Is breast cancer a genetic curse delivered at conception? Is it caused by eating too much fat or taking too many hormones? Is it tied to lack of exercise, lack of vitamins, too much wine, too few babies?
The causes of breast cancer have still not been confirmed, but the consensus among researchers now is that a woman's reproductive cycle is at the root of the disease. The more menstrual cycles during her lifetime, the more her risk of breast cancer, it seems.
Anything that reduces the number of menstrual cycles--pregnancy, breast-feeding, late puberty, early menopause, removal of the ovaries--reduces exposure to the hormones of that cycle and seems to decrease the risk, says Walter Willett, professor of epidemiology at Harvard Medical School. Willett is part of the school's epic health studies of a quarter-million nurses.
"There are some areas where the evidence is very good and some where it's very shaky," Willett says.
Two of the major health threats to women past menopause are heart attack and osteoporosis, a condition in which bones become less dense and easily broken. Taking hormones after menopause, usually the female hormone estrogen, proved to greatly reduce the risk of both ailments.
The bad news came later: Hormone therapy increases the risk of breast cancer.
"With everything we know about the biology of breast cancer and estrogen, it would be amazing if this did not increase the risk," Willett says. "The surprise is that hormone therapy doesn't increase the risk more than it does."
The increase seems to be limited to the time women are taking the hormones, he says. "After stopping, the risk drops off very rapidly over the next two years.
"It's the women who continue to take them into their 60s and 70s who are greatly increasing their risk. They may feel better. It reduces heart disease. Maybe there's a fountain-of-youth attitude toward them."
But it would be better overall, Willett says, for women to approach hormone therapy cautiously.
"There are many ways to reduce the risk of heart attack--staying really lean, active and not smoking, for example. But there are not very many ways to prevent breast cancer."
"This is one of the more controversial areas," Willett says.
Eating too much fat was once thought to cause breast cancer, but subsequent studies show "little or no relation."
Yet your eating habits early in life may be very important. Animal studies show that restricting all kinds of calories early in life reduces breast tumors by 90%.
It appears this happens because diet affects growth, and slower growth means beginning your reproductive cycles later in life.
There are clues, however, that some elements of diet may affect breast cancer, Willett says. So far, he says, there is suspicion that:
* Too little Vitamin A may promote breast cancer. "Just getting up to the government's 'recommended daily allowance' is all you need."
* Too few fruits and vegetables may promote breast cancer. "It may just be the Vitamin A you're not getting, or it may be lack of antioxidants," nutrients such as carotene and Vitamin E that act to prevent gene damage.
* Too little food fiber may be a cause. "There's lots of speculation about that but minimal evidence."
* Broccoli and garlic may contain substances that can prevent cancer, "but perhaps not at levels consumed by humans. This is a research area."
Fat cells can manufacture the female hormone estrogen, and estrogen seems to be a factor in causing breast cancer.
Being overweight after menopause seems to increase a woman's risk of breast cancer, but before menopause it seems to protect against breast cancer.
"This is very puzzling," Willett says. "The hypothesis is that fatter women miss more menstrual periods, and that is what causes the effect."
Being extremely lean can also cause women to miss menstrual periods and thereby lower breast cancer risk.
Studies have been of younger women, and it appears they must exercise as strenuously as an athlete before there is any effect.
Some believe that women who exercise at that level probably did so when they were children and that exercise in childhood is when the real effect takes place by delaying the onset of menstruation.
"What seems to be emerging is that diet, particularly in combination with exercise, early in life may be very important," Willett says.
The statistics are irrefutable: Asians have less incidence of breast cancer than Latinos, who have less than African Americans and whites.
But the tempting conclusion is wrong, Willett says. The differences are not because of race. Lifestyle is the key.
Take women from each group, impose upon them the same lifestyle and environment, and they wind up, after a few generations, with the same breast cancer rate, he says.
"This is a profoundly important finding. It says that the differing risks are not due to genetic factors and therefore are potentially avoidable," Willett says.
Heredity does sometimes play a part, however. All breast cancer results from a series of defects in the genes of individual breast cells. Some of these defects--or the propensity to develop them--can be passed on from a parent.
But even if a woman's mother or aunt has breast cancer, it doesn't mean that breast cancer runs in the family, Willett says. Researchers don't begin suspecting a hereditary link unless a woman has several close relatives with breast cancer.
Even then, "it largely depends on what age the relative had breast cancer," Willett says. "If they had it after age 70, the increased risk is very low. If they had it before 50, then it's quite substantial."
When or if a woman has children has long been established as indicators of a woman's risk of breast cancer. An early first birth--before age 30--seems to protect against breast cancer, Willett says. Having no children increases the risk. Having a first child after age 40 increases the risk even more.
"It can get very complicated," Willett says. Being pregnant seems to increase a woman's risk of breast cancer temporarily--for about 10 to 15 years. After that, it seems to have a protective effect.
"By having an early first birth, you can get that period of risk over before the age at which breast cancer usually occurs," Willett says.
The three best studies have shown no relationship between breast cancer and abortion or miscarriage, Willett says.
Twenty years ago it was feared that birth control pills might double or triple the risk of breast cancer, "but that's definitely not true," Willett says.
"Overall it's pretty clear there's not any major relationship between contraceptives and breast cancer."
"There really is strong evidence that even moderate drinking is a mild risk factor," Willett says. "There are probably 40 studies showing this."
The type of alcoholic drink and the age of the drinker do not seem to matter. The alcohol apparently increases the level of estrogen in the blood stream, he says.
"If someone is really concerned about breast cancer, it probably makes sense to drink only infrequently. Probably one or two drinks a week would not appreciably increase the risk."
Too much X-ray radiation can cause many kinds of cancer, and the breast seems to be particularly susceptible during childhood, Willett says. "But the breast seems to become relatively immune to radiation after age 40."
One promising area of research is the effect artificial light has on humans, Willett says. The secretion of one hormone, melatonin, which regulates our sleep cycles, is affected by darkness. Could artificially extending daylight be affecting other hormones and, indirectly, breast cancer?
"It's an interesting idea, and we should keep an open mind," Willett says. "But it's still only hypothesis."
Breast cancer is one of the few cancers that smoking does not seem to cause, Willett says.