Speciation and bioavailability of iron in plant foods

Iron deficiency is a serious public health problem which affects around 2 billion people worldwide. When left untreated iron deficiency leads to iron deficiency anaemia (IDA) and in the UK IDA affects an estimated 2.4 million adults, 500 thousand pre-school children and up to 1.8 million school-age children. The majority of the iron in our diet is present in plant foods. However, absorption of plant-derived iron is very variable, ranging from 0-15%, due to the presence of other components in the diet that can enhance iron absorption (for example vitamin C) or inhibit iron absorption (for example phytate). However, very little is known about the different forms of iron in plant foods and how different species are absorbed by the body. Also, thrre are limited data on the effects of processing and digestion of plant foods on iron absorption.

Our aim in the proposed project is to identify and characterise the different forms of iron in selected plant foods (for example wheat, peas and broccoli) that are potentially good sources of iron in the UK diet and are believed to contain different iron species. We will use the raw food materials but also cooked and processed foods following simulated digestion.

We have gathered together a multidisciplinary team with a range of expertise, including: (i) state-of-the-art analytical techniques that will be used to characterize the different iron forms, including the distribution in plant tissues and plant cells (ii) whole food imaging techniques that will be used at the UK's national synchrotron facility to identify structures within foods associated with high iron concentrations (iii) high resolution microscopy, spectroscopy and proteomics that will be used to fully characterise these structures and to provide information on proteins in the foods that bind iron (iv) assays using human cells that will be used to study the availability of the different forms of iron for absorption by the body before and after simulated digestion.

This research will provide essential information on the major forms of iron present in plant-based foods and how well they are absorbed by the body. Information on iron-binding proteins will be useful in the long-term to aid crop breeding programmes and select for varieties that are naturally rich in iron. Knowledge gained from this project will enable the development of plant foods and processed food products with higher iron bioavailability and the design of efficacious and safe alternative iron food fortificants that more closely represent the forms of iron naturally present in the diet. Finally, our data will better inform policy makers, regulatory authorities, nutrition consultative groups and health professionals on strategies for improving the iron status of the UK population.

We have selected common classes of plant-based foods that contribute significantly to the UK iron intake and are predicted to contain contrasting chemical forms of iron(cereals, pulses and vegetables). We will investigate the chemical form of iron-complexes in these crops, and the effect of processing and digestion on iron bio-availability and absorption.
To characterize abundant iron species, we will perform whole sample imaging of the plant tissues and of the processed and digested foods, using x-ray fluorescence (xrf) imaging and NanoSIMS. Soluble extracts from raw, processed and digested foods will be frationated by size exclusion chromatography. Iron-rich fractions will be identified by Inductively Coupled Plasma-Mass Spectrometry, and further analysed using advanced spectroscopy techniques (electron paramagnetic resonance, Mössbauer spectroscopy and x-ray based Extended x-ray Absorption Fine Structure). Nanoparticulate (<100 nm) iron will be observed using high resolution Transmission Electron Microscopy.
The bioavailability of different forms of iron will be determined using the Caco-2 cell model. The uptake of iron from digestates will be measured both before and after digestion. Chemical and siRNA-based inhibitory methods will be used to determine the contributions of the three uptake pathways in Caco-2 cells, namely (a) inhibitors of endocytosis, (b) an inhibitor of DMT1 and (c) an inhibitor of Dcyt-B. Endocytosis is the route by which all nanoparticulate (<100 nm) iron accesses the cell, whereas Dcyt-B is required to reduce soluble ferric iron so that it can be transported via DMT1.
The data on the chemical forms of iron in food, effects of processing and digestion, and the bio-availability of Fe, can be used by the food industry to improve nutritional quality of products. Th knowledge on iron speciation and bioavailability will also help in developing improved assays for predicting iron availability, and to state this information on food labels.

Two billion people worldwide remain iron deficient with iron deficiency anaemia being one of WHO top 10 targets for cure and prevention. Children and women of child-bearing age, particularly pregnant women, have high iron requirements and are at increased risk of iron deficiency anaemia with consequent serious adverse effects on health. Traditionally iron deficiency has been tackled through fortification and supplementation programmes with little success. More recently, biofortification and the nutritional quality of food crops has become an important goal for the agricultural sector. Globally, we have to move to a more sustainable diet, taking into account climate change and nutritional (food) security, and therefore diets will contain lower intakes of meat, especially red meat which has the greatest greenhouse gas effect but which provides iron of the highest bioavailability. Alternative plant food sources, such as cereals and green vegetables, will be of growing importance for supplying iron in the diet. Diets consumed by vegetarians and low-income population groups are particularly low in bioavailable iron.
This project will provide information on the major forms of iron present in plant-based foods and how well they are absorbed by the body. It is anticipated that this will inform strategies for increasing the iron content of foods either by (i) controlling food processing or (ii) manipulating plant-food crops with the ultimate aim to improving the iron status of the UK population.
This project has great potential to bring benefit to (a) the health status of the UK population (b) food regulation and policy (c) UK competitiveness in the field.
(a) The understanding of how iron bioavailability in plant foods can be improved can help to inform plant breeders, growers and the food industry to take steps to ensure improved nutritional quality of their crops and food products produced using plant foods as raw materials. Furthermore, the findings from this project will be disseminated to health professionals (doctors, dieticians, nurses) so that they can offer personalised dietary advice for patients. This will in time help to improve the iron status of the population.
(b) The results of this project can be used for developing better food/nutrition policies for the UK (and internationally) and to develop food based dietary guidelines that ensure that the diet of all population groups meets their requirements.
(c) There are major gaps in our understanding of iron speciation in foods, particularly plant-based foods, and starting to fill these, as we propose, can inform on the development of food-like (and therefore safe) iron food fortificants/supplements and greatly enhance UK competitiveness in the field both academically and industrially.