level of estrogen in oral contraceptive agents was three to five times higher than it is currently.

Alcoholics have low plasma PLP concentrations, and this reduced B₆ status is distinct from that caused by liver disease or poor diet. Acetaldehyde but not ethanol decreases net PLP formation by cells and is thought to compete with PLP for protein binding. This may make cellular PLP more susceptible to hydrolysis by membranebound phosphatase (Lumeng and Li, 1974). The extent to which this causes an increased B₆ requirement is not known.

The lowered plasma PLP concentration observed in pregnancy is lowered further in subjects with preeclampsia or eclampsia (Brophy and Siiteri, 1975; Shane and Contractor, 1980). Cord blood PLP of the newborn and placenta enzymes involved in PLP synthesis are also reduced (Gaynor and Dempsey, 1972), suggesting a potentially increased B₆ requirement in preeclampsia.

In controlled studies, clinical symptoms of vitamin B₆ deficiency have only been observed during depletion with very low levels of B₆ and have never been seen at intakes of 0.5 mg/day or more. Most studies of B₆ requirements have focused on adults and have been depletion-repletion studies. Starting with the pioneering studies of the Sauberlich group (Raica and Sauberlich, 1964; Sauberlich, 1964; Sauberlich et al., 1972), studies (described below) have demonstrated the usefulness of B₆ status indicators for tracking relative vitamin status. However, many of the studies are flawed in that requirements have usually been assessed by identifying the B₆ intakes that return status indicators to the prestudy baseline values. Baseline values have been those of motivated healthy individuals on self-selected diets or on diets containing 1.5 to 2 mg of pyridoxine (PN). It is not surprising that the assessed requirements based on this approach of normalizing values are invariably similar to or higher than the baseline B₆ intake. Normalization to baseline should require the same intake as in the baseline period, but studies generally come up with a higher value. This suggests that equilibration was not reached during the study periods for some of the indicators.