Body Composition vs. Weight

A liter of water weighs 1 kg. After a meal high in carbs and salt, your body retains water (each gram of glycogen stores 3 g of water). A single restaurant meal can make the scale jump by 1 to 2 kg the next day, without a single gram of fat being gained.

Conversely, at the start of a low-carb diet, the loss of glycogen and water can cause weight to drop by 2 to 3 kg in a few days, creating the illusion of rapid fat loss. When carbs return, the water returns too, which can be falsely discouraging.

The menstrual cycle causes women's weight to fluctuate by 1 to 3 kg depending on the phase, mainly due to water retention linked to hormonal shifts. Comparing weight at the same point in the cycle is more relevant than comparing day-to-day.

Body recomposition involves losing fat while simultaneously gaining muscle. Weight stays stable or changes little, but the silhouette transforms. This is possible, and is actually the most common situation for strength training beginners.

Favorable conditions for recomposition: being a gym beginner, being moderately overweight, consuming enough protein (1.6-2.2 g/kg), and strength training 3 to 4 times a week. Under these conditions, weight might not move for months even as clothes become looser.

For advanced trainees, recomposition is slower and more difficult. It is often more effective to alternate between phases of slight calorie surplus (muscle gain) and slight deficit (fat loss) in cycles of 8 to 16 weeks.

DEXA (dual-energy X-ray absorptiometry) is the gold standard: it measures fat mass, lean mass, and bone density with 1-2% precision. Costly and available only in medical settings, it's ideal for an annual check-up.

Bioimpedance (connected scales) is accessible and convenient, but its precision varies from 3% to 5%. Results are influenced by hydration, time of day, and the previous meal. Always use it under the same conditions for reliable comparisons.

Skinfold calipers (adipometer) offer good precision (3-4%) when performed by a trained professional. Waist circumference is the simplest indicator: above 94 cm for men and 80 cm for women, cardiovascular risk increases significantly according to the WHO.