Yeah, its an individual thing, carbs are an easy way to lower your calorie count and if youre doing cardio youre most likely depleting your glycogen stores anyway, releasing water. The theory behind the low carb diet (Im not saying its right, so no one lay egg about this) is that their two hormones in charge of fat metabolism. Insulin and glucagon. Insulin helps drive fat into fat cells while glucagon is responsible for liberating it. Insulin overrides glucagon. So the point of low carb is the prevention of insulin production which is done by limiting high gycemic index carbs. (things that raise your blood sugar quickly invoking an insulin response). So when people do the low carb thing (<30g)its not just a matter of limiting calories its trying to get an optimal hormonal situation for fat release. The by product of fat burning is ketones and so when you are eliminating ketones say in your urine is says that youve been using fat as a fuel source. Some say that ketones are the prefered energy source over glucose. Who knows! I think that people feel that they lose muscle more than they do (the ones that have actually tried it) are thinking more the fact of the muscle glycogen depletion which eliminates the water being held in the muscle tissue. The people who enjoy this method of weight loss chalk it up to the muscles being flat. If true, then once youve attained your bodyfat level and add carbs back, you'll get an instant pump in muscle volume. But its whatever works for you and there are alot ways to get from point A to point B and that is usually what's most comfortable for you.