Women who upped their tomato intake made more of a hormone thought to protect against the disease.
For instance, a glass of tomato juice a day would contain the right amount of the lycopene, the plant chemical believed to be behind the effects.
Lycopene gives tomatoes their rich red colour and is already believed to help ward off prostate cancer.
Those who don’t like tomato juice could try a bowl of tomato soup, plus tomato sauce with their pasta and dollop of ketchup a day.
The researchers, from Rutgers University in New Jersey, measured hormone levels in 70 women aged 55-plus before and after upping their tomato intake for ten weeks.
All were either overweight or had a close relative with breast cancer, putting them at higher risk of the disease themselves.
The amount of lycopene found in a glass of tomato juice a day raised levels of the hormone adiponectin by nine per cent, the Journal of Clinical Endocrinology & Metabolism reports.
Adiponectin is involved in regulating fat levels and obesity is known to raise the odds of breast cancer.
Researcher Adana Llanos said: ‘The findings demonstrate the importance of obesity prevention.
Previous work suggests that lycopene also combats breast cancer by stopping cancerous cells from growing and spreading.
Tomatoes are the best source of the compound but small amounts are also found in guavas, watermelons, apricots, papayas and pink grapefruit.
However, British experts said that weaknesses in the study mean that there is no need for women to rush out and buy lots of tomatoes.
Alternative: Those who don’t like tomato juice could try a bowl of tomato soup, plus tomato sauce with their pasta and dollop of ketchup a day
Yinka Ebo, of Cancer Research UK, added: ‘Our diet is made up of lots of different foods and nutrients and many of these affect our risk of cancer, often in combination with one another.
‘Eating any one specific food is unlikely to have a major impact on preventing cancer on its own.