The human body is mostly composed of water (70%). This means that hydrogen and oxygen are the two most common elements in the human body. But carbon, nitrogen, and phosphorus are among the other most common elements in the human body (sulfur is also up there). These three elements are derived from food eaten by a person. Carbon is the principle element in all major classes of biological molecules (proteins, lipids, sugars/starches, and nucleic acids). Nitrogen is used in amino acids and nucleic acids. Phosphorus is used primarily in phosphate groups (nucleic acids and ATP, among others). So the human body incorporates all of these elements from food into these various components of the body. When the body dies and decomposes in the ground, these atoms are reincorporated into the natural cycle through the soil, bacteria, plants, and eventually animals (including humans) again.