Effect of Zinc Depletion and Repletion on Resting Metabolic Rate and Zinc Status

Abstract

1167 Previous research has shown that zinc intakes below recommended values for a prolonged period of time may lead to a decrease in plasma zinc status and resting metabolic rate (RMR) because of zinc's involvement in 1,5'-deiodinase function. 1,5'- deiodinase is an enzyme responsible for converting thyroxine (T4) to triiodothyronine (T3), a major regulator of RMR. PURPOSE: Research is limited to primarily male subjects confined to a metabolic ward, therefore the purpose of this study was to determine if low zinc intakes for 45 days in free-living male and female subjects would negatively impact on RMR and zinc status, and if this would be reversed after 20 days of zinc supplementation. METHODS: The study consisted of four phases: Prebaseline (PB), Baseline (B), Phase I (PI) (45 days of zinc depletion), and Phase II (PII) (20 days of zinc repletion). Blood was drawn at each time point; plasma zinc status was analyzed using atomic absorption spectrophotometry. RMR was assessed at PI and PII. Subjects with plasma zinc levels > 0.7 μg/mL at PB were instructed to follow a low zinc diet (PI) with < 5 mg/day of dietary zinc for at least 45 days to induce deficiency. Subjects who were deficient at PB had blood drawn (B) again for reliability measures. If deficiency was confirmed, supplementation with 24 to 26 mg of zinc gluconate immediately began for 20 days (PII). RESULTS: One 24 year old male (S1) and one 22 year old female (S2) completed the study. At PB, S1 weighed 95.5 kg and was 170. 2 cm tall, while S2 weighed 63.6 kg and was 170.2 cm tall. After 74 and 77 days, respectively on the low zinc diet, plasma zinc decreased from 0.82 ± 0.09 μg/mL to 0.77 ± 0.04 μg/mL for SI and 0.87 ± 0.23 μg/mL to 0.58 ± 0.06 μg/mL for S2. RMR for S1 after PI was 1729.63 ± 170.63 kcal/day and 1265.59 ± 110.57 kcal/day for S2. Following zinc supplementation, plasma zinc increased to 0.87 ± 0.02 μg/mL in S1 and to 0.72 ± 0.00 μg/mL in S2; RMR for S1 and S2 after PII were 2031.25 ± 199.76 kcal/day and 1273.18 ± 112.12 kcal/day, respectively. CONCLUSIONS: Dietary zinc intakes of < 5 mg/day for 45 days resulted in a reduction in plasma zinc (male) and zinc deficiency (female). Following zinc supplementation, plasma zinc levels and RMR increased in both subjects. Further research in a larger sample size is required to ascertain long-term effects of zinc deficiency on these parameters.