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Phytic acid added to white-wheat bread inhibits fractional apparent magnesium absorption in humans^1,2,3

¹ From the Laboratory for Human Nutrition, Institute of Food Science and Nutrition, Swiss Federal Institute of Technology, Zurich.

Background: Phytic acid has been reported to impair the absorption of minerals and trace elements such as calcium, zinc, and iron in humans. However, limited information is available on the effect of phytic acid on magnesium absorption.

Objective: The objective was to evaluate the effect of phytic acid on fractional apparent magnesium absorption in humans.

Design: Two stable-isotope studies were performed with 8-9 healthy adults per study. Test meals were based on 200 g phytic acid-free wheat bread; test meals with and without added phytic acid were served on days 1 and 3 according to a crossover design. Phytic acid was added in amounts similar to those naturally present in whole-meal (1.49 mmol) and in brown bread (0.75 mmol). Each test meal was labeled with 0.7 mmol ²⁵Mg or 1.1 mmol ²⁶Mg. The total magnesium content was standardized to 3.6 mmol in all test meals. Apparent magnesium absorption was based on fecal monitoring.

Results: The addition of phytic acid lowered fractional apparent magnesium absorption from 32.5 ± 6.9% (no added phytic acid) to 13.0 ± 6.9% (1.49 mmol added phytic acid; P < 0.0005) and from 32.2 ± 12.0% (no added phytic acid) to 24.0 ± 12.9% (0.75 mmol added phytic acid; P < 0.01). The inhibiting effect of phytic acid was dose dependent (P < 0.005).

Conclusion: The results show that fractional magnesium absorption from white-wheat bread is significantly impaired by the addition of phytic acid, in a dose-dependent manner, at amounts similar to those naturally present in whole-meal and brown bread.

Key Words: Magnesium absorption • phytic acid • wheat bread • stable isotopes • fecal monitoring

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