“Another good reason to eat your greens: It makes you more optimistic about the future”, is the cheerful news in the Daily Mail.
The Mail reports that optimistic people have higher levels of plant compounds called carotenoids (such as the red/orange pigment in carrots) in their blood. Many carotenoids are thought to act as antioxidants, which could protect against cell damage.
The Mail’s story was based on research looking at self-reported optimism and blood levels of various carotenoids in middle-aged Americans. The researchers found that higher carotenoid levels were associated with higher optimism levels. However, the strength of the relationship was reduced when accounting for demographic and health factors, or lifestyle factors such as diet.
It is possible that having higher levels of antioxidants in the body leads to better physical health and this in turn enhances optimism. However, it’s just as possible that people who have better psychological wellbeing are likely to eat a healthier diet. Any link between carotenoid levels and mood is likely to be down to a complex multi-directional relationship between physical and psychological factors and behavioural choices.
Though the study doesn’t prove that fruit and vegetables make you optimistic, the range of known health benefits associated with eating fresh fruit and vegetables mean that it’s well worth getting your 5 A Day.
Where did the story come from?
The study was carried out by researchers from Harvard and the University of Wisconsin-Madison, US, and was funded by the Robert Wood Johnson Foundation, it was published in the peer-reviewed journal Psychometric Medicine.
The Daily Mail appears to have automatically assumed the cause and effect direction of this relationship – that eating vegetables makes you optimistic. Unfortunately, this cannot be concluded from the research. It could just as easily be the case that some optimisitic people choose to eat more fruit and vegetables.
What kind of research was this?
This was a cross-sectional study looking at self-reported optimism and blood levels of various antioxidants in a sample of 982 men and women who were taking part in an observational study called the Midlife in the United States (MIDUS) study.
The researchers introduce how health is defined by the World Health Organization as being a state of both psychological and physical wellbeing, and not merely the absence of disease.
The researchers say that to understand health, the full spectrum of psychological and physical wellbeing needs to be considered. However, they say that physical wellbeing in terms of ‘positive’ biological processes in the body has rarely been studied.
They say that as various antioxidants (such as carotenoids) are considered to be a sign of biological health and have been associated with various health benefits. They looked to see how levels of antioxidants related to people’s feelings of optimism, which the reserachers used as an indicator of psychological wellbeing.
However, a cross-sectional study such as this is limited as it cannot demonstrate cause and effect and say which way the relationship is going. It could be that having higher levels of antioxidants in your body through eating a diet rich in fruit and vegetables leads to better overall health and feelings of optimism and wellbeing. But it could equally be that people who are in good wellbeing – both physically and psychologically – make healthier lifestyle choices, such as choosing a better diet and exercising more, than people who feel in poorer health.
What did the research involve?
The original MIDUS study was set up to look at a range of factors that influence the mental and physical health of Americans as they age and the first phase of the study (1994 to 1995) included a national sample of 7,108 individuals aged 25 to 74 years.
The current study used data collected as part of the second phase of the MIDUS study 10 years later (2004 to 2005) when the individuals completed various psychological and physical assessments.
This study included 982 individuals who had complete data on psychological assessment and had blood taken to measure their antioxidant levels. Just over a third of the participants were part of a set of siblings or twins taking part in the study.
Optimism (defined as the general expectation that the future will be favourable) was assessed with a validated test – the six-item revised Life Orientation Test.
Participants had to say how much they agreed (on a five-point scale) with positive statements, such as “I expect more good things to happen to me than bad”, and negative statements, such as “I hardly ever expect things to go my way”.
Total optimism scores were calculated, after reverse-scoring responses to positive statements. Therefore larger overall scores indicated higher levels of optimism.
Fasting blood samples were taken at a research centre during a two-day visit. The researchers took measures of nine antioxidants:
- carotenoids (trans-β-carotene, 13-cis-β-carotene, α-carotene, β-cryptoxanthin, lutein, zeaxanthin, and lycopene)
- vitamin E (α-tocopherol and γ-tocopherol)
They looked at individual levels of each carotenoid and at individual levels of the two vitamin E compounds.
Potential confounders taken into account were:
- self-reported demographic factors including age, sex, ethnicity, educational level and household income
- self-reported health status factors including chronic diseases (heart disease, high blood pressure, stroke, high cholesterol, diabetes, or cancer) and body mass index
The researchers also asked the participants questions about how many fruits and vegetables they ate, whether they took multivitamins, how much exercise they took, and if they smoked.
What were the basic results?
The average age of participants was 55 years and the ethnicity of most participants was white. The researchers found that people with higher optimism levels tended to be:
- to be educated to a higher level
- have higher income
- eat more fruit and vegetables
- be less likely to smoke
- be more likely to take exercise
Generally, individuals with higher optimism tended to have higher carotenoid levels. After adjusting for age, each standard deviation increase in optimism was associated with a 3% to 13% increase in the levels of different carotenoids. Optimism was also significantly associated with total carotenoid concentration. However, the strength of the relationship was reduced when controlling for the measured demographic characteristics and health factors. The strength of the relationship was also reduced when taking into account fruit and vegetable intake and smoking status.
Optimism was not significantly associated with vitamin E levels.
How did the researchers interpret the results?
The researchers say that optimism was associated with higher carotenoid concentrations, and this association was partially explained by diet and smoking status. They say that the direction of effects cannot be conclusively determined and that effects may be bidirectional given that optimists are likely to engage in health behaviours associated with more serum antioxidants, and more serum antioxidants are likely associated with better physical health that enhances optimism.
This is well-conducted research which has used a validated measure to assess the optimism of a sample of middle-aged American citizens and measure their blood antioxidant levels.
The researchers found a link between higher carotenoid levels and higher optimism, but as the researchers rightly conclude, their findings do not prove cause and effect and it isn’t possible to say in which direction the relationship is going.
It is possible that having higher levels of antioxidants in the body leads to better physical health and this in turn enhances optimism, but then it is equally possible that people who are in better psychological wellbeing are likely to engage in health behaviours associated with higher antioxidants, such as eating a healthier diet.
There is almost certainly a complex multi-directional relationship between various physical and psychological factors and behavioural choices. This is supported by the fact that taking demographic factors, health factors and diet and smoking into account reduced the significance of the relationship between carotenoids and optimism. A link between diet and carotenoid levels is to be expected, as fruit and vegetables are the main source of carotenoids.
It’s also worth noting that although the study sample was large, the 982 participants were only those with complete data on psychological assessments and antioxidant levels. In order to have the blood tests, participants needed to be healthy enough to travel to the research clinic, and so the included participants may not be representative of the health and optimism of the entire US sample in the larger cohort study.
Overall, this is well-designed research, and is another supporter of the healthy diet, but it does not prove that eating a healthy diet will make you optimistic.