Measuring obesity: BMI, calories and beyond

Covid-19 has made us heavier. Surveyed this summer, 41 per cent of people reported that they have put on pounds since the first lockdown last March, prompting Public Health England to launch its latest Better Health campaign, which includes an updated NHS weight loss plan and an app to help us monitor our BMI.

Meanwhile the pandemic has made it harder for many to access healthy food, shrinking household incomes, making it harder to travel to supermarkets, and limiting choice.

All this brings Nesta’s own Healthy Life mission into even sharper focus – and also underlines that Body Mass Index is still the dominant metric for tracking the nation’s weight. At Nesta we’re using BMI too, as we embark on a decade-long drive to halve obesity rates in the UK. Research suggests we can do that by trimming average calorie intake by 250 a day, roughly the equivalent of one standard-sized Mars bar – and if we succeed it will add two years of healthy life expectancy for ten million Britons.

Our aim is to make the nation’s food environment healthier for everyone, reversing the rise in obesity and reducing health inequalities across the UK. This means making healthy options accessible, appealing and affordable for all. However there is no one standard metric for ‘food environments’, so we’re reliant on measures of people’s health, particularly population-level obesity rates, to help us understand their impact.

Ours is an ambitious goal, and one that can only be achieved if we adopt hard-and-fast measures that are universally understood and can be shared. Measurement is critical to help us define the challenge ahead and identify where disproportionate health burdens lie – by identifying the links between excess weight and economic deprivation, perhaps, or variations by age, gender and ethnicity. It will also help us target our work: for example we might use data that explores regional differences in obesity rates to identify locations for a trial project or experiment. And it will allow us to track our impact: for example we’re already developing an online food takeaway platform to test how we might nudge customers toward choosing lower-calorie meals.

However, there’s a problem here, and it needs to be addressed. While the calorie has long been a standard unit for monitoring food consumption, and BMI is a decades-old diagnostic tool for determining healthy weight, both are imperfect measures and can cause problems when used inaccurately – and BMI in particular has fallen increasingly into disrepute. Just this April, parliament’s Women and Equalities Committee said BMI was dangerous and its use should be discontinued. So how can we argue that they still have merit?

Surprising as it may seem, the BMI formula dates back almost two centuries: it was invented by the Belgian mathematician Adolphe Qeutelet as a means of measuring obesity on a population level. Since its adoption by medical science in the 1970s, it has become the most common tool for classifying whether a person has body fat levels which put them at risk of ill-health and disease. Its advantage is simplicity: by calculating a person’s height-to-weight ratio, it places them in one of four categories: underweight (with a BMI below 18.5); healthy weight (18.5-24.9); overweight (25-29.9); or obese (30-plus). It is used not only by healthcare practitioners to help diagnose patients, but by researchers and policymakers looking to design programmes to improve the nation’s health. We know that on average BMI has been climbing in the UK over recent decades, with the mean figure in adults rising from 25.8 in 1993 to 27.6 in 2019.

At first glance, calories may seem less contentious: a calorie is simply a unit of energy, and if we consume more than we need to live, move and grow, we store the excess primarily as fatty tissue. The calorie first moved from physics to nutrition in the late 1800s, and calorie-counting has become popular worldwide as a means to manage people’s weight. Accurate data on UK calorie consumption is hard to come by, and while some studies suggest there’s been a reduction in average intakes, a 2016 report from the Behavioural Insights Team debunked that. It points to a rise in under-reporting, perhaps caused by people’s self-consciousness about their size, the public shame associated with calorie counting or the general trend toward eating more meals outside the home.

The case against BMI

Despite its widespread use, BMI has been under fire for decades. It does not distinguish between fat and muscle, and doesn’t take into account how fat is distributed around the body. Studies suggest it’s an imperfect predictor of body fat levels, making it ripe for misclassifying people as overweight. Elite athletes can have BMIs of 30, for example, but negligible body fat, whereas older people who’ve lost muscle mass might be carrying lots of fat but register a low BMI. A more useful indicator, it’s been suggested, is how people carry weight: research shows that body fat around the waist could pose a higher risk to health than around the thighs and bottom, as it indicates more fat around vital organs.

What’s more, BMI was never designed to take into account age, gender and ethnicity. For example men and women carry fat differently, and one study found that BMI’s ability to diagnose obesity is especially compromised in men and in the elderly. And BMI can’t be applied universally to all ethnicities, as populations differ physiologically and have varying risk levels for obesity-related diseases, which BMI does not take into account. Research has shown that for the same BMI rating, people of African ethnicity appear likely to carry less fat, and people of South Asian ethnicity more fat, than the general population. It’s why a lower threshold for the overweight classification has been recommended by the UK’s South Asian Health Foundation. Data is scarce on BMI across ethnic groups, but more tailored approaches are clearly needed to accurately identify unhealthy weight categories and support everyone to live a healthier life.

