Lactose absorption capacity was determined by lactose tolerance tests with breath hydrogen determination in 102 healthy, adult, Hungarian pairs of twins in order to test monogenic Mendelian inheritance of the absorptive lactase phenotypes, lactose absorber and lactose malabsorber. Of the total, 52 pairs were monozygous (MZ) and 50 dizygous (DZ) twins of indentical sex. All MZ twins were concordant with respect to lactase phenotype. Among DZ twins, the distribution of lactase phenotypes was in agreement with Hardy-Weinberg expectations derived from the frequencies of the hypolactasia gene in DZ and MZ twins, and in the general Budapest population.
In the second part of the study, three commonly used methods of lactose tolerance testing, the blood glucose, the blood galactose, and the breath hydrogen tests, were compared in 49 pairs of twins concordant for lactase phenotype. Blood galactose concentration showed the greatest and only significant difference between the intrapair correlation coefficients of MZ and DZ, and no overlap between lactose absorbers and lactose malabsorbers. The intrapair correlation coefficients of peak breath hydrogen concentration in MZ and DZ twins did not significantly differ from zero, but the resolution of lactase phenotypes was satisfactory. Differences in glucose absorption and concentration in lactose absorbers and malobsorbers overlapped considerably, and among lactose absorbers correlation coefficients in DZ were higher than in MZ twins. In MZ and DZ twins, the difference in concordance and constancy of lactose intolerance symptoms was not significant.