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Articles

Excessive Ca and P Intake during Early Maturation in Dogs Alters Ca and P Balance without Long-Term Effects after Dietary Normalization¹

Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, The Netherlands

²To whom correspondence should be addressed.

	ABSTRACT

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Calcium (Ca) and phosphorus (P) balance is important for skeletal development. Although the effects of deficiencies are well known, reports on the effects of excessive Ca and P supply are relatively scarce. Epidemiologic data and a few controlled studies have shown that skeletal abnormalities may develop when Ca intake is excessive, particularly in periods of rapid growth. Changes in Ca and P balance during and/or after a high Ca intake are thought to underlie this phenomenon. In this study, the effects of excessive Ca (3.1 g/kg dry matter) or Ca and P (Ca 3.1 g/kg, P 2.8 g/kg) intake on Ca and P balance in young, rapidly growing dogs during (for the period from 3 to 17 wk of age) and after (for the period from 17 to 27 wk of age) high Ca and P intake were compared with findings in age-matched controls with normal Ca and P intakes (Ca 1.0 g/kg, P 0.8 g/kg). Dogs fed a high Ca diet developed hypercalcemia, and food intake and fractional absorption of Ca and P were significantly lower at 15 wk of age, whereas endogenous fecal and renal Ca excretion were significantly higher than in controls. This resulted in significantly higher Ca retention than in controls only at 9 wk of age, and in disproportionate absorption of Ca and P. In dogs fed a high Ca and P diet, normocalcemia was maintained, fractional absorption of Ca and P were significantly lower at 9 and 15 wk of age, but retention of both was significantly higher at 9 wk than in controls. The endogenous fecal Ca and renal P losses were significantly higher, but renal Ca excretion was not different from that in controls. After normalization of Ca and P intake, Ca and P balance did not differ among groups. In conclusion, excessive Ca and P intake during early maturation alters Ca and P balance, but does not influence Ca and P balance after dietary normalization.

KEY WORDS: • calcium • phosphorus • balance studies • diet • dogs

	INTRODUCTION

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In mammals, intestinal calcium (Ca)³ and phosphorus (P) absorption is through paracellular and transcellular pathways. Paracellular transport is a passive process that is dependent upon the concentration gradient, whereas transcellular transport is an active mechanism activated by 1,25 dihydroxycholecalciferol [1,25 (OH)₂ vitamin D-3] (Allen 1982 , Charles 1992 , Murer and Hildmann 1981 ). At a young age, Ca and P absorption is predominantly passive (Ghishan et al. 1980 , Lee et al. 1991 , Younoszai 1981 ). As a result, Ca and P absorption is a function of dietary Ca and P intake (Allen 1982 , Ghishan et al. 1980 , Heaney et al. 1975 , Younoszai 1981 ). Animals that are fed diets with a high Ca content or a disproportionately high Ca to P ratio, and/or are fed excessively, may develop hypercalcemia and consequently hypoparathyroidism and hypercalcitoninism (but not hypercalcitoninemia per se). This could be the cause of the increased incidence of skeletal abnormalities in dogs of large breeds that are fed in this way (Hazewinkel et al. 1987 , Hedhammar et al. 1974 , Slater et al. 1992 , Zentek et al. 1995 ). Chronic hypercalcemia may lead to permanent alterations in the regulation of Ca metabolism (Ahrén and Bergenfelz 1993 , Brown 1983 , Goedegebuure and Hazewinkel 1986 ), particularly if this occurs in an early stage of life when the organism is susceptible to programming (Barker 1997 ). Even after normalization of Ca and P intake, these alterations in the regulation of Ca metabolism may persist. This may have severe consequences for Ca and P balance and skeletal development.

To study the influence on Ca metabolism of an excessive dietary Ca intake with or without a proportionally high P intake from the age of partial weaning, balance studies of ⁴⁵Ca and P were conducted in three groups of dogs. To allow study of permanent changes in Ca and P metabolism, Ca and P intake were normalized (control diet) from 17 to 27 wk of age. Balance studies were performed twice in both parts of the study, first after a short adaptation period (3 wk after complete weaning and 3 wk after dietary change), and then after long-term adaptation to either diet (9 wk after complete weaning and 8 wk after dietary change).

