You’d be forgiven for looking warily at your bacon sandwich this morning, if you’ve seen headlines suggesting that a diet high in animal proteins is nearly as dangerous as smoking. Cheese and meat cause cancer! That carbonara is a time-bomb ticking in your stomach! Quick, go vegetarian!
The news is based on a study in the journal Cell Metabolism, which found that people who got more than a fifth of their daily calories from animal protein were 74 per cent more likely to have died during the study than people who ate less. We’re bombarded with food messages like this, which often seem to change from day to day. Dame Sally Davies, the Chief Medical Officer, has warned about the “addictive” potential of sugar, and the World Health Organisation said yesterday that recommended sugar allowances were too high; an editorial in the journal Open Heart, also published yesterday, suggested that the risk of saturated fats was overblown.
Every Christmas, suddenly, red wine and chocolate become good for us. Last year headlines screamed that eating three sausages a day raises your risk of dying of heart disease by three quarters. It’s hard to know what to make of it all, the what’s-curing-me-and-killing-me-today merry-go-round.
Can eating burgers really be as bad for you as smoking? Before answering that, it’s worth looking at how we know how bad smoking is.
In the late Forties, a man called Richard Doll was given the task of finding out what was behind the dramatic increase in lung cancer deaths. Originally, he and his colleagues thought it was probably the new practice of coating roads with Tarmac. But upon interviewing 649 men with lung cancer in 20 London hospitals, he found one remarkable fact: all but two of them were smokers (he also interviewed a smaller group of women, in which the divide was less dramatic but still very large). He promptly quit smoking. His research had found a simple fact: smoking causes lung cancer, and in fact is the cause of almost all lung cancer.
More on science
This caused great excitement. Researchers wondered if other cancers, or other diseases, could be linked as straightforwardly to lifestyle factors. The science of epidemiology – of the causes of disease in populations – had its greatest success since Dr John Snow showed that unclean water caused London’s cholera outbreaks in the 19th century.
But smoking was a low-hanging fruit. There aren’t very many straightforwardly poisonous things that lots of humans imbibe in large amounts and lots of humans don’t touch at all. Working out whether a particular food is good for you, for instance, is fantastically tricky: you can’t prescribe someone a course of celery for 20 years, and compare how well they do to someone on a celery placebo. You have to rely on people reporting what they eat, which they do only unreliably. And unless you have very large samples, it’s hard to tease out causes from mere correlations: how can we know whether celery makes you live longer, or whether people who eat celery tend to live healthier lives generally?
What’s more, the body is very complex, so plausible hypotheses about what will do you good and do you harm often turn out to be false. This is why you should ignore anyone who tells you that you ought to eat pomegranate or chia seeds because they’re good for your liver, or whatever. They have no idea what they’re talking about.
Epidemiologists have, however, been able to tease out broader-brush factors. Red meat, salt, sugar, fat and alcohol are all bad for you in large amounts; eating plenty of fruit and veg is good for you. But exactly how good and how bad, and how much of each you should have, is all very much in dispute. The Cell Metabolism study found a huge increase in cancer risk from animal-protein-rich diets, but most earlier research on related topics had found a far less dramatic impact, of between 10 and 15 per cent.
And that’s the key. None of these studies is the final truth; science is incremental, it learns by degrees, and epidemiology doubly so. Meat, in large quantities, is probably a bit more dangerous than we previously thought, but to say that it is suddenly as dangerous as smoking is to run far ahead of the evidence. And, of course, there are other differences: it’s very difficult to include cigarettes as part of a balanced diet, for instance.
Richard Doll’s discovery paved the way for a remarkable age of public health research, which has led to us knowing far more about what helps us live longer and what kills us than we did half a century ago. But the picture is usually cloudier than university PR departments like to admit. After decades of study, the best, most well-supported advice is still what your mother told you: eat your greens and get plenty of exercise.