Will lifting weights convert my fat into muscle?

Sort of, kind of, maybe; not really.

What weight lifting will do

Weight lifting can increase muscle mass, primarily through becoming damaged (via weight lifting) and then sending out signals to the body to turn ingested proteins into new muscle tissue as a repair mechanism. Carbohydrates and fats are used as energy to fuel this process.

Weight lifting can also decrease fat mass, by using up body fat stores to fuel both this muscle building process and also using fat stores directly as fuel for the exercise that is needed to damage the muscle.

What weight lifting will not do

The technical, literal, direct conversion of fat into muscle.

Fat is comprised of triglycerides, which are molecules shaped like a capital E comprising a backbone and three chains of fatty acids (shown below). These chains are made up of carbon, hydrogen, and oxygen almost exclusively.

Muscle mass is made up of muscle tissue, glycogen, water, and some intra-muscular fat.[1][2] The muscle tissue (the only tissue able to contract) is made up of chains of amino acids, which can be quite various in their structure. These chains contain nitrogen, and nitrogen is almost exclusively stored in the body as muscle (with some amino acids floating around).

It is impossible for fat to directly turn into muscle, since fat lacks the nitrogen and no mechanism exists in the body to reconstruct fat into amino acids. No evidence has surfaced implying that amino acids can be made in the body from anything other than other amino acids, a process known as transamination.[3][4]

The vast majority of muscle built is from dietary nitrogen intake; dietary protein is the only significant source of nitrogen in the human diet.[5][6]

So although lifting weights can both build muscle and induce fat loss, these should be viewed as two separate results and not one being the result of another.

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  1. Du M, Yin J, Zhu MJ Cellular signaling pathways regulating the initial stage of adipogenesis and marbling of skeletal muscle . Meat Sci. (2010)
  2. Frayn KN Fat as a fuel: emerging understanding of the adipose tissue-skeletal muscle axis . Acta Physiol (Oxf). (2010)
  3. GOLDNER F Jr A review of the transamination reaction and its relationship to acute myocardial infarction . Am Pract Dig Treat. (1957)
  4. Hirotsu K, et al Dual substrate recognition of aminotransferases . Chem Rec. (2005)
  5. Phillips SM The science of muscle hypertrophy: making dietary protein count . Proc Nutr Soc. (2011)
  6. Phillips SM, Hartman JW, Wilkinson SB Dietary protein to support anabolism with resistance exercise in young men . J Am Coll Nutr. (2005)