protein origin (5–8).
 |
FOOTNOTES |
|---|
Manuscript received 30 July 2007.
|
LITERATURE CITED |
|---|
 TOP LITERATURE CITED |
|---|
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. Fairweather-Tait S, Lynch S, Hotz C, Hurrell RF, Abrahamse L, Beebe S, Bering S, Bukhave K, Glahn R, et al. The usefulness of in vitro models to predict the bioavailability of iron and zinc - A consensus statement from the HarvestPlus expert consultation. Int J Vitam Nutr Res. 2005;75:371–4.[Medline]
4. Lynch S. The precision of in vitro methods and algorithms for predicting the bioavailability of dietary iron. Int J Vitam Nutr Res. 2005;75:436–45.[Medline]
5. Hurrell RF, Reddy MB, Juillerat M, Cook JD. Meat protein fractions enhance nonheme iron absorption in humans. J Nutr. 2006;136:2808–12.
6. Storcksdieck genannt Bonsmann S, Hurrell RF. Iron-binding properties, amino acid composition, and structure of muscle tissue peptides from in vitro digestion of different meat sources. J Food Sci. 2007;72:S19–29.
7. Layrisse M, Martinez-Torres C, Roche M. Effect of interaction of various foods on iron absorption. Am J Clin Nutr. 1968;21:1175–83.[Abstract]
8. Monsen ER, Cook JD. Food iron absorption in human subjects. V. Effects of the major dietary constituents of semisynthetic meal. Am J Clin Nutr. 1979;32:804–8.
This article has been cited by other articles:
|
|
 |
|
  C. N. Armah, P. Sharp, F. A. Mellon, S. Pariagh, E. K. Lund, J. R. Dainty, B. Teucher, and S. J. Fairweather-Tait L-{alpha}-Glycerophosphocholine Contributes to Meat's Enhancement of Nonheme Iron Absorption J. Nutr., May 1, 2008; 138(5): 873 - 877. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
