Eating grapefruit every day could raise the risk of developing breast cancer by almost a third, US scientists say.
A study of 50,000 post-menopausal women found eating just a quarter of a grapefruit daily raised the risk by up to 30%.
The fruit is thought to boost levels of oestrogen — the hormone associated with a higher risk of the disease, the British Journal of Cancer reported.
But the researchers and other experts said more research was still needed.
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 Southern 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.
Previous studies have shown that a molecule called cytochrome P450 3A4 (CYP3A4) is involved in metabolising oestrogen hormones.
And 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 higher.
They said: “It is well established that oestrogen is associated with 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.”
And they said this was the first time a commonly eaten food had been linked to an increased risk of breast cancer in older women.
However, they warned that more research was needed to confirm the findings which may have been affected because they only took into account intake of the fruit, but not grapefruit juice.
Breast cancer accounts for almost a third of all cancers in women, and previous lifestyle factors linked to the disease include drinking alcohol and being overweight.
Dr Joanne Lunn, a nutrition 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 a piece of the jigsaw that will eventually help us to understand how our diets affect our health.
“Although we are beginning to get a better awareness 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.”
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
Those poor grapefruit farmers. First they get hit because their high-usage market of elderly people is scared off because of drug interactions and now this.
Yet the publication of this study tells you more about publicity-hungry scientists than it does about grapefruit and cancer.
We’ve pointed out before how tenuous much of the research is that claims specific cancer-prevention properties for individual produce items. Well, the good news is we are equally skeptical about this claim.
The publication of research in this form was really highly irresponsible. It is highly predictable what the headlines will be and that the caveats will be lost as consumers pay attention to the headlines.
Not only is this one study finding something that nobody else has ever found but the researchers admit they did not control for relevant variables.
An obvious one: grapefruit juice consumption.
The basic thesis of the researchers is that consumption of grapefruit may increase the levels of the hormone oestrogen (estrogen) in the bloodstream by inhibiting a molecule called cytochrome 3A4 (CYP3A4) that helps to break down oestrogen.
Yet it is believed that grapefruit juice would have the same effect. So if the consumers that did not get cancer happened to drink grapefruit juice, it would knock out the whole research.
Not that it would settle anything if the grapefruit juice lead fizzled.
It is important to understand several things about this research:
- Nobody actually knows if these consumers eat more grapefruit than any other consumers. It is based on a self-reporting system that may not be accurate.
- The researchers did not actually take any blood tests on people who ate grapefruit to determine the level of oestrogen in the blood stream.
- Having failed to even look at obvious variables such a grapefruit juice consumption, there seems to have been no effort to ascertain what else these individuals who have cancer might have in common besides grapefruit consumption. Maybe people whose taste buds favor grapefruit consumption also like fried calamari and that is the cause. Or, perhaps, grapefruit consumption is concentrated in individuals of specific ethnic heritage, and the genetic predisposition of that ethnic group is to cancer.
- By selecting out one thing to study — breast cancer — and one thing only, the researchers neglect the possibility that even if they are correct, the product might have countervailing benefits to offer. What if it increased the likelihood of getting breast cancer but decreased the likelihood of getting a heart attack?
- The study covered only post-menopausal women. So it tells us nothing about women who have not yet undergone menopause. At the same time this holds open the question of long-term use. Even if the researchers turned out to be correct, how much usage would be required to generate this effect? Are these grapefruit consumers actually lifers — and the relevant statistic is that eating grapefruit daily for 50 years is the problem? Or does an 80-year-old woman who never ate grapefruit in her life increase her risk by starting now? The study tells us nothing.
It is hard to know who is more irresponsible, the academics who submit this kind of half-done research to respected journals, the journal editors and peer review panels that publish it or the consumer publications that run with the story blasting headlines that accentuate the dramatic possibilities more than represent a sober-eyed view of the truth.
There is a kind of unholy trinity at work here. Rarely do these stories come about because reporters are perusing their subscription to the British Journal of Cancer and suddenly leap out of their chairs in a moment of enthusiasm and declare that this is a great story.
Typically the researchers and the journals make sure the reporters know when they have headline-producing stuff. This transforms what could be a noble search to advance human knowledge by incrementally increasing our understanding into a tawdry battle in which academics pursue tenure, grants, prizes and publicity at least as much as they pursue truth.
Already the shame is spreading, Cancer Research UK felt compelled to issue a statement:
CANCER RESEARCH UK CAUTIOUS OVER
GRAPEFRUIT BREAST CANCER
Cancer Research UK has played down reports of a link between eating grapefruit and an increased risk of breast cancer in postmenopausal women.
A study by scientists at the Universities of Southern California in Los Angeles and Hawaii in Honolulu reported that eating as little as a quarter of a grapefruit a day may increase the risk of breast cancer in older women by 30 per cent
But a Cancer Research UK spokesperson pointed out that it was the first study to find such a link.
Researchers analysed data from around 46,000 women taking part in the Hawaii-Los Angeles Multiethnic Study, 1,657 of whom had been diagnosed with breast cancer.
Study participants were asked how much grapefruit they had consumed daily over the previous 12 months.
The researchers found that grapefruit consumption was “significantly associated” with an increased risk of breast cancer, and propose that that the link could be due to the fruit increasing blood levels of the hormone oestrogen, which is known to be associated with breast cancer risk
Previous research has revealed that a molecule called cytochrome P450 3A4 (CYP3A4) helps to break down oestrogen.
Grapefruit is thought to inhibit this molecule, and the team suggests that this could, in theory, raise levels of oestrogen in the blood.
However, Liz Baker, science information officer at Cancer Research UK, said: “Although the chemicals in grapefruit are known to interfere with the action of several drugs, this is the first and only study to show a link between grapefruit and breast cancer risk, and the researchers themselves say that the results need to be confirmed in follow-up studies.
“What we do know is that eating a good mix of at least five fruit and vegetables a day can reduce the risk of many diseases, including some cancers.”
It is good that reputable scientists feel a need to issue explanations and qualifications. But as Mark Twain told us, “A lie can travel halfway around the world while the truth is putting on its shoes.”
The researchers and the journal got their 15 minutes of fame, and the journalists got their headlines. Now that the cautionary notes are out… Where do the grapefruit growers go to get their customers back?