While Iron does play a critical role in nutrition, it is well known now that except in the case of iron deficiency anemia, supplementing or consuming large amounts of iron can be detrimental to one's health. In fact, iron's energy boosting capacity only works in the presence of such deficiency. Due to iron fortification efforts in the U.S. and the fact that adult men need only 8 mg/day and adult women need 15 mg/day before 50 years old and only 8 mg/day thereafter, iron deficiencies today aye very rare.
So Popeye was wrong about taking spinach for iron, but he was on the mark in regards to spinach's natural folic acid and beta-carotene contents. Although it turns out that folic acid's effects on reducing homocysteine, which was long considered a bio-marker for heart disease, is no longer widely accepted, this essential B vitamins is still critical to our health. . On a personal note, many years ago I supplemented with folic acid only to see my homocysteine levels increase. Though I don't remember clearly my dietary habits at the time, my unhealthy eating patterns may have had more to do with the increased homocysteine than the folic acid supplementation. In fact, its often said that those who supplement feel more empowered to eat poorly because they think that supplementation offers some magical benefit that neutralizes poor eating. As is increasingly understood by many (except for supplement marketers), that is simply not the case.
It is well established that folic acid supplementation during pregnancy reduces neuro-tube defects. There are also a plethora of studies that showed or failed to show that folic acid improves cognition. The jury is still out on that one. However, the recommended daily does of folic acid is 400 micrograms/day and eating plenty of green vegetables such as spinach, asparagus, green beans, etc. which is critical to any healthy diet will provide the necessary daily amount of needed folic acid.
So if you want to be like Popeye, then eat your spinach as part of a healthy Mediterranean diet. If you want to find love like Popeye, make sure to include some Olive Oyl in your diet.