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Dietary iron intake based on food consumption data underestimates the contribution of iron from soil

25 March 2015

Research conducted by the universities of Nottingham, East Anglia, Otago and Lilongwe in Malawi has recently shown that estimating the dietary intake of iron based on food composition data may be underestimating the contributions from iron in the soil boosting the dietary intake.

The study was conducted in Malawi with two separate populations, living off a plant based diet where threshing the grains was practiced, but with contrasting soil mineralogy for the cereal crops, one with an acidic soil and the other calcareous. With a plant based diet it is often theorised that there will be a risk of iron deficiency, but one of the conclusions from this study was that this risk was lower than expected in the test populations. Higher levels of the minerals Al (Aluminium) and Ti (Titanium) found in the experiments results indicated that soil contamination (via threshing) had occurred and that in turn this contamination should be considered when determining iron dietary intake of the relevant populations and not just food composition.

The research process involved the collection and analysis of duplicate diet composites from 120 Malawian women in two rural districts where threshing of the crops may contaminate cereals with soil iron and other minerals. Soil and diet composites from the two populations were then subjected to a simulated gastrointestinal digestion and iron availability in the samples was measured, which showed that the iron intakes were higher than calculated intakes in previous studies, with this being attributed to contaminant soil iron. A small portion of iron in acidic soil, but not calcareous soil however, was shown to be bioavailable, and may have contributed to higher plasma ferritin and total body iron for the women than expected. Results showed lower iron intakes, if calculations were based solely on food composition. The reasoning behind this was not simply the soil mineralogy, although the advantage of the acidic soil was shown, but the threshing of smaller grains .i.e. a sorghum over a maize based cereal, was theorised to have a greater contamination effect from the soil.

As the combined universities experiment states, the study of “geophagia”, the bio-availability of minerals from sources extraneous to the food itself, has been documented before with water, cooking pots, dust and soil all contributing. This study reinforces the need that any future methods of calculating iron intake from food composition data would now have to factor in the importance of soil and other extraneous source contamination, where the food product is a plant based diet, and where threshing or another manner of contamination, is prevalent.

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