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Enhanced retention of urea-nitrogen in patients on total parenteral nutrition for intestinal failure
Chical Nurrition (1991) 10: 67-70 0 Longman Group UK Ltd 1991
Enhanced retention of urea-nitrogen in patients on total parenteral nutrition for intestinal failure B.J. MORAN, Departments Southampton Southamptonl
S.J. KARRAN and A.A. JACKSON of Surgery and Human Nutrition, University of Southampton, Bassett Crescent East, SO9 3TU, UK (Correspondence to A.A.J., Department of Human Nutrition, University
of
ABSTRACT-Urea, the main end product of human nitrogen metabolism, is either excreted in the urine or salvaged for further metabolic interaction, through the activity of the colonic microflora. In patients on TPN (total parenteral nutrition) the bowel is ‘defunctioned’ insofar as the microflora are deprived of exogenous substrate, and it might be expected that urea salvaging is reduced to low levels. The recovery of labelled urea-nitrogen was measured in 7 patients on TPN and compared with 4 normals. Following a single intravenous dose of 15N15N-urea, urine was collected for 48h and analysed for 15N. In normals the median retention of 15N was 17% (range 13-17) of the dose compared with 38% (24-52) in 5 TPN patients not on antibiotics (p
at a time when the bowel is, in effect, ‘defunctioned’. With no oral intake there is no exogenous substrate available for the colonic microflora. As urea hydrolysis is dependent upon intestinal floral activity, we reasoned that it would be diminished or absent in patients on TPN.
Introduction One of the major ways in which the body accommodates to changes in protein intake is through alterations in the rates of urea production and excretion (1). In the normal individual proteins are degraded to amino-acids which are oxidised with the amino-group going to form urea in the liver (2). Under normal circumstances only a portion of the urea produced is excreted from the body in the urine (about 70-80’S), with the remaining urea being salvaged and retained in the body (3,4). It would appear that when the metabolic demand for protein exceeds the dietary supply, a process of accommodation takes place, whereby a smaller proportion of the urea produced is excreted in the urine with a greater proportion of the urea nitrogen being salvaged and retained in the body (5, 6). Evidence suggests that the salvage system operates at the level of the colon, being a function of the metabolic activity of the colonic microflora. There is controversy as to the detailed mechanisms whereby this process takes place and its relevance, if any, for general metabolism. However, evidence suggests that the nitrogen from urea hydrolysis is potentially available for further metabolic interaction (7). Patients on total parenteral nutrition (TPN) receive an abundant supply of energy and protein
Subjects and methods Subjects
The study was carried out in 4 healthy normal volunteers and 7 patients on total parenteral nutrition. One patient had been stable on home TPN for 18 months. Six patients were in hospital at the time of the study, but were in a stable clinical state, and no longer had unusual fluid losses through fistulae etc. The study was approved by the Joint Ethical Committee of the Southampton and South West Hampshire District Health Authority. The normal subjects remained on their habitual diet for the duration of the study. The patients on TPN received 1OMJ and lO--14g nitrogen per day (Table). Two of the patients on TPN were on active antibiotic therapy at the time of study; two others had completed a course of antibiotics at least one week before the start of the study. 67
68
RETENTION OF UREA-NITROGEN
IN PATIENTS ON TPN FOR INTESTINAL FAILURE
Table The retention of labelled nitrogen was measured following the intravenous infusion of a single dose of lSNlSN-urea m normal subjects and patients on total parenteral nutrition. Two of the patients on TPN were receiving antibiotics at the time of the study. The retention of labelled urea nitrogen is expressed as a percentage of the dose.
