Some biologically active vitamin B6 is excreted in the urine, and a number of studies have assessed nutritional status by microbiological measurement of this excretion; it is difficult to interpret the results in terms of underlying nutritional status rather than as a reflection of recent intake, although the excretion does fall in deficiency (Sauberlich et al., 1972, 1974). A possibly important source of error here is that minor renal damage, resulting in albuminuria, will result in a considerable increase in urinary albumin-bound pyridoxal phosphate.
About half of the normal dietary intake of vitamin B6 is excreted as 4-pyridoxic acid (see Figure 9.1). Urinary excretion of 4-pyridoxic acid will largely reflect recent intake of the vitamin rather than underlying nutritional status. More importantly, renal clearance of 4-pyridoxic acid is a marker of renal function, irrespective of vitamin B6 status (Bates et al., 1999a; Coburn et al., 2002).
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