A method for dye marking a liquid enteral nutritional product during delivery thereof from a supply container to a feeding tube delivering the nutritional product to the gastrointestinal tract of a patient includes the steps of: (1) providing apparatus such as a formulation chamber, e.g., a drip chamber, connectable to the supply container, with at least one sustained release reservoir containing a physiologically acceptable and dispersible marker dye positioned within the formulation chamber, and, liquid communication means connecting the formulation chamber to the feeding tube of a patient; (2) providing a supply container containing the nutritional product; (3) connecting the formulation chamber to the supply container and the liquid commmunication means to the feeding tube; and (4) flowing the dye-marked nutritional product through the apparatus and into the feeding tube. The sustained release reservoir may be a conventional slow release coated tablet, microspheres, or capsule type or an osmotically driven device. The marker dye may be a colorant dye or a fluorescing dye or a mixture thereof. If quick coloration is desired, the sustained release reservoir may be coated with a soluble layer of the marker dye or marker dye not in a sustained release reservoir may be placed in the formulation chamber along with a sustained release reservoir containing dye.