Subjects
Age Y
Normal 1 2 3 4
31 38 22 23
Duration TPN
Antibiotics
Intake gN/d
Urine urea gN/d
Retention ISN % dose
8.3 1.3 5.4 9.4
15 17 17 13
On TPN Pl P2 P3 P4 P5
Home TPN Fistula post-gastrectomy Fistula post-DU operation Carcinoma oesophagus Post-oesophagectomy leak
67 61 64 77 74
16m 8d 8d 12d 8d
nil nil off for 1 wk nil off for lwk
10 14 14 14 14
4.7 7.9 8.0 7.0 8.6
52 35 38 45 24
64 61
7d 7d
Ampicillin Cefotxime, Metronidazole. Gentamicin
14 14
7.1 10.5
11 1.5
On TPN and antibiotics P6 Post-gastrectomy P7 Post-oesophagectomy
leak
Methods Each subject received a single intravenous dose of lSrng/kg body weight of 15NlSN-urea (97.7 atoms % , MSD Isotopes, Canada). A sample of urine was collected from each subject before administration of the isotope, for the calculation of natural abundance. All urine passed for 48h following the administration of the isotope was collected into acid in 12 hourly aliquots. The volume was measured and a sample was stored frozen for later analysis. The urea and ammonia content of the urine were measured by the Berthelot method (8). Urea was isolated for mass spectrometry by short ion exchange column chromatography (9). The enrichment in urinary urea was measured in a triple collector, isotope ratio mass spectrometer (SIRA 10, VG Isogas, Cheshire). Results None of the patients (apart from the patient on home TPN) passed stool during the study period, and so label not recovered in the urine can reasonably be presumed to have been retained in the body. The recovery and retention of 15N for all the studies are shown in the Table. Most of the label was excreted in the first 12h, and enrichment was
down to background by 48h. The normal individuals retained 17% of the dose of label on average. The patients on TPN fell into two groups. In 5 patients not receiving antibiotics the retention of urea nitrogen, a median of 38% of the dose, was than normal (p < 0.05, significantly greater Wilcoxon rank sum test). In the two patients under active treatment with antibiotics, the retention was 11 and 15% of the dose, not different from the normals (Fig.). Discussion Several groups have confirmed that, in normal man, the excretion of urea is consistently less than that produced (3, 4, 8, 10, 11). Up to 30% of production may be hydrolysed in the bowel with the nitrogen being available for further metabolic interaction. The turnover of urea in the body is relatively slow. Following a single dose of labelled urea, however, the excretion of label in urine is virtually complete by 48 h (12). The demonstration that the normal subjects had retained 17% of the dose of label at 48h is therefore in keeping with the findings of previous investigators. The magnitude of the retention of label in the patientson TPN, median 38%) was an unexpected finding.,’ When Long et al (10) measured urea kinetics in two severely stressed patients who were
CLINICAL NUTRITION
60 t
. 0
2 40-
.
! 30.
; 20. 4 10.
.
+
.
Ls
. 0 (antibiotics)
NORMALS
69
In conclusion, significant retention of urea nitrogen has been demonstrated in patients on TPN compared to normals. The potential implications of urea-nitrogen utilisation are of importance for our understanding of nitrogen economy in the human body and conform with the ideas that the gut has an important role as an organ involved in the processing of nitrogen (17). There is the need to evaluate further the implications of these observations, and to define the fate of the substantial quantities of urea nitrogen that are likely to be salvaged on a daily basis.
TPN PAT1 ENTS
Fig. The retention of labelled nitrogen was measured following the intravenous infusion of a single dose of 15NlSNurea. Patients on total parenteral nutrition retained significantly more label than normal controls (P co.05 Wilcoxon rank sum), but this difference was abolished in those patients who were receiving antibiotics.
Acknowledgements This work was supported in part by B. Braun, The Rank Foundation, The Rank Prize Funds, The Hedley Foundation and the Wessex Medical School Trust.
References receiving no dietary intake, they were unable to demonstrate any hydrolysis of urea. Neither patient was receiving an adequate intake of energy at the time of study and both must have been in a catabolic state. There have been no studies in which urea hydrolysis and retention have been measured in patients receiving TPN, in whom an adequate energy intake has ensured an anabolic state, effectively increasing the demand for protein. In the present study all subjects were either weight stable or gaining weight and apparent nitrogen balance, based upon urea-N excretion, was positive. The primary determinant of urea hydrolysis is the activity of the gastro-intestinal microflora, which has been thought to be dependent upon the availability of exogenous substrate (13). As patients on TPN have no oral intake the microflora are deprived of any exogenous substrate, and urea hydrolysis should be reduced to a minimum. However, there was substantial hydrolysis of urea in these patients. There is now evidence to show that in the defunctioned colon there may be substantial bacterial metabolism, probably utilising endogenous mucus and sloughed cells as a source of energy (14, 15, 16). For the two patients receiving antibiotics retention of isotope was not different from normal. It is well recognised that the use of antibiotics may interfere with the normal activity of the intestinal microflora, and reduce the hydrolysis of urea (3, 13).
CN.
-B
5.
10.
11. 12. 13.
