Bioavailability of Zinc from Cooked Philippine Milled, Undermilled, and Brown Rice, as Assessed in Rats by Using Growth, Bone Zinc, and Zinc-65 Retention JANET R. HUNT,* ,² LUANN K. JOHNSON, ² AND BIENVENIDO O. JULIANO ‡ Grand Forks Human Nutrition Research Center, USDA-ARS, P.O. Box 9034, Grand Forks, North Dakota, 58202 and Philippine Rice Research Institute Los Ban ˜os, Pili Drive, University of the Philippines Los Ban ˜os Campus, 4031 College, Laguna, Philippines The nutritional bioavailability of zinc from cooked milled, undermilled, and brown Philippine rice (variety PSB Rc14) was evaluated in rats, comparing results based on weight gain, tibia zinc incorporation (slope ratio analyses), and zinc radiotracer retention. Milling reduced the phytic acid and mineral content of the rice, resulting in zinc concentrations of 16.5, 19.4, and 27.2 μg/g and phytate/zinc molar ratios of 4, 20, and 28 for milled, undermilled, and brown rice, respectively. Measured zinc bioavailability was similar whether using growth, bone zinc, or radioisotope retention as criteria, at approximately 92, 86, and 77% of zinc sulfate, for milled, undermilled, and brown rice, respectively. However, the higher percent bioavailability of the zinc after milling was insufficient to compensate for the lower zinc content. With respect to zinc, the nutritional value was inversely related to milling, providing approximately 15, 17, and 21 μg bioavailable zinc/g rice, respectively, for milled, undermilled and brown rice of this variety. KEYWORDS: Zinc bioavailability; absorption; retention; slope ratio; zinc-65; radioisotope tracer; brown rice; white rice; milling; phytic acid; weight gain; growth; tibia zinc INTRODUCTION There have been several attempts to popularize brown rice in Southeast Asia in place of milled rice. Brown rice is richer in fat, vitamins, protein, and minerals than milled rice, but also richer in dietary fiber and phytate that may inhibit absorption of minerals such as iron and zinc. This is of concern because, according to the recent 1998 national nutrition survey, 31% of Filipinos suffer from iron deficiency anemia (1). In addition, calculations of the global food supply suggest that 71% of the population in Southeast Asia is at risk for low zinc intake (2). No human data are available comparing zinc bioavailability from brown and milled rice in Filipino diets. In Filipino adults, zinc absorption from two 3-d-germinated brown rice-based weaning foods was determined from the fecal recovery of zinc radioisotope, in comparison to that of unabsorbed chromium radioisotope. Zinc absorption was 14.7 ( 1.2% from a germinated brown rice/mungbean (7:3) mixture, and 12.0 ( 1.0% from a germinated brown rice/cowpea (7:3) formulation (3). Baseline data on rats were desired prior to human studies on Filipinos. Previous data in rats showed a much lower zinc bioavailability in cooked brown rice than in cooked milled rice: 46 vs 92% of zinc sulfate when based on weight gain, and 32 vs 69% when based on total femur zinc, respectively (4). Zinc balance in rats showed an apparent absorption of 14% from brown rice, 23% from undermilled rice, and 22% from milled rice; the resulting femur zinc concentrations were 171, 188, and 194 μg/g, respectively (5). On the basis of whole body scintillation counting of zinc-65 radiotracer, rats retained 56.4% of the zinc from brown rice, which resulted in a bioavailability of 84% relative to that of a zinc chloride control (6). The bioavailability of zinc is affected negatively by the molar ratio of phytate to zinc (7). As estimated in a WHO report (7), diets with a phytate/zinc molar ratio greater than 15 have relatively poor zinc bioavailability (10-15%), those with a phytate/zinc ratio between 5 and 15 have medium zinc bio- availability (30-35%), and those with a ratio less than 5 have high (45-55%) zinc bioavailability. However, limited human data on zinc absorption from practical whole diets suggest a more moderate influence of the phytic acid, resulting in 26 and 33% zinc absorption with phytate/zinc molar ratios of 14 and 5, respectively (8), and 29% zinc absorption with a phytate/ zinc molar ratio of 7 (9). Phytate/zinc molar ratios are reported as 4-11 for milled rice (4, 5, 10), 11 for undermilled rice (5), and 13-47 for brown rice (3-6, 10). In view of these considerations and developments, a col- laborative study was undertaken by the Philippine Rice Research Institute (PhilRice) Los Ban ˜os with the USDA Grand Forks Human Nutrition Research Center to study the bioavailability * To whom correspondence should be addressed. Telephone: 701-795- 8328. Fax: 701-795-8220. E-mail: jhunt@gfhnrc.ars.usda.gov. ² Grand Forks Human Nutrition Research Center. ‡ Philippine Rice Research Institute Los Ban ˜os. J. Agric. Food Chem. 2002, 50, 5229-5235 5229 10.1021/jf020222b CCC: $22.00 © 2002 American Chemical Society Published on Web 08/03/2002