Think back to the last time you went to your doctor or healthcare provider. Odds are if they checked your body weight and height, they also calculated your Body Mass Index (BMI).
BMI, an estimate of a person’s body fat, is used to predict a person’s risk for chronic disease. BMI is calculated by measuring a person’s weight relative to their height, and is highly regarded by medical professionals as a good indicator of a person’s likelihood of developing diabetes, metabolic syndrome, and cardiovascular disease.
BMI is widely used by many healthcare providers and researchers because it is a simple and noninvasive way to collect information about overall health (and can be administered by asking for self-reported information over the phone). More importantly, many studies have found it to be valid and reliable indicator to use when assessing population-wide health and disease risk. This is hugely helpful when looking at population risk because it can help guide public health campaigns and educate health professionals on strategies for diagnosis and treatment.
However, BMI becomes less useful when it comes to assessing the risk of an individual.
Let’s look at some of the limitations:
BMI is a measure of your relative weight. In the fitness industry, for example, BMI is often thought to provide an inaccurate or incomplete picture of an active individual’s health, because it doesn’t take into account a lot of factors, such as body composition, age, and sex.1,2 Those factors, along with ethnicity, can all influence your body weight.3,4,5 BMI also doesn’t take into account weight differences among the body’s tissues such as muscle, bone, organs, stored water and fat. When assessing someone’s body composition, we typically break the overall value into fat mass and fat-free mass. Fat-free mass contributes to overall health, along with essential fat (a component of fat mass necessary for survival and health), whereas an excessive amount of non-essential fat negatively contributes to health.
People who strength train consistently tend to have a greater amount of lean muscle mass than people who don’t strength train. Yet, with BMI all weight — fat mass and lean mass — is created equal. Of course, higher amounts of physical activity and lean muscle mass are protective for health and longevity, so take this into consideration when assessing your BMI value.
Not only do high amounts of excess body fat matter, but also where and how you carry your body fat makes a difference to your health risk. Body fat that is right under the surface of the skin that you can pinch is called subcutaneous fat. This type of body fat poses a lower risk of disease than visceral fat, that is, fat carried in and around the organs.6,7
My grandfather was a good ol’ Irishman. In honor of his Irish heritage he liked to drink beer and joke that he had Dunlap’s disease (where his belly had “done lapped over his belt!”). I don’t know if that is a classic Irish joke, but the truth is I couldn’t have pinched my grandfather’s belly, because his fat was much deeper than the surface of the skin. Fat that is stored around the organs is more metabolically active. It releases fatty acids, inflammatory agents, and hormones that lead to higher LDL cholesterol, triglycerides, blood glucose, and blood pressure.
Another problem with BMI is that it assumes if you’re not overweight, you’re not at risk. This simply isn’t true. Thin individuals who lack of muscle mass and carry excess body fat are at just as much risk for chronic disease complications as their obese counterparts, especially when the excess fat is carried in and around the organs.8,9 Yet, by BMI standards these individuals would not be considered at risk for chronic disease.
If you’re concerned about better understanding your body fat and risk for disease, it is possible to use additional tools to obtain more information and gain greater insight.
BMI is only an estimate of body fat.10 There is no direct way of measuring body fat, short of doing a cadaver analysis (at which point, I might add, the human associated with that body no longer cares, and modern day science has a whole list of other diseases it would like to use that cadaver for to advance treatment).
Thankfully there are a lot of convenient and inexpensive options for measuring body fat including skinfold calipers, bioelectrical impedance and the Bod Pod (air displacement plethysmography).2,11 Many of these options are available in fitness facilities and university settings. Recommended body fat percentages for women for overall health are between 21 and 32 percent, and between 10 and 22 percent for men.
If you are concerned and want to gain more information about your visceral and subcutaneous fat, consider a bone densitometry scan (DXA). These scans are more expensive and may be more challenging to find and schedule, but they are considered the gold standard for measuring body fat because of the associated accuracy and reliability.
If you want to skip the expense of a DXA scan, a simple measure of health risk is waist circumference. A waist circumference greater than 35 inches for women and greater than 40 inches for men is considered as an independent risk factor for cardiovascular disease, cancer, and all-cause mortality.9,12,13 To measure your waist circumference, simply place a cloth tape measure around the smallest part of the waist while standing relaxed.
BMI has great utility in measuring health and disease risk in populations at large, but when it comes to your personal health, BMI may or may not matter given other health behaviors and where you carry your body fat. Excess body weight and obesity don’t typically show up by themselves. Most of the time excess weight is accompanied by other negative health behaviors such as poor diet, smoking, being sedentary, and high stress. By including regular strength training and a healthy diet, you can reduce excess body fat and overall risk for disease, regardless of what happens to BMI.
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