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Dietary calcium does not exacerbate phytate inhibition of zinc absorption by women from conventional diets

¹ From the US Department of Agriculture–Agricultural Research Service, Grand Forks Human Nutrition Research Center, Grand Forks, ND.

² Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the US Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.

³ Supported by the USDA Agricultural Research Service.

⁴ Reprints not available. Address correspondence to JR Hunt, USDA-ARS GFHNRC, 2420 2nd Avenue N, STOP 9034, Grand Forks, ND 58202-9034. E-mail: janetrhunt{at}gmail.com.

Background: Although calcium inhibits zinc bioavailability in rats, especially from high-phytate diets, the effect of calcium on zinc absorption by humans from practical diets remains unclear.

Objective: The objective was to test the inhibitory effect of dietary calcium, in Western diets with high and low phytate content, on zinc absorption.

Design: Using a 2 x 2 factorial design, zinc absorption was determined in 10 healthy women from 1-d diets with moderate and high calcium contents of 700 and 1800 mg/d and low and high phytate contents of 440 and 1800 mg/d. Absorption was measured by using extrinsically added ⁶⁵Zn and subsequent whole-body scintillation counting.

Results: Mean (±SE) fractional zinc absorption was 32.8 ± 2.3% from the moderate-calcium, low-phytate diet; 26.9 ± 2.4% from the moderate-calcium, high-phytate diet; 39.4 ± 2.4% from the high-calcium, low-phytate diet; and 26.2 ± 2.3% from the high-calcium, high-phytate diet. The respective values for absolute zinc absorption were 3.8 ± 0.3, 3.0 ± 0.3, 4.5 ± 0.3, and 3.2 ± 0.3 mg/d. Phytate significantly reduced fractional zinc absorption by 10 percentage points and reduced absolute zinc absorption by 25%, or 1 mg/d. Differences in dietary calcium did not affect zinc absorption, regardless of a high or low dietary phytate content.

Conclusions: In healthy women consuming 1-d menus of ordinary foods (some fortified with calcium), dietary phytate reduces zinc absorption, but calcium does not impair zinc absorption, regardless of whether dietary phytate is low or high.