Do you gain weight or do you lose weight by lifting weights?

Question

Assuming you keep your diet in the range where you neither lose weight nor do you gain weight and keep it steady.

Also assuming that the only exercise you do is lift weights once or twice a week.

So in this situation, suppose you burn 1 lb of fat would that be converted into approximately 1 lb of muscle, i.e. 0.9 or 0.95 lbs of muscle? or would it be substantially less than 1 lb like around 0.1 or 0.2 lbs of muscle? Also what factors does body fat to muscle conversion depend on?

I have studied enough physics to know that 1 lb of fat would never produce 1 lb of muscle because if it did, the fat to muscle conversion process would have been 100% efficient and it did not waste any energy. If 1 lb fat did produce 1 lb of muscle then that would imply that you are eating more than you should.

Answer 1

Assuming you keep your diet in the range where you neither lose weight nor do you gain weight and keep it steady.

Here, you are defining your food intake as "whatever is necessary to ensure that you neither gain or lose weight". With that condition, then by definition if you lose 1 lb of fat, then you must be gaining 1 lb of non-fat mass, in order to satisfy your requirement that you neither gain or lose weight. In order to achieve this, you would need to be eating in a caloric deficit. (Explanation below.)

However, if you actually meant to say that you were eating in a calorie balance, so that you are burning exactly as many calories as you are consuming, then if you were gaining muscle you would also be gaining weight.

The reason for this is that the amount of energy stored in a pound of fat is vastly more than the amount of energy needed to build a pound of muscle. One pound of fat contains about 4280 kcal (39.5MJ/kg), and one pound of lean tissue around 824 kcal (7.6MJ/kg).¹ So it only takes the energy stored in around 0.2 lb of fat to create 1 lb of lean tissue. Therefore in a calorie balance, a person could gain 5 lb of lean tissue for every 1 lb of fat lost.

Finally, it should be noted that you cannot convert fat into muscle. You can gain muscle at the same time as you are losing fat, but there is no conversion happening, you're just taking energy from the fat stores in order to fuel the muscle building process, but the actual muscle is built from proteins consumed in your diet, not from fat.