The answer is simple – it the beginning we often lose a lot of water. I often use a picture of a double humped camel in the dessert – those humps have water stored as fat and other chemicals. Also, 1g of glycogen (a complex carbohydrate our body uses to store energy in the liver and muscles) holds 4-5 g of water. We have around 500 grams of glycogen in our body, so, we can in many cases quickly drop 1.5-2 kg (4-5 pounds) in the first day by depleting this rapid source of on demand energy. It is also a reason why someone on a strict calorie restriction can rapidly gain few pounds over one day by a simple chit – the glycogen and water are rapidly reaccumulated…