selection for high B6 intake may lower CHD incidence, the highest quintile of intake was associated with increased supplement use. Some of these individuals may also exhibit other lifestyle differences that influence CHD risk, some of which were not and others that could not have been considered in the analysis. In addition, vitamin intakes were normalized to energy intake, which may have had an effect.
A study of elderly patients with coronary disease indicated a significantly elevated plasma homocysteine concentration compared with control subjects; homocysteine values were inversely correlated with plasma vitamin concentrations (Robinson et al., 1995). Plasma PLP values below 20 nmol/L were seen in 10 percent of the patients but in only 2 percent of the control subjects (p < 0.01).
Studies of B6-homocysteine-vascular disease relationships were not considered in this analysis if conducted with patients with end-stage renal disease. Because homocysteine is metabolized in the kidney, this condition would exacerbate any effects of vitamin deficiency. Kidney disease may also affect B6 metabolism and turnover.
Several ongoing randomized trials are addressing whether supplementation will decrease risks of CHD. Thus, it would be premature to establish a B6 intake level and corresponding homocysteine value for lowest risk for disease.
The relationship of vitamin status to cognitive function was recently evaluated in the elderly (Riggs et al., 1996). B6 status, as evaluated by plasma PLP concentrations, was related to 2 out of a battery of about 20 tests. The usefulness of these tests for evaluating B6 status will require further validation of the putative relationships.
Vitamin B6 bioavailability was recently reviewed by Gregory (1997). B6 in a mixed diet is about 75 percent bioavailable (Tarr et al., 1981). A mixed diet typically contains about 15 percent pyridoxine (PN) glucoside (Gregory, 1997), which is about 50 percent as bioavailable as the other B6 vitamins. The bioavailability of nonglucoside forms of the vitamin is greater than 75 percent.
The absorption of B6 compounds in the absence of food is comparable, even at very high doses. About 70 percent of a loading dose