Clinical and experimental methods for the determination of cholesterol absorption

The following section summarizes this methodological treatment of cholesterol absorption by considering the advantages and disadvantages of the available methods. As outlined in Table 6, there are certain technical and interpretational disadvantages to all methods which should be weighed in deciding the appropriate method to use for a given experimental protocol. Direct methods are virtually the only ones capable of measuring the total flux of cholesterol between intestinal lumen and lymph. All the other methods allow interpretations relating only to the flux of exogenous cholesterol. On the other hand, the direct methods are invasive--involving surgery--in the case of experimental animals, and intubation, in the case of intestinal perfusion. This could potentially lead to unnatural conditions for the absorptive surface of the gut. Direct methods, balance methods, and the isotopic equilibrium method all measure a mass of cholesterol transferred. Method IV, the plasma isotope ratio method, and the continuous feeding method, on the other hand, only provide an estimate of the percent of an administered dose absorbed, although this can be converted to a mass estimate with an accurate knowledge of intake. The balance methods, continuous feeding, and isotopic equilibrium method all provide an estimate of absorption over time. The plasma isotope ratio method and method IV, however, rely on repeated measurements to measure fluctuations in absorption. In their favor, method IV and the isotope ratio technique have several advantages that make them the methods of choice for human and large-scale animal studies. These are simplicity, low isotope dosage, and the fact that they are readily done on an outpatient basis. In general, then, direct or balance methods are preferable for precise studies where absolute levels of absorbed cholesterol and fluctuations over time need to be assessed. Direct methods certainly provide the greatest interpretational flexibility in this regard by measuring endogenous and exogenous flux. Among balance methods, method II may be the most precise, but method I is the easiest to perform technically, may be used on an outpatient basis, and requires lower isotope dosage. For evaluating differences in percent cholesterol absorption in man and in all animals except the rabbit, the plasma isotope ratio technique is technically the easiest method both for the investigator and the subject. It also appears to compare very favorably with the more tedious balance methods and will undoubtedly be used in more and more experimental studies of cholesterol absorption.