Overview: It is very common for most people to lose a good amount of weight in the first week of a new diet, but this is often followed by slow gains and hard work in following weeks. Some people take this as proof that their current strategy is not working and therefore fall off the ban wagon, but unusual changes from the outset are actually expected and predictable. If your second week is a notable step back from your first, then you should continue with the program that you are on until it becomes clear over the long term whether its working.
Significance: The purpose of weight loss is to lose fat, but you are actually losing a combination of fat, muscle and water. Body fat alone is composed of 50 percent water. In extreme cases it may be possible to lose up to 10 kgs during the first week, most of which is water weight, but this is not a sustainable pace.
Glycogen: According to the website Peak Performance, another area of the body where water is prolific is in glycogen stores. Glycogen is a type of carbohydrate stored in the liver and muscles. It acts as a backup energy source when energy from the diet runs low. About 400g of glycogen can be stored at one time, but for every gram of glycogen, it also stores three times its weight in water. In total there may be a maximum of 1,600 combined grams of glycogen and water in your body, which is about 1.5 kgs. It’s possible to lose all or most of this within the first week, but some of it may be regained during the second.
Warning: Body weight is a misleading way to keep track of fat loss because it can fluctuate throughout the day and week due to water retention or the amount of food you’ve eaten. It may be possible that some of the water that retreated during the first week will come back during the second week, potentially leading to weight gain, but you can actually lose fat and gain overall body weight at the same time. Body fat percentage is a better measure overall of fat levels. A number of tools including calipers can measure body fat.
Considerations: The dramatic change to your diet and exercise regimen may also cause large fluctuations of weight during the first few weeks. This is a period when the metabolism is adjusting to the new routine. In the short term, your weight may change in unpredictable ways, but over time you should see more consistent and predictable losses.
Recommendation: During the second week, your weight may still be fluctuating in odd ways. Starting around the third week, you should start to settle into a steady routine that produces a consistent loss of 0.5 or 1kg a week. Anymore than this may actually make you sick, and a severe calorie restriction opens up biochemical pathways that result in eating into your muscle tissue for its energy and nutrients.