Chandarika Persaud

The transfer of 15N from urea to lysine in the human infant.

To explore the nutritional significance of urea hydrolysis for human subjects, male infants being treated for severe undernutrition were given oral doses of 10 mg [15N15N]urea every 3 h for 36 h, on admission, during rapid growth and after repletion with either moderate or generous intakes of protein. Urea hydrolysis was calculated from the 15N enrichment of urinary urea, and where possible, lysine, alanine, glycine and histidine were isolated from urine by preparative ion-exchange chromatography for measurement of 15N enrichment. Sufficient N was obtained for 15N enrichment of lysine to be measured on fifteen occasions from six children. Urea hydrolysis accounted for half of all urea production with 130 (SD 85) mg N/kg hydrolysed per d, most of which appeared to be utilized in synthetic pathways. Of the samples analysed successfully, nine samples of lysine were enriched with 15N (mean atom percent excess 0.0102, range 0.0017-0.0208) with relative enrichment ratios with respect to lysine of 1.63 (range 0.18-3.15), 1.96 (range 0.7-3.73) and 0.9 (range 0.4-1.8) for glycine, alanine and histidine respectively. Enriched samples were identified at each treatment phase and 68% of the variation in lysine enrichment was explained by the variation in urea enrichment with 54% explained by the overall rate of delivery of 15N to the lower gastrointestinal tract. The results indicate a minimum of 4.7 mg lysine per kg body weight made available by de novo synthesis with the more likely value an order of magnitude higher. Thus, urea hydrolysis can improve the quality of the dietary protein supply by enabling an increased supply of lysine and other indispensable amino acids.

Comparison of urinary 5-L-oxoproline (L-pyroglutamate) during normal pregnancy in women in England and Jamaica

Urinary 5-L-oxoproline was measured during normal pregnancies in Southampton, England and Kingston, Jamaica. The CV of 5-L-oxoproline excretion in urine, determined over 7 d in a non-pregnant woman and three pregnant women, was 10-36%. Compared with non-pregnant women, urinary 5-L-oxoproline increased three to four times from early pregnancy in women in Southampton, a highly significant difference, and remained elevated at similar levels during mid and late pregnancy. For women in Kingston, the excretion of 5-L-oxoproline was similar to that of Southampton women in the non-pregnant group and during early pregnancy. However, there was a progressive increase in the excretion of 5-L-oxoproline as pregnancy advanced and by late pregnancy excretion was from three to ten times greater than the average for the non-pregnant women. There was a significant difference between the women in Southampton and the women in Kingston during mid and late pregnancy, with women in Kingston excreting twice as much 5-L-oxoproline during late pregnancy. If the excretion of 5-L-oxoproline is a measure of glycine insufficiency, the results would indicate that in some pregnancies the ability of the mother to provide glycine for herself and the developing fetus is marginal or inadequate and the constraint appears more marked in Jamaica than in England.

Urea kinetics varies in Jamaican women and men in relation to adiposity, lean body mass and protein intake

Objective: We have measured urea kinetics in normal adult men and women of different body composition to determine whether adiposity is associated with differences in the rate of urea production or endogenous urea hydrolysis.Design: Urea kinetics were determined from the excretion of [15N15N]urea in urine over a period of 48 h following a single oral dose of [15N15N]urea, in nine lean and nine obese women and in seven light and seven heavy males while they were consuming their habitual diets. Urinary 5-L-oxoproline was measured as an index of glycine metabolic status.Setting: The studies were carried out in the research ward of the Tropical Metabolism Research Unit, University of the West Indies.Results: Successful studies were completed in eight obese and five lean women and in six heavy and five light men. When compared with lean women, in obese women the rate of urea production and hydrolysis was significantly greater and this difference could not be accounted for by the greater fat-free mass alone, and was in part associated directly with the increase in fat mass. The rate of urea production and hydrolysis was greater in heavy men than in light men, a difference which was attributed to an increase in dietary protein. In obese women and heavy men there was a significantly higher rate of excretion of 5-L-oxoproline in urine when compared with lean women and lean men respectively.Conclusion: This paper highlights the difficulty in identifying an appropriate reference with which to express results in people of different body composition. In obese women urea production and the hydrolysis of urea are increased, in part related to the increased fat-free mass, but also related to the increased fat mass itself. In obese women and men on high protein diets the greater rate of hydrolysis urea may be a reflection of an increased demand for the synthesis of non-essential amino acids, especially glycine.Sponsors: This work was carried out as a part of an elective study period of SCC, which was supported by the Wellcome Trust, Cow and Gate/Nutricia and Lederle. We acknowledge support from BBSRC and the Wessex Medical Trust.

