Details about magnesium absorption

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Challenges in the Diagnosis of Magnesium Status

"Due to the complex nature of magnesium absorption, segments of the GI tract can vary in their contribution to absorption; however, under normal physiologic conditions the general guide is the duodenum absorbs 11%, the jejunum 22%, the ileum 56%, and the colon 11% (Figure 3) [3,58]."

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"Magnesium is a divalent cation, which plays a critical role in how the mineral is absorbed [83,84]. Magnesium is the most densely charged of all the biological cations, due to a high charge to radius ratio, resulting in high hydration energy for the Mg2+ cation [83]. This hydration energy results in tight coordination with a double layer of water molecules, increasing the hydrodynamic radius 400 times that of the dehydrated radius [83,85], resulting in an aquated cation that is too large to transverse typical ion channels (Figure 5) [86]. The removal of the hydration shell around magnesium is a precondition for absorption and can be accomplished by both TRPM6 and TRPM7 [⇈] and the magnesium associated paracellular claudins [64,67,87,88]."

It can be presumed that low doses would skew the absorption site distribution towards the upper parts, but it's counterintuitive: the active process of magnesium absorption, that prevails at lower intakes, occurs primarily farther down the small intestine.

Segmental transport of Ca2+ and Mg2+ along the gastrointestinal tract

"Although occasionally debated, paracellular absorption is generally considered to be predominant under normal physiological circumstances (3, 4, 32, 40, 44). When paracellular absorption cannot meet the daily need of the body, transcellular absorption becomes essential (6, 40). Specific transport proteins, located in the luminal and basolateral membrane of the epithelial cells, govern this latter process (23, 26, 32)."

"Transcellular absorption of Ca2+ and Mg2+ from the intestinal lumen is facilitated by members of the transient receptor potential (TRP) ion channel family, which represents the rate-limiting factor in the transcellular transport process. Intestinal ~~absorption of Ca2+ is mediated via TRP vanilloid type 6 (TRPV6), whereas~~ absorption of Mg2+ takes place via TRP melastatin type 6 (TRPM6) (20, 30, 43)." "For transcellular Mg2+ transport in the intestine TRPM7 and cyclin M4 (CNNM4) seem to be important in addition to TRPM6, since they are though to play a role in apical and basolateral transport of Mg2+, respectively (38, 47)."

"TRPM6 is expressed in the distal regions of the gastrointestinal tract of human subjects and mice (Fig. 2B). Interestingly, like TRPV6, TRPM6 in mice is expressed strongly in the cecum (Fig. 2B)."

"TRPM7 was evenly expressed throughout the gastrointestinal tract (Fig. 2D). Expression of the basolateral Mg2+ transporter CNNM4 was most prominent in the distal gastrointestinal segments, resembling the expression profile of TRPM6 (Fig. 2F). However, in contrast to TRPM6, CNNM4 expression was not completely absent in the proximal gastrointestinal tract."

Dark bars: human
Gray bars: mice
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"To investigate whether Mg2+ absorption is regulated by dietary Mg2+ content, mice were fed a diet with a low (0.02% wt/wt Mg2+), a normal (0.25% wt/wt Mg2+), or a high (0.48% wt/wt Mg2+) Mg2+ content for 2 wk."

"Urinary excretion of Na+, K+, and Cl− did not differ significantly between the groups, excretion of Pi was significantly higher in the low-Mg2+-diet group compared with the groups receiving normal and high-Mg2+ diets (~~Table 2~~)."

"Dietary Mg2+ restriction significantly stimulated the rate of intestinal absorption of Mg2+ (Fig. 3G). Conversely, intestinal absorption of Mg2+ in the high-Mg2+-diet group was diminished compared with the normal group. The absorption of Mg2+ in the low-Mg2+-diet group was significantly higher compared with the normal group (110.0 ± 3.0% and 100.0 ± 0.1%, respectively, P < 0.01) and in the group on the high-Mg2+ diet absorption was significantly lower compared with the normal group (95.0 ± 1.0 vs. 100.0 ± 0.1%, respectively, P < 0.05) (Fig. 3H). TRPM6 mRNA expression levels in the colon were slightly, but significantly, regulated by dietary Mg2+, showing an increase in the group on the low-Mg2+ diet and a decrease as a consequence of the high-Mg2+ diet (Fig. 3I)."

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"The proximal Mg2+ absorption was higher compared with the distal part of the intestine at a high luminal concentration of Mg2+ (44 mM) (Fig. 4E), but not at lower luminal concentrations (1.6 and 10 mM Mg2+) (Fig. 4, A and C). The relationship between the absorption of 25Mg2+ and the luminal Mg2+ concentration is displayed in Fig. 4G. In the proximal intestine, the absorption of 25Mg2+ resembles a linear relation to the buffer Mg2+ concentration, whereas in the distal part of the intestine the absorption of Mg2+ seems saturable at higher Mg2+ concentrations."

"Paracellular and transcellular absorption of Mg2+ in intestinal segments. Solid lines represent mice with a cannula placed in the stomach, dashed lines represent mice with a cannula placed in the cecum. 25Mg2+ absorption by using a buffer containing 1.6 (A), 10 (C), or 44 (E) mM Mg2+." "Relationship between luminal Mg2+ concentration and 25Mg2+ absorption (G)."

"The findings in our study, therefore, suggest that ~~transcellular Ca2+ absorption takes place in the proximal region of the intestine, whereas~~ transcellular Mg2+ absorption mainly occurs in the distal gastrointestinal segments." "Paracellular absorption of Ca2+ and Mg2+ mainly takes place in the duodenum and jejunum."

A representation of how much of the daily amount is absorbed and the route distribution (transcellular or through the cell; paracellular or between cells):

Intestinal absorption of magnesium

Amazoniac

Intestinal absorption of magnesium from food and supplements

A suggestion when dealing with gut microbial imbalances:

	Supplemental Mg
Meals with problematic ingredients	Topical
Typical everyday meals	Oral 100 mg
Meals with safe 'prebiotic' ingredients	Oral >100 mg

This is considering it a microbial growth factor that's worth restricting or exceeding in each extreme.

The food content will add some more magnesium per meal and the total must not be too off from the efficient absorption range.