Waterlow J C 1968 Observations on the mechanism of adaptation to low protein intakes. Lancet ii: 1091-1096 Harper A E, Benevenga N J, Wohlhueter R M 1970 Effects of ingestion of disproportionate amounts of amino acids. Physiological Review 50: 428-458 Walser M, Bodenlos L J 1959 Urea metabolism in man. Journal of Clinical Investieation 38: 1617-1626 Jackson A A, Picou D, Lindman J 1984 The noninvasive measurement of urea kinetics in normal man by a constant infusion of 15NlSN-urea. Human Nutrition: Clinical Nutrition 38C: 339-354 Picou D. Phillips M 1972 Urea metabolism in malnourished and recovered children receiving a high or low protein diet. American Journal of Clinical Nutrition 25: 1261-1266 Jackson A A. Landman J P, Stevens M C G et al 1988 Urea kinetics in adults with sickle cell disease. European Journal of Clinical Nutrition 42: 491-496 Jackson A A 1983 Amino acids: essential and nonessential? Lancet i: 1034-1037 Kaplan A: Urea nitrogen and ammonia nitrogen. In Standard Methods of Clinical Chemistry, S Meites (ed.) 5: 245-256.1965 Jackson A A, Golden M H N, Jahoor P F et al 1980 The isolation of urea nitrogen and ammonia nitrogen from biological samples for mass spectrometry. Analytical Biochemistry 105: 14-17 Long C L, Jeevenandam M, Kinney J M 1978 Metabolism and recycling of urea in man. American Journal of Clinical Nutrition 31: 1367-1382 Jones E A, Smallwood R A, Craigie A et al 1969 The enterohepatic circulation of urea nitrogen. Clinical Science 37: 82.5-836 Moran B J, Jackson A A 1990 15N-urea metabolism in the functioning human colon: luminal hydrolysis and mucosal permeability. Gut 31: 454-457 Richards P 1972 Nutritional potential of nitrogen recycling in man. American Journal of Clinical Nutrition 25: 615-625
70
RETENTION OF URE,A-NITROGEN
IN PATIENTS ON TPN FOR INTESTINAL FAILURE
14. Rubinstein R, Howard A V, Wrong 0 M 1969 In vivo dialysis of faeces as a method of stool analysis. IV The organic anion component. Clinical Science 37: 549-564 15. Wrong 0 M 1988 Bacterial metabolism of protein and endogenous nitrogen compounds. In Role of the Gut Flora in Toxicity and Cancer. I R Rowland, ed. Academic Press, pp 327-362
16. Moran B J, Jackson A A 1990 Metabolism of 15Nlabelled urea in the functioning and defunctioned human colon. Clinical Science 79: 253-258 17. Souba W W 1988 The gut as a nitrogen processing organ in the response to critical illness, Nutrition Support Services 8: 15-22
Submission dute: 10 April 1990; Accepted after revision: 4 January 1991
Enhanced retention of urea-nitrogen in patients on total parenteral nutrition for intestinal failure B.J. MORAN, Departments Southampton Southamptonl
S.J. KARRAN and A.A. JACKSON of Surgery and Human Nutrition, University of Southampton, Bassett Crescent East, SO9 3TU, UK (Correspondence to A.A.J., Department of Human Nutrition, University
of
ABSTRACT-Urea, the main end product of human nitrogen metabolism, is either excreted in the urine or salvaged for further metabolic interaction, through the activity of the colonic microflora. In patients on TPN (total parenteral nutrition) the bowel is ‘defunctioned’ insofar as the microflora are deprived of exogenous substrate, and it might be expected that urea salvaging is reduced to low levels. The recovery of labelled urea-nitrogen was measured in 7 patients on TPN and compared with 4 normals. Following a single intravenous dose of 15N15N-urea, urine was collected for 48h and analysed for 15N. In normals the median retention of 15N was 17% (range 13-17) of the dose compared with 38% (24-52) in 5 TPN patients not on antibiotics (p
at a time when the bowel is, in effect, ‘defunctioned’. With no oral intake there is no exogenous substrate available for the colonic microflora. As urea hydrolysis is dependent upon intestinal floral activity, we reasoned that it would be diminished or absent in patients on TPN.
