Can you tell how healthy you are from your weight?

Why weight isn't the whole picture

Weight is often treated as shorthand for health. We are encouraged to calculate our body mass index (BMI), are constantly being exposed to images of a “healthy” aesthetic, and may judge our own success by the number on the scales. But what if we are measuring the wrong thing?

There are plenty of good reasons to pay attention to our weight. Carrying excess body fat can increase the risk of conditions including type 2 diabetes, cardiovascular disease, fatty liver disease, and a number of cancers.

However, weight cannot tell us everything about our health, it cannot show how much of the body is fat or muscle, where fat is stored, or whether blood glucose, blood pressure, and cholesterol are healthy.

Why metabolic health matters

This is where metabolic health comes in. Metabolic health describes how well your body is able to regulate blood glucose, blood fats, blood pressure, and energy.

Poor metabolic health can develop without obvious symptoms, which is one reason weight is so often used as a convenient, but incomplete, way of estimating risk.

Two people of the same height and weight can have very different metabolic health. Equally, someone with a BMI in the healthy range may have metabolic issues, while a person with a higher BMI may currently have healthy metabolic markers.

For this reason, looking at metabolic health alongside weight can give us a more meaningful picture of our health and risk than weight alone.

What can BMI tell you?

BMI compares weight with height, and is commonly used to categorise adults as underweight, healthy weight, overweight, and obese. It is quick, inexpensive and useful for identifying patterns of health risk across large populations.

For an individual, however, BMI is better regarded as a screening tool than a diagnosis. Like weight alone, it cannot distinguish fat from muscle or tell us where body fat is stored.

A muscular person may have a high BMI despite carrying relatively little body fat, while someone else may have a BMI within the healthy range but more body fat and less muscle than the number suggests.

The reason this matters is because muscle plays such an important role in metabolic health. It is one of the main places where sugar (glucose) is taken up from the bloodstream, helping the body respond to insulin and keep blood sugar steady.

Importantly, BMI also needs to be interpreted in the context of age, sex, and ethnicity - the same BMI is not always equal. People from some ethnic backgrounds - including South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds - often develop metabolic conditions at a lower BMI.

The National Institute for Health and Care Excellence (NICE) therefore recommends using lower BMI thresholds when assessing health risk in these groups.¹

Why does the location of body fat matter?

Subcutaneous fat, which is stored beneath the skin, is different from visceral fat, which is found deeper in the abdomen around internal organs.

Higher levels of visceral fat are associated with insulin resistance, raised blood pressure, abnormal blood fats and cardiovascular disease, independently of BMI.² This means that two people with the same BMI may have different metabolic risks depending on where they carry fat.

Waist-to-height ratio can be a helpful alternative to BMI when assessing metabolic risk. NICE advises adults to aim for a waist measurement of less than half their height.¹ For example, someone who is 170 cm tall would aim for a waist circumference below 85 cm.

Can you have poor metabolic health at a healthy weight?

You can have a BMI in a healthy range and still have poor metabolic health.

Some people still develop insulin resistance, abnormal blood fats, high blood pressure, or fatty liver disease at a relatively low weight.

Being metabolically unhealthy is associated with a higher risk of type 2 diabetes and cardiovascular disease, even in people with a BMI within the healthy range. A slim appearance should not therefore be equated to good health.

Can you have a higher weight and good metabolic health?

Some people living with overweight or obesity have healthy blood glucose, blood pressure, and blood fats, with no current evidence of metabolic disease.

Whilst their risk of diabetes and cardiovascular disease may be lower than in people of a similar weight who already have metabolic abnormalities, it may still be higher than that of people with both a lower BMI and healthy metabolic markers.³

Crucially, metabolic health isn’t static and can change over time. Normal results today do not guarantee ongoing health, but equally poorer metabolic health can be improved by changes in diet, lifestyle, and with certain medications.

It’s important not to make assumptions in either direction. Weight and metabolic health are not synonymous.

Which measurements tell us more about metabolic health?

So, if BMI alone isn’t a reliable marker of health, what can help to paint a fuller picture?

There are numerous factors used to assess metabolic health.

What about insulin resistance tests?

Early insulin resistance doesn’t always show up on tests such as blood glucose or HbA1, as the pancreas can initially compensate by producing more insulin.

HOMA-IR

A Homeostatic Model Assessment of Insulin Resistance (HOMA-IR ) is a test that compares fasting insulin to fasting glucose, with a higher result suggesting that the body needs more insulin to keep glucose under control.

Whilst HOMA-IR is used widely in research and is available through some specialist and private services, it is not currently offered routinely in NHS primary care.

OGTT

An oral glucose tolerance test (OGTT) shows how effectively your body deals with a measured glucose drink. After fasting overnight, your blood glucose is checked before the drink and again two hours afterwards, revealing any impaired glucose tolerance.

It is available on the NHS when clinically indicated and is most commonly used to test for diabetes during pregnancy.

Look beyond weight when measuring progress

Whilst weight and metabolic health cannot be used interchangeably, weight loss may improve several aspects of metabolic health for someone carrying excess body fat. But it’s important to remember that the number on the scales is not the only sign of progress.

Being more physically active can improve insulin sensitivity, fitness, and blood pressure, while strength-based activity helps preserve or build muscle. Changes to diet, sleep, smoking, or alcohol intake can also improve blood glucose and cholesterol.

Some of these benefits can occur even when weight loss is modest, so it’s worth making lifestyle changes even when weight doesn’t change significantly.⁴

Other measures of progress might include a smaller waist, lower blood pressure, improved HbA1c or cholesterol, and better strength, fitness, sleep, or energy.

What to remember

• Weight can help identify health risk, but it’s not a direct measure of metabolic health.

• It does not tell us enough about body composition, fat distribution or how well the body is regulating glucose, blood pressure, and blood fats.

• The clearest picture comes from combining weight and waist measurements with appropriate metabolic markers, personal risk factors, and changes over time.

• Weight certainly plays a role, but alone it cannot tell the whole story.