Mineral balance is difficult to calculate for many dietary minerals because of the multiple compartments in which they can be temporarily stored. Although it can be a relatively easy to estimate the intake of a particular mineral, its route of metabolism may be complex and this makes even tracer studies of questionable reliability. In addition, multiple routes of excretion also exist and this complicates the matter further. The skin is one route of excretion that is not often considered when measuring mineral losses, but substantial amounts of certain minerals can be lost through sweat. As well as the common macrominerals such as sodium and chloride, a myriad of trace minerals can also be lost. In some cases sweat can account for significant losses from the body stores. Iron losses through sweat have been reported to be substantial with some studies showing several milligrams per day lost to sweat. However, environmental contamination and desquamated epithelial cells containing iron may have caused an overestimation of iron losses in these studies.
In one of the more rigorously controlled studies1, researchers used a careful cleaning and double sample collection procedure to reduce the risk of iron contamination during the sampling of sweat from healthy male volunteers. Sampling from the men was performed in a sauna in order to induce perspiration, and sweat was collected in plastic drip trays trays and from the skin during a 30 minute period. To remove desquamated cells, the authors centrifuged the collected sweat to remove any cells and their iron contents prior to analysis. Samples were also taken in consecutive periods in order to minimise the influence of environmental iron present on the skin at baseline. The authors analysed the volume of fluid lost through sweat and the iron content, and determined that the total iron losses were 22.5 μg per litre of sweat. Normal sweat loss in a human is around half a litre, but can increase to 20 litres in extreme condition. Under normal conditions the typical losses of iron from sweating may therefore be around 11.25 μg.
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