Interpreting Weight Fluctuations

Body fat changes slowly: a ~0.5 kg fat change requires roughly a ~3,500 kcal energy imbalance, so genuine fat gain or loss of more than a few hundred grams in a single day is physiologically implausible. Yet the scale can jump 1–2+ kg overnight. That movement is almost entirely non-fat mass shifts:

• Glycogen + bound water. Each gram of stored muscle/liver glycogen holds roughly 3 g of water. A high-carb day or a rest day refills glycogen and adds water weight; a low-carb day or hard training depletes it and drops weight — neither is fat change. This is why starting a low-carb diet shows a fast initial "whoosh." (See Carbohydrates — Types, Glycemic Index, and Requirements.)
• Sodium and hydration. High-sodium meals and lower water intake cause water retention; the scale rises without any fat gain (see Hydration and Fluid Replacement).
• Gut contents. Undigested food and stool add measurable weight until cleared.
• Hormonal / menstrual. Water retention varies across the menstrual cycle, often peaking premenstrually (see Menstrual-Cycle Tracking).
• Glycogen after detraining, alcohol, heat, illness — all move water.

The practical consequence: never react to a single weigh-in. Use standardized conditions and read the smoothed weekly trend (see Bodyweight Tracking, Moving Averages). Educating users on this prevents both false panic ("I gained 2 kg overnight") and false hope.

• ~0.5 kg of fat ≈ ~3,500 kcal imbalance — fat changes slowly.
• Glycogen binds ~3 g water per gram; carb/training swings move water weight.
• Sodium and low fluid intake drive water retention independent of fat.
• Gut contents and menstrual-cycle hormones add measurable swings.
• A single overnight 1–2 kg move is water/glycogen/food, not fat.