Introduction One of the major ways in which the body accommodates to changes in protein intake is through alterations in the rates of urea production and excretion (1). In the normal individual proteins are degraded to amino-acids which are oxidised with the amino-group going to form urea in the liver (2). Under normal circumstances only a portion of the urea produced is excreted from the body in the urine (about 70-80’S), with the remaining urea being salvaged and retained in the body (3,4). It would appear that when the metabolic demand for protein exceeds the dietary supply, a process of accommodation takes place, whereby a smaller proportion of the urea produced is excreted in the urine with a greater proportion of the urea nitrogen being salvaged and retained in the body (5, 6). Evidence suggests that the salvage system operates at the level of the colon, being a function of the metabolic activity of the colonic microflora. There is controversy as to the detailed mechanisms whereby this process takes place and its relevance, if any, for general metabolism. However, evidence suggests that the nitrogen from urea hydrolysis is potentially available for further metabolic interaction (7). Patients on total parenteral nutrition (TPN) receive an abundant supply of energy and protein
Subjects and methods Subjects
The study was carried out in 4 healthy normal volunteers and 7 patients on total parenteral nutrition. One patient had been stable on home TPN for 18 months. Six patients were in hospital at the time of the study, but were in a stable clinical state, and no longer had unusual fluid losses through fistulae etc. The study was approved by the Joint Ethical Committee of the Southampton and South West Hampshire District Health Authority. The normal subjects remained on their habitual diet for the duration of the study. The patients on TPN received 1OMJ and lO--14g nitrogen per day (Table). Two of the patients on TPN were on active antibiotic therapy at the time of study; two others had completed a course of antibiotics at least one week before the start of the study. 67
68
RETENTION OF UREA-NITROGEN
IN PATIENTS ON TPN FOR INTESTINAL FAILURE
Table The retention of labelled nitrogen was measured following the intravenous infusion of a single dose of lSNlSN-urea m normal subjects and patients on total parenteral nutrition. Two of the patients on TPN were receiving antibiotics at the time of the study. The retention of labelled urea nitrogen is expressed as a percentage of the dose.
Subjects
Age Y
Normal 1 2 3 4
31 38 22 23
Duration TPN
Antibiotics
Intake gN/d
Urine urea gN/d
Retention ISN % dose
8.3 1.3 5.4 9.4
15 17 17 13
On TPN Pl P2 P3 P4 P5
Home TPN Fistula post-gastrectomy Fistula post-DU operation Carcinoma oesophagus Post-oesophagectomy leak
67 61 64 77 74
16m 8d 8d 12d 8d
nil nil off for 1 wk nil off for lwk
10 14 14 14 14
4.7 7.9 8.0 7.0 8.6
52 35 38 45 24
64 61
7d 7d
Ampicillin Cefotxime, Metronidazole. Gentamicin
14 14
7.1 10.5
11 1.5
On TPN and antibiotics P6 Post-gastrectomy P7 Post-oesophagectomy
leak
Methods Each subject received a single intravenous dose of lSrng/kg body weight of 15NlSN-urea (97.7 atoms % , MSD Isotopes, Canada). A sample of urine was collected from each subject before administration of the isotope, for the calculation of natural abundance. All urine passed for 48h following the administration of the isotope was collected into acid in 12 hourly aliquots. The volume was measured and a sample was stored frozen for later analysis. The urea and ammonia content of the urine were measured by the Berthelot method (8). Urea was isolated for mass spectrometry by short ion exchange column chromatography (9). The enrichment in urinary urea was measured in a triple collector, isotope ratio mass spectrometer (SIRA 10, VG Isogas, Cheshire). Results None of the patients (apart from the patient on home TPN) passed stool during the study period, and so label not recovered in the urine can reasonably be presumed to have been retained in the body. The recovery and retention of 15N for all the studies are shown in the Table. Most of the label was excreted in the first 12h, and enrichment was
down to background by 48h. The normal individuals retained 17% of the dose of label on average. The patients on TPN fell into two groups. In 5 patients not receiving antibiotics the retention of urea nitrogen, a median of 38% of the dose, was than normal (p < 0.05, significantly greater Wilcoxon rank sum test). In the two patients under active treatment with antibiotics, the retention was 11 and 15% of the dose, not different from the normals (Fig.). Discussion Several groups have confirmed that, in normal man, the excretion of urea is consistently less than that produced (3, 4, 8, 10, 11). Up to 30% of production may be hydrolysed in the bowel with the nitrogen being available for further metabolic interaction. The turnover of urea in the body is relatively slow. Following a single dose of labelled urea, however, the excretion of label in urine is virtually complete by 48 h (12). The demonstration that the normal subjects had retained 17% of the dose of label at 48h is therefore in keeping with the findings of previous investigators. The magnitude of the retention of label in the patientson TPN, median 38%) was an unexpected finding.,’ When Long et al (10) measured urea kinetics in two severely stressed patients who were
CLINICAL NUTRITION
60 t
. 0
2 40-
.
! 30.
; 20. 4 10.
.
+
.
Ls
. 0 (antibiotics)
NORMALS
69
In conclusion, significant retention of urea nitrogen has been demonstrated in patients on TPN compared to normals. The potential implications of urea-nitrogen utilisation are of importance for our understanding of nitrogen economy in the human body and conform with the ideas that the gut has an important role as an organ involved in the processing of nitrogen (17). There is the need to evaluate further the implications of these observations, and to define the fate of the substantial quantities of urea nitrogen that are likely to be salvaged on a daily basis.
