Fruit, Vegetables, Health and Happiness

Whilst perusing the Daily Mail today (my excuse is that it's next to the espresso machine) I saw this interesting health advice:

Forget five a day: Now scientists say you'll be healthier and happier eating seven daily portions of fruit and veg.

The phrase "scientists say" is always a red light. 
The article says that there's been a study which shows that people who eat more fruit and vegetables are happier than everyone else.  It runs through all the classically bad newspaper science reporting clichés.  The old wisdom has been turned on its head, the government has been wrong all these years, those crazy scientists have gone and done it again!  For example, it says:

But now scientists claim if  we upped it to seven, we’d also be far happier.

There is a clear causal implication: if we change our diet to increase our intake of fruit and veg, we will become happier.

Here's a preprint of the paper referred, titled Is Psychological Well-being Linked to the Consumption of Fruit and Vegetables?  The abstract contains the important sentence which is omitted from the newspaper article:

Reverse causality and problems of confounding remain possible.

They remain very possible indeed!  All the paper does is look at some associational studies, where they ask people about their fruit and veg intake and their happiness.  No randomised studies were carried out

It's well documented that poorer people eat less fruit and fewer vegetables, and it also seems pretty likely that your income and wealth have a substantial effect on your happiness, however measured.  Thus income is potentially a very strong confounder.  Needless to say the authors do try controlling for variables like this (and smoking status, BMI, etc), and still find a correlation.

Just as important is the problem of reverse causation: if I'm feeling bad about myself, do I reach for an apple?  Well in my case, it's more likely to be the chocolate or the Pralines and Cream.  So it may be that happiness has an effect on fruit and vegetable intake.

Another difficulty is that these studies rely upon people remembering how much fruit and veg they've been eating, and their mood at the time of asking might affect their memory.

In addition, the effect size is not that large.  Taking all my estimates from Table 1 in the paper (based on Scottish data), people who eat 8 or more portions per day have a mean life satisfaction 0.27 higher than those who eat none at all; this is measured on a scale from 0 to 10.  So it certainly isn't correct to say that scientists claim you'll be far happier if you eat a few more grapes.

There is quite a lot of variation in the estimates, so for example eating 5-6 portions appears to result in a higher increase over eating none (0.23), than does eating 6-7 (0.17).  It seems to me that a better idea would be not to treat the intake as a discrete variable taking lying in these different categories, but as something continuous and fit a single regression line; this would avoid these problems and allow the whole sample to do the work.

Lastly the DM question whether the health advice given by the government should be changed on the basis of this paper.  It mentions that various other countries suggest a higher intake of fruit and veg, though they omit to mention that the definition of 'fruit and veg' and a 'portion' are different in each case. 

Needless to say nothing should (or will) be changed because of this one paper, but it's worth pointing out that health advice is partly based upon its actual effect on the population - if the Ministry of Health started saying that you should eat ten portions of fruit a day, people might dismiss the advice as unrealistic, and ignore it altogether.  There are ethical questions about how untruthful one can be in this case, but the presentation of such advice is certainly important.