The short answer is “no”. Fat tissue and muscle tissue are two separate types of tissue. The only way fat turns to muscle is through catabolism (breakdown) of the fat and anabolism (building) of muscle tissue, through stress being placed on the muscle (most effectively weight training, but also to some extent some types of cardio). Metabolism is a process made of both catabolism and anabolism, although usually when we refer to our ‘metabolism’ we mean our ability to catabolise food and anabolise it into energy to support our body processes.
I think there’s somewhat of a grammatical joke in there, saying that one thing weighs more than the other, but yes, muscle is heavier by volume than fat.
The density of human adipose tissue (fat) is 0.92 g/cm3. The density of human muscle tissue is 1.06g/cm3. So if you have 1 kg of muscle, it will take up 943 cubic centimeters (cc’s) of space, whereas 1 kg of fat will take up 1087 cc. 1000 cc is a liter or roughly equivalent to one quart.
Muscle is also more metabolically active than fat – it uses more energy than fat just to exist in a resting state. 1lb of fat burns 2 calories a day just to exist, whereas 1lb of muscle burns 6 calories or 3 x that amount. This still doesn’t add up to much if you’re counting in bowls of icecream, but it’s still better to have more muscle. Yes, it makes you heavier, but your body shape will be more attractive and your fitness level will be greater which should give you greater enjoyment out of life. I know I have more fun when I am fit – there’s more that I can do.
There are plenty of other benefits to weight training as well:
It prevents the loss of lean body mass which happens from dieting alone, as well as the natural loss of lean body mass that occurs as we age.
Weight training workouts burn calories too and help to raise your metabolism for up to 24 hours after you stop working out!
It strenghtens your bones and connective tissues along with muscles – important for preventing osteoporosis and sprains/weak joints.
It does take QUITE a bit of effort and time to put on muscle weight though.