When we look at how the body uses different organic compounds such as carbohydrates, lipids, and proteins to produce energy for function and maintenance of the body's cells, tissues, and systems, explain why a person who has been starved of food and nutrients is extremely emaciated with little muscle or fat on their body (from the chemistry standpoint).
All such organic compounds take part in different metabolic pathways (for instance in glycolysis, Crebs cycle,cell respiration). This processes are necessary for the maintenance of normal vital activity. Proteins are the main structural blocks of human muscles. After destruction in the small intestine under the fermentation process proteins turning into amino acids and are absorbed into bloodstream. Then they should be rearranged into the new proteins with help of protein-synthesis machinery to form new body-specific proteins which is used to form muscle fiber (actin, myosin etc.) Human body accumulative activity of fat also depends from it's consuming or from carbohydrates income which can turn into the fat molecules through Acetyl-CoA metabolism.