	ANIMALS AND METHODS

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Animals and diets.

Twenty-four Great Dane dogs, originating from four different litters (bred at our laboratory), all with comparable body weight (BW) and blood chemistry, were randomly divided into three groups at 3 wk of age. The dogs were fed a complete dry pelleted diet formulated to meet the requirements of dogs according to the NRC 1974 [diet normal Ca, normal P (NCaNP) n = 9 dogs] or with an identical composition as diet NCaNP, except for the Ca content [diet high Ca, normal P (HCaNP) n = 9] or the Ca and P contents [diet high Ca, high P (HCaHP) n = 6] (Table 1). Ca, P, protein, fat, inorganic content (ash remaining after combustion) and dry matter were analyzed in the foods at the start of the experiment, as previously described (Nap et al. 1991 ) (Table 1) . Dietary vitamin D content was analyzed by using the procedure described by Borsje et al. (1982) . The ingredients of the diets were identical those of the diets used earlier (Hazewinkel et al. 1987 , Nap et al. 1993 ).

Calcium kinetics.

Metabolic studies were performed at 9, 15, 21 and 25 wk of age. Dogs were kept individually in metabolic cages. All feces and urine were collected for 7 d. Kinetic analysis was performed using a two-compartment model (Heaney 1969 and 1973 ). The turnover of Ca was calculated from the disappearance of ⁴⁵Ca from the circulation in a 6-h period after the intravenous (iv) administration of ~0.18 MBq ⁴⁵Ca as ⁴⁵CaCl₂ (the equivalent of 5 µCi, specific activity 0.39 GBq/mg CaCl₂, Du Pont de Nemours, Boston, MA) and the stable total plasma Ca concentration, as described previously (Hazewinkel et al. 1987 and 1991 , Nap et al. 1993 ). For determination of the endogenous fecal fraction of ⁴⁵Ca and absorption of stable P, feces were collected for 3 d after the iv administration of ⁴⁵Ca. On d 4 of each metabolic study, ~0.18 MBq ⁴⁵Ca was mixed with the daily ration of food. This represents 2.6–5.3 x 10^-3% of the total daily dietary Ca intake. Feces were collected for the four consecutive days for the determination of the nonabsorbed ⁴⁵Ca.

Analysis of feces, urine and blood.

Individual pooled feces were weighed and homogenized with a known volume of distilled water. Four samples were taken, weighed, dried for 24 h at 60°C and ashed for 6 h in an oven at 600°C. The ash was dissolved in 10 mL hydrochloric acid (1 mol/L); distilled water was added to a volume of 100 mL. The specific activity of ⁴⁵Ca was determined in a liquid scintillation counter (1212 Rackbeta, LKB Wallac, Turku, Finland), using a 0.4-mL sample thoroughly mixed with 4 mL scintillation fluid (Ultima Gold, Packard Instrument, Meridan, CT). Duplicate samples were used for the chemical determination of P concentration.

Individual urine collections were pooled and acidified with hydrochloric acid, weighed and sampled for chemical analysis of Ca and P. The excretion of Ca in the urine was measured only by analysis of the total Ca concentration because the specific activity in urine was too low for reliable counting of ⁴⁵Ca (Schoenmakers, H. Hazewinkel and W. van den Brom, unpublished results).

Blood samples for the counting of ⁴⁵Ca and analysis of stable Ca were collected from the jugular vein. Blood was immediately transferred to heparin-coated tubes, and the specific activity of ⁴⁵Ca in plasma was determined as described for the fecal samples.

Chemical analysis.

