Efficacy of the rat bioassay for the determination of biologically available vitamin B-6.

Research was conducted to evaluate the merits of rat bioassays in studies concerning the bioavailability of vitamin B-6. A protocol was devised, which included pair-feeding and prevention of coprophagy. Plasma pyridoxal 5'-phosphate (PLP) was measured by a rapid chromatographic method as an indicator of vitamin B-6 status. Pyridoxamine and pyridoxal exhibited 70% molar activity relative to pyridoxine at suboptimal dietary levels. Selected foods (spinach, cornmeal and potato) were evaluated at three dietary levels with this protocol. Analysis of dose-response curves (dietary vitamin B-6 vs. plasma PLP) by slope-ratio methods yielded imprecise estimates of the relative vitamin B-6 bioavailability. This low precision, in addition to the potential for interference by direct absorption of B-6 vitamers synthesized by intestinal microflora, indicates the need for improved methods for measurement of biologically available vitamin B-6. It was concluded that rat bioassay methods, even as modified here, may be frequently unsuitable for use in studies of vitamin B-6 bioavailability. Results presented provide further support for the use of plasma PLP as an indicator of vitamin B-6 nutriture.