Zinc, copper and magnesium absorption from a fibre-rich diet.
Level 4 - case-series / case-control
Uncontrolled single-arm metabolic balance and stable isotope study in a small healthy cohort.
PubMed 8829129 · doi:10.1016/S0946-672X(96)80014-9
What was done
Mineral absorption and retention were evaluated in 8 healthy participants consuming a high-fiber, high-phytate diet of conventional foods (29 g dietary fiber, 1 mmol phytic acid, 140 µmol zinc, 13 µmol copper, and 9 mmol magnesium per 10 MJ). Fractional absorption was measured by fecal monitoring of zinc, copper, and magnesium stable isotopes added on one day. Apparent absorption, retention, and endogenous losses were calculated from fecal and urinary excretion across the final 16 days of a 21-day constant intake period.
What was found
Fractional absorption was 29 ± 12% (48 ± 20 µmol) for zinc, 44 ± 7% (8 ± 1 µmol) for copper, and 46 ± 6% (5 ± 1 mmol) for magnesium. Estimated endogenous intestinal losses were 40 ± 20 µmol for zinc, 12 ± 4 µmol for copper, and 3 ± 1 mmol for magnesium. Retention was negative for all three minerals: -7 ± 19 µmol for zinc, -5 ± 4 µmol for copper, and -3 ± 1 mmol for magnesium.
Why it matters
This study demonstrates that high-fiber, high-phytate diets may impair the bioavailability of zinc, copper, and magnesium to levels below those required to offset natural endogenous excretion.
Limits
The study included only 8 subjects with no baseline or low-fiber control diet group for comparison. Total duration of intake was limited to 21 days, so potential long-term homeostatic adaptations were not evaluated.
Cited by
- supports Phytic acid found in legumes and whole grains binds to divalent cations including zinc, copper, magnesium, and selenium, inhibiting their absorption.