Magnesium phosphate, tribasic, pentahydrate.

Tribasic magnesium phosphate delivers phosphate along with the magnesium, so it adds to total phosphate load rather than sitting outside it. Phosphate and magnesium form poorly soluble complexes at intestinal pH, which is part of why this salt dissolves slowly. Anyone already taking a separate phosphate source is stacking two contributions to the same pool.

Pyridoxal kinase, the enzyme that converts pyridoxine to its active phosphate form, is a magnesium-ATP dependent enzyme. Supplying the already-phosphorylated vitamer bypasses that step. The two are frequently formulated together because magnesium status affects how much active B6 a body generates from the plain vitamin.

Tribasic magnesium phosphate has low water solubility and depends on stomach acid to liberate free magnesium ions. An acidifying agent lowers gastric pH and increases the dissolved fraction available for absorption. This matters more for a tribasic phosphate salt than for a soluble organic salt such as citrate or glycinate.

Bicarbonate raises gastric pH, and a mineral salt that needs acid to dissolve gives up less free ion at higher pH. Taking an alkalinising agent at the same time as this salt therefore reduces the dissolved magnesium fraction. Separating the two in time is the usual answer, and the same logic applies to any acid-suppressing agent.

Activated charcoal is non-selective and adsorbs minerals along with everything else in the lumen. Taken with a mineral salt it reduces the amount available for absorption. Spacing by several hours is standard practice with charcoal and any nutrient.

The galacturonic acid units in pectin are negatively charged at intestinal pH and bind divalent cations including magnesium and calcium. Binding within the gel phase lowers the free ion concentration available at the absorptive surface. Some of the bound mineral is released again when colonic bacteria ferment the pectin, so the effect is a shift in where absorption happens rather than a total loss.

Guar gum forms a viscous layer that slows diffusion of dissolved ions to the intestinal wall. For an already poorly soluble salt this compounds the limiting step. The interaction is about rate and contact rather than chemical binding, and it fades as the gum is fermented in the colon.

Glucomannan hydrates to a high-viscosity gel that entraps luminal contents. Minerals held in that gel reach the brush border more slowly and less completely within the absorptive window. As with other viscous fibres, taking the mineral apart from the fibre dose sidesteps the issue.

Bentonite is a layered aluminosilicate whose interlayer sites exchange cations, and magnesium is one of the cations it will take up. Co-ingestion therefore removes a share of the mineral from solution. Clays are used for their binding capacity, so the interaction is inherent to what they do rather than incidental.