What are superfoods, and are they really so good for our health?
The term superfoods entered the language in the 1990s for foods packed with nutrients that supposedly have health-giving properties. Some are exotic, such as alfalfa, spirulina and wheatgrass. And some prosaic, such as broccoli, beans and beetroot.
The latest addition, watercress, was hailed this week by researchers at the University of Ulster, who fed large quantities of the peppery salad leaf to 60 men and women daily for eight weeks. This resulted in increased antioxidants in their blood and decreased DNA damage to white blood cells.
The researchers concluded, in the American Journal of Clinical Nutrition: "The results support the theory that consumption of watercress can be linked to a reduced risk of cancer."
Are the watercress claims credible?
Not really. The research was funded by British watercress suppliers. Karol Sikora, professor of cancer medicine at Imperial College said: "The real problem is that it's not watercress specific - there's nothing magic there. We know that fruits and vegetables all do affect DNA damage, hence the five-a-day strategy to prevent cancer. There is absolutely nothing special about watercress."
What does the term superfood mean?
There is no definitive list. Candidates are regularly put forward, usually backed by marketing hype. Among the best known are oily fish (salmon, mackerel, tuna) for omega 3 fatty acids, blueberries for vitamin C, brazil nuts for selenium, carrots for beta-carotene, tomatoes for lycopene, olive oil for the anti-inflammatory compound oleocanthal, and red wine for resveratrol and garlic.
Their benefits are often overstated but they are worthwhile additions to any diet.
Why not take vitamin pills instead?
Because eating is a pleasure _ swallowing pills is not. Research has yielded confusing results, with claims showing they protect against heart disease or cancer soon contradicted by studies showing the opposite.
The argument for superfoods, which contain the vitamins in their raw, unprocessed state, is that they are natural sources, safe and easily absorbed. Calcium, for example, sold as calcium carbonate (chalk) is difficult to digest but in a glass of milk it is easily absorbed.
Does designating something as a superfood have an effect?
Yes. Sales of blueberries soared a couple of years ago after claims that they helped protect the body from a range of illnesses. Nutritionists say blueberries are bursting with vitamin C and offer one of the best sources of the antioxidant anthocyanin, believed to help keep the heart healthy and maintain youthful skin. In 2004, United States Department of Agriculture researchers revealed that blueberries contained pterostilbene, which could be as effective as prescription drugs in helping lower cholesterol. Blackcurrant growers hit back with a campaign to promote the benefits of their "forgotten fruit", saying the berries contained more antioxidants.
Which was the first superfood?
Perhaps spinach. Sales peaked in the 1950s helped by the popularity of Popeye, the cartoon character who gulped down tins of the stuff to give him strength. A generation of children forced to eat it were turned off the dark green mush. But now it's trendy.
Which is the most widely used superfood?
Tea. It is drunk by millions, not because it is healthy but because it is soothing, thirst-quenching and delicious.
Research shows it is high in antioxidants and may offer protection against cancer and heart disease. But adding milk and, worse, sugar, may negate its health-giving benefits.
Do scientists always get it right about superfoods?
No. One of the biggest upsets, scientifically, was last year when research published in the British Medical Journal suggested that the advice to eat more oily fish to benefit the heart was wrong. Studies of omega 3 fatty acids, the key constituent of fish oils thought to protect the heart, found no clear evidence they were any help.
The claim that a diet high in fibre protects against bowel cancer, a theory promulgated since the 1970s, was based on the observation that the cancer was rare in Africa, where the staple diet is fibre-rich vegetables and grains. But researchers now think the presence of sugar in the gut, rather than low fibre, may be the key factor.
What should we do about superfoods?
Eat them, enjoy them, but don't regard them as medicine. So-called superfoods are best included as part of a balanced diet. The surest route to better health is to alter this balance gently. Up to one third of cancers and a high proportion of heart disease are thought to be associated with diet, and modifying the food we eat is one of the best defences. The standard advice is still the best - eat more fruit and vegetables, more starch and carbohydrate and less animal protein. Then try a square of chocolate - just for the pleasure.