Effect of dietary water intake on urinary output, specific gravity and relative supersaturation for calcium oxalate and struvite in the cat

Abstract

It has been reported that daily fluid intake influences urinary dilution, and consequently the risk of urolithiasis in human subjects and dogs. The aim of the present study was to investigate the role of dietary moisture on urinary parameters in healthy adult cats by comparing nutritionally standardised diets, varying only in moisture content. A total of six cats were fed a complete dry food (6.3 % moisture) hydrated to 25.4, 53.2 and 73.3 % moisture for 3 weeks in a randomised block cross-over design. Urinary specific gravity (SG), urine volume, water drunk and total fluid intake were measured daily; relative supersaturation (RSS) for calcium oxalate (CaOx) and struvite was calculated using the SUPERSAT computer program. Cats fed the 73.3 % moisture diet produced urine with a significantly lower SG (P < 0.001) compared with diets containing 53.2 % moisture or lower. Mean RSS for CaOx was approaching the undersaturated zone (1.14 (sem 0.21); P = 0.001) for cats fed the diet with 73.3 % moisture and significantly lower than the 6.3 % moisture diet (CaOx RSS 2.29 (sem 0.21)). The effect of diet on struvite RSS was less clear, with no significant difference between treatment groups. Total fluid intake was significantly increased (P < 0.001) in the 73.3 % moisture diet (144.7 (SEM 5.2) ml, or 30 ml/kg body weight per d) compared with the 6.3 % (103.4 (SEM 5.3) ml), 25.4 % (98.6 (SEM 5.3) ml) and 53.3 % (104.7 (SEM 5.3) ml) moisture diets, despite voluntary water intake decreasing as dietary moisture intake increased. Cats fed the 73.3 % moisture diet had a higher total daily fluid intake resulting in a more dilute urine with a lower risk of CaOx when compared with the lower-moisture diets.

Full text

Effect of dietary water intake on urinary output, specific gravity and relative
supersaturation for calcium oxalate and struvite in the cat
Catherine M. F. Buckley, Amanda Hawthorne, Alison Colyer and Abigail E. Stevenson*
Waltham Centre for Pet Nutrition, Freeby Lane, Waltham-on-the-Wolds, Melton Mowbray, Leicestershire LE14 4RT, UK
(Received 13 October 2010 – Revised 27 January 2011 – Accepted 13 February 2011)
Abstract
It has been reported that daily fluid intake influences urinary dilution, and consequently the risk of urolithiasis in human subjects and dogs.
The aim of the present study was to investigate the role of dietary moisture on urinary parameters in healthy adult cats by comparing nutri-
tionally standardised diets, varying only in moisture content. A total of six cats were fed a complete dry food (6·3 % moisture) hydrated to
25·4, 53·2 and 73·3 % moisture for 3 weeks in a randomised block cross-over design. Urinary specific gravity (SG), urine volume, water
drunk and total fluid intake were measured daily; relative supersaturation (RSS) for calcium oxalate (CaOx) and struvite was calculated
using the SUPERSAT computer program. Cats fed the 73·3% moisture diet produced urine with a significantly lower SG (P,0·001) com-
pared with diets containing 53·2 % moisture or lower. Mean RSS for CaOx was approaching the undersaturated zone (1·14 (SEM 0·21);
P¼0·001) for cats fed the diet with 73·3% moisture and significantly lower than the 6·3 % moisture diet (CaOx RSS 2·29 (SEM 0·21)).
The effect of diet on struvite RSS was less clear, with no significant difference between treatment groups. Total fluid intake was significantly
increased (P,0·001) in the 73·3 % moisture diet (144·7 (SEM 5·2) ml, or 30 ml/kg body weight per d) compared with the 6·3 % (103·4
(SEM 5·3) ml), 25·4 % (98·6 (SEM 5·3) ml) and 53·3 % (104·7 (SEM 5·3) ml) moisture diets, despite voluntary water intake decreasing as dietary
moisture intake increased. Cats fed the 73·3 % moisture diet had a higher total daily fluid intake resulting in a more dilute urine with a lower
risk of CaOx when compared with the lower-moisture diets.
The extent to which cats are able to adjust the amount of
water they drink in relation to the moisture content of their
diet has been a source of investigation over the last 30
years. Cats fed an all meat or wet diet, which typically contains
in excess of 75 % moisture, will voluntarily drink only small
quantities of water
(1,2)
. Although cats drink more water
when offered dry diets, they do not drink amounts that
would be expected to compensate for the significant
reduction in moisture content of these diets
(2)
.
It is believed that cats adjust their water intake such that
they effectively excrete their renal solute load. Since cats fed
dry food have a lower total water intake than when fed wet
food, it would suggest that the absolute water requirement
to dilute the renal solute load is lower than that resulting
from a wet-food diet. This means that cats fed wet or natural
diets may consume water in excess of any physiological
requirement.
Dry cat foods have been implicated as a risk factor for
cats susceptible to feline lower urinary tract disease
(3)
, while
feeding wet diets has been shown to reduce the recurrence
of calculi and signs of idiopathic cystitis compared with
dry food
(4)
.
While the benefits of wet food are thought to be attributed
to the increased water intake and related increase in urine
volume, additional nutritional differences exist between wet
and dry diets that may also be responsible for some of these
benefits. Only a single study has previously been conducted
to investigate solely the effect of dietary moisture in cats,
although this used a semi-purified diet
(5)
.
The aim of the present study was to investigate the role of
dietary moisture on urine parameters and total daily fluid
intake in healthy adult cats by comparing nutritionally standar-
dised diets, varying only in moisture content.
Materials and methods
A panel of six healthy neutered adult cats was studied, consist-
ing of three males and three females, ages ranging from 2
years and 7 months to 6 years and 9 months. The experimental
procedures were approved by the WALTHAM
w
(Melton
Mowbray, Leicestershire, UK) ethical review committee. A
single batch of complete and balanced dry diet was used in
the present study (6·3 % moisture) and this was soaked with
de-ionised water where necessary to achieve dietary moisture
*Corresponding author: A. E. Stevenson, fax þ44 1664 415440, email abigail.stevenson@effem.com
Abbreviations: CaOx, calcium oxalate; RSS, relative supersaturation; SG, specific gravity.
British Journal of Nutrition (2011), 106, S128–S130 doi:10.1017/S0007114511001875
qThe Authors 2011
British Journal of Nutrition

