Bioaccessibility of iron and zinc in selected complementary foods fortified with micronutrient powders in Kenya

Fortification with micronutrient powders (MNPs) is recommended as a strategy for increasing the micronutrient content in complementary foods. However, plant-based diets commonly consumed in developing countries are rich in phytates and tannins, which decrease the micronutrient bioavailability. The present work analysed the relationship between the antinutrient content, and also iron and zinc bioaccessibility, in home-made MNP-fortified complementary feeding porridges refined with white rice, maize, white sorghum, finger millet, pearl millet, Irish potato, and banana samples, which were obtained from the local market and milled into flour. Porridges were prepared using the flour, cooled to 50°C, fortified with MNPs, and subjected to in vitro digestion. Total and bioaccessible zinc and iron were quantified using atomic absorption spectrometry. Tannins and phytates were analysed using Folin-Denis and high-performance liquid chromatography methods, respectively. Porridges were classified as having poor bioavailability if their phytate-zinc and phytate-iron molar ratios were > 15 and > 0.4, respectively. Only pearl millet and soybeans showed tannin levels higher than the recommended values. The lowest phytate level was observed in refined white rice (0.11 ± 0.04 g/100 g), and the highest was in pearl millet (2.83 ± 0.10 g/100 g). Zinc bioaccessibility ranged from 7.31% (finger millet) to 26.05% (corn-soy blend). Only pearl millet porridge was classified as having poor zinc bioavailability. Iron bioaccessibility ranged from 20.73% (refined white rice) to 0.62% (pearl millet). Refined white rice and Irish potato were the only foods with the phytate-iron ratio within the recommended range. Iron bioaccessibility decreased significantly with an increase in both tannin (r = -0.31, p = 0.045) and phytate (r = -0.39, p = 0.01) contents. Zinc bioaccessibility showed a significant positive relationship with tannin levels (r = 0.41, p = 0.008), but an insignificant inverse relationship with phytate levels (r = -0.072, p = 0.700). Iron bioaccessibility was adversely affected by phytate and tannin levels. To improve iron and zinc bioavailability in complementary foods, strategies for lowering the phytate and tannin contents at the household level are recommended.