Another day, another health story to wake up to on the radio. It turns out that five a day is not enough. We need to eat at least seven portions of fruit and/or vegetable a day. More if you can. Oh, and fruit isn’t as healthy as vegetables. But the good news is you will live 7.5 years longer if you eat seven or more. You’ll probably get onto your greengrocer’s Christmas card list as well.
My first thought was to groan. Very few health initiatives ever really work, and the fact that the five a day one has caught on and resonated with the public in the UK is a rare achievement. Food habits are tough to shift both on an individual level and on a society one. But the good news is that we are eating more fresh fruit and vegetables and we are on the whole living longer and healthier lives. So something is going right. But I wonder how many people who might have eaten an apple instead of a Mars bar today to get up to the 5 mark now won’t bother. There is something really dispiriting about unrealistic goals.
But I thought I had better look at the science behind it. Correlating food choices and health benefits is a really tough job. Disentangling cause and effect and using that information to make predictions is pretty much what science is about. It is easier in some areas of study than others, and nutrition is one of the hardest. For a start you can’t do the kinds of experiments you would need. Even though some might jump at the chance, you can’t feed people on a diet of fizzy drinks and microwave meals to confirm that these are as unhealthy as we think they are. In particular, you can’t keep them there until they die to see the effect on lifespan.
And there are so many things that affect health that working out what contribution diet makes to it is a challenge to the most dedicated statistician.
Looking at the latest study it looks very much like they have tried to overcome these difficulties by adopting a really simple methodology and having lots of people involved to counteract any biases. They surveyed 65,000 people, which is a large sample by any standards. They worked out their fruit and vegetable consumption by simply asking them on a particular day what they had eaten. They then sorted the results into 5 categories and compared death rates.
There is nothing wrong with this approach. In fact it is a useful exercise. The trouble with it is that it really is just one piece of the jig-saw and doesn’t prove much on its own. There is the obvious objection that we already know that wealthy people live longer, and that wealthy people eat more greens and other fresh food. Perhaps the reason rich people live longer is that they have a better diet, but maybe not. It makes no more sense to say that eating 5 a day more makes you live longer than that eating 5 a day or more makes you rich, based purely on this survey.
So on the whole although there doesn’t seem to be much doubt that fresh vegetables are a good thing, there doesn’t seem to be much more we can say on the matter. I like the 5 a day idea personally, and I’ll be sticking with it. The actual NHS guideline is 5 portions of 80g, but that is a pretty small portion. I think most people who follow it will be exceeding the minimum. I am not convinced that eating even more will really make that much difference.
In the future we might be able to do much better surveys than this one. This survey was very wide but not very deep. Now we all carry powerful computers around with us which we use mainly to chat and check Twitter and Facebook. We could conceivably keep a much more detailed record of what we actually eat. And the phones can also monitor our movements, so we can also get a better picture of the amount of exercise we get. This means we might be able to analyse the relationship between our lifestyles and our health a lot more reliably than we can at the moment. I bet somebody will do exactly that soon, and that the results won’t be what anyone would predict. We have some surprises in store.