The biggest culprit is water (and the water in the food you eat). The calories in a litre of soda would add about 45g if you didn’t burn them off, but step on the scale immediately after drinking it and you’ll be up around a kg; go to the bathroom and you’ll likely drop 450g to 680g. You even lose water weight – about 1kg a day – just by breathing and sweating. Day-to-day fluctuationscan be the result of a high-sodium meal or your level of hydration, while your menstrual cycle can cause changes all month long. “It’s important to keep the bigger picture in mind,” says Mohr. No one meal or single splurge will move the scale’s needle in a lasting way unless it becomes a habit. However, a difference of 100 calories at every meal could add up to more than 13kg in a year – in either direction.