Lowering dietary phosphorus concentrations reduces kidney calcification, but does not adversely affect growth, mineral metabolism, and bone development in growing rabbits

Abstract

New Zealand White rabbits were used to investigate the influence of increasing dietary P concentrations on growth performance, mineral balance, kidney calcification and bone development. The minimum dietary P requirement of 0·22 % (National Research Council) is usually exceeded in commercial natural-ingredient chows, leading to undesirable kidney calcifications. In order to study the optimal dietary P level, rabbits were fed semi-purified diets with four different P levels (0·1, 0·2, 0·4, and 0·8 %; w/w) at a constant dietary Ca concentration (0·5 %) during an 8-week period. Body weight and growth were not influenced by the dietary P level. During two periods (days 20 -23 and 48 -51), faeces and urine were collected quantitatively for the analysis of Ca, Mg and P and balances were calculated. Increased dietary P intake caused increased urinary and faecal P excretion and P apparent absorption and retention. Faecal Ca excretion increased with higher dietary P levels, whereas urinary Ca excretion reacted inversely. The apparent absorption of Ca became reduced at higher dietary P concentrations, but Ca retention was unchanged. The response of Mg was in a similar direction to that of the Ca balance. Kidney mineral content increased with higher dietary P levels, indicating the presence of calcified deposits. Nephrocalcinosis became more severe in kidney cortex and medulla at increasing dietary P levels, as was confirmed by histological analysis. Femur bone length was not differentially influenced by dietary P. Bone density (g/cm 3 ) of the femur diaphysis became significantly lower at the 0·8 % dietary P level as compared with the 0·2 % P group only. The bone Mg content was significantly increased on the 0·8 % P diet, both in the diaphysis and epiphysis. Plasma P concentration increased and plasma Ca decreased with higher dietary P levels, whereas plasma Mg levels were unaffected. The present study shows that the current recommended minimum dietary P level of 0·2 % for rabbits, as advised by the National Research Council in 1977, leads to a normal growth and bone development, but also causes some degree of kidney calcifications at a dietary Ca level of 0·5 %. As the dietary P level of 0·1 % virtually prevented kidney calcification and at the same time did not give evidence for any deleterious effects on growth and bone development, this indicates that the current recommended dietary P level for rabbits should be regarded as a maximum advisable concentration, and that a lower P level may be more optimal.

Influence of diet on calcium metabolism, tissue calcification and urinary sludge in rabbits (Oryctolagus cuniculus)*

“…The apparent digestibility of Ca increased, that of P decreased, and urinary Ca levels increased, with increasing dietary Ca content; and as reported previously (Kamphues et al, 1986;Ritskes-Hoitinga et al, 2004), the degree of urinary sludge, as assessed by ultrasonography and by urinary DM concentrations, also increased with increasing dietary Ca. Free-ranging lagomorphs and other small hindgut fermenters such as hyraxes (Procavia capensis) also excrete urine with Ca sludge (Leon and Belonje, 1979;Cheeke, 1987; fig.…”

“…In relation to other studies, in which more distinct effects of dietary Ca or P supplementation on organ mineral content and calcification were achieved (Kamphues et al, 1986;Ritskes-Hoitinga et al, 2004), the range of these minerals in the diets used in our study appear moderate. This is also the most likely reason why differences between the groups in plasma Ca levels were not significant at the end of the experiment, although our data in general is within the range reported by other authors (Kennedy, 1965;Chapin and Smith, 1967a,b;Kamphues et al, 1986;Ritskes-Hoitinga et al, 2004). An important limitation of our study is that although diets were designed without vitamin D supplementation, the actual content of this vitamin in the diets was not quantified.…”

“…Urinary sludge and tissue calcification depend on a variety of factors; dietary Ca and phosphorus (P) are among those. In controlled studies, tissue calcification, urinary sludge and uroliths were triggered by both an increasing Ca:P ratio when Ca was added (Kamphues et al., 1986) and by a decreasing Ca:P ratio when P was added (Cramer et al., 1998; Ritskes‐Hoitinga et al., 2004). For the veterinary practitioner, it is important to know what dietary recommendations should be given to rabbit owners, and what additional factors play a role in urolithiasis prevention.…”

Mineral absorption in the black rhinoceros (Diceros bicornis) as compared with the domestic horse

“…Low levels of available P can reduce microbial growth, organic matter and cellulose digestibility as well as the production of volatile fatty acids (Breves and Höller, 1987; Komisarczuk et al., 1987a,b) – a major energy source for herbivores. High intraluminal levels of Ca could, theoretically, render P unavailable through the formation of insoluble Ca–P complexes (Ritskes‐Hoitinga et al., 2004). A species adapted to a diet with a particularly high Ca:P ratio, such as the black rhinoceros (c.f.…”

Rabbit dental disease and calcium metabolism – the science behind divided opinions

“…In rabbits, increasing dietary phosphorus content (from 0·1 to 0·8%) at a constant calcium concentration (0·5%) results in a dramatic decrease in calcium excretion through the kidney and increased faecal calcium output because of the formation of CaPO 4 complexes (Ritskes‐Hoitinga et al . ). With excessively high‐dietary calcium intake, intestinal absorption in rabbits is not reduced (as in other mammals) but calcium renal excretion is increased (Whiting & Quamme ).…”