The results of the largest study ever concerning diet and cancer in women are in. 

And some doctors are disappointed that there appears to be no link between eating a low fat diet and lowering the risk of breast and colorectal cancer or heart disease.

Most of the women who took part in the study were overweight or obese, and 60 percent were told to continue eating what they wanted. The rest were put on a low-fat diet for a study that lasted 10 years.

Joanne Sether Menard

Joanne Sether Menard was one of the participants. She was surprised by the study findings. "I would think that it certainly helps in preventing heart disease,  and maybe even some types of cancer."

But she is wrong. So was Doctor Elizabeth Nabel, Director of the National Heart, Lung and Blood Institute.

Dr. Elizabeth Nabel

"We were surprised by the findings. We went into the study thinking that a diet low in total fat would reduce the risk definitively for breast cancer, colon cancer and heart disease."

The study found that women on the low fat diet had only a slight decrease in breast cancer and heart disease.

Likewise, the diet did not protect women from colorectal cancer.

The results come from the Women's Health Initiative and are published in the Journal of the American Medical Association.

Researchers from many institutions tracked nearly 49,000 older women who have already been through menopause.

Ross Prentice

Ross Prentice was one of the researchers. "Our study, even though it has yet to yield the dramatic results that some might have hoped for, does include some trends toward positive health benefits."

For instance, some women who ate high fat diets before the study, but changed to low-fat diets during the study, saw a reduction of breast cancer risk of up to 20 percent. 

Regarding heart disease, Mr. Prentice says the type of fat the women ate made a difference. "Among the women who made larger reductions in saturated and transfat, we saw trends toward reduction in heart disease."

Mr. Prentice says women who were on a low fat diet had no adverse effects. "So we think women who are currently on a low fat diet can confidently continue on such a diet."

Researchers now want to find out if the types of fat eaten may be more of a factor for heart disease.

And that the amount of fat eaten may be less important than a person's overall weight and exercise level.