Measurement of ¹⁴C-lactose absorption in the diagnosis of lactase deficiency

A ¹⁴C-lactose absorption test, with the use of a simple CO₂ collection apparatus, was validated as a new useful diagnostic test for lactase deficiency. The method consisted of measurement of the specific activity of ¹⁴CO₂ in the exhaled air after oral administration of 5 μCi lactose-1-¹⁴C together with carrier lactose (50 Gm.). The ¹⁴CO₂-specific activity in the breath 1 2 to 4 hours after ingestion of ¹⁴C-lactose relates to the adequacy of lactose digestion. Thirty-five Japanese adults, who live in Japan, and 18 American adults, who live in the United States, were studied. The mean of the area under the curve relating ¹⁴CO₂-specific activity to time after lactose ingestion in 11 milk-tolerant Caucasian subjects (Group I) was 22.77 ± 7.22 (1 S.D.), whereas that of 7 milk-intolerant Negroes (Group II) was 9.86 ± 3.17; the difference was statistically significant (p < 0.01). The ¹⁴CO₂ results correlated well with jejunal lactase activity (r = 0.74) and with the lactose tolerance tests (r = 0.87). The mean of the area under the ¹⁴CO₂ curve in 6 milk-intolerant (Group A), 19 subjects suspected of milk intolerance (Group B), and 10 milk-tolerant Japanese subjects (Group C) were 8.75 ± 2.34, 11.93 ± 3.14, and 14.66 ± 3.44, respectively. This test separated Group A and Group C (p < 0.01), while jejunal lactase activity and lactose tolerance tests among the Japanese subjects were generally low and could not be used to separate milk-intolerant from milk-tolerant Japanese subjects. The mean of the area under the ¹⁴CO₂ curve of milk-tolerant Japanese was significantly (p < 0.02) lower than that of milk-tolerant Caucasians, although there was no significant difference between milk-intolerant Japanese and milk-intolerant Negroes. Our study showed that the ¹⁴C-lactose absorption test was a useful diagnostic tool for detecting lactase deficiency. Our data suggested that Japanese subjects in general do not digest lactose as effectively as Caucasian subjects.