Blog · Diet and body composition

BMI and body composition: an educational article

Educational article: BMI relates weight to height, but it cannot tell fat from muscle or see visceral fat. It is not a study.

Updated October 20, 2026

For decades BMI (Body Mass Index) has been the standard measure for assessing body weight and metabolic risk. According to an educational article by MultiCare Health System, it misclassifies at least one person in three.

It does not distinguish fat mass from muscle mass, does not show how fat is distributed in the body, and does not see visceral fat, the fat stored around the organs.

The scientific problem with BMI

BMI = kg/m². An athlete and a sedentary person who both weigh 90 kg can have the same BMI, yet very different risk profiles: one carries mostly muscle, the other mostly abdominal fat.

Equally, a person with a normal BMI can have low muscle mass and high visceral fat (TOFI: Thin Outside, Fat Inside).

A couple in an elegant hotel bar in the evening: he smiles while she shows him the Digital Twin figure on her phone.

Ai-generated illustration

Changes in body shape: composition is needed

According to the article, change in body shape depends on variation in two body composition characteristics (adiposity and muscularity), not one alone.

Someone who diets and exercises may gain muscle while losing fat, with weight unchanged. BMI would say nothing has changed; body composition would show fat being traded for muscle.

Normal weight obesity: when BMI is not enough

Normal weight obesity means a normal BMI (18.5–24.9) with low muscle mass and high visceral fat. It is associated with insulin resistance, metabolic syndrome and increased cardiovascular risk, which BMI alone does not show.

A number on the scale does not say what your weight is made of.

The Digital Twin and this research

The Digital Twin lines up the data your devices write to your phone, such as weight, body fat percentage and lean mass when your scale records them, and shows you how they change over time. It does not calculate risk indices and it does not diagnose: what you read here describes groups of people, not you. These are data to read together with your doctor.

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