Measurement of urea kinetics with a single dose of [15N15N]-urea in free-living female vegetarians on their habitual diet

Abstract

Urea kinetics were measured in a group of six healthy female vegetarians with no restrictions on lifestyle. Following a single oral dose of [¹⁵N¹⁵N]-urea, all urine was collected for 48h and the amounts of [¹⁵N¹⁵N]-urea and [¹⁵N¹⁴N]-urea were measured and used to determine the rates of urea production and urea salvage by the colonic microflora. Dietary intake was recorded over the same period and the intake of energy and nitrogen was derived from food composition tables. There was wide inter-individual variability within the group. Body mass index (BMI) ranged from 18.8 to 25.6 kg/m², energy intake between 7.1 and 10.7MJ/day and protein intake between 46.4 and 73.6g/day. When expressed in relation to body mass, nitrogen intake ranged between 106 and 209mgN/kg/day, urea production between 114 and 248 mgN/kg/day and urinary urea excretion between 78 and 174 mgN/kg/day. The proportion of the urea produced which was excreted in the urine varied with the protein intake from 50 to 70%, with 30 to 50% of the urea nitrogen produced being salvaged in the colon. When the pattern of salvage/production relative to the intake of protein was compared with that seen for 100 reference measurements made in individuals on a mixed omnivorous diet, there was little difference. It is concluded that the single oral dose of [¹⁵N¹⁵N]-urea method for measuring urea kinetics provides a simple and non-invasive approach suitable for use in free-living individuals. Differences ina urea kinetics between vegetarians and omnivores are more likely to be a consequence of differences in the amount of protein ingested, rather than the quality of the protein.