They are the fruits, vegetables and juices widely believed to provide you with a nutritional shot in the arm far superior to anything else you might get from the fresh-produce aisle. Consume a superfood often enough, and the belief is that it will supercharge the immune system, stave off illness and even make you live longer.
Blueberries, broccoli and brazil nuts are among the best known, but wheatgrass, wolfberries and kefir (from fermented grains) also make it onto the list. Nutritionists at the Baylor University Medical Center in Dallas recently added kiwi fruit and barley to the catalogue of so-called wonder foods.
Proponents believe that a food becomes "super" if it is particularly dense in certain nutrients that have been shown in studies to help combat disease. But British experts have now issued a sobering warning to the nutrient-obsessed: superfoods don't actually exist.
"There's no scientific definition of a superfood, and no food is super if it's not consumed as part of a healthy, varied and balanced diet," says Bridget Aisbitt, nutrition scientist with the British Nutrition Foundation (BNF). "It is a myth that a specific food can improve brain power or boost energy levels. That's not the case. You might as well eat an apple or orange as some blueberries - they are just as good for you."
Others agree. Catherine Collins, chief dietician at St George's Hospital in south London, says the hype surrounding superfoods "is meaningless and potentially harmful". Even if a food is rich in a nutrient, that does not necessarily mean we should eat more of it. "Consuming too much of a particular food or nutrient is not a healthy thing to do," she adds. "If you overload on specific vitamins and minerals, they are excreted in urine when the body has taken sufficient amounts. Other nutrients, which are not excreted as easily, are potentially toxic."
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There are even doubts whether some acclaimed superfoods have much benefit at all. The Australian Consumers' Association (ACA) has found many "fashionable" foods are a waste of money. Wheatgrass, for instance, the bright-green cocktail of nutrients extracted from the pulped shoots of grasses, may comprise up to 70% chlorophyll, but claims that it heals tissues and purifies the liver are farfetched. There is little evidence for many of its purported benefits, and a 30g serving of cooked spinach, broccoli or a garden salad contains more vitamins and minerals than a shot of wheatgrass juice. Similarly, blue-green algae and spirulina were found to be lacking in substance.
Aisbitt concedes that the concept of superfoods encourages some people to eat more fruit and vegetables, which, considering the average Briton falls well short of the recommended five portions a day, is something in their favour. But following an investigation by the BNF, it is urging consumers to broaden their horizons when it comes to their weekly shop. "It is healthier to eat bananas, oranges, carrots and sprouts on a regular basis than it is to consume large amounts of berries or baby kale every so often," Collins says. "There are important nutrients and bioactive compounds in fruit and vegetables, and the wider the variety you eat, the better."