A study conducted by US scientists found that eating grapefruit every day could raise the risk of developing breast cancer by almost a third.
A study of 50,000 post-menopausal women found that eating just a quarter of a grapefruit every day raised the risk by up to 30%.
The fruit is thought to boost levels of the hormone oestrogen which is associated with a higher risk of the disease, according to the report published in the British Journal of Cancer.
The women had to fill in questionnaires saying how often they ate grapefruit and how big their portions were.
The researchers, at the universities of South California and Hawaii, found that women who ate one quarter of a grapefruit or more every day had a higher risk of breast cancer than those who did not eat the fruit at all.
Data from previous studies have shown that a molecule called cytochrome P450 3A4 (CYP3A4) is involved in metabolizing oestrogen hormones.
It is thought that grapefruit may boost blood oestrogen levels by inhibiting this molecule, allowing the hormones to build up.
The researchers found that in women who ate at least a quarter of a grapefruit daily, levels of oestrogen were much higher.
They said: “It is well established that increased levels of oestrogen is associated with an increased breast cancer risk.
“Therefore, if grapefruit intake affects oestrogen metabolism leading to higher circulating levels, then it is biologically plausible that regular intake of grapefruit would increase a woman’s risk of breast cancer.”
They also said that this was the first time a commonly eaten food had been linked to an increased risk of breast cancer in older women.
They also warned however that more research was needed to confirm the findings which may have been affected because they only took into account intake of the whole fruit, but not grapefruit juice.
Breast cancer accounts for almost a third of all cancers that occur in women, and previous lifestyle factors linked to the disease include drinking alcohol and being overweight.
Dr Joanne Lunn, scientist at the British Nutrition Foundation said: “This is an interesting study of a large group of post-menopausal women whose diet and health have been followed for many years.
“However, this study is simply one piece of the jigsaw that will eventually help us to understand how our diets affect our health.
“Although we are beginning to gain a better understanding of how our diets can modify the risk of diseases such as cancer, we are still a long way from identifying particular foods that might specifically increase or decrease risk of disease.”
However, she said that some dietary patterns are associated with a reduced risk of certain cancers and that a diet rich in a variety of different fruits and vegetables could help reduce the risk of heart disease and some cancers.