Obesity one of leading risk factors for premature death

It is one of the world’s largest health problems – one that has shifted from being a problem in rich countries, to one that spans all income levels.

02 March 2021 | Health

Obesity is most commonly measured using the body mass index (BMI) scale. The World Health Organization (WHO) define BMI as: “a simple index of weight-for-height that is commonly used to classify underweight, overweight and obesity in adults.

BMI values are used to define whether an individual is considered to be underweight, healthy, overweight or obese. The WHO defines these categories using the cut-off points: an individual with a BMI between 25.0 and 30.0 is considered to be ‘overweight’; a BMI greater than 30.0 is defined as ‘obese’.

The Global Burden of Disease is a major global study on the causes and risk factors for death and disease published in the medical journal The Lancet. These estimates of the annual number of deaths attributed to a wide range of risk factors.

Risk factor

Obesity is a risk factor for several of the world’s leading causes of death, including heart disease, stroke, diabetes and various types of cancer. It does not directly cause any of these health impacts but can increase their likelihood of occurring.

According to the Global Burden of Disease study 4.7 million people died prematurely in 2017 as a result of obesity. To put this into context: this was close to four times the number that died in road accidents, and close to five times the number that died from HIV/AIDS in 2017

At a basic level, weight gain – eventually leading to being overweight or obesity – is determined by a balance of energy. When we consume more energy – typically measured in kilocalories – than the energy expended to maintain life and carry out daily activites , we gain weight. This is a called an energy surplus. When we consume less energy than we expend, we lose weight – this is an energy deficit.


This means there are two potential drivers of the increase in obesity rates in recent decades: either an increase in kilocalorie intake i.e. we eat more; or we expend less energy in daily life through lower activity levels. Both elements are likely to play a role in the rise in obesity.

To tackle obesity it’s likely that interventions which address both components: energy intake and expenditure are necessary. Over the past century – but particularly over the past 50 years – the supply of calories has increased across the world. In the 1960s, the global average supply of calories (that is, the availability of calories for consumers to eat) was 2200kcal per person per day. By 2013 this had increased to 2800kcal.

Across most countries, energy consumption has therefore increased. If this increase was not met with an increase in energy expenditure, weight gain and a rise in obesity rates is the result.

Overall we see a strong positive relationship: countries with higher rates of overweight tend to have a higher supply of calories.

How do we measure obesity in adults?

The most common metric used for assessing the prevalence of obesity is the body mass index (BMI) scale. The World Health Organization define BMI as: “a simple index of weight-for-height that is commonly used to classify underweight, overweight and obesity in adults. It is defined as the weight in kilograms divided by the square of the height in metres (kg/m2). For example, an adult who weighs 70kg and whose height is 1.75m will have a BMI of 22.9.”

Measured BMI values are used to define whether an individual is considered to be underweight, healthy, overweight or obese. The WHO defines these categories using the cut-off points in the table. For example, an individual with a BMI between 25.0 and 30.0 is considered to be ‘overweight’; a BMI greater than 30.0 is defined as ‘obese’.

How do we measure obesity in children and adolescents?

The metric for measuring bodyweight in children and adolescents is also the body mass index (BMI) scale, measured in the same way described above. However, interpretation of BMI scores is treated differently for children and adolescents. Whilst there is no differentiation of weight categories in adults based on sex or age, these are important factors in the body composition of children. Factors such as age, gender and sexual maturation affect the BMI of younger individuals. For interpretation of individuals between the ages of 2 and 20 years old, BMI is measured relative to peers of the same age and gender, with weight classifications judged as shown in the table.

Is BMI an appropriate measure of weight-related health?

The merits of using BMI as an indicator of body fat and obesity are still contested. A key contention to the use of BMI indicators is that it provides a measure of body mass/weight rather than providing a direct measure of body fat. Whilst physicians continue to use BMI as a general indicator of weight-related health risks, there are some cases where its use should be considered more carefully:

muscle mass can increase bodyweight; this means athletes or individuals with a high muscle mass percentage can be deemed overweight on the BMI scale, even if they have a low or healthy body fat percentage;

muscle and bone density tends to decline as we get older; this means that an older individual may have a higher percentage body fat than a younger individual with the same BMI; women tend to have a higher body fat percentage than men for a given BMI.

Physicians must therefore evaluate BMI results carefully on a individual basis. Despite outlier cases where BMI is an inappropriate indicator of body fat, its use provides a reasonable measure of the risk of weight-related health factors across most individuals across the general population. Sources: Sciencedaily, ourworldindata, The Lancet

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