Lipodensitometry and BMI: what are the differences and why does it matter?

Key takeaways

• BMI is a calculation based on weight and height. It helps assess body size, but does not provide information about body composition ;
• Lipodensitometry is a medical imaging exam that precisely measures fat mass, muscle mass, and bone mass ;
• A person can have a normal BMI and excess visceral fat. This is what lipodensitometry can detect ;
• Visceral fat, located around the organs, is more strongly associated with cardiometabolic risk than subcutaneous fat ;
• Normal body fat levels vary depending on sex, age, and clinical context. Only your doctor can interpret your results ;
• Bioelectrical impedance analysis and lipodensitometry both measure body composition, but they rely on different principles and offer different levels of precision ;
• Lipodensitometry is an exam performed with a doctor’s prescription and is available at Radiologie Dix30 clinic in Brossard.

Lipodensitometry and BMI: what exactly are they?

BMI and lipodensitometry can both help assess metabolic health, but they are fundamentally different in nature. 

Body mass index (BMI) is a mathematical calculation. It is obtained by dividing weight (in kilograms) by height squared (in meters). The result places a person into a body weight category : underweight, normal weight, overweight, or obesity. It is a simple tool, widely used to assess cardiometabolic risk. The Canadian classification, established by Health Canada, defines a normal BMI as between 18.5 and 24.9 kg/m². These thresholds apply to adults aged 18 and over, excluding pregnant or breastfeeding women.

Lipodensitometry, on the other hand, is a medical imaging exam. It is based on dual-energy X-ray absorptiometry. A very low-intensity X-ray beam scans the body and allows software to distinguish three separate compartments : bone mass, muscle mass, and fat mass. It provides an objective and precise measurement of body composition, whereas BMI offers only a weight-to-height ratio.

📖 Definition — Cardiometabolic risk : the combined risk of developing cardiovascular disease (heart attack, stroke) and metabolic disorders (type 2 diabetes, metabolic syndrome). It is assessed based on several combined factors : excess visceral fat, high blood pressure, dyslipidemia (an abnormal level of lipids in the blood), elevated blood sugar, and physical inactivity. The more of these factors are present, the higher the overall risk. 

Why is BMI not always enough to assess metabolic health?

BMI does not distinguish muscle from fat. Two people of the same height and weight can have very different body compositions. One may have high muscle mass with little fat. The other may, by contrast, have little muscle and a high amount of fat. Their BMI will be identical, but their metabolic risk profiles will be very different.

This limitation is especially evident in two situations :

• Very muscular people (athletes, manual workers) may have a BMI in the overweight range even though their body fat level is completely normal ;
• People referred to as “skinny fat” — meaning slim individuals with low muscle mass and excess visceral fat — may have a normal BMI while still presenting a high metabolic risk.

📋 Good to know — Health Canada recommends supplementing BMI with other indicators, particularly waist circumference, to better assess the health risks associated with excess abdominal fat.

What lipodensitometry reveals that BMI does not show

Where BMI provides a single number without distinguishing how tissues are distributed, lipodensitometry measures total fat mass, muscle mass, and bone mass. It also identifies where fat is located (visceral or subcutaneous), which has a major impact on cardiometabolic risk.

It also makes it possible to track changes in body composition over time. During a weight loss program or after bariatric surgery, weight may plateau while body composition improves. Muscle mass increases while fat mass decreases. BMI calculation does not make it possible to objectively capture this change.

Visceral fat and subcutaneous fat: what is the difference?

The body stores fat in two distinct locations, with very different implications for health.

Subcutaneous fat lies directly under the skin. It is visible and can be felt, especially around the abdomen, hips, and thighs. It serves as an energy reserve and thermal insulation. Excess subcutaneous fat contributes to weight gain, but its impact on metabolic health is less significant than that of visceral fat.

Visceral fat is deeper. It accumulates around the abdominal organs, such as the liver, pancreas, and intestines. It is invisible from the outside and cannot be measured by waist circumference alone. Excess visceral fat is closely associated with a risk of type 2 diabetes, cardiovascular disease, high blood pressure, and metabolic syndrome.

📋 Good to know — A person can have a flat stomach and a high level of visceral fat. This is precisely what lipodensitometry can detect, whereas neither BMI nor waist circumference can show it.

What is a normal body fat percentage?

Unlike BMI, there are no universal thresholds for body fat percentage. Reference values vary depending on sex and age.

As a guideline, the ranges commonly used for adults place the acceptable body fat percentage at:

Acceptable body fat percentage

Women 21 % to 31 %

Men 14 % to 24 %

These values naturally increase with age, in connection with the gradual loss of muscle mass. They are reference points, not fixed standards, and they also vary depending on ethnic background and level of physical activity.

📋 Good to know — The interpretation of your body fat percentage is always individualized. Your doctor is the one who analyzes your results while taking into account your age, sex, medical history, and health goals. An isolated number, without clinical context, is not enough to define a risk.

Lipodensitometry or bioelectrical impedance analysis: what is the difference?

Both of these exams measure body composition, but they are based on very different principles.

Bioelectrical impedance analysis measures the electrical resistance of tissues. A painless, low electrical current passes through the body. Muscles, which contain a lot of water, conduct electricity well. Fat, which is low in water, offers greater resistance. Software then uses this data to estimate body composition. 

Lipodensitometry uses X-rays to directly analyze tissue density. It precisely distinguishes bone, muscle, and fat mass, and segmentally locates visceral fat.

Each of these two methods has its own advantages and limitations :

• Bioelectrical impedance analysis : easy to access, quick, and available in many medical offices and pharmacies. However, it remains sensitive to variations in hydration, which can affect the reliability of results depending on the clinical context ;
• Lipodensitometry : a highly precise diagnostic tool, particularly for measuring visceral fat and bone mass. It requires advanced equipment and a specialized radiology team.

💡 Tip — The two methods are not opposed to one another. Your doctor chooses the tool best suited to your clinical situation and the purpose of the assessment.

In what cases can your doctor prescribe lipodensitometry?

Lipodensitometry is prescribed when your doctor needs a precise and objective assessment of your body composition. 

It is mainly indicated in the following situations :

• Abdominal obesity ;
• Cardiometabolic risk factors (high blood pressure, dyslipidemia, or type 2 diabetes) ;
• Metabolic syndrome ;
• Follow-up after bariatric surgery ;
• Sarcopenia (loss of muscle mass related to age or illness) ;
• Monitoring a weight loss or nutritional rehabilitation program.

Radiologie Dix30 clinic welcomes you in Brossard, on Montreal’s South Shore, for your exam. Send your prescription by fax to 450-904-0405 or by email to info@radiologiedix30.com to schedule your appointment. 

Is lipodensitometry painful or restrictive?

Lipodensitometry is completely painless. You remain lying on the table for about ten minutes, without direct contact with the machine. No injection is required. The radiation dose used is extremely low.

Preparation involves a few simple precautions to follow in the days leading up to the exam, including fasting for 4 hours and stopping calcium supplements 48 hours beforehand. These instructions are provided when your appointment is scheduled.

You can visit our lipodensitometry page for preparation details and all practical information about the exam.

Sources

Health Canada. Canadian guidelines for body weight classification in adults. https://www.canada.ca/fr/sante-canada/services/aliments-nutrition/saine-alimentation/poids-sante/lignes-directrices-classification-poids-chez-adultes/guide-reference-rapide-intention-professionnels.html

American Council on Exercise (ACE). Body Fat Percentage Categories. https://www.acefitness.org/resources/everyone/tools-calculators/percent-body-fat-calculator/