The concentrations of Ca in urine and plasma were analyzed colorimetrically after complexing with Arseno-III (Beckman Instruments, Brea, CA). The concentrations of P in urine and ashed feces were analyzed by the molybdate method with reduction (Beckman Instruments). The analysis of fecal P concentration by this method was validated by preparation of a solution of purified tricalciumphosphate [Ca₃(PO₄)₂] (Merck, Darmstadt, Germany) in hydrochloric acid and distilled water, as described for the ashed fecal samples. Serial dilutions in triplicate, with each sample determined in duplicate, revealed parallelism with the calculated standard line, with a mean deviation in slope of 2%.

Data processing.

The intake of Ca (V_ICa) and P (V_IP) was determined by means of daily weighing of the food given to each dog and subtraction of the uneaten remainder. The total fecal content of ⁴⁵Ca after its iv (R₃) and oral (R_F) administration was determined from the specific activity of ⁴⁵Ca in the ashed fecal samples. The total fecal content of P (V_FP) was determined from the P content of the ashed fecal sample. The turnover of Ca (T_Ca) was calculated from the disappearance of ⁴⁵Ca from the circulation after iv administration. The total urinary Ca (V_uCa) and P (V_uP) losses were calculated from the urine volume and the stable Ca and P concentrations in the urine.

From these data, the following parameters were calculated: the total daily loss of Ca in the feces (V_FCa); the endogenous intestinal excreted Ca that is not reabsorbed (V_fCa); the absolute quantity of Ca absorbed from the diet, i.e., the true calcium absorption (V_aCa); the fraction of Ca absorbed from the total Ca intake (_Ca) and the calcium retention (_Ca), i.e., the difference between the calcium accretion (V_o⁺_Ca) and resorption (V_o^- _Ca) from bone (Table 2).

Finally, all parameters were converted to mmol Ca or P per kg BW per day, except for _Ca and _P.

Statistical analysis and data presentation.

All statistical analyses were performed using SPSS for Windows 7.0 (SPSS, Chicago, IL). Differences between groups were analyzed by multiple comparisons (Tukey's test) after a one-way ANOVA and testing for homogeneity of variance with Barlett's test. The one-tailed Pearson's correlation coefficient was used to analyze the correlation between the absorbed fraction of ⁴⁵Ca and P. Values were considered different when P < 0.05. Results are presented as means ± SEM.

	RESULTS

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Calcium: 3–17 wk of age.

Plasma Ca concentration was significantly higher in group HCaNP than in groups NCaNP and HCaHP at both 9 and 15 wk of age (Table 3). Growth of dogs of groups HCaNP and HCaHP remained significantly behind the growth of their littermates in group NCaNP, which was consistent with growth charts for Great Danes (Kirk 1966 , Lewis et al. 1987 ) (Table 3) . Food intake (thus energy intake) was according to the calculated requirements in groups NCaNP and HCaHP, whereas dogs of group HCaNP consumed significantly less per kg MBW (Table 3) . Because the dogs were fed according to the expected MBW with regard to age and breed, the dogs in groups HCaNP and HCaHP were allowed to consume more food per kilogram actual MBW than were dogs of group NCaNP. As a consequence, V_ICa in group HCaHP was 3.2 and 3.6 times the V_ICa measured in group NCaNP at 9 and 15 wk of age, respectively, whereas in group HCaNP V_ICa was only 2.4 times V_ICa in group NCaNP at 9 wk of age and 2.7 times at 15 wk of age (Fig. 1 ).

View larger version (33K): [in this window] [in a new window]   Figure 1. Ca intake (VICa), the fraction of true Ca absorption (Ca), the true Ca absorption (VaCa), endogenous fecal Ca excretion (VfCa), renal Ca loss (VuCa) and Ca retention (Ca) in three groups of dogs fed diets with different Ca and P concentrations from 3 to 17 wk of age [normal Ca, normal P (NCaNP; n = 9): 1.04 g Ca, 0.82 g P; high Ca , normal P (HCaNP; n = 9): 3.11 g Ca, 0.87 g P; high Ca, high P (HCaHP; n = 6): 3.10 g Ca, 2.77 g P per 100 g dry matter] and identical diets (all dogs 1.04 g Ca, 0.82 g P per 100 g dry matter) from 17 to 27 wk of age, as revealed by balance studies at 9, 15, 21 and 25 wk of age. Data are presented as means + SEM. Significant differences (with P 0.05), are indicated by A (different from group NCaNP) and/or B (different from group HCaNP). The solid bar represents group NCaNP, the open bar, group HCaNP and the hatched bar, group HCaHP.

