BMI. To measure or not to measure

BMI: To Measure or Not to Measure

Following a recent report, calls have been made to immediately scrap the use of BMI as it’s said to ‘contribute to health issues such as eating disorders and people’s mental health by disrupting body image and inviting social stigmas.’

But is scrapping it completely a good call to make?

What the report says

Parliament’s Women and Equalities Committee released a report investigating the effects BMI has on body image, concluding that BMI should be discarded as a measure of individual health.

Branding the Government’s Obesity Strategy as ‘dangerous’ for people with negative body image and found to potentially trigger eating disorders, Caroline Stokes, Committee Chair, said:

“The use of BMI as a measure of healthy weight has become a kind of proxy or justification for weight shaming.

“Anyone can suffer from body dissatisfaction. Over the past ten years, there has been a wealth of research and recommendations on how to tackle negative body image but Government action in this area is limited – we need to see urgent action.”

The committee argues that using BMI as a one-size-fits-all approach is “at best ineffective and at worst perpetuating unhealthy behaviours,” such as disordered eating and weight shaming with negative impacts on body image and mental health.

The report proposed that Public Health England should focus on a “Health at Every Size” approach instead to account for additional factors such as age, gender, and ethnicity and prioritise healthy lifestyle choices over correcting weight.

BMI, mental health, and weight stigma

When it comes to mental health, some research supports associations between higher BMI and poorer mental health.

Evidence using Mendelian randomisation found that a higher BMI was associated with depressive symptoms and lowered well-being.

The study examined bidirectional, casual effects between BMI and mental health using summary-level data from genome-wide association studies, finding no consistent evidence in the other direction, such that depressive symptoms or well-being were not associated with higher BMI.

The underlying mechanisms may be due to weight-related physical illness and weight stigmatisation negatively affecting mental health.

Further research suggests that weight bias and stigma have also been shown to have significant mental health impacts on an individual, with one BMC Medicine review, in particular, outlining that stigma can drive weight gain and BMI levels.

The review suggests that the negative characterisation of those classed as overweight or obese has led to people perceiving themselves as overweight, and the effect of weight stigma itself can “trigger physiological and behavioural changes linked to poor metabolic health and increased weight gain.”

The authors note that stigma can induce additional eating, decrease self-regulation, increase cortisol levels compared to controls – especially those who perceive themselves as overweight – and avoiding exercise is a common behaviour.

The opinion review further highlights how weight stigma experiences predict future weight gain and increases the likelihood of reaching an ‘obese’ BMI.

As some research supports the negative impact of BMI and weight stigma on mental health, researchers have long disputed BMI to measure individual health, whether it’s either beneficial or presents too many limitations to use.

Why use BMI to evaluate health?

BMI was developed as a straightforward calculation and reasonable estimate of an individual’s adiposity, based on measuring their height and weight and is used as a risk indicator of disease.

Measures of adiposity are calculated differently depending on whether you’re an adult, adolescent, or child, and BMI ranges do not apply to pregnant women or for use in some medical conditions.

According to the World Health Organisation, a BMI between 18.5 kg/m2 and 24.9 kg/m2is considered healthy, with a BMI above his range indicating an individual carries excess body fat and a BMI lower than the healthy range, showing they carry less than may be beneficial.

BMI has allowed researchers to identify trends that body fat can have within a large population concerning disease risk, with strong associations linking certain BMI levels to particular diseases.

Risks associated with high BMI

Raised BMIs are categorised as overweight (between 25 and 29.9kg/m2), obese (between 30 and 39.9kg/m2) and severely obese (40+kg/m2).

Research has found that BMIs above the healthy range can heighten an individual’s risk of developing disease, and sometimes by a significant amount.

EASO’s statistics show that “overweight and obesity are the fifth leading risk of global deaths” and that “44% of the diabetes burden, 23% of the ischaemic heart disease burden and between 7% and 41% of certain cancer burdens are attributable to overweight and obesity.”

Further studies have shown that the chance of developing serious diseases increases with a raised BMI, including arthritis, several types of cancer (such as those of the breast, colon, and prostate, hypertension, high cholesterol, and heart failure.

One 2020 study, of 2.8 million adults, found evidence to show that the higher the BMI, the higher the risk of developing certain health conditions, including Type 2 Diabetes and sleep apnoea. Compared with healthy weight individuals, the study found:

BMI in kg/m2  Increased risk of Type 2 Diabetes Increased risk of sleep apnoea
25 – 30 more than double more than double
30 – 35 five times higher six times higher
35 – 40 nine times higher 12 times higher
40 – 45 12 times higher 22 times higher

The study also found that those with a BMI of 40-45 kg/m2 had triple the risk of heart failure, high blood pressure and dyslipidaemia, as well as a 50% higher risk of dying prematurely from any cause.

Other research studies have also shown that the chance of developing disease increases with a raised BMI, including conditions like arthritis, several types of cancer (such as those of the breast, colon, and prostate), hypertension, high cholesterol, and heart failure.

Risks associated with low BMI

Similarly, those with a BMI below the healthy range also have an increased risk of developing disease.

A BMI below the healthy range (<18.5kg/m2), classified as underweight, has been associated with developing certain conditions as the body’s immunity can be compromised by the lack of food and nutrient intake.