contents of 25·4, 53·2 and 73·3 %, with cats allocated to each of
the four diets in a randomised block design. The cats were
offered 188·3 kJ/kg (45 kcal/kg) body weight in each of the
four phases split into three meals over the course of the
day. Each trial phase lasted for 3 weeks during which food
intake, water intake (water drunk plus dietary water), faeces
quality, urine volume and specific gravity (SG) were recorded
daily and urinary pH was measured using a 24 h monitoring
system. The first 7 d data from each trial phase were discarded
from any analysis, allowing for a period of adaptation. A 48 h
urine sample was collected on days 17–19 in each phase. The
urine sample was frozen immediately upon collection using
dry ice and titrated down to pH 2 with concentrated HCl.
The sample was then analysed for Na, K, Mg, Ca, Cl, sulphate,
phosphate, oxalate, citrate, pyrophosphate, NH
3
and urate.
These data were analysed using the computer-based program
SUPERSAT
(6)
to calculate relative supersaturation (RSS) of the
urine for calcium oxalate (CaOx) and struvite.
Mean trial data for each cat were used in all analyses rather
than individual data points. Error is expressed as standard
errors of the mean. For the RSS measurements, ANOVA was
performed using cat and diet as main effects. For SG, urine
volume, water drunk and total water intake, a mixed model
analysis was performed to allow for the repeated measures
on each cat, within a diet. Thus, diet in cat was added
as a random effect. The day £diet interaction was added
as a fixed effect to investigate whether any effect of diet chan-
ged from day 8 to 21. Adjustments for the water drunk, as a
covariate, were investigated but dropped from the model as
appropriate. To allow for a six pairwise comparison of the
data, a Bonferroni correction was made and is included
where it alters the data interpretation. Statistical analyses
were performed using Statgraphics Centurion XVI and
GenStat
w
version 12.2 statistical software.
Results
The effect of phase was found to be non-significant and was
subsequently dropped from the model. The interaction
effect of day £diet was not significant overall for SG,
water drunk, urine volume and water intake and was there-
fore dropped from the model. Over the course of the present
study, the cats showed a mean increase in body weight of
8·03 þ3·16 %, despite being offered only 188·3 kJ/kg per d
(45 kcal/kg per d). Faeces quality was excellent, with 100 %
of faeces voided being acceptable across each phase.
As dietary moisture level increased, the cats compensated
by significantly reducing the amount of water they drunk
voluntarily. There was a significant increase in total water
intake (P,0·001) for the diets with 73·3 % moisture com-
pared with all other diets. For the highest moisture diet,
mean total water intake was 144·7 (SEM 5·2) ml or 30 ml/kg
body weight per d compared with 103·4 (SEM 5·3), 98·6 (SEM
5·3) and 104·7 (SEM 5·3) ml for the 6·3, 25·4 and 53·3 %
moisture diets, respectively (Fig. 1).
The average 24 h urine volume produced by cats fed
the diet containing 73·3 % moisture was 86·7 (SEM 6·93) ml,
which was significantly higher than all other diets
(P,0·001). Average urine volume did not differ significantly
for any of the other diets (P.0·05).
Mean 48 h urinary pH showed no statistically significant
differences according to the diet group (6·22 (SEM 0·053);
6·23 (SEM 0·034); 6·24 (SEM 0·061) and 6·19 (SEM 0·042), respect-
ively for the treatment groups as described previously). For
SG, the day £diet interaction and the main effect of day
were found to be non-significant and were dropped from
the model. Following this, the effect of diet was interpreted
as significant (P,0·001). There was a significant reduction
in mean SG for the 73·3 % moisture diet (1·036 (SEM 0·002))
compared with all other diets (1·052–1·054 (SEM 0·002);
P,0·001). All other diets were not significantly different
(P.0·05).
The mean CaOx RSS for the 73·3 % moisture diet was signifi-
cantly reduced (1·14 (SEM 0·21)) compared with the 6·3 % (2·29
(SEM 0·21)) and 53·2 % moisture diets (2·06 (SEM 0·21)). The
73 v. 25·4 % diet was non-significant, following a Bonferroni
correction.
The effect of diet on struvite RSS was not significant overall
(P¼0·155). By performing pairwise comparisons, the average
RSS struvite for the 73·3 % diet was significantly lower than the
53·2 and 6·3 % diets (P¼0·045 and 0·035, respectively). All
other diets were not significantly different (P.0·05). When
a Bonferroni correction to the data was made, the test level
became 0·008, making none of the diets significantly different.
Discussion
The key findings from the present study were that high dietary
water intake is related to a significant increase in urine
volume, reduction in SG and decrease in CaOx RSS, demon-
strating a beneficial effect of high-moisture diets on cat urinary
parameters.
Dietary moisture level (%)
Total water intake (ml)
6·3 25·4 53·2 73·3
87
107
127
147
167
a
a
a
b
Dietary moisture level (%)
Total water intake (ml)
6·3 25·4 53·2 73·3
87
107
127
147
167
a
a
a
b
Fig. 1. Effect of dietary moisture on total water intake. Values are means,
with 95 % CI represented by vertical bars.
a,b
Mean values with unlike letters
were significantly different (n6).
Effect of dietary water on urine parameters S129
British Journal of Nutrition

