Popeye ate spinach, thinking it contained plenty of iron to make him strong. However, oxalates in spinach reduce the bioavailability of iron. A Swedish study has found that spinach also contains nitrates which boost the production of 2 key proteins in muscles involved with calcium and muscle contractions, making them stronger and more efficient. Although the study was done in mice, Popeye seems to have had the right idea.
Hernández A, Schiffer TA, Ivarsson N, Cheng AJ, Bruton JD, Lundberg JO, Weitzberg E, Westerblad H. (2012). Dietary nitrate increases tetanic [Ca2+]i and contractile force in mouse fast-twitch muscle. J Physiol.. 590(Pt 15): 3575-83