What makes a strong Popeye after eating spinach?

To my knowledge, spinach contains iron that is needed, so we do not anemia or deficiency of red blood cells. If no shortage of red blood cells, our body can smoothly "provide transportation" for oxygen to move through the body. With the presence of oxygen, the body can burn for energy so that we can be strong.Perhaps because the current burning of calories in their bodies, since Popeye has a unique super body that can be moved at the Popeye soon so have more energy.

core of the story is, be strong Popeye after eating spinach because Popeye is the man planet Spinach

It was the spinach itself that made Popeye so strong. Probably all the vitamins and other nutrients found in spinach. It was a good way to make kids at least want to try spinach.