After breast cancer - do dietary fats make a difference?

Fats can be divided into three types: saturated (mostly of animal origin such as meat and dairy products); monounsaturated (found in avocados, nuts, olive oil and seed oils for example); and polyunsaturated (which includes the essential omega-3, -6 and -9 fatty acids). There are also trans-fats which are the result of hydrogenation - a food manufacturing process that converts unsaturated fats into fats which have a higher melting point and extended shelf-life. Trans-fats are found in some margarines and in manufactured and fast foods such as cakes, biscuits and french fries.

While many studies have investigated the effect of fats on the risk of breast cancer, very few have looked at their role in the diet of women who have had breast cancer and whether they affect recurrence and long term survival.

Among these, animal studies showed that the consumption of omega-6 polyunsaturated fats increased the incidence of lung metastases whereas omega-3 consumption decreased it.

In humans, two large studies in the US - Women's Healthy Eating and Living (WHEL) and Women's Intervention Nutrition Study (WINS) - reached different conclusions. WHEL found no difference in additional breast cancer events or deaths between women who ate different amounts of dietary fat while WINS found that breast cancer-free survival was 24% higher in women who ate a low fat diet. The results seen in WINS could have been due to weight loss rather than lower fat intake as their effects are difficult to distinguish.

A recent study found that diets with higher levels of monounsaturated fats than saturated fats (i.e. a high ratio) appeared to be associated with a decreased risk of any cancer. It is thought that the specific levels were unimportant, as long as the ratio was high. Further studies are needed to investigate whether this finding applies to breast cancer specifically.

There is still much to be learned about the role of dietary fat in breast cancer prevention and prognosis and it is widely acknowledged that to separate the effects of diet from other lifestyle factors is difficult. However, the available evidence shows that fat consumption does have an effect on overall good health. The Food Standards Agency advises that consumption of saturated fats should be kept low by swapping for the healthier monounsaturated and polyunsaturated fats while trans-fats should be avoided as much as possible.

Against Breast Cancer's Diet and Lifestyle Study (DietCompLyf) is a multi-centre clinical study investigating the diet and lifestyle, complementary and alternative treatments they may use. For more information see our Diet & Lifestyle Study page.