Reading between the lines of the dietary-fat studies

Three reports in last week’s Journal of the American Medical Assn. examined the relationship between a reduced-fat diet and breast cancer, colon cancer and cardiovascular disease. The results, based on one study, showed that after eight years of reducing fat consumption by 8% to 10%, women had no decreased risk in any of the three disease categories.

But don’t load up on Velveeta and ice cream just yet. That’s not the full story.

If my risk is just as great whether I eat a lot of fat or not, is this the green light to butter my bread and chow down on burgers?

Not a chance. The women in the study were ages 50 to 79. That means they’d spent decades eating a typical American diet before half of them tried, for eight years, to cut down on fat.

What is the right amount of fat to eat?

The recommended range runs from a painfully low 10%, recommended by Dean Ornish’s Lifestyle Program, to a range of 20% to 35%, recommended by the Institute of Medicine. But don’t get too hung up on counting only fat grams. Total calories count -- a lot.

When we do eat fat, does it matter what kinds we eat?

Yes, a lot. The study of 48,835 women let subjects eat any kind of fat, as long as they lowered their total fat consumption.

Nutritionists now know that saturated fats and trans fats are closely linked to heart disease risk.

Long-term studies of Americans have found that people who eat a lot of meat, full-fat dairy products, and processed foods made with trans fats are at higher risk for heart disease. Polyunsaturated and monounsaturated fats are actually beneficial. The good fats are found in liquid oils such as olive and canola, and in fish and nuts.

The JAMA studies did not disprove these things. But they may have suggested that to begin, at middle age, to try to reverse the damage done from lifelong bad habits could well take longer than eight years.

If we know that there are good fats and bad fats, why did this study let women choose any kind of fat, as long as they lowered total fat consumption?

The study began enrolling women in 1993. Since then, there has been a sea change in dietary recommendations and knowledge about nutrition. The biggest change has been the recommendation to reduce trans fats and saturated fats while eating moderate amounts of healthy monounsaturated and polyunsaturated fats to reduce cardiovascular risk. As yet, there’s no solid evidence that the kind of fat consumed affects cancer development.

It’s confusing. If some studies say that a Western diet carries greater risk for cancer than an Asian diet, why didn’t the recent study show a reduction in breast cancer?

Again, America’s obesity epidemic could well skew the results. Obesity itself increases the risk of breast cancer by 50%, probably by increasing the amount of estradiol in the blood, which, with other hormones, plays a key role in triggering breast cancer. There are contradictory results in breast cancer studies, some showing that being overweight at various times in life either increases or lowers the risk of future breast cancer.

“With breast cancer, nothing is straightforward,” says Walter Willett of the Harvard School of Public Health. “It makes sense for most people to make their dietary decisions based on what it does for heart disease. That’s where the data are most strong.”

The JAMA studies found that women who cut down on fat reduced their risk of breast cancer by 9%, a result deemed statistically not significant, though an encouraging trend.

Were there any other important trends?

Yes. Women who ate the most fat when the study began, and therefore reduced their fat intake by a higher percentage, showed greater reductions in breast cancer risk -- up to 20%, says Ross Prentice, principal investigator on the breast cancer study and researcher at the Fred Hutchinson Cancer Research Center in Seattle. Also, colorectal cancer can take many years to develop and is preceded by polyps in the colon. While the women who reduced their fat consumption didn’t have lower rates of cancer, they did have fewer polyps.

What does this study tell us about children’s nutrition?

Directly, nothing. But the studies’ between-the-lines message is that overcoming the damage set in motion by a lifetime of bad eating habits is very difficult. “The changes that we need to make are best if they’re lifetime changes,” says Prentice.