Absorbability of calcium from concentrates
Cereals and vegetable protein sources are important determinants of ACa to non-ruminants, because the phytate which they contain forms unabsorbable complexes with calcium in feeds, as well as with any added inorganic calcium. Thus animals with high calcium requirements might be prevented from absorbing according to need. The only data for ACa in a foodstuff for non-ruminants, published in a review by Soares (1995), indicated a relative value for sesame seeds only 65% of that of calcium carbonate (CaCO3) when given to rats. The review abounds with relative values for inorganic sources when added to various diets but gives no indication of the quantitative influence of dietary phytate on calcium availability. Hill (1984) suggested that, as a rule of thumb, 1.3 g Ca should be added per gram of phytate phos- phorus (P) present in excess of 2 g kg21 in rations for poultry: the calculation was based on the formula of calcium phytate. Improvements in apparent ACa have been reported when phytase was added to a maize–soybean meal diet low in phosphorus (4.3 g kg21 DM) for young pigs (Pallauf et al., 1992);
further examples are given in Chapter 5. Correction of Pallauf et al.’s data for endogenous faecal calcium (FECa) at 32 mg kg21live weight (LW) (ARC, 1981) gives an ACa coefficient of 0.58 for the basal diet and similar calculations for data from Han et al. (1997) give a coefficient of 0.72. With other papers indi- cating even higher ACa values for cereal/vegetable protein diets, with inor- ganic sources providing most of the calcium (e.g. Kornegay and Qian, 1996), an average absorbability of 0.70 will be used in later calculations of require- ment. As far as absorbability of calcium in concentrates to ruminants is con- cerned, phytate is not a threat, because it is degraded in the rumen. Low absorption, associated with negative calcium balances, has been reported in normocalcaemic lactating ewes given energy-rich diets (Braithwaite, 1983b), but, as with forages, this is unlikely to represent a limitation imposed by the feed (AFRC, 1991).
The priority of all mammals is to maintain calcium concentrations in plasma and extracellular fluids (ECF) close to 2.5 mmol (100 mg) l21 in the face of large fluctuations in demand and lesser fluctuations in supply (Hurwitz, 1996). Such constraints are necessarily relaxed in egg-laying avian species, and the processes for achieving the contrasting objectives of the dairy cow and the laying hen were reviewed by Horst (1986) and Gilbert (1983), respectively.
Homeostasis is achieved partly by the hormonal regulation of absorption.
Calcium is absorbed according to need up to the limits set by the absorb- ability of the mineral in the diet (Schneider et al., 1985; Bronner, 1987); this is close to 90% for milk and probably rarely < 50% of the total calcium supply
from solid diets, although few studies have been conducted with farm animals given marginal supplies of calcium to prove the point (AFRC, 1991).
Small amounts of calcium may be absorbed from the rumen (Yano et al., 1991), but the major absorptive site is the small intestine. Control of absorp- tion is achieved by two hormones, parathyroid hormone (PTH) and the physiologically active form of vitamin D3, 1,25-dihydroxycholecalciferol (calcitriol, 1,25-(OH)2D3). The parathyroid gland is acutely sensitive to small deviations in the ionic calcium concentration in ECF and, when concentra- tions fall, PTH is normally secreted (Brown, 1991) and activates vitamin D3 (see Omdahl and DeLuca, 1973; Borle, 1974). Vitamin D3 is hydroxylated to 25-hydroxy-D3 (25-OHD3) in the liver and further in the kidney to two compounds, 24,25-(OH)2D3 or 1,25-(OH)2D3. While the primary site of 1,25- (OH)2D3synthesis is the kidney, it is now clear that other tissues, such as the bone marrow and skin, can synthesize 1,25-(OH)2D3 for local calcium needs, i.e. it is a paracrine rather than an endocrine activity (Norman and Hurwitz, 1993). In the intestinal mucosa, 1,25-(OH)2D3 acts relatively sluggishly, open- ing up calcium channels and facilitating calcium uptake and transfer, with the help of the calcium-binding protein, calbindin (Hurwitz, 1996). The role of calbindin in facilitating absorption of calcium according to supply and demand is illustrated by data for fast- and slow-growing chicks given diets varying in calcium (Fig. 4.2; Hurwitz et al., 1995). Provision of vitamin D in a
Fig. 4.2. Mechanism for absorbing calcium according to need: as dietary Ca
concentrations decrease, synthesis of a potentiator of absorption (calbindin) increases;
if high energy diets are fed, demand for Ca rises and the broiler chick synthesizes yet more duodenal calbindin (from Hurwitz et al., 1995).
hydroxylated form overrides the normal absorptive control mechanisms, enhancing the efficiency of absorption from diets providing plentiful calcium (Braithwaite, 1983a). When the supply of calcium is excessive, the homeo- static mechanisms are reversed by the secretion of calcitonin (Beckman et al., 1994). The vast quantities of calcium required by the laying hen are mostly absorbed by passive mechanisms (Gilbert, 1983).
