Body Composition vs. Weight

One 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 go up 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 wrongly discouraging.

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

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

Conditions favorable for recomposition: being a beginner at weightlifting, having moderate excess weight, 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 while clothes become looser.

For advanced trainees, recomposition is slower and harder. It's often more effective to alternate between phases of slight caloric surplus (muscle gain) and slight deficit (fat loss) in 8 to 16-week cycles.

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

Bioelectrical impedance (smart scales) is accessible and practical, but its precision varies by 3% to 5%. Results are influenced by hydration, time of day, and previous meals. 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.