You ate well all week. You trained three times. On Friday the scale said 78.2 kg. Saturday morning it says 80.1 kg. Nothing has changed except you went to a dinner. This is one of the most demoralising experiences in dieting, and one of the most misunderstood.

The 1.9 kg you "gained" overnight is not fat. It is physiologically impossible to store that much fat in 24 hours. What you're measuring is almost entirely water, glycogen, and digestive contents, all of which fluctuate enormously.

Glycogen and its water

Every gram of glycogen (carbohydrate stored in your muscles and liver) is stored with approximately 3 grams of water. Your body can store roughly 400–600g of glycogen in total. If you ate a high-carb meal after a period of lower-carb eating, your body rapidly tops up its glycogen stores. That process alone can account for 1–2 kg on the scale overnight, with no change in fat mass whatsoever.

This is also why low-carb diets produce dramatic initial weight loss. You're not losing fat in the first week. You're depleting glycogen stores and shedding the water that comes with them. Similarly, returning to carbs replenishes those stores and the weight comes straight back.

Sodium and water retention

Sodium is a primary regulator of water balance. When you eat a high-sodium meal (restaurant food, processed snacks, takeaway), your kidneys retain extra water to maintain blood sodium concentration. The effect can be significant: studies show that a high-sodium meal can increase bodyweight by 0.5–1.5 kg within 24 hours compared to a low-sodium equivalent meal of identical calories.

This is entirely independent of caloric content. You can be in a caloric deficit and see the scale go up due to sodium alone.

Hormonal fluctuations

Cortisol, the stress hormone, causes water retention directly. A hard training session, poor sleep, or a stressful day all elevate cortisol and cause the scale to rise the following morning. This creates a perverse pattern where the days after your most intense workouts often show the highest scale weight.

For women, hormonal cycles cause significant bodyweight fluctuations across the month, often 1–3 kg, driven by oestrogen and progesterone's effects on fluid balance. These fluctuations have nothing to do with fat and are entirely predictable once you understand the cycle.

Digestive contents

Your digestive tract can hold 1–2 kg of food and waste at any given time. If you ate a large, late meal or if your bowel movements are irregular, the difference in digestive contents alone can account for 1 kg of apparent bodyweight change. This is why weighing yourself at the same time each morning, after using the bathroom, before eating, is the only way to get any consistency from daily weigh-ins.

What to actually measure

Daily scale weight is noise. Weekly averages are signal. Instead of reacting to each individual reading, calculate your 7-day average weight each week. Fat loss progresses slowly, roughly 0.25–0.5 kg of actual fat per week in a meaningful deficit. Looking at weekly averages filters out the glycogen, sodium, and hormonal noise and shows the actual trend.

  • Weigh daily, at the same time. After waking, after bathroom, before eating.
  • Track the 7-day average. This is your real progress metric.
  • Ignore single-day spikes. A 1.5 kg overnight gain after a social dinner is water, not fat.
  • Look for trend direction over 3–4 weeks. Not day-to-day movement.
You cannot gain 2 kg of fat overnight. But you can gain 2 kg of water. Your scale cannot tell the difference.