No, in fact you are wrong for exercise like intense weight lifting.

Of course proper diet does have a huge effect in weight loss though.

I'm not going to say you haven't lost eight pounds in three days, but I will say that most of that was water weight and the rest was probably muscle.

A pound of fat is roughly 3500 calories. In order to lose eight pounds of fat in three days, you would have had to induce a caloric deficit of over 9000 calories per day.

A note to everyone: The most weight you can healthily lose is about two pounds per week. More than that and you're going to start losing a measurable amount of muscle mass. The exception is when you first start out exercising and eating properly, but that should only last for a couple of weeks.

When you cover huge distances all day the weight/calorie arithmetic is not linear. Your body rapidly sheds large amounts of weight to become more efficient.

I specifically said it was not water.

You body stores about 1 pound of glycogen (a quickly available starch energy source) in the muscles and the liver. With it is about 5 pounds of water. When you eat a low carb diet, the first thing your body does is burn that off. That, plus dehydration, means you can expect to lose about 6-8 pounds in a few days. I can and have done that many times. And then I can gain it back in two more days with a couple of big plates of pasta. Competitive athlethes like wrestlers who need to make weight can routinely cut and gain a temporary 15-20 pounds in less than a day.

In other words, sorry if you don't believe me, but much of what you lost was water.

I know all this. But when the weight stays off, and you in fact continue to lose weight in the following weeks, then it is not water loss. A lot of muscle, yeah.