Abstract: A method for measuring fat assimilation, such as fat digestion and fat absorption, in a person is provided comprising feeding the person labeled fat, nonabsorbable marker, and a means for coloring stool; allowing the fat, marker and stool coloring means to travel through the digestive tract of the person; monitoring stool from the person for the appearance of the coloring means; collecting stool containing the coloring means; and measuring the amount of marker and labeled fat in the stool to determine the portion of fat digested and/or absorbed by the system. Also provided is a formulation to facilitate one-step administration and specimen collection of a fat-digestion and fat-absorption determinant.