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Magnesium is an essential mineral with numerous physiological functions and, hence, Mg deficiency and hypomagnesaemia cause a variety of clinical symptoms including neurological disorders such as tetany in ruminants. Since Mg homeostasis in the extracellular compartment (ECC) and blood is not regulated by hormones, the steady-state concentration of Mg in the blood depends on Mg efflux from the (ECC) into milk or tissues including bones and on absorption of Mg (influx) from the gastrointestinal tract (GIT). Mg absorption (influx) from the rumen is a) active, b) essential for Mg homeostasis and c) includes two transport mechanisms: a potential difference (PD) dependent or K-sensitive and potential difference independent or K-insensitive mechanism. The kinetic data (Km and Vmax) of the two parallel working uptake mechanisms are not known but there are convincing evidence for a ?job sharing? between these two mechanisms: Mg uptake with high affinity/low capacity via the PD-dependent mechanism at low Mg concentrations, and Mg uptake with low affinity/high capacity for high Mg concentrations (PD-independent) (for details see review Martens and Schweigel 2001).
If the proposed model of ?job sharing? of the two Mg uptake mechanisms is correct, two consequences regarding possible effects of K can be predicted. (i) An increase of K should reduce Mg absorption to a large extent at low Mg concentrations, because Mg absorption at low ruminal Mg concentration primarily depends on PD-dependent or K-sensitive uptake. (ii) An increase of Mg intake could compensate for the possible negative effects of high K intake, because the PD-independent or K-insensitive uptake is mainly active at high ruminal Mg concentrations. These assumptions are indeed confirmed by experimental observations. Ram et al. (1998) fed increasing amounts of Mg at two levels of K-intake. It was interesting to learn that the absolute amount of reduced Mg absorption was almost identical at all Mg intakes. However, the relative change decreased with increasing Mg intake (table 1).
Table 1: Effect of K intake (1 or 3.6 %) on Mg absorption in sheep at increasing Mg intake (Ram et al. 1998). Number in parenthesis: Apparent digestibility of Mg
g/d Mg-Absorption (g/d)
K 1 % K 3.6 % Decrease
Mg-Absorption (g/d) Change
1.64 0.58 (35) 0.27 (16) 0.31 54
3.14 1.17 (37) 0.81 (26) 0.36 31
4.66 1.56 (33) 1.14 (24) 0.42 27
This observation was confirmed by a meta-analysis of Weiss (2004). The magnitude of Mg-digestibility depends on Mg intake and K content, which takes into account the two Mg transport mechanisms as explained above, and is described by the equation: Digestible Mg = 4.5 (± 4.0) + 0.24 (± 0.07) x Mg intake (g/d) ? 4.4 (± 2.2) x K (% dry matter). Hence Mg requirement (digestible Mg) can be calculated according to this equation.