There’s a reason Popeye reached for a can of spinach rather than a fresh bunch. He knew that he could get more bang for his buck. You can squeeze a lot of spinach into a can or a box, delivering more spinach in less volume. (You would have to eat a mountain of fresh to get what you can in a 10-ounce box of frozen.) We prefer frozen spinach over canned—it’s got better flavor and is lower in sodium—but the same principle applies. One cup of frozen spinach has more than four times the amount of nutrients, such as fiber, folate, iron and calcium, than a cup of fresh spinach, so if you want to power up, do it with frozen spinach.