Originally posted by Andy Dreisch
I'm just curious to know if the body will go after fat first, or muscle, when it realizes it's under a severely restricted caloric intake.
Personally, I go for the weight loss over time approach, like a pound or so a week or every two weeks. I hear of multiple pounds lost per week on some diets, though, and thought of this question.
I wonder if the body finds it easier to burn fat in these instances, or muscle.
Anybody know?
The thing is that "that depends".
The body burns 2 types of fuel. It burns carbohydrate or fat. Carbohydrate is great for high levels of exertion. Burning carbohydrate gives twice the energy yield per O2 molecule and power output is generally oxygen limited. However, you only have a limited amount of carbohydrate (about 2000 calories).
Why is carbohydrate so limited if it is such a great fuel? Well it weighs more then fat - twice as much for the energy content. So you body has a nearly endless supply of fat which is great at long sustained moderate exercise.
Now the body HAS TO HAVE some carbohydrate. The nervous system can only burn carbohydrate. So if you are not eating it, it has to come from somewhere. Where? Well you body takes muscles, breaks them down into proteins, and converts the proteins to carbohydrates.
So if your diet is overly carbohydrate restricted, you will lose muscle mass. In facts, in many if not most diets, people lose 1 lb of muscle for every lb of fat lost. People who exercise quite a bit and who eat enough carbohydrate can actually gain muscle mass whilst losing fat. However, on most fast weight loss diets, I bet that nearly all the weight lost is muscle. Fat is 3500 calories per pound and it takes a sedentary person days to burn off a lb of fat so losing weight by losing fat is just not feasible quickly unless you are exercising quite a bit.
Now, I did manage to do this once. I spend 2 weeks on a road bike tour which featured over 70 miles per day and over 4,000' of climbing per day. I avoided fats in my diet and consumed ample carbohydrates to replenish my glycogen stores. I also exercised at a reasonably moderate pace. I mean I was faced with climbs in excess of 7 miles and I did not think I could hammer up a 7 mile long hill without my legs giving out before I ran out of hill. I managed to lose 10 lbs in 2 weeks. Not too shabby. It can be done, but that was a rather special situation.
As a rule of thumb, losing a lb per week is a more realistic goal.