Magnesium in the skeleton
when dietary supplies are inadequate (Gardner, 1973). Lability of the skeletal magnesium pool declines with age (Blaxter and McGill, 1956), but this may reflect the waning indirect influence of calcium-regulating hormones on bone resorption (see Chapter 4). Magnesium is unavoidably mobilized from the skeleton when calcium and phosphorus are withdrawn (see Table 4.3), and there is pronounced seasonal cycling of bone magnesium reserves in associa- tion with lactation (Fig. 5.6c, d). The changes in Ca : Mg ratio in rib reported by Engels (1981) were not attributable to magnesium deficiency, because plasma magnesium concentrations were normal throughout the study. The feeding of gross excesses of magnesium (24 g kg21DM) to lambs and steers increased magnesium concentrations in the rib from 3.6–4.1 to 5.8–6.4 g kg21 DM (Chester-Jones et al., 1989, 1990), but smaller supplements (5.5 g kg21 DM) have also increased bone magnesium by 22% (Chicco et al., 1973).
Assumptions that magnesium retention is insignificant in mature animals may not always be safe.
There was general agreement that the faeces is not used as a route of excretion for magnesium absorbed in excess of need. Faecal endogenous losses of magnesium (FEMg) were estimated to vary little from 3 mg Mg kg21 live weight (LW) in ruminants by ARC (1980), who set a widely followed precedent by using this value as a constant to correct faecal magnesium excretion data and thus derive true from apparent absorption values. It has been recently argued that there is appreciable secretion of magnesium in saliva and, since absorptive efficiency is far from complete, FEMg should not be constant but increased by factors, such as potassium, which reduce absorption from the primary site, the rumen (Dua and Care, 1995). However, earlier work with sheep found no such effect of potassium (Newton et al., 1972). Allsop and Rook (1979) found evidence of decreased FEMg in magnesium-deprived sheep, while Grace et al. (1985) found that 65% less magnesium was recycled to the rumen via parotid and mandibular saliva when sheep were fed fresh ryegrass, which was high in potassium and low in magnesium, rather than lucerne hay, which was low in potassium and high in magnesium (saliva flow was not greatly affected by forage type); furthermore, the amounts of magnesium secreted remained relatively small (0.85 and 0.30 mg Mg kg21 LW, respectively). Potassium-loaded sheep might therefore secrete less salivary magnesium than unsupplemented sheep, because they absorb less magnesium from their diet, offsetting any tendency to reabsorb less of the magnesium which is secreted. The constancy of FEMghas yet to be disproved.
Magnesium is less readily filtered at the glomerulus than most macrominerals, but sufficient is filtered and escapes tubular reabsorption, once the renal threshold of 0.92 mmol l21 is exceeded, to allow urine to be the major route of excretion for excessive dietary supplies (Ebel and Gunther, 1980).
Faecal endogenous losses
Urinary losses
The early magnesium nutrition of the newborn deserves comment. The magnesium concentration in main milk is low (0.3 mmol l21) and only one- tenth that of calcium, but it is maintained during depletion and represents a continuous drain on maternal reserves. Colostrum contains 0.1 g Mg kg21, two to three times more magnesium than main milk, giving a heightened maternal demand for magnesium at calving (Goff and Horst, 1997).
Magnesium is absorbed very efficiently from milk by the young lamb and calf, with absorption from the large intestine making a large contribution to that high efficiency (Dillon and Scott, 1979), but values fall in calves from 87% at 2–3 weeks to 32.3% at 7–8 weeks of age (ARC, 1980). Prolonged rearing of calves on a simulated milk diet will induce hypomagnesaemic tetany (Blaxter et al., 1954), as will the artificial maintenance of adult ruminants on a similar diet (Baker et al., 1979). The magnesium concentration of milk cannot be raised by dietary magnesium supplements.
Magnesium metabolism is influenced by animal as well as dietary factors.
Repeatable individual variation has long been identified as a potential stumbling-block to research workers, which must be minimized by appro- priate experimental designs (e.g. the Latin square) if reliable results are to be obtained. Studies with identical twin cows showed that such variation was partly heritable (Field and Suttle, 1979). In a comparison of beef breeds on a common diet, Aberdeen Angus were found to have relatively low serum magnesium concentrations (Littledike et al., 1995). Marked bovine breed differences in magnesium metabolism have been linked to differences in susceptibility to grass tetany (Greene et al., 1986), but the small numbers representing each breed or cross (n= 5) and inconsistent rankings for crosses raise the need for confirmatory studies. Shockey et al. (1984) reckoned that sheep were 1.75 times more efficient at absorbing magnesium from their diet than cattle and they explained their findings by the higher ratio of rumen surface area : rumen contents in sheep. The regression techniques used to make the comparison were not ideal for such purposes, but evidence for a species difference in AAMg has now been confirmed (Table 6.2). The magnesium concentration required to saturate absorption from the rumen can be three times greater in cattle than in sheep (12.5 vs. 4 mmol l21; Martens, 1983): this may partially compensate for any limitation of surface area when magnesium supplements are used but leaves cattle more susceptible to the K 3 Mg antagonism than sheep. The validity of using sheep as a model for magnesium in cattle requires urgent study. Individuality is seen in sheep given their nutrients by abomasal infusion (Baker et al., 1979) and is not merely a reflection of variation in rumen metabolism.
Maternal transfer of magnesium
Genetic variation in magnesium metabolism
Although most of the body magnesium (60–70%) is present in the skeleton (Todd, 1969; Fig. 6.3), magnesium ranks second to potassium in quantity in the intracellular fluids and organelles. Unlike potassium, it is largely (80%) protein-bound and associated predominantly with the microsomes (Ebel and Gunther, 1980). Magnesium also occurs in relatively low but life-sustaining concentrations in extracellular fluids, including the cerebrospinal fluid (CSF), and in the blood it is present in both plasma and erythrocytes. Magnesium is vitally involved in the metabolism of carbohydrates, lipids, nucleic acids and proteins, mostly as a catalyst of a wide array of enzymes (Wacker, 1969), facilitating the union of substrate and enzyme by first binding to one or the other (Ebel and Gunther, 1980). Magnesium is thus required for: oxidative phosphorylation, leading to ATP formation, which sustains processes such as the sodium (Na+)/K+pump; pyruvate oxidation and conversion of a-oxoglu- tarate to succinylcoenzyme A; phosphate transfers, including those effected by alkaline phosphatase, hexokinase and deoxyribonuclease; and the b-oxi- dation of fatty acids (Shils, 1997). The transketolase reaction of the pentose monophosphate shunt is also magnesium-dependent (Pike and Brown, 1975).
Magnesium also performs functions which do not rely on enzyme activation:
the binding of magnesium to phosphate groups on ribonucleotide chains influences their folding; magnesium ions modulate neuromuscular activity and affect autonomic control in the heart; low ionic concentrations accelerate the transmission of nerve impulses; and muscle contraction depends partly on exchanges with calcium (Ebel and Gunther, 1980; Shils, 1997). Finally, magnesium affects cell membrane integrity by binding to phospholipids.