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Dietary calcium does not exacerbate phytate inhibition of zinc absorption by women from conventional diets^1,2,3,4

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.

ABSTRACT

Background: Although calcium inhibits zinc bioavailability in rats, especially from high-phytate diets, the impact 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.

Received for publication October 29, 2008. Accepted for publication December 9, 2008.