Moderate exercise did not cut breast cancer risk. Vigorous exercise did, but only in women who were not overweight. However, it's possible that overweight and obese women found moderate exercise more taxing and misreported it as strenuous exercise.
The findings suggest that exercise itself protects against breast cancer, regardless of whether it leads to weight loss, note Michael F. Leitzmann, MD, and colleagues at the National Cancer Institute.
The researchers analyzed data on more than 32,000 postmenopausal women collected over 11 years as part of the Breast Cancer Detection Demonstration Project.
Women were asked about their exercise habits, and Leitzmann's team rated their exercise as "moderate" or "vigorous."
Activities rated as "moderate" were rated "non-vigorous." They included:
- Light housework
- Washing clothes
- Home repairs
- Lawn mowing
- General gardening
- Light sports or exercise
- Light jogging
- Recreational tennis
- Bicycling on level ground
Activities rated as "vigorous" include:
- Housework such as scrubbing floors or washing windows
- Heavy yard work
- Digging in the garden
- Chopping wood
- Strenuous sports or exercise
- Fast jogging
- Competitive tennis
- Bicycling on hills
- Fast dancing
Overall, women who got a lot of vigorous exercise had only a small decrease in breast cancer. But the researchers saw a much stronger effect in women who were neither overweight nor obese.
"When we evaluated the relation of vigorous activity to breast cancer among women who were of normal weight ... the risk among women reporting the highest amount of vigorous activity decreased by about 30% compared with women with no vigorous activity," Leitzmann and colleagues found.
The study appears in the Oct. 31 online issue of the open-access journal Breast Cancer Research. Leitzmann is now with University Hospital Regensburg, Germany.