Magnesium

Fibre-fed bacteria make minerals easier to absorb

Magnesium absorption happens mainly in the small and large intestine, and gut bacteria play a direct role in how much the body actually takes up. When gut bacteria ferment certain fibres, particularly inulin and other fructans, they produce short-chain fatty acids that lower the local pH in the colon. This more acidic environment makes magnesium more soluble and easier to absorb across the gut lining, alongside other changes like an enlarged absorptive surface and increased mineral-binding proteins.

Tested directly in a randomised trial

A randomised, placebo-controlled, crossover trial gave postmenopausal women a prebiotic fibre blend for 6 weeks and measured magnesium absorption directly using stable isotopes. Magnesium absorption increased by 5.2 percentage points with the prebiotic, compared with a decrease of 4.3 percentage points on placebo, a real, measurable swing of over 9 percentage points between the two. The same trial found a similar effect for calcium, suggesting the two minerals may benefit from the same fibre-driven mechanism.

Source: Effects of oligofructose-enriched inulin on intestinal absorption of calcium and magnesium and bone turnover markers in postmenopausal women

Results vary between studies and populations

Not every trial testing prebiotic fibre and magnesium has found the same benefit. Some studies in adolescents and young men found no significant change in magnesium absorption over shorter treatment periods, while postmenopausal women, who tend to have lower baseline mineral absorption, have shown the clearest response. This suggests fibre's effect on magnesium absorption may depend on age, sex, baseline mineral status, and how long the fibre is taken for, rather than being a universal effect.

What this means for you

Magnesium deficiency can cause muscle cramps, fatigue, and, in more severe cases, heart rhythm problems, so persistent unexplained cramping or fatigue is worth a blood test rather than a magnesium supplement bought on spec. People with conditions that affect gut absorption, including coeliac disease, Crohn's disease, and chronic diarrhoea, are at higher risk and may need magnesium levels checked more proactively. Alongside dietary sources like leafy greens, nuts, and whole grains, the trial evidence above is a reasonable basis for including prebiotic fibre sources in the diet as a genuine, food-first way to support magnesium absorption, particularly for postmenopausal women. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.