How sarcopenic obesity is diagnosed: beyond body mass index
For years, body mass index — commonly known as BMI — played a central role in the assessment of obesity. Its statistical utility and simplicity made it a widely used tool in both clinical medicine and public health. However, contemporary metabolic medicine has begun to question an idea that for decades seemed sufficient: that body weight could summarise a person’s true state of health.
Because the human body is far more complex than a single figure.
“For many years, we reduced obesity to a question of weight,” explains Dr Susana Monereo, a specialist in endocrinology and head of the Obesity and Cardiometabolic Risk Unit at the Públio Cordón Memorial Hospital. “Today we understand that body composition, visceral fat and muscle mass have a huge impact on metabolic health and ageing.”
Two people may have exactly the same weight, the same height and even the same BMI, and yet have profoundly different metabolic profiles. One may retain adequate muscle mass and a relatively healthy distribution of body fat. The other, by contrast, may accumulate visceral fat whilst progressively losing muscle. At first glance, they might appear similar. Clinically, they are not.
This phenomenon defines a condition of growing importance in preventive medicine and healthy ageing: sarcopenic obesity.
What we understand by obesity today
For decades, obesity was defined almost exclusively in terms of body weight and body mass index. However, modern medicine understands obesity in a much more complex way.
It is now considered a chronic condition involving metabolic, hormonal, inflammatory and functional changes that go far beyond the visible accumulation of body fat.
Modern metabolic medicine recognises that obesity does not depend solely on how much a person weighs, but also on the amount of body fat, how it is distributed and the impact it has on organs, metabolism and physical function.
In advanced body composition analysis, body fat percentages persistently above 25 per cent in men and 35 per cent in women are usually associated with a significant increase in cardiometabolic risk. However, the real problem is not just how much adipose tissue there is, but where it accumulates.
Visceral fat — the fat surrounding internal organs such as the liver, pancreas or intestines — is associated with intense inflammatory and metabolic activity. In many body composition studies, areas of visceral fat exceeding 100 cm² are associated with an increased risk of insulin resistance, type 2 diabetes, cardiovascular disease and progressive metabolic deterioration.
“There are patients who are not extremely obese in terms of weight and yet show significant metabolic abnormalities,” notes Dr Monereo. “The distribution of body fat and the progressive loss of muscle often provide a better explanation of metabolic risk than the scales.”
It is precisely within this new understanding of obesity that sarcopenic obesity stands out.
Sarcopenic obesity: when muscle begins to lose quality
Sarcopenic obesity combines two processes that were long studied separately: an increase in adipose tissue and a progressive decline in muscle mass and function.
Far from cancelling each other out, these two phenomena tend to reinforce one another. Muscle loss reduces energy expenditure and encourages a sedentary lifestyle; at the same time, visceral fat maintains a chronic inflammatory state capable of further deteriorating muscle and accelerating metabolic decline.
The result may go unnoticed for years.
Some patients maintain seemingly stable body weight figures whilst their bodies slowly begin to lose muscle quality, strength and metabolic capacity.
And therein lies one of the main limitations of BMI.
Body mass index does not distinguish between fat, muscle and bone. Nor does it provide information on a crucial factor in the medicine of ageing: the functional quality of muscle.
In an international consensus published in *Clinical Nutrition*, the European scientific societies ESPEN and EASO specifically warn that BMI is insufficient to adequately identify sarcopenic obesity and recommend incorporating body composition and functional assessment tools into clinical diagnosis.
Muscle as a metabolic organ
One of the most significant developments in modern metabolic medicine has been the realisation that muscle is not merely for walking, supporting our bodies or engaging in physical activity.
We now know that it also functions as a metabolic and endocrine organ, playing a decisive role in glucose regulation, insulin sensitivity, inflammatory balance, energy expenditure and even cardiovascular health.
An article recently published in *The Lancet Diabetes & Endocrinology* highlights that muscle is actively involved in functions as diverse as immune regulation, vascular health and the maintenance of energy metabolism. The authors summarise this idea with a particularly revealing phrase: “Muscle matters”.
“Muscle loss is not just a question of strength,” explains Dr Monereo. “Many patients begin to notice tiredness, reduced exercise tolerance or difficulty in regaining functional capacity long before they notice significant changes in weight. In reality, the body may be progressively losing metabolic quality.”
Body composition and metabolism: a new way of assessing obesity
Precision medicine has driven a profound change in the way metabolic health is studied. Today, the focus is on understanding the body’s actual composition: how much muscle the patient retains, where body fat is distributed, and how these parameters evolve over time.
This analysis offers a far more sophisticated view of metabolism.
Because when muscle mass decreases, the impact goes beyond aesthetics. It changes the way the body utilises glucose, regulates inflammation, responds to physical exertion and maintains physical autonomy.
Thanks to advanced multi-frequency bioimpedance systems, it is possible to observe something that for years remained hidden behind a simple weight figure: how muscle is actually distributed, where visceral fat begins to accumulate, and what metabolic changes start to develop even before any obvious symptoms appear.
“Body composition often reveals a metabolic reality that is completely different from what the scales show,” says Dr Monereo. “There are patients with seemingly normal weights who already have advanced muscle loss and excess visceral fat.”