The data from the present study show that water intake
alone has a strong influence on urine parameters. This is in
agreement with Gaskell
(5)
who demonstrated that addition
of water to a semi-purified diet to achieve a moisture level
of 75 % produced a similar water intake and urine SG to the
present study. Increasing water intake has been identified as
an important strategy in the management of patients with
urolithiasis, primarily because of its role in increasing urine
volume. The effect of this is a decrease in saturation through
dilution of calculogenic material
(7)
, and an increase in the
minimum supersaturation required to elicit initiation of crystal-
lisation
(8,9)
, and these benefits outweigh the potential negative
effects of dilution of inhibitors of crystallisation or growth
(10)
.
In the present study, each dietary moisture level would have
offered the same calculogenic load, because the cats were
offered the same amount of food differing only in the levels
of water added. This suggests that any effects observed on
urine can be attributed to varying degrees of urine concen-
tration as dietary moisture levels are altered, rather than any
nutritional differences.
When considering the natural behaviour of the cat’s ances-
tor, much of their daily water requirement is obtained from
their prey (which typically contains 70– 75 % moisture) and
cats have evolved to drink very little water. As a result, cats
naturally have a very low thirst drive and are therefore slow
to respond to changes in their hydration state. This reluctance
to adapt their voluntary water intake sufficiently is illustrated
in the present study where cats fed the dry diet (6·3 % moist-
ure) consumed approximately 30 % less water overall com-
pared with cats fed on the highest-moisture diet, despite the
dry-diet group increasing their voluntary drinking by approxi-
mately sixfold in comparison. This behaviour puts cats at a
higher risk of lower urinary tract disease when fed low-moist-
ure diets, based on a more concentrated urine and an associ-
ated increase in CaOx RSS. Since energy intake remained
constant across all four diets, these data suggest that the
high-moisture (and less energy-dense) diet may drive cats to
ingest more dietary moisture than is required in order to
meet their daily energy requirements with the consequence
of positive effects on urinary parameters.
The significant reduction in CaOx RSS in cats fed the high-
est-moisture diet demonstrates that increased water turnover is
of real benefit in cats. In terms of the biological relevance of
the observations made in the present study, the high-moisture
diet resulted in the production of urine approaching the
undersaturated zone for CaOx RSS, which is defined as ,1,
reducing the risk of both homogeneous and heterogeneous
CaOx crystallisation
(11)
. The other diets resulted in mean
CaOx RSS values ranging from 1·99 to 2·29, classified as
within the metastable zone (RSS ¼1– 12), indicating that
new CaOx stone formation in these cats is possible if fed
these diets in the long term. The present study was underpow-
ered, and it is expected that statistically significant differences
in struvite RSS would be found in the highest-moisture diet in
a suitably powered study. Since literary evidence shows that
urinary pH correlates with struvite RSS
(12)
, the lack of signifi-
cant differences in urinary pH across dietary groups in the
present study may explain the similar findings for struvite
RSS and indicates that urinary pH may be a more important
driver of struvite RSS than SG.
Acknowledgements
There are no conflicts of interest to disclose. The study
received no specific grant from any funding agency in the
public, commercial or not-for-profit sectors. C. M. F. B.
prepared the manuscript. A. C. prepared the statistical
analyses. A. E. S. and A. H. conducted the study.
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