Campus & Community

Cutting calories cuts breast cancer risk

6 min read

Researchers see it as another reason to stay thin

Severe dieting and pregnancy protect anorexic women from breast cancer, Karin Michels has found. She hopes this discovery can be used to lower the risk of that killer disease for all females. (Staff photo Stephanie Mitchell/Harvard News Office)

Reducing calories protects mice and rats against breast tumors, a number of studies have shown. Can it do the same for humans? A natural experiment in Norway during World War II hints that it can. Under famine conditions, prepubertal girls who consumed an average of 22 percent fewer calories than normal enjoyed a lower rate of breast cancer.

Karin Michels and Anders Ekbom, Harvard University researchers, were discussing such things one day over a beer in Boston. They wanted to do an experiment that would definitely prove, or disprove, whether voluntarily cutting calories would protect women against breast cancer. The best way to do that would be to recruit two large groups of women; one would agree to drastically cut their food intake, then their cancer incidence could be compared with that of the other group. This kind of experiment would take many years, they agreed, and Harvard was not going to be part of semi-starving thousands of women.

Michels, an associate professor of obstetrics, gynecology, and reproductive biology at Harvard Medical School, thought of a way to get around these obstacles. “What if we used women who voluntarily starve themselves, anorexics with an intense fear of being fat?” she asked Ekbom, an adjunct professor of epidemiology at the Harvard School of Public Health. Ekbom liked the idea.

What is more, he knew a way to get the information they wanted without waiting years. He is from Sweden and that country keeps meticulous records of hospitalizations for problems like anorexia and for every case of newly found breast cancer, information not available in the United States. Another Swedish database holds records of every childbirth in that country, which turned out to be a very important aspect of their research.

Ekbom and Michels, who is also an associate professor at the Harvard School of Public Health, identified 7,303 women who were hospitalized for anorexia between 1965 and 1998. All were less than 40 years old at the time. They then mixed and matched these women with a list of those diagnosed with invasive breast cancer. Compared with the general population of Sweden, the anorexics enjoyed, on average, a 53 percent lower incidence of the cancer.

“These results provide good evidence that eating less in early life can protect women against breast cancer,” Michels notes.

What is more surprising, former anorexics who became pregnant had the most profound reduction in risk for the disease, amounting to 76 percent.

Of the women, 1,942, or 27 percent, became pregnant and enjoyed the most protection against a major killer. The remainder, 5,361 women, boasted 23 percent less incidence of breast cancer than the general population.

Stay slim

Michels doesn’t recommend that young women starve themselves, then get pregnant, to help prevent breast cancer. “Anorexia is a life-threatening disease,” she says emphatically.

“At this stage in the research,” she continues, “we can sum up our recommendation in two words: ‘Stay slim.’ Avoiding or embracing particular foods may not be as important as reducing total caloric intake. This may be especially true during adolescence and early adulthood.”

Regular physical activity at these ages should also help. There have been studies indicating that young women athletes enjoy a decreased risk of breast cancer.

Before more detailed advice can be given, a better understanding of how calorie reduction works against breast tumors is needed. One idea Michels is exploring goes all the way back to the womb. Previous research she did found that low-weight babies have less chance of getting breast cancer as adults. In this study, women who weighed less than 5.5 pounds at birth had about half the risk of those who weighed 8.8 pounds or more.

Bigger babies get more exposure to estrogen and other growth hormones that spur rapid cell division. More cell division increases the risk for genetic mutations, which can set women up for breast cancer later in life. Michels doesn’t recommend that pregnant women strive for lower-weight babies, but more detailed knowledge of how these early life factors protect against breast cancer could lead to more practical advice.

Researchers at Columbia University reported on May 25 that aspirin, which interferes with the activity of estrogen, can cut the incidence of breast cancer. They found that women who took seven or more tablets a week were about 28 percent less likely to develop the cancer than those who don’t take the popular painkiller regularly. That’s about the same protection as anorexia gives to women who don’t become pregnant.

Other factors during early life have been and are being studied, including timing of menses and a woman’s height. Late puberty, leading to fewer ovulations in a lifetime, may provide some protection by lessening exposure to estrogen. “We’re going to look into this,” notes Michels, “but ovulations are difficult to count.”

Tall people are at a higher risk for all types of cancers. “Food intake during their early years may be involved,” Michels speculates.

“Low caloric intake also slows metabolism, reduces expression of cancer genes, and enhances expression of genes that suppress tumors,” she continues. In other words, there could be a combination of many things – hormones, growth factors, cell-development changes, and the turning on and off of genes – that are involved in the calorie-cancer connection.

Michels and Ekbom are now examining the role that the density of breast tissue may play. Other studies have concluded that the lower the density, the less risk of breast cancer. They want to determine if density can be lowered by eating less.

Calories and aging

Meanwhile, parallel research goes on in trying to learn if eating less helps people live longer. It works for worms and rodents, even for monkeys, but starving yourself into old age may not be workable for humans. Calorie cuts of 30 percent seem to be required, and most people can’t stay on such a sparse diet for very long.

Michels thinks there also may be an association between substantially lower food intake and the markedly lower rate of breast cancer in poor nations. “Women in developing countries often don’t have enough to eat, work hard, and give birth,” she notes. “This parallels conditions in our study. Maybe the underlying mechanisms are similar.”