Phosphorus: 3–17 wk of age.

V_IP in group HCaNP did not differ significantly from V_IP in group NCaNP, whereas V_IP in group HCaHP was significantly higher. The _P was significantly lower at both 9 and 15 wk of age in groups HCaNP and HCaHP than in group NCaNP (Fig. 2 ). V_AP was significantly higher in group HCaHP than in groups NCaNP and HCaNP at 9 wk of age. When the dogs were 15 wk old, V_AP in group HCaHP had decreased and was no longer significantly different from V_AP in group NCaNP. V_AP was significantly lower in group HCaNP than in group NCaNP and group HCaHP at both 9 and 15 wk of age (Fig. 2) . The concentration of P in the urine was almost undetectable in group HCaNP at 9 and 15 wk of age, resulting in a significantly lower V_uP than in groups NCaNP and HCaHP. V_uP was significantly higher in group HCaHP than in group NCaNP at both 9 and 15 wk of age (Fig. 2) . _P was higher at 9 wk in group HCaHP than in groups NCaNP and HCaNP. At 15 wk, _P in groups HCaNP and HCaHP was not significantly different from _P in group NCaNP (Fig. 2) .

View larger version (13K): [in this window] [in a new window]   Figure 2. P intake (VIP), the fraction of apparent P absorption (P), the apparent P absorption (VAP), renal P loss (VuP) and P retention (P) in three groups of dogs fed diets with different Ca and P contents from 3 to 17 wk of age and identical diets from 17 to 27 wk of age, as revealed by balance studies at 9, 15, 21 and 25 wk of age. The solid bar represents group NCaNP, the open bar group HCaNP and the hatched bar group HCaHP. See legend of Figure 1 for further details.

Plasma Ca concentration did not differ among groups at 21 and 25 wk of age (Table 3) . Food intake improved in dogs of groups HCaNP and HCaHP as soon as they were fed diet NCaNP (Table 3) . BW increased but was still significantly lower at 21 and 25 wk of age than in group NCaNP (Table 3) . Because the dogs were fed according to expected MBW, V_ICa in groups HCaNP and HCaHP was significantly higher than in group NCaNP at 21 wk of age. At 25 wk, V_ICa in groups HCaNP and HCaHP was comparable to V_ICa in group NCaNP (Fig. 1) . The _Ca in groups HCaNP and HCaHP increased to values not different from _Ca in group NCaNP at both 21 and 25 wk of age (Fig. 1) . V_aCa was slightly, but significantly higher in group HCaHP, but not in group HCaNP compared with dogs of group NCaNP at 21 wk of age. At 25 wk, V_aCa in groups HCaNP and HCaHP did not differ significantly from V_aCa in group NCaNP (Fig. 1) . V_fCa in groups HCaNP and HCaHP was comparable to V_fCa in group NCaNP at 25 wk of age, but at 21 wk, V_fCa in group HCaHP was significantly lower than in group HCaNP, but not group NCaNP (Fig. 1) . V_uCa was higher in group HCaHP than in groups NCaNP and HCaNP at 21 wk of age. At 25 wk, V_uCa in groups HCaNP and HCaHP did not differ from that in group NCaNP (Fig. 1) . _Ca was higher in group HCaHP at 21 wk of age than in group NCaNP, but not in group HCaNP. _Ca in groups HCaNP and HCaHP was not different from that in group NCaNP at 25 wk of age (Fig. 1) . At 21 wk, Vo^-_Ca was significantly lower in group HCaHP (12.3 ± 3.47 mmol Ca/kg BW) than in group HCaNP (20.3 ± 1.00 mmol Ca/kg BW), but not in group NCaNP (17.2 ± 0.52 mmol Ca/kg BW). There were no differences in Vo⁺_Ca at 21 wk of age (in group NCaNP, Vo⁺_Ca = 20.7 ± 0.65 mmol Ca/kg BW; in HCaNP, Vo⁺_Ca = 24.8 ±1.05 mmol Ca/kg BW; and in HCaHP, Vo⁺_Ca = 17.2 ± 3.17 mmol Ca/kg BW), nor were there significant differences among groups for values of Vo⁺_Ca and Vo^-_Ca at 25 wk of age (for group NCaNP, Vo⁺_Ca = 21.9 ± 2.92 mmol Ca/kg BW, Vo^-_Ca = 17.9 ± 3.13 mmol Ca/kg BW; for group HCaNP, Vo⁺_Ca = 21.6 ± 0.75 mmol Ca/kg BW, Vo^-_Ca = 16.3 ± 0.73 mmol Ca/kg BW; and for group HCaHP, Vo⁺_Ca = 19.3 ± 0.37 mmol Ca/kg BW, Vo^-_Ca = 14.0 ± 0.95 mmol Ca/kg BW).

