I'm doing an analysis, and retinol and beta carotene are involved. I need to know :
1) Normal levels of dietary beta carotene and Retinol consumed (mcg per day);
2) Normal levels of plasma retinol and Beta carotene (ng/ml);
3) Is it possible a plasma beta carotene equals zero?
I'm talking about intervals. Which values are considered to be too high or too small?