D-Threitol can be regarded as the main end product of D-xylose metabolism in hummans. Threitol is a C4-polyol (tetritol); the total C4-polyol concentration of threitol decreases with age. Several inborn errors of metabolism with abnormal polyol concentrations in body fluids are known to date (such as pentosuria and galactosemia). Most of these defects can be diagnosed by the assessment of urinary concentrations of polyols. Several studies have revealed that urinary levels of some polyols may vary in diseases associated with carbohydrate metabolism derangements such as diabetes mellitus and uremia. The abnormal occurrence of various polyols in diseases with a specific enzyme deficiency such as pentosuria and galactosemia has also been reported , 6(5), 657-66.). Moreover, D-Threitol is found to be associated with ribose-5-phosphate isomerase deficiency, which is also an inborn error of metabolism. Threitol in the urine is a biomarker for the consumption of apples and other fruits.