Tag: tomatoes

A tomato a day keeps depression at bay

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:

  • height
  • 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.”

Conclusion

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.

Notes:

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.

Do tomatoes prevent strokes?

“Tomatoes are ‘stroke preventers’,” BBC News has claimed.

The news is based on a study looking at the levels of various chemicals called carotenoids in men’s blood and their long-term risk of stroke.

Carotenoids are naturally occurring chemicals which give fruit and vegetables their colour. They can act as antioxidants. Antioxidants are believed to help protect against cell damage from molecules known as “free radicals” and “singlet molecular oxygen”. Antioxidants are thought to work by reacting with an unstable molecule and bringing it under control.

 

Some have suggested that antioxidants may have a protective effect against stroke by reducing damage to blood vessels.

In this study, the researchers found that men with the highest levels of a chemical called lycopene (known to be an antioxidant) in their blood had a 55% reduced risk of stroke compared with those who had the lowest levels. Lycopene is the chemical that gives tomatoes their distinctive red colour.

An important limitation of this study is that, although it included 1,000 men, only 67 strokes occurred. This makes for a very small sample size, which decreases the reliability of the risk calculations.

Overall, this research cannot show that the levels of lycopene were directly responsible for the differences in stroke risk, and it is also unclear how lycopene could prevent strokes. However, the findings of this study support the recommendation to eat a balanced diet rich in fruit and vegetables.

Where did the story come from?

The study was carried out by researchers from the University of Eastern Finland, Lapland Central Hospital and University Hospital of Kuopio, Finland. It was funded by Lapland Central Hospital.

This study was covered accurately by the BBC. However, the researchers looked at blood levels of lycopene, which is a marker of tomato intake, but they did not directly look at tomato consumption itself, which the BBC headline seems to suggest.

That said, it is reasonable to assume that most middle-aged Finnish men would be getting their lycopene intake from tomatoes, rather than from more exotic sources such as papaya or pink guava.

The researchers were also looking more generally at all the major types of carotenoids, rather than lycopene only.

What kind of research was this?

This was a prospective cohort study. It aimed to determine the association between blood concentrations of carotenoids at the start of the study and the risk of stroke during follow-up.

Carotenoids studied by the researchers included:

  • lycopene
  • a-carotene, found in vegetables such as carrots and spinach
  • β-carotene, the substance that gives carrots their orange appearance
  • a-tocopherol, more commonly known as vitamin E
  • retinol, more commonly known as vitamin A

A prospective cohort study is the ideal study design to investigate whether carotenoids influence stroke risk.

However, this type of study cannot show direct cause and effect (causation), since there may be other factors that explain any associations seen (called confounding factors).

For example, people who eat a diet high in carotenoids may also have other healthy lifestyle habits such as not smoking. These other factors could be associated with reduced risk, rather than the carotenoids themselves.

A more ideal design, which would balance out other confounding factors, would be a randomised controlled trial which randomised people to diets high or low in carotenoid-containing fruits and vegetables.

However, as a diet low in fruit and vegetables is known to be bad for health, such a trial may not be practical or ethical to perform.

What did the research involve?

The researchers recruited into the study a representative sample of 1,031 men living in and around the city of Kuopio in Finland who were aged between 42 and 61 and had no history of stroke. At the start of the study, fasting levels of carotenoids were measured from blood samples. The researchers also measured participants’:

  • blood levels of low density lipoprotein (“bad” fat)
  • high density lipoprotein (“good” fat)
  • cholesterol and triglycerides (specific fats)
  • blood pressure
  • body mass index (BMI)

They also collected information on known risk factors for stroke, such as:

  • alcohol consumption
  • physical activity
  • diabetes
  • smoking

The researchers then followed up the men until they had a stroke or until the study came to an end. They followed the men for an average of 12.1 years. The number of strokes was determined from the FINMONICA stroke register, the Finnish national hospital discharge registry and death certificate registers.

The researchers then looked to see whether there was an association between blood levels of carotenoids at baseline and the risk of stroke, adjusting for some possible cofounders (age, examination year, BMI, blood pressure, smoking status at baseline, levels of serum low-density lipoprotein and cholesterol, diabetes and history of stroke).

What were the basic results?

During the study, 67 men had a stroke, 50 of whom had the most common type of stroke – an ischaemic stroke – which is caused by a blood clot. The men who had a stroke were older, had higher blood pressure, were more likely to have diabetes and had lower blood concentrations of lycopene.

The researchers divided blood carotenoid levels into quarters, and compared men with the highest levels with men with the lowest levels.

Men with the highest levels:

  • had a decreased risk of stroke by 55% (hazard ratio 0.45, 95% confidence interval [CI] 0.25 to 0.95)
  • had a decreased risk of ischaemic stroke by 59% (hazard ratio 0.41, 95% confidence interval 0.17 to 0.97)

Blood levels of the other carotenoids studied were not associated with stroke risk.

How did the researchers interpret the results?

The researchers concluded that: “High serum [blood] concentrations of lycopene, as a marker of intake of tomatoes and tomato-based products, decrease the risk of any stroke and ischaemic stroke in men.” They also stated that a balanced diet including fruits and vegetables “may prevent stroke”.

Conclusion

In this study, men in Finland with the highest blood levels of lycopene at the start of the study had a 55% reduced risk of stroke and a 59% reduced risk of ischaemic stroke (caused by a blood clot) over the following 12 years. However, levels of other carotenoids studied were not associated with stroke risk.

This study has the limitation that serum levels of lycopene cannot be shown to be responsible for the differences in stroke risk, as it is possible that other factors could explain the reduction in stroke risk. In particular, the researchers did not collect any information on dietary intakes. Therefore their analyses did not account for dietary factors, and also, although they adjusted for several other potential confounders that could influence the association, they did not adjust for physical activity. However, the researchers did collect information on baseline physical activity.

These things are important, as higher carotenoid levels could be associated with a healthy lifestyle, including healthy diet and regular physical activity, and it could be these things that are influencing stroke risk, and not the carotenoid themselves.

Information on serum levels of carotenoids and fats, smoking, physical activity and other factors were only measured at baseline. This is important because it is possible that these could have changed over the course of the study.

Another important limitation is that, although the study included 1,000 men, only 67 strokes occurred. When these men with stroke were further divided into four categories depending on their carotenoid level, this made very small sample sizes, which decreases the reliability of the risk calculations. The results of this study will therefore need to be confirmed in a larger group of people, and other population groups such as women or other ethnic groups.

While this study cannot prove that lycopene reduces stroke risk, it certainly lends support to the recommendation to eat a balanced diet rich in fruit and vegetables. Whether or not such a diet can reduce stroke risk, a large body of evidence shows that it can cut your risk of heart disease as well as some types of cancer.