Phosphorus: 17–27 wk of age.

The intake of food, thus V_IP, was significantly higher in groups HCaNP and HCaHP than in group NCaNP at 21wk of age (Fig. 2) . V_IP in groups HCaNP and HCaHP did not differ from that in group NCaNP at 25 wk of age. Values for _P in groups HCaNP and HCaHP were not different from those in group NCaNP at both 21 and 25 wk of age (Fig. 2) . V_AP was higher in group HCaHP, but not in group HCaNP compared with group NCaNP at 21 wk of age, whereas at 25 wk of age, V_AP in groups HCaNP and HCaHP did not differ from V_AP in group NCaNP (Fig. 2) .V_uP was significantly higher in group HCaHP than in groups NCaNP and HCaNP at 21 wk of age, but V_uP in groups HCaNP and HCaHP did not differ from V_uP in group NCaNP at 25 wk of age (Fig. 2) . _P in groups HCaNP and HCaHP was not different from _P in group NCaNP at both 21 and 25 wk of age (Fig. 2) .

Correlation between the fraction of absorption of calcium and phosphorus.

The correlation coefficient between _Ca and _P was significant at 15 (r = 0.82, P < 0.001) and 21 (r = 0.53, P = 0.008) wk of age, but not at 9 (r = -0.23, P = 0.331) and 25 (r = -0.18, P = 0.401) wk of age.

	DISCUSSION

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High Ca feeding, initiated at the time of partial weaning, led to decreased growth velocity, food intake and _Ca and an increased V_fCa. Increased V_uCa appears to be dependent on elevation of plasma Ca concentration, which occurred only in the dogs fed the high Ca diet without a proportionally high P content. Despite these changes, V_aCa and _Ca were higher in both groups with high Ca intake than in controls at 9 wk of age. At 15 wk of age, however, the significantly lower _Ca in these groups led to values of V_aCa and _Ca that were not different from controls. Fractional P absorption appeared to follow the same pattern as _Ca, independent of P intake. In dogs of group HCaNP, this led to a significantly lower V_AP at both 9 and 15 wk of age, together with reduced V_uP, whereas dogs of group HCaHP excreted a significant part of the absorbed P in the urine. At 9 wk of age, _P was elevated in group HCaHP, but at 15 wk, there were no differences in _P among the three groups. These alterations in Ca and P balance at a young age appeared to be fully reversed after 8 wk of consuming a normal Ca diet.

Ca absorption and retention in dogs of group NCaNP (controls) were comparable to data previously reported for large- and small-breed dogs (Hazewinkel et al. 1987 , Nap et al. 1993 ) fed diets of similar composition and studied at a comparable age.