Urea kinetics in healthy women during normal pregnancy

Urea kinetics were measured in normal women aged 22-34 years at weeks 16, 24 and 32 on either their habitual protein intake (HABIT) or a controlled intake of 60 g protein/d (CONTROL), using primed-intermittent oral doses of [15N15N]urea and measurement of plateau enrichment in urinary urea over 18 h (ID) or a single oral dose of [15N15N]urea and measurement of enrichment of urea in urine over the following 48 h (SD). The intake of protein during HABIT-ID (80 g/d) was greater than that on HABIT-SD (71 g/d); urea production as a percentage of intake was significantly greater at week 16 for HABIT-ID than HABIT-SD, whereas urea hydrolysis at week 16 was greater for HABIT-SD than HABIT-ID and urea excretion at week 32 was greater for HABIT-ID than HABIT-SD. The combined results for HABIT-ID and HABIT-SD showed a significant reduction in urea production at week 32 compared with week 24. Urea excretion decreased significantly from week 16 to week 24 with no further decrease to week 32 and urea hydrolysis was significantly greater at week 24 than either week 16 or week 32. Compared with HABIT, on CONTROL there was a decrease in urea production at week 16, and urea excretion was significantly reduced at week 16. For all time periods urea production was closely related to the sum of intake plus hydrolysis. Hydrolysis was greatest at week 24 and closely related to urea production. There was a significant inverse linear relationship overall for hydrolysis as a proportion of production and excretion as a proportion of intake. The results show that on HABIT N is more effectively conserved in mid-pregnancy through an increase in urea hydrolysis and salvage, and during late pregnancy through a reduction in urea formation. Lowering protein intake at any stage of pregnancy increased the hydrolysis and salvage of urea. The staging of these changes was later than that in pregnancy in Jamaica.

Infants in Trinidad excrete more 5-L-oxoproline (L-pyroglutamic acid) in urine than infants in England: an environmental not ethnic difference

The demand for glycine to satisfy normal growth during early life is considerable and most has to be made endogenously. The extent to which adequate glycine is available can be assessed by measuring the urinary excretion of 5-L-oxoproline. The excretion of 5-L-oxoproline at 6 weeks of age for infants in Trinidad of African, Indian or mixed parentage (398 mumol/mmol creatinine) was significantly greater than for infants born in England of Caucasian parentage (194 mumol/mmol creatinine). There was no relationship between 5-L-oxoproline excretion and either sex or pattern of feeding. There were significant inverse relationships between 5-L-oxoproline/creatinine and birth weight, and head circumference either at birth or 6 weeks of age, suggesting that limited availability of glycine is associated with poorer growth before and after birth. For a group of infants born in England of Indian parentage, excretion of 5-L-oxoproline (155 mumol/mmol creatinine) was not different to infants of Caucasian parentage, but significantly less than infants born in Trinidad. The demonstration that 5-L-oxoproline/creatinine was similar in infants born in England, regardless of parentage, shows that the differences between England and Trinidad are related to environment and are unlikely to be accounted for by genetic differences or ethnicity.

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

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 ure...

Urinary 5-L-oxoproline (pyroglutamic acid) excretion is greater in infants in Jamaica than in infants in England.

Objective: To determine the pattern of excretion in urine of 5-L-oxoproline, as a measure of glycine status, during the first six weeks of life in Jamaican infants. Design: Spot samples of urine were collected from term and preterm infants at birth and longitudinally to four weeks of age, or at six weeks of age. 5-L-oxoproline was isolated by column chromatography and hydrolysed to L-glutamic acid, which was measured enzymatically and the results expressed relative to creatinine excretion. Setting: Maternity wards and postnatal clinic of the University Hospital of the West Indies. Subjects: African-Caribbean infants, 19 term and 21 preterm, from birth to four weeks of age, and 79 term infants at six weeks of age. Results: There were no differences between term and preterm infants. Excretion of 5-L-oxoproline increased progressively from birth, 141 μmol/mmol creatinine, to 270 μmol/mmol creatinine at four weeks of age. At six weeks of age, excretion was significantly greater than at birth or four weeks of age, 525 μmol/mmol creatinine. Compared with infants born in England, the excretion of 5-L-oxoproline was not different at birth, but was significantly greater in Jamaican infants at six weeks of age. Conclusions: Glycine status, indicated by increased excretion of 5-L-oxoproline, is marginal in Jamaican infants at six weeks of age, and this possibly reflects a limitation in the endogenous biosynthesis of glycine due to a dietary limitation of folate or vitamin B-12. Sponsorship: Nestlé Nutrition Research Grant Programme.

Protein quality and urea kinetics in prepubertal Chilean schoolboys

Urea kinetics were measured non-invasively in 12 Chilean schoolboys aged 8-10 years who were receiving one of two diets, either predominantly animal protein or predominantly vegetable protein. Both the diets provided an equivalent level of gross protein, 1.2 g/kg/day. The study diets were given for 10 days to enable adaptation to take place. On the eighth day a single oral dose of 15N15N-urea, 100 mg, was given and the amount of label excreted as 15N15-urea in urine over the subsequent 48 hours was measured. There was little difference in any aspect of urea kinetics between the two diets with urea production (animal, 173 +/- 50 mgN/kg/day; vegetable 179 +/- 53 mgN/kg/day), urea excretion (animal, 86 +/- 19 mgN/kg/day; vegetable, 105 +/- 13 mgN/kg/day), urea nitrogen hydrolysis (animal, 87 +/- 49 mgN/kg/day; vegetable, 74 +/- 42 mgN/kg/day), and the salvaged urea-nitrogen derived from hydrolysis which returned to urea formation (animal, 12 +/- 5 mgN/kg/day; vegetable, 17 +/- 9 mgN/kg/day) all being similar. A very high proportion of the salvage nitrogen derived from urea hydrolysis was maintained within the metabolic pool, about 80%, which was equivalent to 0.4 g protein/kg/day. This is the first time urea kinetics have been measured in children of this age and shows that 57% of the ura produced is excreted in urine on average with about 43% of the urea-nitrogen being salvaged for further metabolic interaction. It is concluded that the vegetable based protein diet taken habitually by Chilean children is metabolically equivalent in terms of urea kinetics to a diet based upon animal protein at this level of intake, but that high rates of salvage of urea nitrogen are found on both diets.