Posted on 08 April 2022

Body mass index (BMI) has long been the gold standard for assessing overall risk for diseases that plague so many of us in western civilisation, including diabetes, heart disease, cancer and strokes. Many of us whom have attended general practice will recall their weight and height being measured as this is used to describe, more specifically, the ratio of body mass to surface area. BMI was conceived in the 1830s by Lambert Adolphe Jacques Quetelet (1796-1874), a mathematician and renowned sociologist. Although niche at first, the clinical applicability grew as more research was undertaken and by the mid-20th century the utility of BMI to assess clinical populations expanded.

On the scale with BMI, a normal weight falls between a BMI of 18.5 and 25, you are overweight if it is between 25 and 30 and beyond that a BMI of 30 or more is classed as obese. Although the clinical application is unquestionable, BMI has flaws which do not generally account for other bodily characteristics which affect the weight of an individual including medication status, the time of day, muscle mass, bone density and other factors. Yet, this metric persists even though some clinicians bemoan the use of BMI to assess all patient’s disease risk as often it wrongly assigns a healthy individual into underweight or obesity status and is thus a black and white metric.

This view is reflected broadly by a number of clinicians in the field, as confirmed from a recent article in the journal Science, Dr Ahima mentions, “There is an urgent need for accurate, practical and affordable tools to measure fat and skeletal muscle, and biomarkers that can better predict the risks of diseases and mortality’. To measure yourself, the calculation is as follows; take your weight in kg and then divide that by the square of your height in metres. You can assess yourself against these markers;

  • A BMI below 18.5 is considered "underweight."

  • A range of 18.5 to 24.9 is "normal or healthy."

  • A range of 25 to 29.9 is "overweight."

  • A BMI of 30 and above is "obese”.

The biggest flaw in BMI is that it fails to address the person’s body fat versus lean mass. As muscle is much denser than fat, (with one cubic inch of muscle weighing more than a cubic inch of fat), incorrect assessment can occur. Thus, a common failing is that BMI will measure only the two factors it has (weight and height) and almost always classifies musclular, athletic people as fatter than they really are. For example, a 6’5 Olympic weightlifter with a weight of 90kg (200lbs) may have the same BMI (26) as a someone who has never exercised in their life and thus both would be designated overweight. There, the assessment is correct for the clinical population of the individual neglecting to exercise, but wrongly classifies the extremely fit Olympian as at high risk of mortality and disease, despite a clean bill of health. ​

It’s not what you have, it’s where you have it. The location of fat deposition in the body has a serious impact on disease risk, as subcutaneous (under the skin) fat is much more healthy than the insidious depositing of fat around the organs (visceral fat). Indeed, when CT scans show fat deposition around the liver, for example, pear shaped bodies and fat around the waist are a better predictor of heart attack and stroke than your BMI. Similarly, waist to hip ratio is a step in the right direction in observing disease risk as research has elucidated a closer link to heart attack risk than BMI.

In clinical patients, BMI probably still remains the best measure. However, this is only useful if taken in context with other diagnostic testing including for blood pressure, resting heart rate, cholesterol, fasting sugars and non-alcoholic fatty liver disease. Then, a patient who is presenting with symptoms but has normal testing may undergo a body composition analysis such as DEXA scans (dual x-ray absorptiometry) to locate fat deposition and assess water weight, bone mass, lean mass and other factors. Used in tandem, a much more thorough assessment can be made instead of sticking only with BMI and wrongly assigning underweight or obesity to patients within the clinic. Of course, handing an overweight or obese assignment to someone who believes they are otherwise healthy due to the incorrect assessment can be damaging to patient self esteem and trust with their clinician.

However, the fact remains that almost two thirds of adults in the UK are overweight or obese, meaning that the majority of patients are suitable for BMI assessment. Many believe obesity and eating behaviours to be linked more closely to satisfaction, happiness and quality of life, something which is echoed by World Health Organisation reports. In countries with a greater quality of life, measured by happiness or life satisfaction, there is often lower prevalence of obesity. Indeed, many articles have been written during the 2020 and 2021 lockdown in the UK about individuals eating to cope, as food has become a powerful mechanism used by many to quell anxiety and entertain. 

In summary, though, there are things you can enact to achieve a healthy BMI, but one should look at other measures before feeling disheartened. Although these following advice points are general to heart disease, diabetes, obesity and other related diseases (known as metabolic syndrome), they are applicable still in the maintenance of normal bodily weight;

  • Exercise and physical activity – even modest amounts of aerobic exercise such as walking can reduce systolic and diastolic blood pressure by as much as 3-5mmHg in as little as 8 weeks – a greatly effective method for non-pharmacological intervention.

  • Stress management – being aware of work stress levels and planning for stressful situations is a great way to manage your blood pressure – a rise in the level of stress hormones such as cortisol raises your blood pressure and can increase the risk of heart attacks and strokes.

  • Diet – managing the amount of lipids and fats you consume in your diet is also a key part of staying in a healthy range, as well as reducing your risk for coronary artery disease. Make a conscious effort to increase the amount of healthy fats you consume, as well as taking on good, complex carbohydrates, green cruciferous vegetables and to reduce the intake of saturated fat and cholesterol.

  • Weight management – Even modest weight loss can greatly improve your blood pressure readings, along with reducing your changes for other non-communicable diseases such as metabolic disease and diabetes.

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