TPN PAT1 ENTS
Fig. The retention of labelled nitrogen was measured following the intravenous infusion of a single dose of 15NlSNurea. Patients on total parenteral nutrition retained significantly more label than normal controls (P co.05 Wilcoxon rank sum), but this difference was abolished in those patients who were receiving antibiotics.
Acknowledgements This work was supported in part by B. Braun, The Rank Foundation, The Rank Prize Funds, The Hedley Foundation and the Wessex Medical School Trust.
References receiving no dietary intake, they were unable to demonstrate any hydrolysis of urea. Neither patient was receiving an adequate intake of energy at the time of study and both must have been in a catabolic state. There have been no studies in which urea hydrolysis and retention have been measured in patients receiving TPN, in whom an adequate energy intake has ensured an anabolic state, effectively increasing the demand for protein. In the present study all subjects were either weight stable or gaining weight and apparent nitrogen balance, based upon urea-N excretion, was positive. The primary determinant of urea hydrolysis is the activity of the gastro-intestinal microflora, which has been thought to be dependent upon the availability of exogenous substrate (13). As patients on TPN have no oral intake the microflora are deprived of any exogenous substrate, and urea hydrolysis should be reduced to a minimum. However, there was substantial hydrolysis of urea in these patients. There is now evidence to show that in the defunctioned colon there may be substantial bacterial metabolism, probably utilising endogenous mucus and sloughed cells as a source of energy (14, 15, 16). For the two patients receiving antibiotics retention of isotope was not different from normal. It is well recognised that the use of antibiotics may interfere with the normal activity of the intestinal microflora, and reduce the hydrolysis of urea (3, 13).
CN.
-B
5.
10.
11. 12. 13.
Waterlow J C 1968 Observations on the mechanism of adaptation to low protein intakes. Lancet ii: 1091-1096 Harper A E, Benevenga N J, Wohlhueter R M 1970 Effects of ingestion of disproportionate amounts of amino acids. Physiological Review 50: 428-458 Walser M, Bodenlos L J 1959 Urea metabolism in man. Journal of Clinical Investieation 38: 1617-1626 Jackson A A, Picou D, Lindman J 1984 The noninvasive measurement of urea kinetics in normal man by a constant infusion of 15NlSN-urea. Human Nutrition: Clinical Nutrition 38C: 339-354 Picou D. Phillips M 1972 Urea metabolism in malnourished and recovered children receiving a high or low protein diet. American Journal of Clinical Nutrition 25: 1261-1266 Jackson A A. Landman J P, Stevens M C G et al 1988 Urea kinetics in adults with sickle cell disease. European Journal of Clinical Nutrition 42: 491-496 Jackson A A 1983 Amino acids: essential and nonessential? Lancet i: 1034-1037 Kaplan A: Urea nitrogen and ammonia nitrogen. In Standard Methods of Clinical Chemistry, S Meites (ed.) 5: 245-256.1965 Jackson A A, Golden M H N, Jahoor P F et al 1980 The isolation of urea nitrogen and ammonia nitrogen from biological samples for mass spectrometry. Analytical Biochemistry 105: 14-17 Long C L, Jeevenandam M, Kinney J M 1978 Metabolism and recycling of urea in man. American Journal of Clinical Nutrition 31: 1367-1382 Jones E A, Smallwood R A, Craigie A et al 1969 The enterohepatic circulation of urea nitrogen. Clinical Science 37: 82.5-836 Moran B J, Jackson A A 1990 15N-urea metabolism in the functioning human colon: luminal hydrolysis and mucosal permeability. Gut 31: 454-457 Richards P 1972 Nutritional potential of nitrogen recycling in man. American Journal of Clinical Nutrition 25: 615-625
70
RETENTION OF URE,A-NITROGEN
IN PATIENTS ON TPN FOR INTESTINAL FAILURE
14. Rubinstein R, Howard A V, Wrong 0 M 1969 In vivo dialysis of faeces as a method of stool analysis. IV The organic anion component. Clinical Science 37: 549-564 15. Wrong 0 M 1988 Bacterial metabolism of protein and endogenous nitrogen compounds. In Role of the Gut Flora in Toxicity and Cancer. I R Rowland, ed. Academic Press, pp 327-362
16. Moran B J, Jackson A A 1990 Metabolism of 15Nlabelled urea in the functioning and defunctioned human colon. Clinical Science 79: 253-258 17. Souba W W 1988 The gut as a nitrogen processing organ in the response to critical illness, Nutrition Support Services 8: 15-22
Submission dute: 10 April 1990; Accepted after revision: 4 January 1991