March is National Nutrition Month, so it would make sense for us to share a blog focused on “healthy weight.” However, I’m not going to discuss how a high BMI is harmful or share healthy recipes. I will discuss how BMI is being used as an “easy” but murky indicator of health and how the relationship between overweight, obesity, and breast cancer is not as clear-cut as it is often made out to be.
First, a primer on the BMI, or body mass index. It is supposed to give someone a calculation of how much body fat they have. Someone’s BMI is calculated by dividing their weight by their height in inches squared, multiplied by 703. If someone’s BMI is between 18.5 and 24.9, they are deemed to be at a healthy weight. BMI above 25 is considered overweight, and over 30 is considered obese. For example, someone who is 5’3″ and 152 pounds has a BMI of 27. If you want to know how to calculate your BMI, you can visit this site, but wait until you finish reading this because you might not want to by the end. Also, though we don’t have space to get into this here, the formula for BMI was based on white men, so another reason to consider its relevance to anyone else’s health. For more information on the history of the BMI, check this out.
BMI is used as shorthand for health when in reality it is very flawed when applied so broadly. For one, BMI does not take into account muscle mass. There are multiple stories of top athletes with high BMIs that would put them in the range of obesity. This story features a graph that indicates some athletes, including Tom Brady and LeBron James, would be classified as overweight, according to their BMI. There is also this story featuring women in the military who fail the military’s height and weight standards because muscle mass is not taken into account with these measurements for women recruits, but it is for the men. When I googled “athletes and high BMI,” many of the article titles said things like “Why Athletes Shouldn’t Worry About Their BMI.” If BMI is applied as an indicator of health, but athletes don’t have to pay attention to it, then why does it apply to the rest of us who also also have varying levels of muscle mass and cardiovascular fitness?
You might say, “well, most of us aren’t pro-athletes or in the military, so it should be a more accurate measure of health for the rest of us.” If BMI is a reliable indicator of health, then how can we explain the results of this study from the International Journal of Obesity:
It looked at the proportion of healthy individuals by BMI and six other heart and metabolic measures like blood pressure, cholesterol, and insulin resistance. Out of a national database of more than 40,000 people, about 70% of people with normal-weight BMIs were in the healthy range for all the other measures. So were 47% of people with an overweight BMI, 30% of those considered obese, and 16% of those labeled extremely obese. NPR
To quote from the abstract of the article, “[u]sing BMI categories as the main indicator of health, an estimated 74,936,678 US adults are misclassified as cardiometabolically unhealthy or cardiometabolically healthy.” So when it comes to our overall health, the BMI is an unreliable indicator. But what about when it comes to breast cancer?
The relationship between BMI and bodyweight to breast cancer is not very clear either. We have written previously about how some recommendations for breast cancer prevention are difficult to follow, including that being overweight prior to menopause is seemingly protective against breast cancer, but being overweight after menopause increases risk. Turns out, some more important indicators for breast cancer risk than weight are waist-to-height ratio, muscle mass, and levels of insulin sensitivity, none of which BMI nor the number on the scale can tell you.
But we do know that more body fat can increase risk of breast cancer. Yes, increased body fat is connected with increased rates of breast cancer diagnosis and recurrence, but this cannot be accurately measured by the BMI or the scale. One study followed women with a “normal” BMI but a higher percentage of body fat according to a DEXA scan (which can separate mass of bones, fat, and muscle), and these women did have a higher risk of breast cancer compared to “normal” BMI women with lower levels of body fat. So it is NOT the weight; it is the actual fat cells that increase estrogen and inflammation levels in the body that can increase the risk of breast cancer. Interestingly, weight training may be one of the best exercises for reducing breast cancer risk and for breast cancer survivors in preventing lymphedema, and it could actually end up increasing your BMI because muscle weighs more than fat.
What can you take away from this information? Exercise and a healthy diet are important for reducing your risk of breast cancer, but your BMI and the number on the scale aren’t great tools to measure that risk.
Sources:
https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmi_tbl.htm
