Inositol Phosphates. Ion Chromatography Methods, Intestinal Degradation and Influence on Iron Bioavailability
Doctoral thesis, 1998
Inositol phosphates are presently a topic of intense investigation, owing to their nutritional and physiological importance for plants and animals. This thesis presents new methods for the analysis of inositol mono- to hexaphosphates (IP1-IP6) and their isomers. The methods are based on extraction with diluted HCl, purification on anion-exchange resins, separation using high-performance ion chromatography and detection with optical or electrochemical techniques. The analytical systems permit determination of inositol phosphates in food, digesta and other biological samples with high sensitivity and separation of close to 30 of the 39 possible isomers (discounting enantiomers).
Another part of the thesis concerns the effects of feed treatments on the degradation of IP6 in the feed and gastrointestinal tract of pigs. The intentions were to increase the phosphorus utilization of IP6 by pigs and, accordingly, to decrease the phosphorus level in the manure. It was concluded that steeping as compared to pelleting of a mixed diet reduced ileal IP6 content and enhanced the apparent phosphorus absorption in the ileum. Calcium carbonate supplementation impaired IP6 degradation in the colon of pigs, but did not affect IP6 degradation in the stomach and small intestine. The best result with respect to reduced amount of fecal IP6 (by 64%) was demonstrated in pigs fed a barley-rapeseed meal diet, both steeped with whey and supplemented with microbial phytase. Depending on the origin of the phytase enzymes (microbial and/or cereal) present during the IP6 degradation, different isomeric forms of the IP6 degradation products were formed and pathways for IP6 degradation proposed.
The thesis also considers the influence of inositol phosphates on iron bioavailability in vitro using human intestinal Caco-2 cells and in vivo in humans. The addition of isolated inositol phosphates showed that IP6 and IP5 inhibited iron absorption, while IP4 and IP3 did not. However, in breads containing a mixture of inositol phosphates with different degrees of phosphorylation, IP4 and IP3 contributed to the negative effect on iron absorption in humans. The inhibitory effect of these less phosphorylated inositol phosphates was likely the result of stable complexes formed by interactions with the more phosphorylated inositol phosphates through iron. No significant variation in iron uptake between the addition of various isomers of IP4 and IP3 was shown in Caco-2 cells.