In the first 1-2 weeks of low carb eating, people tend to lose weight very quickly. The main reason for this is reduction in water weight.
The mechanism behind it is two-fold:
- Insulin: When insulin goes down, the kidneys start shedding excess sodium from the body. This also lowers blood pressure.
- Glycogen: The body stores carbs in the form of glycogen, which binds water in the muscles and liver. When carb intake goes down, glycogen levels in the body go down, and the water follows along.
This does not happen to nearly the same extent on a higher carb diet, even if calories are reduced significantly. Even though some people use this as an argument against low-carb diets, reduced water weight should be considered an advantage. I mean, who wants to carry around excess bloat and water weight all the time?
Anyway, despite claims to the contrary, this is far from being the main weight loss advantage of low-carb diets.
The studies clearly show that low-carb diets lead to more fat being lost as well, especially the “dangerous” belly fat found in the abdominal cavity.
So, part of the weight loss advantage of low-carb diets is explained by reductions in water weight, but there is still a major fat loss advantage as well.
The bottom line is, when people go low-carb, they lose significant amounts of excess water from their bodies. This explains the rapid weight loss seen in the first week or two.