A lot of it was probably water. But, you probably also shifted about a lbs of body fat. What happens when we go low carb is that we deplete the glycogen stores in out liver and muscles, and that glycogen is stored with a grams of water, so as its depleted we lose water weight. The first two weeks, while your body adapts to the lost glycogen stores is where you see all thos pounds falling off. After that, it will slow to 1 to 2 lbs a week.
The reason why low carb works so well with PCOS sufferers is due to the increased glycemic control and decreased insulin response to blood sugar spikes. Insulin is the hormone that drives fat storage in our bodies, and many things can impact insulin response, mostly a high carb consumption, but also too much protein or just too many calories in general.
Good luck, even if the initial loss was water it still feels good to have lost that water weight. Now, remember, you will experience a stall here either this week or next while your body adjusts. Don't freak out, just keep on keepin on, you might stall on he scale but you will still be losing fat. This is where measuring is a good idea because the scale will stall from time to time.
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