While all vegetables contain a wide variety of phytonutrients, one of the benefits of spinach is its especially high phytonutrient content, which gives it anti-inflammatory, anti-cancer and blood-pressure-lowering properties as well as many other positive health effects.
Spinach is an excellent source of antioxidants, as well as bone-supportive, healthy-heart and pro-digestive nutrients: vitamin C, vitamin E, beta-carotene, vitamin K, calcium, magnesium, zinc, manganese, selenium, folate, potassium, vitamin B6, vitamin A, iron, copper, niacin, omega-3 fatty acids, and fiber. One cup of raw spinach has only 7 calories; a cup of cooked spinach has 40 calories, 7 grams of carbohydrate, 4 grams of fiber, and 5 grams of protein. With more nutrients than lighter-colored greens, spinach adds punch to a well-balanced meal plan.
You can find spinach throughout the year in your regular grocery store. Fresh whole leaf and baby spinach are the best buys for maximum nutrient content. You're likely to get the best deals on fresh spinach from March through October, when spinach is in season. Look for dark green leaves without signs of yellowing, wilting, or a slimy coating. If you're into gardening, I have found that baby spinach is especially quick and easy to grow, and very economical -- a $1-2 seed packet can provide you with fresh spinach for an entire season.
Don't wash your spinach after harvesting or when you get home from the store -- store it dry in an airtight container for a shelf life of about five days. Do wash fresh spinach before you use it. If you're using whole leaf spinach, one way to wash it is to place the leaves in a large bowl of water, swish them around, and then discard the water and sand. You can repeat this step until all the sand is gone, or just rinse the leaves under running water to remove the sandy residue. If you bought baby spinach or whole leaf spinach in a bag then it will already have had the sand removed and you just need to rinse it off under running water.
Another good form of spinach to buy is frozen chopped spinach -- especially useful because it can be stored in the freezer and used periodically. I use a fine cheese grater to shred frozen spinach into rice, sauces, and to top off pizzas; then I store the remainder in a zip-lock bag in my freezer for later use. Canned spinach is another option, but the nutrient content of canned spinach is less than that of fresh and frozen spinach.
Spinach is high in oxalates, which is a naturally occurring substance found in plants and animals. High concentrations of oxalates can cause them to crystalize. If you have a history of kidney or gallbladder problems you should probably limit the amount of spinach you eat. Oxalates also can interfere with calcium and iron absorption, although less so with the form of iron found in spinach (non-heme iron).
Spinach also contains naturally occurring substances called purines. Excess accumulation of purines in the body can lead to excess accumulation of uric acid. In some people this can lead to a condition known as gout. If you are prone to gout then you should limit your intake of spinach.