The Daily Mail today reported that there is, “yet another good reason to tuck into that salad: eating tomatoes could ward off depression”.
Tomatoes are rich in lycopene, the chemical that gives them their distinctive colour. Lycopene is an antioxidant, a type of naturally occurring chemical believed to help protect against cell damage.
Previous research suggests foods high in antioxidants could have a preventative effect against physical diseases such as strokes. In this study, the researchers were interested in seeing if a similar preventative effect could also apply to depression.
The researchers assessed the mental health and dietary habits of 986 Japanese people aged over 70 years. They found that those who reported eating tomatoes two to six times a week were 46% less likely to report mild or severe symptoms of depression than those who said they ate tomatoes less than once a week. No such association was found for other vegetables.
This study has many limitations to consider, including a potential error in the way they measured dietary intake. Crucially, an inherent weakness of this type of research (a cross-sectional study) is that it can’t prove a direct cause and effect between reported tomato consumption and mental health.
It can also be subject to confounders. For example, it could be that in some cases, people who eat a lot of fresh fruit live a healthier lifestyle and take lots of exercise – and the exercise could be having the beneficial effects on mental health.
With those caveats in mind, this study is consistent with the advice that eating a healthy, varied and balanced diet is beneficial for both physical and mental health.
Where did the story come from?
The study was carried out by researchers from Japanese and Chinese universities and was funded by grants from the Japanese Ministry of Education and Ministry of Health and the Japan Arteriosclerosis Prevention Fund. No conflicts of interest were declared.
The study was published in the peer-reviewed Journal of Affective Disorders.
The media coverage of the study was balanced and included a useful testimony from the researchers indicating that they could not be sure if lycopene in tomatoes directly affects the mind.
What kind of research was this?
This was a cross-sectional survey looking at the potential link between intake of vegetables and tomato products and depression.
The researchers state that defective antioxidant defences are related to symptoms of depression. That is, people who are more vulnerable to cell damage caused by ‘rogue’ molecules called free radicals, may also be more prone to experiencing depressive symptoms.
They were interested to investigate whether vegetables, which are known to be good sources of antioxidant chemicals, may have a protective effect. They were particularly interested in lycopene, a powerful antioxidant present in high levels in tomatoes.
Cross-sectional studies can only highlight associations – they cannot prove cause and effect (in this case, they can’t prove that eating tomatoes causes less depression or protects against it).
Depression and its causes are complex. The causes may include genetics, environment, and personal circumstances. Additional factors, outside of antioxidant intake, influence this relationship and this type of study is unable to account for them all.
What did the research involve?
Information on 986 ‘community dwelling’ (not in hospital or residential care) elderly Japanese individuals aged 70 years and older was analysed in this study. Participants were living in one of the major cities in the Tohoku area of Japan.
The participants’ dietary intake was assessed using a validated self-administered diet history questionnaire. This required participants to indicate the average frequency they ate each of a list of 75 food items over the past year, ranging from “almost never” to “two or more times per day”.
The questions on tomatoes included fresh tomatoes as well as tomato products such as tomato ketchup and “tomato stew” – a Japanese dish consisting of beef stewed in tomato juice.
Other vegetables were categorised into:
- green-leaf vegetables
- cabbage and Chinese cabbage
- carrot, onion, burdock, lotus root and pumpkin
- Japanese white radish (daikon) and turnips
Tomato and tomato product consumption was then categorised into three separate consumption groups:
- one or fewer servings per week
- two to six servings per week
- one or more servings per day
Depressive symptoms were evaluated using a Japanese version of a 30-question Geriatric Depression Scale (GDS). The scale used two cut-offs: 11 (mild and severe depressive symptoms) and 14 (severe depressive symptoms). Participants were also categorised as having mild or severe depression if they used anti-depressive drugs.
Numerous other measures were taken, including:
- body weight
- blood pressure
- indicators of past health
- current medication intake
- sociodemographic variables such as age, gender, and educational level
- perceived social support – for example, was there a friend or relative readily available if a participant fell ill
Participants with no information on diet, or who had a history of cancer or impaired mental ability, were excluded from the study.
The analysis compared differences in tomato and vegetable intake to see if they were significantly related to reports of depressive symptoms. Depressive symptoms were defined as mild or severe depressive symptoms (GDS of 11 or more) or use of antidepressants.
What were the basic results?
The prevalence of mild and severe depressive symptoms in the group was 34.9% when combined and 20.2% for only those categorised as severe.
There were significant differences in the baseline characteristics of those reporting different tomato consumption levels for a range of variables, including gender, smoking status, education level and marital status, and others.
Tomato consumption seemed high in this population as there were:
- 139 (14%) people in the one or fewer servings per week group
- 325 (33%) in the two to six servings per week group
- 522 (56%) in the one or more serving per day group
After adjustment for potentially confounding factors, the relative risk of having mild and severe depressive symptoms (combined) was 52% less in those eating tomatoes or tomato products once or more each day, compared with those reporting consumption of once a week or less (odds ratio (OR) 0.48 95% confidence interval (CI) 0.31 to 0.75).
