BMI is not black and white

Though a key measure of health risk since the 1970s, Aela O’Flynn asks if BMI is misleading medicine.

Body Mass Index (BMI) is described by the World Health Organisation as “the most commonly used tool to correlate risk of health problems with the weight at population level”. In spite of this powerful statement, emerging research indicates that BMI does little but scratch the surface in assessing an individual’s risk of disease.

BMI is a useful indicator for quick analysis of adult nutritional status. It’s very easy to measure, only using weight in kilograms divided by the height in metres squared. The normal range is between 18.5 and 24.9 and until recently those in this range were considered to be “healthy”. Health, however, has revealed itself to be much more complicated.

a high BMI does not necessarily indicate poor health

If you are in the normal range for BMI, you are considered to be an ‘ideal’ weight for your height. Other factors, such as body composition, are completely disregarded. This means that BMI is all but irrelevant for anyone with a high muscle mass, for instance. This is a major issue when assessing athletes. For example, Irish rugby international Cian Healy’s BMI of 32.7 (2017) would have him classified as obese.

It follows that a high BMI does not necessarily indicate poor health. Research indicates that the converse is also true. A recent study published in the American Journal of Cardiology found that people with a normal BMI who led sedentary lifestyles had a comparable risk of cardiovascular disease to those who were ‘overweight’. Another study in the International Journal of Obesity found that 30% of normal weight individuals were considered “cardiometabolically unhealthy”, or at high risk of developing cardiovascular disease, while nearly half of overweight and 29% of obese individuals were at low risk.

Cardiovascular disease is a broad term for any disease of the heart or circulation. According to the Irish Heart Foundation, it is the most common cause of death and disease in Ireland. This research is of particular interest to the medical community as it could open the door to lifestyle medicine as the first-line in medical intervention for a whole range of diseases, not least cardiovascular disease.

Doctors all over the world use BMI as their primary method to assess the risk of cardiovascular disease. Though useful for both doctor and patient as a preliminary guideline, BMI must be used in conjunction with other tests in order to provide an accurate picture of an individual’s health. The CDC (Centre for Disease Control and Prevention) in the US published a report for health practitioners in 2009 to highlight the limitations of BMI. BMI is described in this report as a “surrogate measure of body fat” because it actually measures excess weight, which may not necessarily be fat.

There are a number of other measures that can be used to improve the diagnostic usefulness of BMI in various categories. To measure body fat, the easiest methods are waist circumference, which measures fat distribution using a tape measure, and skinfold thickness, which uses a callipers to gently pinch the skin to measure excess skin and fat on the body. To measure metabolic health, it is important to examine blood pressure, the levels of triglycerides, cholesterol and glucose in the blood, and insulin resistance. This revelation about the relationship between BMI, exercise, and cardiovascular disease could have a major impact on public health and the future of preventative medicine.

Medical professionals are becoming increasingly interested in lifestyle medicine interventions. Lifestyle medicine has been hailed as the future in preventing the incidence of chronic disease due to lifestyle-related factors. It is focused around promoting health and prevention of disease by educating and prescribing a healthy balanced diet, physical activity, adequate sleep, and positive lifestyle choices such as minimal alcohol consumption, minimising stress, and being a non-smoker.

One of the key facets of lifestyle medicine is encouraging patients to prevent disease through being physically active. Exercise is now being termed “the magic pill” because of its unique ability to prevent an incredibly diverse range of diseases. Science is only beginning to uncover the extent of the impact that exercise can have.


Most of us are familiar with the guidelines to engage in at 150 minutes of moderate aerobic activity each week, such as brisk walking or cycling, and two strength sessions. Though this equates to less than 30 minutes a day, these 150 minutes has been shown to add an average of three to four years to your life. In the American Journal of Cardiology study, it was further noted that individuals with a healthy BMI who had characteristics related to lack of fitness, such as shortness of breath and inactivity, increased their chances of being at high-risk of cardiovascular disease to a similar level as those in the overweight BMI cohort. Prioritising physical activity will not only extend your life and reduce your risk of chronic disease, it will also maintain and enhance your quality of life as you age, due to the mobility, muscle mass and cardiac health promoted by an active life.

In addition to moving more, sitting less has an impact on health. A 2016 study coordinated by the American Heart Association linked an increased risk of cardiovascular disease with 7.7 hours of sitting per day or more, a figure easily reached by office workers, students, or even anyone with a long commute. However, physical activity maintains a link to reduced risk of cardiovascular disease regardless of sedentary hours in the day. Despite this, both lack of activity and sedentary behaviour remain relevant risk factors for cardiovascular disease.

The evidence overwhelmingly suggests that using BMI as the only method of analysing the risk of cardiovascular disease is simply not enough. Other tests must be used in tandem with BMI to accurately assess the risk of individual. Additionally, the sole use of weight-loss treatment plans to reduce the risk of adverse cardiovascular events may not be sufficient. It is here where lifestyle medicine can come to the fore and use physical activity to maximise overall health and wellness, thereby reducing the risk of cardiovascular disease. So please, get moving and protect your heart, promote your health, or you might just pay the price.