Eating a tomato just two days a week could cut the risk of depression by half, according to a new study.

Japanese researchers analyzed the metal health records and diet habits of 986 people who were 70 or over and found that participants who regularly ate the fruit were significantly less likely to suffer depression.

Results from the study, published in the Journal of Affective Disorders, found that those who ate tomatoes two to six times a week were 46 percent less likely to develop depression than those who ate them less than once a week.

Furthermore, researchers found that participants who ate tomato products every day reduced their depression risk by 52 percent.

Researchers said that other fruits and vegetables did not have the same benefits as tomatoes.

The findings show that eating healthy vegetables like cabbage, carrots, onions, pumpkins and green-leaf vegetables appeared to have little or no effect on the participants' mental well-being.

Researchers noted that a 34.9 percent of the participants had either mild or severe depressive symptoms, and up to 20.2 percent of the participants suffered severe depression at some point in their lives.

Experts noted that the elderly are significantly more likely to develop depression because of the effect on mood from declining health, bereavements and loneliness.

Chinese and Japanese scientists led by Dr Kaijun Niu from China's Tianjin Medical University wanted to see whether tomatoes, which are rich in antioxidant chemicals believed to protect against some diseases, can also help promote psychological well-being.

Tomatoes, which is a good source of lycopene, an antioxidant that gives tomatoes their deep red color, has been tied to a lower risk of prostate cancer and heart attacks.

Researchers said that besides protecting the prostate and heart, lycopene might also protect mental and physical health by reducing oxidative stress or damage to healthy brain cells.

It is unclear whether lycopene in tomatoes directly affects the brain or whether it protects against the resulting depression caused when people develop potentially fatal disease like cancer.

However, researchers conclude that the findings suggest that "a tomato-rich diet may have a beneficial effect on the prevention of depressive symptoms," compared to diets rich in other kinds of vegetables.