Evidence is strong that exercise in general promotes and enhances health (Fentem, 1994). Additional studies support the concept that exercise may play a protective role in the development of cancer in some populations. Researchers have found that strenuous exercise more than five hours per week lowered the risk of invasive breast cancer, particularly of estrogen receptor–negative invasive breast cancer, when compared with less activity. These findings were supported even as other cancer risks were controlled for in the study population (National Cancer Institute, 2007).