The risk reduction was slightly less (46%) for those eating two to six servings of tomatoes or tomato products compared to those reporting consumption of once a week or less (OR 0.54, 95%CI 0.35 to 0.85).
The analysis showed a statistically significant trend (p<0.01) linking higher tomato consumption to lower levels of depressive symptoms.
Similar results were obtained when they considered only severe depressive symptoms (GDS of 14 or more) which showed a 40% reduction in those eating tomatoes or tomato products once or more each day compared with those reporting consumption of once a week or less (OR 0.60, 95% CI 0.37 to 0.99).
The analysis reported here was adjusted for the confounders discussed above, as well as:
- smoking and drinking habits
- physical activity
- cognitive status
- self-reported body pain
- total energy intake
- reported intake of all kinds of fruits, green tea, and vegetables
No significant relationships were observed between intake of other kinds of vegetables and depressive symptoms.
How did the researchers interpret the results?
The researchers concluded that, “this study demonstrated that a tomato-rich diet is independently related to lower prevalence of depressive symptoms. These results suggest that a tomato-rich diet may have a beneﬁcial effect on the prevention of depressive symptoms. Further studies are needed to conﬁrm these ﬁndings.”
This cross-sectional study examined the relationship between the intake of various vegetables and tomato products (a major source of lycopene) and depressive symptoms in elderly Japanese people.
They found a statistically significant trend indicating higher levels of tomato or tomato product were associated with less risk of depressive symptoms.
By contrast, no other vegetable groups were found to be significantly linked with depressive symptoms.
This study had some strengths, including its adequate size and that it adjusted for a large number of variables that may have influenced the link between diet and depression in its analysis. However, there are also some important limitations to consider, including the following points.
The type of study
An inherent limitation of cross-sectional studies is that they can only highlight associations between diet and disease – they cannot prove cause and effect, for example, whether eating lots of tomatoes (lycopene) causes fewer signs of depression, or whether people displaying more signs of depression eat less tomato products. Depression and the causes of it are likely to be complex and so there will be many additional factors, outside of antioxidant intake through tomatoes, that influence this relationship and which this type of study cannot account for. If all these factors were adequately accounted for, there may be no link between tomatoes and depression found. Further studies would be needed to see if this is the case.
How depression was assessed
The measure used in the study (the Japanese version of the Geriatric Depression Scale), was just that, a measure of the severity of symptoms of depression. There was no attempt to clinically diagnose depression. So, those reporting mild or severe depressive symptoms may have been a mix of people who had been formally diagnosed with depression and those that hadn’t.
Tomato eating habits were self-reported
The measure of diet was self-assessed by asking people to recall their consumption of various foods from the previous year. This may be prone to significant error in recalling this information accurately which could bias the overall results.
Do Japanese people eat more tomatoes than us?
The majority (56%) of the Japanese participants reported eating tomato products once or more per day, which may be considered a high level of consumption by other countries’ standards. This highlights the fact that diets across the world vary greatly and the results of studies linking diet to disease in other countries are not always directly relevant or applicable to the UK.
Differences between tomatophobes and tomatophiles
There were significant differences in the baseline characteristics of those reporting different tomato consumption levels for a variety of variables. This suggests people who eat different amounts of tomatoes are quite different from each other in many other ways. These other factors may influence how likely they are to exhibit depressive symptoms. While the researchers made every effort to adjust their analysis for many factors that may have influenced the diet-depression link (level of education, etc), they are unlikely to have measured or adjusted for them all. This limitation is common in this type of study design and is known as the risk of “residual confounding”.
The study authors hypothesised that the lycopene in the tomatoes may have a protective effect for developing depression. However, lycopene intake was not assessed directly (for instance through giving a supplement) or estimated through nutritional analysis of the food the participant ate. This type of cross-sectional study design is not set-up to be able to prove that lycopene is protective.
However, it did tell us that tomato consumption may be associated with depression in some way.
One such explanation is that eating fewer tomatoes may be an indicator of a generally less healthy lifestyle or a sign of being generally worse off. This group may be more prone to depression later in life as they have tougher lives. This is just one of many possible explanations that are not proven. It is equally possible that people who are suffering from depression may be more likely to slip from following a healthy diet (including tomatoes) and lifestyle.
Further research (such as a randomised control trial, where one group are given a tomato-rich diet and the other a tomato-free diet) will need to be done to further explore this potential relationship, or come up with other explanations for the result.
This study does not change the general advice to eat a varied and balanced diet and take regular exercise to promote good (physical and mental) health and wellbeing.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
Daily Mail, 4 December 2012
Daily Mirror, 5 December 2012
Links to the science
Journal of Affective Disorders. Published online July 25 2012