Google the search terms ‘happiness’ + ‘research’ and you’ll retrieve about 56,300,000 results. Yet, as economists David Blanchflower and Andrew Oswald, and Professor of Public Health, Sarah Stewart-Brown, pointed out in a recent study, of the many thousands of scientific papers published on the various determinants of people’s happiness and psychological health, virtually none have investigated the role played by what people eat.
To address this glaring deficiency in our understanding of what makes people feel happy, the 3 researchers mined data from 3 large, representative, cross-sectional studies of random samples of adults in England, Scotland, and Wales – a total of over 80 000 UK citizens.
Each survey included questions on intake of fruit and vegetables, measuring it in standardised portions of up to eight or more a day; as well as on 7 different measures of mental health, from mental wellbeing (WEMWBS) through mental illness (GHQ-12), life satisfaction, self-reported health, happiness, nervousness and feeling low.
So what did they find?
Very simply,
“happiness and mental health rise in an approximately dose-response way with the number of daily portions of fruit and vegetables.”
(Translation: the more fruit and veg the survey participants ate, the happier they were.)
The researchers were aware, of course, that there are many potential confounders (factors that may make certain other factors appear causally related, when in fact they are simply correlated. For example,
- Wealthier people can afford to eat more fruit and vegetables, AND they are also likely to be happier than poorer people who can’t afford as much of them – so having more money is a ‘cause’ both of being happier and of eating more fruit and veg; and
- People who are happier generally take better care of themselves, for example by eating more healthfully, than unhappy people – so in this case, happiness would be the cause of eating more fruit and veg rather than the higher fresh produce consumption being the cause of happiness.
So they used standard statistical methods to take account of a wide range of potential confounders including age, sex, ethnic group, marital status, having or not having children, socioeconomic and educational circumstances, work and unemployment status, disability and major illness, being religious, smoking, exercise level, body mass index, intake of meat, fish and alcohol, and being sexually active.
And even after taking into consideration all of those factors, they still found
“a remarkably monotonic dose-response relationship between mental health and the number of portions of fruit and vegetables consumed.”
In other words, even after all the statistical adjustments they made to cancel out the effect of potential confounders, the association between better mental health and higher fruit and veg consumption remained strong.
How large was the effect of fresh produce consumption on mental health? Well, for example,
- On the Life Satisfaction Scale, when compared to those who ate almost no fruit and veg, individuals who consumed 8 or more portions per day had an uplift in their life satisfaction score that was only slightly less than the uplift seen in people who were married vs those who were single – and being married is one of the most powerful positive influences on life satisfaction known to happiness researchers!
- On the WEMWBS (Warwick-Edinburgh Mental Wellbeing Scale), which measures positive indicators of wellbeing, people who ate 7-8 portions per day had a wellbeing score approximately 3 points greater than those eating virtually none. If this doesn’t sound like much of a difference, consider this: unemployed people scored 2.4 points lower on average than those in employment, and people with a disability scored 6.4 points lower than the able-bodied; both unemployment and disability are considered by all researchers in the field, to be strong predictors of lower wellbeing.
One of the authors of the study, Dr Sarah Stewart-Brown, suggests that fruit and vegetable consumption may influence mental health through the nutrients fresh produce provides, which makes perfect sense: The brain is just like all the other organs in our body – it requires nutrients to function optimally; and if nutrient intake is inadequate, it will malfunction, resulting in emotional and mental symptoms.
Other researchers have found clear linkages between deficiency of nutrients found abundantly in plants, and various aspects of brain function. For example
- A high potassium diet was found to ease depression and tension and increase vigour and the POMS global mood state score; while
- Low folate levels are linked with depression – possibly because folate plays a crucial role in the activity of receptors for the neurotransmitters (brain chemicals) adrenaline and serotonin, which profoundly afffect mood.
Crucially, since many of the key nutrients in plant foods are water-soluble, they are not stored in our bodies for very long, and therefore must be consumed regularly. Hence, missing out on fruit and veg for even a few days can leave you feeling down in the dumps; but on the bright side, boosting your consumption if it’s been low, can lift your mood very rapidly.
As Dr Stewart-Brown points out, increased fruit and vegetable consumption is already known to reduce the risk of cardiovascular disease and cancer, and many governments already have public health goals to increase consumption of fresh produce. However, on the basis of this recent research, current recommendations for 5 servings of fruit and vegetables per day are probably inadequate to improve mental health (and I would argue, too low to achieve meaningful protection against cancer and heart disease too).
The bottom line: when it comes to fruit and veg, it seems you can’t get too much of a good thing!
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