It's usually pretty obvious if a food item is healthy. An apple or a sprig of iceberg lettuce: healthy. A double bacon cheeseburger: not so much. What isn't so obvious, however, is if a food is exceptionally healthy.

These foods have become known as "superfoods" -- nutrient-dense edibles that many doctors and nutritionists believe help prevent diseases. Examples of superfoods include spirulina, spinach and salmon. And joining the ranks of superfoods are items you wouldn't necessarily think of as healthy at all. In some cases, these foods have gotten a bad rap for far too long.

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Obvious or not, most of these foods are worth paying attention to. Why? Because they offer more "bang for your buck" in the nutrient department, says Susan Kraus, a dietitian at Hackensack University Medical Center in Hackensack, N.J. For example, salmon is packed with protein, omega-3 fatty acids and vitamin D. While other proteins -- like chicken -- might taste great, health professionals believe that salmon can help prevent heart disease, some types of cancer and arthritis.

There's no official definition of what a superfood is, so consumers are urged to be wary of pre-packaged snacks and other products that aggressively market themselves as super. "Take the marketing with a little grain of salt," says David Wolfe, nutritionist and author of the upcoming book Superfoods: The Food and Medicine of the Future.

So how do you figure out which foods are superfoods?

Steven G. Pratt, M.D., author of several books on nutrition, including SuperFoods Rx: Fourteen Foods That Will Change Your Life, says superfoods, in order to qualify as such, should be tasty, readily available and demonstrably nutritious over centuries.

Take cinnamon. This slightly sweet spice helps slow down the rush of sugar to the blood, which in turn helps prevent weight gain. It's also an anti-inflammatory, meaning it can help delay the onset of aging.

Pratt says that, in general, spices are an unsung superfood. "They make food taste better, they're calorie-free and most are anti-inflammatory," he says.

Chocolate, another sweet treat, is also considered super. However, one must eat it in its pure state -- the cacao nut -- to reap the benefits. "Any chocolate that's below 70 per cent cacao has very little nutritional value," says Pratt. Along with omega-3 fatty acids, which help to reduce the risk of heart disease, the bitter nut contains magnesium, iron and phenylethylamine, said to enhance one's mood.

Wolfe says that people who try the slightly bitter nut love the taste immediately. He suggests replacing a lower-quality protein, like whey, with cacao in liquid shakes and smoothies.

Even some canned foods are considered ultra-healthy, such as no-salt or low-salt tomato paste, which has the highest concentration of lycopene of all tomatoes or tomato products, according to Pratt. The phytochemical in the paste is a natural blood thinner and a strong defense against certain diseases, such as prostate cancer.

Beneficial as superfoods are, the most important thing is maintaining a balanced diet. "Eating only these particular foods would be partially beneficial," says Kraus. "However, we need many nutrients daily to maintain optimal health."

Rather than concentrating only on superfoods, she says to consider these three qualities when devising a meal and snack plan: variety, color and processing.

"Try various types of fruits, veggies, greens and lean protein in a rainbow of colors every day, if possible," says Kraus. "And try avoiding foods that are processed. Natural foods are more satisfying (to the body)."