By having a weakened immune system, the risk of infection is heightened and being underweight increases the risk of:

  • Digestive disorders
  • Bone loss
  • Immune deficiency
  • Cardiac abnormalities (such as arrhythmia and heart failure)
  • Cardiovascular disease
  • Anaemia
  • Malnutrition
  • Osteoporosis
  • Anorexia
  • Susceptibility to infectious diseases

It’s also been found that those with lower BMIs risk a shorter life expectancy, according to a 2020 study.

Of 3.6 million adults in the UK, results showed that underweight men (below a BMI of 18.5kg/m2) had a shorter life expectancy by 4.3 years and underweight women had a shorter life expectancy by 4.5 years.

Most causes of death, including cancer, cardiovascular diseases, and respiratory diseases, had a J-shaped association with BMI, with the lowest risk occurring in the range 21–25 kg/m2.

Why is BMI used within healthcare?

As BMI is a simple, inexpensive, and non-surgical measure, it can be a useful screening tool within clinical settings.

The measure can quickly calculate body fat to assist practitioners and determine who may or may not be at risk of developing certain conditions based on their BMI.

BMI guidance can prompt further examination into an individual’s body composition, including fat mass, using additional measures including skinfold thicknesses, weight circumference, biochemical markers, measuring weight loss over time, and evaluating nutritional status.

Using BMI categories and additional methods, it’s possible to gauge the level of intervention required for those with high and low BMIs to help prevent disease. Interventions tend to involve general healthy eating and lifestyle advice, diet and physical activity and the use of drugs and/or surgery.

What are the limitations of BMI?

As helpful as BMI can be to identify weight and give a general idea of body fat within the population, it’s argued that BMI is an inaccurate assessment of an individual’s health.

BMI itself doesn’t include additional factors that may contribute to someone’s level of BMI, such as age, sex, ethnicity, and muscle mass, nor does it consider where that weight comes from – whether that’s down to excess fat, muscle, or bone mass.

As BMI is not a direct measure of adiposity, caution should be taken when interpreting BMI, and NICE guidelines suggest that using waist circumference in individuals with a BMI of less than 35 kg/m2 should be used as an additional measure.

As body composition varies from person to person, there are some general differences between genders and ages.

Females naturally carry more body fat than men, with 25% body fat classed as normal for women and 15% for men, and elders typically carry less muscle mass as age-related muscle loss, sarcopenia, occurs naturally with ageing. From the ages of 30, individuals usually begin to lose 3-5% muscle mass per decade.

BMI is also a less accurate measure of adiposity in individuals with a high muscular mass. Since muscle is much denser than fat, a very muscular person will weigh more, and so their BMI may be classed as ‘overweight’ or ‘obese’, despite their body fat percentage showing as below average.

Even if someone’s BMI is classified within the healthy range, as many as 20% of normal-weight adults could be metabolically unhealthy (increased visceral fat and fatty liver), which wouldn’t be captured by solely using BMI as a measure of risk. If unidentified, this can position an individual as high risk when it comes to developing disease, cardiovascular events and possibly death.

Similarly, certain ethnic groups of the population may be at higher risk of comorbidities at different BMIs.

For example, a BMI of 23 kg/m2 is usually considered healthy, but in adults of South Asian origin, the risk of diabetes is much higher at this BMI, and it’s advised that extra care should be taken when interpreting BMI in these groups.

So, should BMI be scrapped?

BMI is a complex topic of discussion.

It’s important to note that mental health and body image should be considered when it comes to using BMI as a measure of health.

Understanding the Women and Equalities Committee’s report and studies supporting associations between BMI and poor mental health, the stigma associated with BMI is an issue.

Investigations have demonstrated how BMI can contribute to weight stigma and perpetuates negative body image and mental health issues.

By presenting overweight and obesity as unfavourable, individuals risk suffering worse in the future by continuing to gain weight, raising their BMI, and forming unhealthy physiological and behavioural responses, including increased food consumption, disordered eating and reducing exercise.

However, in many respects, BMI is a valuable screening tool for practitioners to use quickly when determining a person’s disease risk.

It’s clear that BMI is not a single or perfect measure of health, fat, diet, and more, but it does provide a good basis for assessment. By the time an individual’s BMI is flagged as too high or too low, whilst intervention will be needed and should still be given, it may be too little too late.

Prevention or earlier intervention to reduce the risk of the associated health impacts from happening may be a better approach to take, and so a different measure from BMI is required to support this.

One crucial measure lies within nutritional history and analysing dietary intake. Using accurate nutritional analysis tools, like myfood24, professionals can monitor nutrient intake over time and identify common problems linked to poor nutrition to show early on that dietary changes need to be made to improve health and reduce the risk of comorbidities and diet-related disease.

These tools also help the individual to understand more about their balance of food and nutrients and its impacts on their health, to prevent too much or too little being consumed.

Overall, it’s important to understand the implications of BMI – its strengths, its limitations, and the effect it has on mental health and body image.

As a general healthcare practise, practitioners using BMI should approach individuals sensitively and carefully to lessen the negative impacts of weight stigma, and rather than relying on BMI as a conclusive measure of health, more factors, including dietary intake, should be considered.

When it comes to improving health and preventing disease, BMI might not be the ultimate solution, but we do need somewhere to start.

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