Public health policy has an enormous impact on national wellbeing (Delaney, Smith and McGovern 2011). A study recently published in Social Indicators Research (Blanchflower, Oswald and Stewart-Brown 2012) investigated the relationship between fruit and vegetable consumption and mental health. The study drew upon three robust, representative, cross-sectional studies of random samples of adults in three UK countries; England, Scotland, and Wales. Each of these surveys gathered self-reported intake data, measured in portions of fruit and vegetables of up to eight or more a day. Most surveys stop at the recommended five or more. The study also gathered data on seven different measures of mental health, from mental wellbeing (WEMWBS) through mental illness (GHQ-12), life satisfaction, happiness, nervousness and downheartedness.
Together these surveys captured information from more than 80,000 people, taking account of a wide range of other potential explanatory factors such as age, sex, ethnic group, socioeconomic and educational circumstances; and other lifestyle factors, such as smoking. They show a remarkably monotonic dose-response relationship between mental health and the number of portions of fruit and vegetables consumed. That is, the more fruit and vegetables consumed, the greater the mental wellbeing. In models based on indicators of positive mental health (WEMWBS, Life Satisfaction and Happiness) the corresponding coefficients continued to increase by up to seven or more portions of fruit and vegetables. In models based on mental health problems (GHQ-12, nervousness, feeling downhearted) they increased by up to five or more.
A strong and consistent dose-response relationship, as shown in these studies, acts as evidence that fruit and vegetable consumption is influencing mental health. Yet, the possibility remains that we could just be documenting a simple correlation; people with better mental health tend to look after themselves – by eating more fruit and vegetables – than those worse mental health.
Traditionally, public health researchers strengthen evidence for causality by analysing cohort studies. Cohort studies allow us to see whether the putative cause (fruit and vegetable consumption) precedes the putative effect (mental health). This approach gets round what is known as ‘recall bias’ in which individuals who have a problem, such as depression, reflect on possible causes and may therefore be more accurate reporters of their dietary intake. Cohort studies depend on the right data being gathered and, until recently, few of these studies have collected good data on diet. Even fewer have included measures of mental wellbeing, as opposed to mental illness. With cross-sectional data as strong as this, in an area of great relevance to public health policy, longitudinal or cohort studies will soon be carried out. Yet, even these may not be definitive.
How do fruit and vegetables influence mental health?
The main mechanism through which fruit and vegetable consumption is likely to influence mental health is through the absorption of water-soluble minerals such as potassium (Torres, Nowson and Worsley 2009) and vitamins such as folic acid (Gilbody, Lewis and Lightfoot 2007; Gilbody, Lightfoot and Sheldon 2007; Fava et al. 1997) which have an impact on adrenaline and serotonin receptors. This may create problems for anyone wanting to prove causality. Water-soluble vitamins and minerals are not stored in the body in the same way as fat soluble vitamins; reserves last for months not years. Intake at one point in time would not therefore be expected to have an influence on health at a later date. What would matter with regard to fruit and vegetable consumption is current or very recent intake, which would be measured in cross-sectional studies.
If it could be shown that fruit and vegetable consumption made a difference to how happy people felt, public health messages to increase their consumption would probably be greatly enhanced. We already know that increasing fruit and vegetable c