The decreasing _Ca and _P in the high Ca groups can possibly be explained by a decline in 1,25 (OH)₂ vitamin D-3 mediated active Ca and P absorption (Kayne et al. 1993 , Nordin 1976 , Schaafsma and Visser 1980 ), together with decreasing passive absorption due to intestinal maturation, as described in rats (Ambrecht et al. 1980 , Ghishan et al. 1980 ) and infants (Allen 1982 , Barltop et al. 1977 , Younoszai et al. 1981 ). However, the regulation of the decline in 1,25 (OH)₂ vitamin D-3 mediated absorption of Ca and P may be essentially different for the two high Ca groups. In group HCaNP, hypercalcemia-induced hypoparathyroidism may be expected, resulting in decreased 1-hydroxylation of 25-hydroxycholecalciferol in the kidney (Horst and Reinhardt 1997 ), which may have been reversed by the high tubular maximum (Tmax) for P (Drezner 1997 ) and the hypophosphatemia observed in these dogs (not shown) (Baxter and DeLuca 1976 , Bushinsky et al. 1989 , Fox and Care 1978 , Slatopolsky and Brown, 1997 ). In group HCaHP, the decreased Tmax for P may have decreased renal production in the presence of parathyroid hormone (PTH) secretion, as reported by Drezner (1997) in humans.

It appears that intestinal absorption of P is dependent on Ca metabolism, whereas a dietary intake of excessive amounts of P did not influence the absorption of Ca in rats, cats, and men (Charles 1992 , Pastoor et al. 1995 , Schaafsma and Visser 1980 , Spencer et al. 1986 ). This is in agreement with our findings and can be explained by the dependence of active P absorption on 1,25 (OH)₂ vitamin D-3. In addition, especially in dogs of group HCaNP, the availability of P may be altered by the presence of the excessive quantities of Ca in the gastrointestinal tract, precipitating with P to insoluble Ca-P complexes at a pH >6.1, as found in the middle and distal parts of the small intestine (Allen 1982 , Charles 1992 , Pastoor et al. 1994 , Schünemann et al. 1989 ).

The relative deficit or excess of P absorbed is partly corrected for by modulation of renal P excretion, whereas for Ca, this is limited to a maximum (Broadus, 1993 ). Renal Ca and P excretion was regulated mainly by PTH. In the hypercalcemic dogs of group HCaNP, a high glomerular filtration of Ca may be expected. Due to hypoparathyroidism in these dogs, Ca reabsorption would decrease and Tmax for P would increase (Hazewinkel et al. 1991 , Lemann 1993 , Nordin 1976 , Sutton et al. 1976a and 1976b , Yanagawa and Lee 1992 ). In the normocalcemic HCaHP-fed dogs, PTH-mediated Ca reabsorption can be expected, whereas the Tmax for P decreases.

Although V_uCa and V_uP are under hormonal control, the endogenous excretion of Ca is probably not directly under that influence because variations in Ca intake did not influence the secretion of Ca in the digestive juices (Heaney and Recker 1982 ). Rather, V_fCa may be determined by the degree of reabsorption of the endogenously excreted Ca and is thus inversely related to _Ca, as revealed by this and other studies (Heaney 1969 , Heaney and Recker 1994 , Levine et al. 1982 ).

Food intake in the group HCaNP, and to lesser extent in group HCaHP, was lower than might be expected, and was allowed for on the basis of the MBW of the matched controls in group NCaNP. This may have been due to enhanced calcitonin excretion, especially postprandially, which influences the satiety center (Azria 1989 , Morley 1987 ), together with the hypercalcemia as observed in group HCaNP (Thys-Jacobs et al. 1997 ). A decreased rate of growth in dogs fed a diet with a high Ca content has been demonstrated previously (Hazewinkel et al. 1987 , Kealy 1977 , Stephens et al. 1985 , Voorhout et al. 1987 ) and also occurs in other species (Azria 1989 ). It remains unclear whether BW gain in groups HCaNP and HCaHP was decreased because of insufficient energy intake or whether the mineral imbalance was responsible, i.e., decreased food intake was the result (Jasper and Cassinelli 1993 , Klein and Simmons 1993 ). In addition, when P supply is insufficient, P is preferentially used for soft tissue growth and energy metabolism; consequently skeletal mineralization and growth rate are slower (Baylink et al. 1971 , Loughead and Tsang 1991 ). This might have played a role in group HCaNP and was reflected by signs of rickets in these dogs (I. Schoenmakers, H. Hazewinkel, G. Voorhout, C. Carlson and D. Richardson, unpublished results).

