Obesity and BMI: what the numbers mean

How the number is worked out

Body mass index is your weight in kilograms divided by the square of your height in metres. Someone weighing eighty kilograms and standing one metre seventy-five has a body mass index of about twenty-six. Any online calculator does the arithmetic, and so does a pharmacy.

Measure rather than estimate. Adults commonly overstate their height by a couple of centimetres, and because height is squared in the formula, a small error there moves the result more than a similar error in weight does. Weigh yourself at a consistent time of day on the same scales.

The number was designed to describe populations, and that is where it works best. Applied to one person it is a starting point for a conversation, which is all it was ever meant to be.

What the bands mean

The bands conventionally used for adults are: below 18.5 underweight; 18.5 to 24.9 healthy weight; 25.0 to 29.9 overweight; and 30.0 and above obesity, which is then commonly divided into three classes as the number rises further.

The bands are cut-offs on a continuous scale, which has two consequences people miss. Nothing changes biologically as you cross a line, so 29.8 and 30.1 are the same situation described with different words. And where you sit within a band matters: the risk associated with 39 is not the risk associated with 30.

The bands also do not apply as written to everyone. For people of South Asian, Chinese and some other backgrounds, health risks appear at lower values, and lower thresholds are used in practice. They do not apply to children and adolescents at all, who are assessed against age and sex charts, nor during pregnancy, nor sensibly in adults over about sixty-five, where a little extra weight is not associated with the same risk.

What body mass index does not measure

It does not distinguish muscle from fat. A muscular person can land in the obesity band while carrying little fat, which is the objection most often raised and is entirely valid for that individual.

It does not say where the fat is, and location matters more than total quantity. Fat around the abdomen and around the organs carries more metabolic risk than the same amount on the hips and thighs, and two people with an identical index can have very different risk profiles for that reason alone.

It says nothing about fitness, diet quality, blood pressure, blood sugar, lipids, sleep or how you feel, all of which affect health independently. Someone active with a raised index can be metabolically healthier than someone inactive within the healthy band.

Used sensibly, none of this makes the measure useless. It makes it one line in a picture, which is why a doctor looks at it alongside the measurements in the next section rather than on its own.

Which other numbers add something

Waist circumference is the most useful addition, because it captures the abdominal fat that the index ignores. Measure at the midpoint between the lowest rib and the top of the hip bone, against the skin, at the end of a normal breath out. The thresholds that indicate raised risk differ by sex and by ethnic background, and a doctor will use the ones that apply to you rather than a single figure.

Waist-to-height ratio is a simple alternative that works reasonably across groups: many clinicians use the rule of thumb that your waist should be less than half your height. It needs a tape measure and nothing else.

The measurements that actually decide treatment are the ones that show what the weight is doing: blood pressure, blood sugar or glycated haemoglobin, a lipid profile, and liver tests. Sleep apnoea, joint pain, reflux, fertility and mood belong in the assessment too, because they are often what the patient came about.

Body composition scales that claim to report fat percentage are not accurate enough to make decisions on, though the trend they show over months can still be motivating.

Why weight is treated as a medical matter

Excess weight, and abdominal fat in particular, is associated with type 2 diabetes, high blood pressure, raised cholesterol and triglycerides, fatty liver disease, obstructive sleep apnoea, osteoarthritis of the knees and hips, gallstones, several cancers, reduced fertility and low mood. The association is strongest with abdominal fat, which is the part that tends to respond first to treatment.

That last point is the encouraging one. Modest weight loss, sustained, improves blood sugar, blood pressure and sleep apnoea measurably, and it does so well before anyone reaches a band on a chart. The clinical target is rarely a particular index; it is the improvement in the things the weight is driving.

Obesity is now understood as a long-term condition with strong biological regulation rather than a simple matter of willpower. Appetite, satiety and energy expenditure are defended by the body, which is why weight regain after a diet is the rule rather than a personal failure, and why treatment is thought of as ongoing rather than as a course.

What treatment involves

The foundation is unchanged and unglamorous: a sustainable change in what and how much you eat, regular physical activity including some resistance work to preserve muscle, attention to sleep, and support for the behavioural side. Structured programmes do better than advice alone, and keeping records of intake is one of the few habits consistently associated with success.

Medicines are an addition to that, not a replacement for it. The GLP-1 medicines have changed what is achievable: our pages on /medicines/wegovy/, /medicines/saxenda/ and /medicines/mounjaro/ describe the individual products, and /weight-loss-injections/ explains how the group works, what the common side effects are and what happens when treatment stops. Each product has a licensed indication set out in its own product information, which includes a weight-related threshold together with other conditions; a doctor checks those against your case rather than against a general figure.

Weight regain when treatment stops is the rule rather than the exception, and any honest conversation about these medicines includes it. Nausea and other digestive effects are common early on and are the usual reason people stop.

Surgery remains the most effective option for some people, is arranged through specialist services in Malta, and is not something any assessment on paper can start.

What to be wary of

Injectable weight loss medicines are now sold by sellers who do no assessment at all, and by sellers offering compounded or unlicensed versions. Those products sit outside the authorised supply chain that the Medicines Authority oversees, and the risk is not theoretical: the strength in the vial, the sterility and the device are all unverifiable.

Be equally sceptical of supplements marketed for weight loss, of detox programmes, and of anything promising a rate of loss. Very low calorie diets have a place, but under supervision rather than from a website, particularly for anyone taking medicines for diabetes or blood pressure, where doses often need to change as weight falls.

One medical point that gets missed: if you take insulin or a sulfonylurea, or medicines for blood pressure, losing weight can make those doses too high, and the result is hypoglycaemia or dizziness. Tell the doctor managing those medicines that you are starting to lose weight.

Finally, weight that is falling without your trying is not good news and needs investigating, not celebrating.