Advanced bioimpedance: much more than just a set of scales
Multi-frequency electrical bioimpedance has become one of the most useful tools for assessing body composition in advanced metabolic medicine. Although many people regard it as simply a ‘smart scales’, its diagnostic capabilities go far beyond body weight.
These systems work by using very low-intensity electrical currents that pass through the body imperceptibly and safely. Based on the resistance offered by different tissues to the passage of this current, the device can estimate how much muscle mass is present, what proportion corresponds to body fat, how body water is distributed, and where visceral fat begins to accumulate.
The physiological principle is relatively simple: as muscle contains large amounts of water and electrolytes, it conducts electricity better; fat, on the other hand, offers greater resistance. By analysing these differences in various body segments — the torso, arms and legs — advanced systems enable the creation of a much more accurate metabolic map of the body.
And therein lies one of its greatest clinical benefits.
This is because two people of the same weight may have completely different body compositions: one may retain adequate muscle mass and low levels of visceral fat; the other, by contrast, may show progressive muscle loss and metabolic changes that are not yet visible on the scales.
“Body composition often reveals a metabolic reality that is completely different from that indicated by weight,” points out Dr Susana Monereo. “There are patients with seemingly normal figures who are already showing advanced muscle loss and excess visceral fat.”
In advanced body composition analysis, body fat percentages persistently above 25 per cent in men and 35 per cent in women are usually associated with an increased cardiometabolic risk, although the interpretation must always be tailored to the individual’s age, fat distribution and clinical context.
Furthermore, various studies have shown a strong correlation between advanced multi-frequency bioimpedance systems and gold-standard techniques used to assess body composition, such as dual-energy X-ray absorptiometry (DXA).
Although DXA continues to be regarded as a gold standard and is of great value in research and advanced metabolic assessment, advances in clinical bioimpedance have made it possible to incorporate body composition assessments that are far more streamlined, accessible and easily repeatable in everyday clinical practice.
Muscle strength and functional tests
One of the most interesting aspects of sarcopenia is that a loss of strength can occur even before muscle loss becomes evident in body composition tests.
For this reason, modern clinical assessment incorporates functional tests that are seemingly simple but of enormous diagnostic value.
One of the most widely used is handgrip dynamometry, which measures handgrip strength using a specialised device. Although it may seem like a simple test, numerous studies have shown that handgrip strength is closely related to frailty, functional autonomy and the risk of physical decline.
The European EWGSOP2 consensus states that a grip strength of less than 27 kg in men and 16 kg in women may suggest clinically significant muscle weakness.
Walking speed and the ability to repeatedly stand up from a chair without using the arms are also assessed; these tests provide a simple way of estimating how the muscles are actually functioning in everyday life.
“The loss of muscle strength affects mobility, independence and quality of life,” notes Dr Monereo. “And it often begins long before there is any noticeable weight loss.”
Muscle ultrasound and muscle quality
Modern medicine is no longer concerned solely with the quantity of muscle, but also with its quality.
In this context, muscle ultrasound has taken on an increasingly important role. These techniques allow us to observe phenomena such as fat infiltration within the muscle, a finding that is particularly common in sarcopenic obesity and metabolic ageing.
Unlike other tests that focus mainly on muscle volume, ultrasound allows the architecture of the muscle to be analysed directly and functional alterations to be detected even before there is any obvious muscle loss.
Put simply, it is not always just a question of how much muscle there is, but of the quality of that muscle.
“We are paying increasing attention to muscle quality,” explains Dr Monereo. “Fat infiltration within the muscle can alter its metabolic and mechanical function even before the patient notices a clear loss of muscle mass.”
Furthermore, ultrasound offers particularly valuable advantages in clinical practice: it does not use radiation, is quick and dynamic, and allows for straightforward monitoring of changes over time.
According to the European EWGSOP2 criteria, values for the appendicular muscle mass index below 7 kg/m² in men and 5.5 kg/m² in women may suggest clinically significant low muscle mass.
Losing fat without losing muscle: the new challenge for metabolic medicine
The emergence of new pharmacological treatments for obesity has profoundly transformed metabolic medicine in recent years. However, specialists are increasingly emphasising a fundamental point: not all weight loss necessarily leads to a complete improvement in metabolic health.
When a person loses weight rapidly, the body does not lose fat alone. Part of that weight loss may also correspond to lean body mass and muscle tissue, particularly if there is no comprehensive approach that includes adequate nutrition, strength training and specialised metabolic monitoring.
A sub-analysis of the STEP-1 trial found that, although semaglutide improved body composition overall, part of the weight loss involved a reduction in fat-free mass.
This phenomenon is particularly significant in people with sarcopenic obesity or at risk of muscle frailty, where preserving muscle becomes just as important a goal as reducing visceral fat. “We now know that the real goal is to preserve muscle, maintain functional capacity and improve the body’s metabolic health throughout the process,” summarises Dr Monereo.
Much more than just an aesthetic issue
Sarcopenic obesity reflects one of the major conceptual shifts in modern metabolic medicine: the realisation that physical health does not depend solely on weight, but also on muscle quality, fat distribution and the body’s functional capacity.
Modern medicine no longer seeks merely to alter the figures on a set of scales. It seeks to preserve strength, independence and quality of life.
This approach is particularly important in an increasingly long-lived society, where the real challenge lies not merely in living longer, but in reaching that age with greater metabolic, functional and physical strength.
The information in this article is provided for general awareness and does not replace an individualised medical assessment.
