What do button-down dietitians have in common with brawl-happy cartoon sailors? They all love their spinach! And for good reason. One serving of these leafy greens is loaded with fiber, calcium, and virtually your entire day's recommended dosage of beta carotene, a nutrient vital for immune-system health, good vision, but not-as far as we know-huge wrist flexors. If you can't stand spinach plain, Katherine Tallmadge, R.D., author of Diet Simple, suggests dropping it into burritos, pasta dishes and canned soup.