Due to the lower food intake in groups HCaNP and HCaHP, V_ICa and V_IP were reduced, which decreased V_aCa and V_AP independently of, and in addition to the decreased fractional absorption of these minerals. All of these changes resulted in values for _Ca and _P in the high Ca groups that were not different from those in the control dogs at 15 wk of age. At 9 wk of age, the predominantly passive diffusion of Ca and P can be held responsible for the high V_aCa in both high Ca groups and for the discrepancy between V_aCa and V_AP, resulting in an altered _Ca to _P ratio in group HCaNP.

From 17 wk of age, all three groups were fed the control diet, to allow for the study of the long-term consequences of a high Ca intake at an early age. Despite slight differences at 21 wk of age, all groups developed the same pattern of Ca and P metabolism, together with normalization of the plasma Ca concentration in group HCaNP.

From this study, it can be concluded that feeding dogs a diet high in Ca, with or without a proportional increase in P content, from the beginning of food intake in addition to the dam's milk, led to a higher Ca retention than in those fed a diet having a Ca content according to the requirements of dogs, only at a young age. At a later stage of maturation and after a longer period of adaptation, Ca retention did not differ from that in dogs fed a normal Ca diet. This was a result of decreased Ca absorption, probably via both the passive- and the active-mediated pathway of absorption. As a consequence, P absorption also decreased, with disproportionate Ca and P absorption in the group fed the high Ca, but normal P diet. Although this decreased Ca absorption might protect the animal from the deleterious effects of high Ca uptake, the hormonal changes and P deficiency, when Ca and P intake are not elevated proportionally, disturb normal skeletal development (I. Schoenmakers, H. Hazewinkel, G. Voorhout, C. Carlson and D. Richardson, unpublished results). The Ca and P balance was not permanently altered by excessive intake at a young age, as revealed by studies performed after the reconstitution of Ca and P intake according to the requirements of dogs.

	FOOTNOTES

 
¹ Supported by the Mark Morris Institute and the Utrecht University.

³ Abbreviations used: BW, body weight; Ca, calcium; ⁴⁵Ca, the radioactive isotope ⁴⁵calcium; HCaHP, high calcium, high phosphorus; HCaNP, high calcium, normal phosphorus; iv, intravenous; MBW, metabolic body weight; MER, maintenance energy requirement; NCaNP, normal calcium, normal phosphorus; P, phosphorus; PTH, parathyroid hormone; R₃, total fecal content of ⁴⁵calcium after intravenous administration; R_F, total fecal content of ⁴⁵calcium after oral administration; T_Ca, turnover of calcium; Tmax, tubular maximum; V_aCa, true calcium absorption; V_AP, apparent phosphorus absorption; V_FCa, daily fecal excretion of stable calcium; V_fCa, endogenous fecal calcium fraction; V_FP, daily fecal excretion of stable phosphorus; V_ICa, calcium intake; V_IP, phosphorus intake; 1,25 (OH)₂ vitamin D-3, 1,25 dihydroxycholecalciferol; V_o⁺_Ca, calcium accretion; V_o^-_Ca, calcium liberated by bone resorption; V_uCa, urinary loss of calcium; V_uP, urinary loss of phosphorus; _Ca, fractional absorption of calcium; _P, fractional absorption of phosphorus; _Ca, calcium retention; _P, phosphorus retention.

Manuscript received August 24, 1998. Initial review completed September 14, 1998. Revision accepted January 13, 1999.

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