Regular exercise has been associated with a reduced risk of developing breast cancer. The relationship between physical activity and breast cancer risk is complex and can be influenced by various factors. Here are some ways in which exercise may play a role:

1. **Hormone Levels:** Physical activity can affect hormone levels in the body. Higher levels of estrogen have been linked to an increased risk of breast cancer, especially in postmenopausal women. Regular exercise may help regulate hormone levels, potentially reducing the risk.

2. **Body Weight:** Maintaining a healthy weight through regular exercise is important in reducing the risk of breast cancer. Being overweight or obese is associated with an increased risk, particularly in postmenopausal women.

3. **Insulin Sensitivity:** Exercise improves insulin sensitivity, which is important for regulating blood sugar levels. Some studies suggest that high insulin levels may be associated with an increased risk of breast cancer, and exercise may help mitigate this risk.

4. **Immune System Function:** Regular physical activity has been shown to enhance immune system function. A strong immune system is crucial for detecting and eliminating potentially cancerous cells in the body.

5. **Inflammation:** Chronic inflammation is thought to play a role in the development of various cancers, including breast cancer. Exercise has anti-inflammatory effects, which may contribute to a lower cancer risk.

6. **Improved Circulation:** Exercise improves blood flow and oxygen delivery to tissues, which can have overall positive effects on health. This improved circulation may contribute to a reduced risk of cancer.

It's important to note that while exercise can be a protective factor, it's not a guarantee against breast cancer. Other factors, such as genetics, family history, and hormonal factors, also contribute to a person's risk. Additionally, the intensity and duration of exercise may influence its impact on breast cancer risk.

The American Cancer Society recommends at least 150 minutes of moderate-intensity or 75 minutes of vigorous-intensity exercise per week for adults, along with strength training exercises at least two days per week. However, it's always advisable to consult with a healthcare professional before starting a new exercise regimen, especially for individuals with existing health conditions.