The humble potato is one of the best sources of long-acting energy, but the key is slow cooking: The high temperatures used in frying and baking reduce the nutritional value of spuds, so opt for boiling or steaming. Cooled slow-cooked potatoes (as in potato salad—with light mayo, please) have more resistant starch, a type of starch that acts like fiber when we eat it, than hot spuds. Resistant starch contains 2 to 3 calories per gram, compared to regular starches that pack 4 calories per gram. Potatoes are also surprisingly good sources of vitamin C, potassium and folate.
Onions were used by Egyptian Pharaohs to pay the workers for building the pyramids—that's how valuable they once were. Now, not so much. But these root veggies are sources of such important cancer-fighters as quercetin and allyl sulfides, as well as protein (who knew?), fiber, potassium, calcium, vitamin C and folate. And in case you're concerned about the social fallout of feasting on onions, slow roasting or sautéing removes the breath factor altogether.
Shrimp, oysters and other shellfish are some of the best sources of protein you can find. They are also rich in zinc—not only necessary for immunity but reproduction, too—plus calcium and omega-3 fatty acids. Shrimp also contain astaxanthin, a compound good for healthy eyes and skin.
The folate in legumes—dried beans and peas, chickpeas and lentils—fights against birth defects and heart disease, while their resistant starch provides long-lasting energy. Beans are among the cheapest protein providers and one of the most ecologically sustainable crops, enriching soil with nitrogen and using less water than most other plant foods.
It's pretty white for a "green" vegetable, but cauliflower is loaded with vitamins A, C and K, as well as protein and calcium. It also has lots of fiber, folate and cancer-fighting phytochemicals. As with all its cabbage cousins, boiling and draining cauliflower gets rid of a lot of the B and C vitamins. But using it in a soup or mashing it with the cooking liquid gives you the full benefits (and a spud alternative even kids love).
The ultra-low-calorie watermelon isn't just the perfect diet food. This summer favorite contains fiber, folate, potassium and calcium and also is rich in lycopene, a powerful antioxidant that protects against certain cancers and osteoporosis.