We’ve known for decades that obesity can significantly increase your odds of getting breast cancer. Study after study have shown a solid link between weight and breast cancer. These studies have estimated that being obese can increase your risk of breast cancer by over 33%, though this can vary by study.
However, a new study shows that even lower weight women may be affected by fat-driven breast cancer.
Researchers have examined the data from 3,460 normal weight women in the Women’s Health Initiative, one of the largest and most comprehensive studies on women’s health, to see whether normal weight women (as determined by BMI) could have an increased risk of invasive breast cancer based on body fat.
Overall, they found that a higher fat content as measured by % body fat is associated with a significantly higher risk of breast cancer. Researchers estimated that high body fat elevates the risk of breast cancer by around 50%.
In the chart above, the odds of breast cancer in each group of higher body fat patients compared to lower body fat patients is represented by the horizontal colored bars, while the margins of error are represented by the pale white surrounding bars.
Trunk fat content, which includes belly fat, was shown to be particularly dangerous. Patients with more than 20 pounds were estimated to have double the odds of invasive breast cancer compared to patients with low fat composition.
Researchers compared the effect of trunk fat in breast risk to all other types of fat, and found a clear increase of risk in patients with high belly fat compared to patients with other types of fat.
The data showed that having higher leg fat or having higher overall body fat is not nearly as dangerous as having high belly fat. High leg fat and high overall body fat were associated with a 50% increase in your odds of getting breast cancer, while high belly fat was associated with a near 100% increase.
This data shows that in setting your fitness goals, body composition may be just as important, if not more important than BMI, for your overall health. BMI has always been an imperfect measure of health.
Professional athletes can have high BMIs while being in peak physical condition. Likewise, as shown in this study, patients can have normal BMIs with a relatively high body fat composition.
There are several available metrics for body fat, we have selected the body fat composition goals from the American College of Sports Medicine to create the chart below as these criteria account for age as well as gender. An alternative metric that aligns well with these criteria can be found from the American Council on Exercise.