Methods for diagnosing impaired absorption of amino acids, monosaccharides and fatty acids
Provided herein are methods for diagnosing impaired nutrient absorption or determining a nutrient absorption level in an individual. A first stable absorption isotopologue of a nutrient and a second stable absorption isotopolog of the same nutrient are administered orally and intravenously, respectively, to the individual. Amounts of the first and second isotopologs are both measured in blood samples drawn periodically and a ratio of the first isotopolog to the second isotopolog is calculated providing a level of nutrient absorption. The results are compared with a healthy control or an ideal value of 1.0. The ratio less than that of a healthy control or significantly lower than 1.0 indicates an impaired nutrient absorption in the individual.