Spinach: Low in calories and high in vitamins, spinach is one of the most nutrient-dense foods in existence. One cup of the leafy green vegetable contains far more than your daily requirements of vitamin K and vitamin A, almost all the manganese and folate your body needs and nearly 40 percent of your magnesium requirement. It is an excellent source of more than 20 different measurable nutrients. Cancer-fighting antioxidants abound in fresh spinach. This dark green leaf will protect your brain function from premature aging and slow old age's typical negative effects on your mental capabilities.