Comparison of four methods for phytate analysis in plant-based foods

Plant-based staple diets common in low-income countries (LICs) often contain high levels of phytic acid (myoinositol hexaphosphate, IP6), an inhibitor of zinc absorption. Depending on food source, handling and preparation, varying amounts of IP6 may be broken down to less phosphorylated inositol phosphates such as inositol tetra- and pentaphosphates (IP4, IP5) which differ from IP6 in their impact on zinc absorption. IP4 has little impact, whereas IP5's impact is moderately reduced as compared with IP6. Therefore to accurately estimate the risk of zinc deficiency, values both for phytic acid and IP5 may be required. High-Performance Liquid chromatography (HPLC) can separate and quantify IP6 and its breakdown products but requires specialized equipment and expertise and is relatively time-consuming and expensive. We compared HPLC and three low-tech and widely accessible alternative methods via an analysis of a sample of typical LIC staple foods. The "ferric precipitation" method and the much simpler "HL" colorimetric assay provide accurate data but they only assay total inositol phosphates. A Polyacrylamide Gel Electrophoresis (PAGE) method can separate IP5 and IP6 from IP4 and other breakdown products, and can provide valid quantitative data for these compounds. Therefore PAGE, alone or in combination with a simple colorimetric assay like the HL method, is an accessible, low-tech alternative to HPLC for estimating dietary total phytate, IP6 and IP5, and represents an appropriate technology for LIC laboratories.