The answer isn't yet clear. Generally, research shows that breast cancer is less common in countries such as China and Japan where diets tend to be low in total fat, polyunsaturated fat, and saturated fat. However, studies of American women have not found a link between breast cancer risk and their typical higher fat intake.
Why the mixed findings?
Some scientists say that studies comparing fat intake and breast cancer risk in different countries don't account for other factors that may influence risk more than dietary fat. These factors include genetics, physical activity level, and amounts of other nutrients consumed.
Some studies suggest that certain types of fat may influence cancer risk. For example, saturated fat in foods such as meat and whole milk products may increase risk. The unsaturated fats in foods such as nuts and vegetable oils and the omega-3 fatty acids (a type of unsaturated fat) abundant in fatty fish such as salmon, sardines and mackerel may help reduce risk. However, more research is needed to better understand how the type and amount of fat consumed influence breast cancer risk.
Until then, it's best to follow recommendations from the Dietary Guidelines for Americans to keep total fat intake from 20% to 35% of calories (about 44 to 78 grams for a 2,000-calorie diet), choose mostly unsaturated fats, and limit saturated and trans fats.
One last point about dietary fat and breast cancer: Because fat is calorie-rich, eating too much might lead to weight gain. And women who gain weight in adulthood seem to be at higher risk of breast cancer, especially after menopause. So keep your weight at a healthy level by balancing the calories you eat with regular physical activity.