Understanding BMI and Setting Realistic Weight Loss Goals
Body mass index is the most widely used screening measure for excess weight in UK clinical practice, and it is the primary eligibility criterion for GLP-1 weight loss medication. But BMI has significant limitations that every patient should understand before using it as their sole measure of success. This guide explains what BMI is and what it measures accurately, where it falls short, how to set realistic and clinically meaningful weight loss goals, and what a healthy rate of weight loss actually looks like on medication.
What BMI Measures and How It Is Calculated
Body mass index is calculated by dividing a person’s weight in kilograms by the square of their height in metres (kg/m2). The result is a single number that provides a population-level indicator of whether a person’s weight is in a healthy range relative to their height. The UK NHS and NICE use the following classification thresholds for adults of White, Black, and mixed ethnicity: below 18.5 kg/m2 is underweight; 18.5 to 24.9 kg/m2 is healthy weight; 25.0 to 29.9 kg/m2 is overweight; 30.0 to 34.9 kg/m2 is obesity Class I; 35.0 to 39.9 kg/m2 is obesity Class II; and 40.0 kg/m2 or above is obesity Class III (sometimes referred to as severe or morbid obesity).
For adults of South Asian, Chinese, Japanese, and other Asian ethnic backgrounds, NICE recommends using lower BMI thresholds, as the same cardiovascular and metabolic risks occur at lower BMI values in these groups. The adjusted thresholds typically used are: overweight at or above 23 kg/m2; and obesity at or above 27.5 kg/m2.
The Significant Limitations of BMI
BMI does not distinguish between fat mass and lean mass (muscle and bone). A highly muscular individual may have a BMI in the overweight range despite having very low body fat. Conversely, an individual with a BMI in the normal range can have excess visceral (abdominal) fat and significant metabolic risk, a pattern sometimes called normal-weight obesity or TOFI (Thin Outside, Fat Inside). BMI also does not account for fat distribution: abdominal fat carries substantially higher cardiovascular and metabolic risk than fat stored in the hips and thighs, but BMI treats both equivalently.
| WHAT BMI MEASURES REASONABLY WELL | WHAT BMI DOES NOT CAPTURE |
| Population-level screening for excess weight. Eligibility criteria for clinical weight management interventions. Approximate correlation with cardiovascular and metabolic risk at a group level. A baseline measure to track change over time. | Body composition (fat vs muscle ratio). Fat distribution (visceral vs subcutaneous). Metabolic health (blood pressure, cholesterol, blood glucose). Fitness level, cardiovascular capacity. Ethnic group risk differences without adjustment. |
Setting Clinically Meaningful Weight Loss Goals
The clinical target for meaningful health benefit from weight loss is a reduction of five to ten percent of initial body weight, sustained over time. This modest weight loss produces disproportionately large benefits: a five percent reduction in body weight reduces triglyceride levels, improves insulin sensitivity, lowers blood pressure, and reduces the risk of progression from pre-diabetes to type 2 diabetes. A ten percent reduction produces further improvements in cholesterol, liver function, joint load, and sleep apnoea severity.
For patients on GLP-1 medication such as semaglutide or tirzepatide, the average weight loss achievable in clinical trials substantially exceeds this five to ten percent threshold, making clinically meaningful health improvements highly achievable. Setting an initial goal of five percent of body weight within the first three months is a realistic and evidence-based target for most patients.
What a Healthy Rate of Weight Loss Looks Like
NICE guidance and NHS clinical standards consider a weight loss rate of 0.5 to one kilogram per week (approximately one to two pounds) to be a safe and sustainable rate for most adults. More rapid weight loss, while sometimes achievable on higher doses of GLP-1 medication, is associated with a higher proportion of lean mass loss if dietary protein intake is insufficient. Patients on GLP-1 medication at Curelo Care should aim for a gradual, steady reduction rather than maximising short-term loss at the expense of muscle preservation.
A weight loss of five to ten percent of your starting body weight is the clinical threshold at which meaningful and measurable improvements in cardiovascular risk, blood glucose, blood pressure, and joint health become evident. For a person who weighs 100 kg, this means a target of five to ten kilograms as the first meaningful milestone, not a target shaped by clothing sizes or aesthetic goals. Using percentage of body weight as your primary measure is more clinically meaningful than kilograms alone.
Clinical Disclaimer: This guide is for general informational purposes only and does not constitute personalised medical advice. Curelo Care is regulated by the General Pharmaceutical Council (GPhC). All weight management medication supplied by Curelo Care is provided following a structured online clinical assessment and prescriber review. If you have any questions about your treatment, please contact our clinical team.
