A question came in from a reader: "Why do I lose weight so much faster on Atkins? It seems to work better for me right away, and then it doesn't work as well and I stop losing as much. It's frustrating."
Great observation. On diets like Atkins, you consume very low amounts of carbohydrates (see blog post 8/25/10.) The key to your quicker initial weight loss is the water loss you first experience...
Your body stores energy in three basic places. When your body needs energy, the easiest place to get it is in the blood where it is stored as a simple carbohydrate, or sugar: blood glucose. This is kind of like carrying cash around with you. It's easy, quick, and everyone takes it as a form of payment. Every cell in your body needs a constant supply of energy, and glucose supplies the quickest, easiest form to use.
The second place your body stores energy is glycogen, which is found in muscles and in the liver. Glycogen is a carbohydrate that is stored with over 2 1/2 times its weight in water. The only way to fill these stores is to consume carbs. You cannot fill glycogen by eating protein or fat. Carbo loading is what athletes do to maximize their glycogen stores. Glycogen is like the money you keep in your checking account; not cash, but readily accessed by ATM or check. Your body will use glycogen and convert it to glucose as your blood glucose diminishes---a great overdraft protection plan, which helps provide constant energy.
Your third energy store is in body fat. It's a highly condensed form of energy that acts like your savings account. It's harder to access this account for cash. Body fat is harder to utilize as immediate fuel, but will be accessed if blood glucose and glycogen stores are not sufficient and your body needs energy.
On a very low carb diet, your glycogen reserves become depleted. As each gram of carb gets used up, over 2 1/2 grams of water are released. Think of this as the "liquid assets" in your checking account, so to speak. The water that is released provides a big part of the initial weight loss on the scales. Once these reserves are diminished, and no more carbs are coming in, that part of the weight loss stops. At that point, weight loss tends to be similar other diets of similar calorie content: a higher percentage of the weight loss is likely body fat.
If a person has been on a low carb diet for awhile and depleted their glycogen stores and then goes back to eating higher carb foods, they will experience a quick weight gain. Even a few slices of bread will trigger the body to reload the glycogen stores, putting back those carb units along with the water they require. Results? The scales go up. This can really mess up your head if you step on the scales every day. Remember, the scales are not just measuring body fat.
I hope this helps!