Clinical diagnosis and population surveys for lactose digestion capabilities present several challenges. Clinical symptoms are discovered by self-reporting; thus, double-blind studies, in which neither the experimenter nor the subject knows if a challenge dose contains lactose or a placebo, are most useful. Direct lactase assay using biopsy specimens of intestinal mucosa is obviously an expensive and invasive method, practical only in particular clinical cases. Indirect assays require subjects to fast for several hours before being given doses of lactose in solution.
Then various tests are used to measure the splitting and subsequent metabolism of the disaccharide. Many of the older methods are cumbersome and imprecise. Blood samples may be tested for glucose before lactose challenge and at intervals afterward. High blood glucose levels after lactose ingestion indicate that lactose is being split in the intestine. A variant of this method is to measure blood galactose. Since the liver metabolizes galactose, a dose of ethanol is given shortly before the experimental lactose to inhibit liver action. Another approach is to measure hydrogen gas excreted through the lungs. Subjects who cannot digest lactose will have hydrogen produced by colonic bacteria. Respiratory hydrogen can be conveniently and efficiently measured by gas chromatography. The ethanol-galactose and hydrogen methods are considered the most reliable; the hydrogen technique is cheaper, easier, and noninvasive (Flatz 1987).
Not only must the population studies of lactase activity be methodologically correct, the subjects must also be truly representative of their populations. Studies done on very small numbers of subjects or on hospital patients or other special groups may be unrepresentative. Indeed, some older studies may be unreliable due to poor techniques or sampling problems. Intermarriage and genetic interchange also complicate analysis of the distribution of lactase persistence. Nonetheless, there has been great interest in the geographical and ethnic distribution of adult lactase persistence and the evolution of this unusual phenotype.
World History