Equally important to the regulation of circulating concentrations of ionic calcium is the net flow of calcium from the enormous reserve in the skeleton.
Bain and Watkins (1993) describe the situation thus: ‘The skeletal morphology of the adult animal represents an elegant compromise between structural oblig- ation and metabolic responsibility, serving the organism in support and loco- motion while actively participating in the regulation of calcium homeostasis’;
this view is equally true of the growing animal. The hormonal partnership which facilitates absorption also facilitates calcium mobilization from bone, but the mechanisms are more complex and involve nuclear receptors for 1,25- (OH)2D3 on cartilage- and bone-forming cells, the chrondrocytes and osteoblasts. Bone-resorbing cells (osteoclasts) respond indirectly to 1,25- (OH)2D3 via cytokines released by the osteoblasts (Norman and Hurwitz, 1993). The balance between calcium accretion and resorption can be set to mobilize around one-fifth of calcium from the skeleton in late pregnancy and lactation in species as diverse as the rat (Brommage, 1989), sheep (Braithwaite, 1983b) and dairy cow (Ramberg et al., 1984); this process may be obligatory, since it is not abated by supplying more dietary calcium (AFRC, 1991) and not accompanied by hypocalcaemia (Braithwaite, 1983b). Protein depletion during pregnancy can also lead to calcium resorption, which is not preventable by supplying more calcium in the diet (Sykes and Field, 1972). Thus rapidly grow- ing offspring can be reared successfully by dams on diets which are deficient in calcium or protein. Bone resorption matters little if the bones are fully calcified at the outset, if depletion is restricted to some 10–20% of the total bone miner- als and if repletion takes place before the next period of intensive demand.
Capacity for resorption decreases with age (Ramberg et al., 1976).
The modulation of excretion by faecal and urinary routes generally plays little part in calcium homeostasis and in determining risk of calcium dysfunction.
The FECa is unaffected by marked reductions in dietary calcium supply and calcium status, remaining consistently low, at about 16 mg Ca kg21LW in the ruminant (ARC, 1980). Increases in dry-matter intake from dry diets gives rise to proportional rises in FECain sheep (AFRC, 1991) and a similar relationship applies to grass diets (Chrisp et al., 1989a, b). High FECavalues (up to 50 mg Ca kg21 LW) have been reported when animals with low requirements (6.5-month-old lambs) were given a frozen, grass–clover diet rich in calcium (12.1 g Ca kg21 DM; Chrisp et al., 1989b). Urinary calcium excretion also tends to remain low and constant, regardless of calcium status, although it Bone resorption
Excretion
may rise significantly in the alkalotic dairy cow and in the laying hen on days when no shell is formed (Gilbert, 1983).
Calcium is the most abundant mineral in the body and 99% is found in the skeleton. A basic function of calcium is therefore to provide a strong framework for supporting and protecting delicate organs, jointed to allow movement and malleable to allow growth.
Bones grow in length by the proliferation of cartilaginous plates at the ends of bones (Vaughan, 1970). Cells towards the end of the regular columns of chondrocytes that constitute the growth or epiphyseal plate become progres- sively hypertrophic and degenerative. They concentrate calcium and phosphorus at their peripheries and exfoliate vesicles rich in amorphous calcium phosphate (Ca3(PO4)2) (Wuthier, 1993). Thus a calcium-rich milieu is provided to impregnate the osteoid (organic matrix) laid down by osteoblasts. The crystalline bone mineral hydroxyapatite, Ca10(PO4)6(OH)2, accumulates in a zone of provisional calcification around the decaying chondrocytes, replacing them with an apparently disorganized and largely inorganic matrix of trabecular bone. Bones grow in width by inward deposi- tion of concentric shells or lamellae of osteoid beneath the surface of the bone shaft or periosteum. Mammalian young are born with poorly mineralized bones and, while they suckle, they do not receive enough calcium to fully mineralize the bone growth that energy-rich milk can sustain (AFRC, 1991). After weaning, there is normally a progressive increase in bone mineralization (for data on lambs, see Field et al., 1975; Spence et al., 1985:
for data on pigs, see Chapter 5), stimulated by increased load-bearing, thus providing added strength and reserves of both calcium and phosphorus. On average, bone contains 36% calcium and 17% phosphorus but there are wide differences between different bones (see Chapter 5). Activity increases bone growth in broilers, which also develop increasingly mineralized bones as they age (Bond et al., 1991); providing a perch increases bone mineralization in hens. Avian species show unique changes in bone morphology with the onset of sexual maturity. Under the influence of oestrogen, the formation of cancellous or structural bone ceases. Instead, new medullary bone is laid down or woven on the surface of existing cortical bone and in the bone marrow to provide a labile calcium reserve to maintain plasma calcium concentrations during shell formation (Gilbert, 1983; Whitehead, 1995).
Formation of medullary bone may also occur during the preparation for lactation in the dairy cow (B. Thorp, personal communication).