Inositol Phosphates. Ion Chromatography Methods, Intestinal Degradation and Influence on Iron Bioavailability

Sammanfattning: 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.