Eat beetroot! Eat kale! Eat mackerel and pomegranate and chard! These are not just foods: they are superfoods. They are health-giving. If you eat them in sufficiently large quantities, you will definitely live forever.
I exaggerate: but media nutritionists are often telling us what foods are healthy, and why. Spinach is full of antioxidants; tomatoes are packed with lycopene. Your body needs these things; therefore, goes the hypothesis, foods with lots of it should be good for us. This morning, the Royal Horticultural Society is telling us all how to grow superfoods in the privacy of our own garden. (Probably not mackerel, but the other ones.)
The trouble is, no one knows what a superfood is, and there's no good evidence that any of the things we call superfoods are actually any better for us than the normal fruit and veg we should be eating as part of a balanced diet anyway. The NHS, in a 2011 report on the subject (PDF), says "There is no official definition of a superfood and the EU has banned the use of the word on product packaging unless the claim is backed up by convincing research."
If you look at the stories, they're almost always unfounded. Worried about diabetes? Or breast cancer? Or lung disease? Try broccoli, nature's panacea! But on a closer inspection, the study into broccoli and diabetes was looking at the effect of a chemical taken from broccoli on some cells in a petri dish. The breast cancer study and the lung disease study were the same sort of thing (looking at the same chemical, as it happens). There's just no way you can translate the effect of a chemical in a dish to the effect of a vegetable, a vast conglomerate of thousands of chemicals, on a human being, an even vaster conglomerate of even more chemicals.
This is an ongoing theme. "Curry could save your life." But, no, it's not a study of humans who've eaten curry, it's a study of rats who've been given a diet high in capsaicin, a component of curry. Green tea prevents prostate cancer! Well, no. Beetroot makes you live longer: no. Beetroot makes you sprightly! No.
The reason all of these headlines are misleading is because it is extremely difficult getting good evidence for which foods are good for you and which foods aren't. As I've written before, "you can’t prescribe someone a course of celery for 20 years, and compare how well they do to someone on a celery placebo." Instead, researchers have to use alternative methods - methods such as trying celery-derived chemicals in dishes, or asking people how much celery they've eaten over the last 20 years and relying on their memory not to be too faulty, or seeing the effect of eating celery on some measure like how dilated your blood vessels get. But none of these translate well into the wider question of "will eating lots of this food make me live longer?"
And, in fact, the evidence for the super-ness of pretty much any single food is simply not there. Of course you should eat your broccoli or your celery or your kale, along with other things; there is pretty good evidence that a balanced diet with plenty of fruit and veg will make you live longer. You should also do plenty of exercise, which is also well-supported by the evidence. (Also you should try to be rich and well-educated, which are the things that really correlate to long life.)
But there's very little evidence that any one of these things is any better for you, really, than runner beans or lettuce or apples, and since so many of these superfoods are relatively expensive things like kale and chard and pomegranate, all these stories do is guilt people into spending money on food that won't really do them any more good than the cheaper equivalents. Sadly, the way to live a long and healthy life is to not drink too much, not eat too much fat and salt, not smoke, and, of course, eat your greens and do your exercise. "Superfoods" are a false shortcut; actually changing your lifestyle is what is required.
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