Sulphur containing amino acids such as cysteine in garlic and onions may promote the bioavailability of iron and zinc from food grains (eg rice, sorghum, chickpea and green gram pulses).
Smita Gautam, Kalpana Platel, Krishnapura Srinivasan (2010). Higher Bioaccessibility of Iron and Zinc from Food Grains in the Presence of Garlic and Onion. J. Agric. Food Chem. 58 (14):8426–8429