BMI has also come under attack over its clinical use in identifying the risk of eating disorders. Conditions such as bulimia or binge eating don’t necessarily cause individuals to shift into particular BMI categories – so there’s a danger people could go undiagnosed and miss out on vital treatment.

Finally there is a question of psychology. Defining people crudely by their BMI band can have a negative impact on mental health, especially in the context of society's changing attitudes about acceptable body shapes.

The case against calories

A scientific measure it may be, but not all calories are created equal. Merely totting up the energy levels in food does not account for the range of nutrients, carbs, proteins and fats we all need to maintain a healthy diet – and research tells us a meal’s calorie count is not a very sophisticated way to assess its healthiness. The source of a calorie affects how you digest and retrieve energy from it – for example metabolising protein takes more energy, so diets high in fat and refined carbohydrates tend to prompt more weight gain than high-protein ones. Different foods also affect how hungry we feel and whether our bodies burn energy or store fat, so people consuming similar calorie counts can end up gaining or losing very different amounts of weight.

As well as playing off the main food types, we must also think about the range of micronutrients we’re consuming, including the vitamins and minerals we need to maintain a strong immune system and overall health. And counting calories not only undervalues the importance of a varied diet, it is bad for our overall mental wellbeing, particularly for those with eating disorders. It’s why the Government’s plan to display calories on restaurant menus, for example, has provoked criticism for its potential to cause harm.

Weighing the positives

No question about it, both BMI and calories are blunt instruments when measuring an individual’s risk of overweight and the health risks that can follow, and alternative metrics may offer more accuracy. They include waist to height ratio, underwater weighing to measure body density and composition, and waist circumference, which has been independently linked with diabetes risk.

At Nesta we believe BMI and calories shouldn’t be the sole tools used by practitioners, nor the starting point for interventions – they should only ever be considered alongside other factors which give a broader picture of an individual’s health, taking into account age, gender, ethnicity, body composition and lifestyle. BMI isn’t alone in this, standardised measures in other areas are often criticised for not capturing the nuance of individual experiences, for example using standardised test scores to measure children’s learning. We also know from years of experience that putting people into weight bands and focusing mainly on individual action will not reduce obesity long-term – which is why Public Health England’s new campaign focusing on fitness plans and BMI apps may once again be doomed to fail. The key to success will instead be innovative thinking and radical policy change that improve the UK’s wider food environment, making healthy choices more affordable and appealing for everyone.

So why are we using BMI and calories at all? Because although they may be imperfect indicators at an individual level, and even counterproductive, they remain useful when probing the wider picture on obesity across the UK. BMI data can be easily gathered at a GP surgery, clinic or at home, giving robust population-level datasets such as the Public Health England Obesity Profile. Data on other measures, such as waist circumferences, is more intrusive to collect and much less widely available. And because of BMI’s long pedigree, we’ve decades of information to paint a picture of how it relates to health outcomes for different groups of people, analysing trends over time.

More importantly, and despite all its drawbacks, research does reveal that BMI measures up well when assessing obesity levels on a population-wide scale, especially when used alongside other methods. Similarly, calorie intake remains useful as a general indicator of healthy eating and a good predictor of obesity across a population – and again it’s simpler data to collect than the nutrient breakdowns of individual diets. It can be found in the National Diet and Nutrition Survey, for example, which makes calories a worthwhile measure when evaluating which interventions might work to reduce obesity overall.

What does this mean for Nesta?

We are not interested in telling individuals they are obese and need to lose weight – evidence exposes this approach as counterproductive. Instead we aim to tackle the issue by making healthy eating more available and accessible to all. That means we’re interested in measuring obesity at the population level, and in that context both BMI and calories are fit for purpose.

Nonetheless, we want to be part of the solution, learning from others who are trying out alternative measurement approaches. Where data exists or we can collect it for ourselves, we plan to explore the potential of complementary measures like waist circumference. Where we’re training our efforts on particular social groupings, we will investigate whether adjusted BMI thresholds would be appropriate. And when designing or evaluating interventions, we’ll seek to calculate not only the total calorific value of food but where those calories come from – by using the Department of Health’s Nutrient Profiling Model, for example, which scores foods based on total energy plus saturated fat plus sugar plus sodium.

Finally, where there’s any question our work might negatively impact people with eating disorders, we’ll use self-reported surveys like the Eating Disorder Examination Questionnaire (EDE-Q), rather than relying on BMI.

We know very well that BMI and calories are imperfect, but they give us a starting point to set about transforming food environments and increase life expectancy for those most affected by health inequalities – it is a national emergency which won’t brook delay.

Could we do more?

Nesta is interested in interventions which bring positive health impacts for all. We want to develop new ways to measure healthy food environments and share that data with policymakers, researchers and communities to bring about change. Are you working in this area? Get in touch and let us know what else you think we should be considering to measure obesity and track progress towards our goal: email [email protected].


Lucy Turner

Lucy Turner

Lucy Turner

Senior Analyst, healthy life mission

Lucy is a senior analyst on the healthy life mission team.

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