Vitamins are brought by meals. Some of them are cofactors in homocysteine metabolism, and, if plasma homocysteine values are not known to have a circadian rhythm, little is known about meal influence on blood folate, and vitamins B12 and B6. The aim of this study is to analyze the effect of breakfast and lunch on plasma folate, vitamins B12 and B6, homocysteine and red cell folate (RCF) in 13 subjects, and, in 6 of them, to compare plasma folate and vitamin B12 values with three analytical methods. In the 13 subjects, folate, vitamins B12 and B6 and RCF were analyzed by RIA, homocysteine by fluorescence polarization immunoassay and vitamin B6 by HPLC. In 6 of the subjects, additionally, plasma folate and vitamin B12 were analyzed by microbiology and by enzymoimmunoassay (EIA). Mean plasma folate levels at 11AM, 2PM, and 4PM were not significantly different, compared with 9AM mean values, independent of the method, while, when analyzed by microbiology, mean vitamin B12 concentrations decreased significantly (-12%), but not by RIA or by microbiology. Plasma folate concentrations, obtained by the different methods, and RCF concentrations were correlated, as for vitamin B12 concentrations. But, when analyzed by EIA, vitamin B12 values were higher than by the two other methods. We conclude that plasma folate levels were constant at the different sampling hours, while vitamin B12 levels were decreased at 11AM, 2PM and 4PM, when compared with the fasting concentrations, and when assayed by microbiology. No difference was seen by RIA or by EIA. The evaluation of clinical data is not easy, due to the lack of a standardized analytical method, and international standards are needed.
