Abstract
The development of the nonabsorbable marker technique has given impetus to the study of intestinal absorption and secretion in man. l - 9 Total volume recovery is not required, and different levels of the intact intestine can be studied without the use of unphysiologic devices such as inflated balloons. The most widely used marker substance is a polyethylene glycol (PEG), which is water-soluble and has a moleculaI' weight of approximately 4000. Absorption of PEG is apparently negligible,I°-12 and a reliable method is available fo r its quanti tative analysis. l3 1'he basic assumption of the nonabsorb able marker technique is that concentration changes of the reference substance accurately reflect dilution or concentration of the test solution. Serious doubt has recently been cast on the validity of this assumption when the technique has been used to study fat absorption, since it has been shown that the water- and fat-soluble phases of a meal dissociate in the stornach and are propelled at different rates through the rest of the intestinal tract. l4 • l5 Und er such circum stances the concentration of the water
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