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Amirabdollahian, Farzad (2008) Zinc adequacy in the UK population. Doctoral thesis, London Metropolitan University.
Zinc (Zn) is an essential trace element with wide range of biochemical and physiological functions. Human Zn requirements are estimated by a consideration of the physiologic Zn requirement, coupled with an estimate of the proportion of Zn absorbed from the usual diet.
An accurate assessment of Zn status in a population is difficult because there is no reliable and globally accepted biomarker and/or index of adequacy. An initial analysis of the National Diet and Nutrition Survey (NDNS) showed that in the UK population average Zn intake ranged from 4.18 mg/day to 10.56 mg/day; and that, based on the current UK Zn Dietary Reference Values (DRVs), a substantial proportion of population sub groups appear to have inadequate Zn intake.
UK DRVs have not been revised since publication, in 1991; however, since then both the knowledge about Zn requirements and the characteristics of the UK population (e.g. average body weight) have substantially changed. In light of this, there is a need to re-consider the UK DRVs. Following the approaches used by recent national and international expert committees, this thesis has developed new estimates for the physiologic Zn requirements of the UK population. In order to develop DRVs, based on these, new estimates of Zn absorption were also needed, to take into account upto date data on the average phytate and Zn intakes of the population.
There is little data on the phytate content of UK foods so tables of the phytate content of foods were constructed from the best available international data. These were used with the average weight of foods consumed in the NDNS to estimate the median phytate intake in the UK population (ranging from 463 mg/day in toddlers to 948 mg/day in adults). In order to validate the phytate tables for use in the UK, an experimental protocol using Ion Pair High Performance Liquid Chromatography (HPLC) was used. It was found that the method was not precise, linear or accurate for the measurement of the phytate content of foods. The validation of these tables remains a priority for further research.
The validity of two recently published models to estimate Zn absorption in the UK population was investigated. Although there was evidence to support validity of the most recently developed mathematical model, it proved unable to estimate proportion of Zn absorption, using parameters for the UK population. A logit regression model was found to be valid and this, used together with estimates of dietary Zn and phytate intake, gave estimates of Zn absorption for the different subgroups of UK population.
Using the new estimates of physiologic Zn requirement and Zn absorption determined in the thesis, a revised version of UK DRVs was developed. The application of these revised DRVs to NDNS data demonstrated that the risk of inadequate zinc intake for many subgroups of the UK population was higher than estimates of risk made on the basis of the existing DRVs. ‘Inadequate Zn intake’ was associated with a poorer quality diet and the selection of a diet of low Zn density. The health and nutritional status of the population classified with Inadequate zinc intake’ was considered; biomarkers indicated that they had poorer Zn status and that they had significantly more CHD risk factors than those classified with ‘adequate Zn intakes’.
However, these findings must be treated with caution, as higher Zn DRVs would lead to a larger proportion of the UK population being considered Zn inadequate and there is little independent data to suggest that this is a problem. Additionally, the study did not take account of the possibility of adaptation to low zinc intakes, the role of other factors in the absorption process, the validity of phytate data for the UK and the robustness of the studies used in the theoretical model of Zn requirements. Current results suggest that UK DRVs need to be revised. There is a need for systematic national review of the current UK zinc DRVs.
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