N05014: The bioavailability of iron, zinc and copper in meat-containing and vegetarian diets in the UK

Study Duration: January 1999 to March 2002

Contractor: Central Science Laboratory

Dietary habits of the UK population are changing, in part, due to public perception of diet-related health issues and the need for convenience. A current trend, particularly amongst women and young girls, towards vegetarianism or the substitution of red meat with fish or poultry has resulted in concern regarding the dietary intake of nutritionally-important minerals, iron in particular. This has been further exacerbated by the reduced calorific intakes also exhibited by these population sub-groups. A study was therefore designed to clarify the nutritional significance of these dietary changes by providing data on the bioavailability of iron (Fe), copper (Cu) and zinc (Zn) from the three main types of diet in the UK.

In addition to the above, consideration was also given to how the different chemical forms of iron in the three types of diet would affect the element's bioavailability.

This project was funded in collaboration with a program of work carried out by researchers at the Institute of Food Research, Norwich who performed the human volunteer study from which the samples analysed and reported in this study originated.

To assess how the three different diet types affect Fe, Cu and Zn absorption, 90 volunteers were given an oral dose of stable isotopes (⁵⁸Fe, ⁶⁵Cu and ⁷⁰Zn) with a test meal. The alteration to the natural isotope ratios of the three elements in the faeces collected over subsequent days enabled the elements' absorption to be calculated. The isotope ratios were measured by Inductively Coupled Plasma - Mass Spectrometry (ICP-MS), to a precision that was sufficient to allow the 'true' absorption of the elements from the different diets. Although ICP-MS is recognised as being a very powerful multi-element and multi-isotope measurement technique, it can be subject to spectral interferences. Because the technique relies on a very high energy argon plasma to bring about the ionisation of the dissolved sample, other elements can combine with the argon to produce species with similar mass/charge ratios to those being investigated e.g. ⁴⁰Ar¹⁴N¹⁶O interferes with the measurement of ⁷⁰Zn and ⁴⁰Ar¹⁶O on ⁵⁶Fe. Hence, a significant proportion of the work carried out in this project was focused on overcoming these difficulties so that suitable measurements of the Fe and Zn isotope ratios in the human study samples could be achieved.

Once the methodologies were developed and validated, they were used to measure both the isotopic ratios of the elements in the samples and also the total amount of each analyte present, as both pieces of information were required to allow accurate and precise kinetic modeling of the elements' in vivo behaviour.

In addition to the faecal samples, a duplicate of all the food consumed by each volunteer was also collected. These samples were analysed for the total levels of the three elements, iron, copper and zinc, and were also investigated with regard to the chemical form in which the iron was present, i.e. haem-iron versus non-haem-iron. This was done because metabolic behaviour of this element is reported to be chemical-form dependent.

Although, historically, much effort has been invested in the elucidation of iron speciation, the methods available remain very non-specific, making data very difficult to interpret. Therefore, within this project the development of a suitable chromatography-based methodology was made a core objective, with a number of different approaches being assessed. However, a definitive method for determining the haem-iron content of the cooked and processed foods, of which the duplicate diets were mainly comprised, remained elusive due to significant interactions between the analyte and the additives and preservatives that are commonly included in our foods.

Within this project, a high throughput, high accuracy ICP-MS methodology was developed for the measurement of iron and zinc isotope ratios in human physiological material. Two publications have been submitted for publication in a peer-reviewed journal and a third is currently being drafted.

The methods have been successfully applied to the characterisation of the 991 faecal samples (digested and analysed in duplicate), the data from which has been combined with the 'totals' data from the duplicate diets and used to answer the nutritional questions posed within FSA Project N05015.

Our understanding of the chemistry relating to the different chemical forms of iron has increased but, due to the complexity of the issue, a definitive methodology for its determination in processed food eludes us.

Contact: Ms Mamta Singh
Tel: 020 7276 8919
Email: mamta.singh@foodstandards.gsi.gov.uk