NUTRITION
Effects of Dietary Protein
on the Kidneys of Normal
Cats and Those With CKD
Liesel van der Merwe BVSC MMedVet (Med)Small Animals
Cats are obligate carnivores and require high dietary
protein levels and provision of the amino acids, arginine taurine, methionine and cysteine. Protein has
also been demonstrated to be an integral component
of glucose and lipid metabolism as well as a vital component in the management of diabetes mellitus, obesity and hepatic lipidosis in this species. 1
Due to increased awareness of the benefits of feeding felines higher protein diets, these diets are being
fed with greater frequency. There is thus a need to
evaluate the biochemical effects of these increased
protein levels. A study by Backlund (2011) evaluated
23 healthy spayed female cats fed, sequentially, high
protein or low protein diet (random single cross over
design). Each cat thus acted as its own control. Statistically significant changes in serum values were seen
with BUN (urea), creatinine, phosphorous, albumin ,
ALT and USG within 1 month of starting the diet.1
In high protein diets, meat is typically the protein
source, causing increased creatine and creatinine intake which can result in mildly increased serum creatinine concentrations. Plasma creatinine is the end
product of protein degradation and is dependent on
the total muscle mass of the individual as the primary source is the skeletal muscle. Creatinine is freely
filtered through the glomerulus and is neither reabsorbed nor secreted, making it a rough estimate for
GFR. Extra-renal factors which can influence creatinine are age – young animals have higher levels, diet,
and exercise (greyhounds – increased levels.1
Creatinine concentrations in these cats were significantly (p<.05) lower when they were fed the HPdiet
- which was unexpected. It is theorised that this may
be due to a higher GFR which is caused by the increased dietary protein content.1 The average pre-trial
and washout creatinine level was 154 nmol/L. In cats
fed the high protein diet the creatinine was 131 nmol/L
and 139 nmol/L in cats fed the lower protein diet.
The majority of urea is synthesised by the liver which
metabolises ammonia , a waste product of protein
degradation on the GIT. Urea is not a good estimation
of GFR as a large portion of that which is filtered is
reabsorbed in the tubules and there are also pre-renal
causes of increases: haemorrhage into the small intestine providing protein substrate, haemolysis, dehydration and high protein diets. It was thus expected
that the urea was significantly higher in cats fed the
high protein diet (approximately 50% above the baseline values).1 The pre feeding urea in the cats was approximately 5.6 mmol/L and post high protein diet
urea levels averaged 8.16 mmol/L and those in the
low protein diet group were 6.8 mmol/L .
Serum phosphorous levels are dependent on the balance between dietary intake and renal excretion. The
phosphorous levels were also lower (p<0.05) in the
high protein group, in contrast to expectations. The
increased GFR would also explain this as phosphorous is primarily controlled by renal excretion.1
Dietary protein is not a contributor to either initiation
nor progression of chronic renal disease in dogs and
cats. High protein feeding can however, exacerbate
clinical signs by worsening azotaemia in patients with
existing renal failure.This occurs because the loss of
renal function leads to an accumulation of nitrogenous and non-nitrogenous end products of protein
metabolism in the blood, not because of damage to
the renal tissue.
Protein restriction in stage 3 naturally occurring kidney
failure has a positive long term effect on outcome.
Cats with CKD receiving renal diets instead of normal
food survived significantly longer is several studies
published: 20.8 months versus 8.7 months (Elliot et
al 2000) and 16 months versus 7 months (Platinga et
al 2005).2
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