The latest study, presented this week at the International Research Conference on Food, Nutrition and Cancer in Washington, DC, builds on previous evidence that breast cancer risk can be reduced by diet.
Earlier this year UCLA made public the findings of an epidemiological study involving more than 2,400 women with early-stage breast cancer. They were seen to have significantly less chance of their cancer return within five years if they ate a low-fat diet with plenty of fruit and vegetables, compared with eating typical American diet high in fat.
However lead investigator Dr James Barnard, professor of physiological science at UCLA, said that his study is a departure in one key respect:
"This is the first study to my knowledge to show that lifestyle changes can induce apoptosis, or cell death, in breast cancer cells."
Twenty-six postmenopausal women attended a 13-day program at the Pritkin Longevity Center and Spa, during which they adhered to an exercise regime and consumed a low-fat, high-fiber diet rich in fruits, vegetables and whole grains.
The researchers took blood samples at the start and end of the trial, and placed these samples in three different sets of culture dishes, each with a different line of breast cancer cells.
In all three they noted a 20 to 30 percent increase in apoptosis (tumor cell death) when they compared the samples from the start of the trial to those at the end.
They also measured changes in serum estradiol (a form of estrogen), insulin and insulin-like growth factor (IGF-1) - all of which are independent risk factors for breast cancer development. In women taking hormone replacement therapy estradiol levels fell by an average of 34 percent, and in those not taking it by 27 percent. Insulin levels fell by 29 percent and IGF-1 levels by 19 percent in all the women.
Overall Barnard concluded that the combined effect of the two processes resulted in serum changes that slowed the growth of breast cancer cells by as much as 19 percent.
Dr William McCarthy of UCLA's School of Public Health said: "This is exciting research because it shows that women can make changes in a very short period of time that can have a dramatic impact on their health - in this case, on the growth and death of breast cancer cells."
The US has one of the highest incidences of breast cancer in the world, where it accounted for 41,250 deaths in 2002. The American Cancer Society estimates that in 2005 approximately 211,240 women will be diagnosed with the disease.
In Asian countries such as Japan, where the typical diet contains much less fat and more fresh, high-fiber foods, breast cancer rates are much lower.
But Barnard warned that they may not maintain their advantage for long:
"As Asian countries like Japan are now becoming more Westernized, their breast cancer rates are going up substantially," he said.
Not all research into the link between breast cancer and diet has yielded positive results, however. A large, European-wide study published in JAMA this January concluded that fruit and vegetables do not in fact reduce the risk, despite previous evidence to the contrary.
Nonetheless, the authors wrote: "This does not exclude the possibility that protective effects may be observed for specific nutrients or in specific subgroups of women, such as those with a family history of breast cancer or oestrogen-receptor positive tumours."