They rank close behind carbohydrates as major sources of energy. They also are important carbon sources for biosynthesis of cholesterol and other steroids. Moreover, plant Triacylglycerols provide essential fatty acids. Proteins Proteins have three major roles in nutrition. They provide both essential and non-essential amino acids as building blocks for protein biosynthesis, not only for the growth of infants and children but also for the constant replacement and turnover of body proteins in adults.
Amino acids are also precursors of hormones, porphyrins and many other biomolecules.
Oxidation of the carbon skeletons of amino acids also furnishes a minor but significant fraction of the total daily energy requirement. Together they are consumed in hundreds of grams per day, depending upon the body weight, age and sex. They serve as essential components of specific coenzymes or enzymes participating in metabolism and other specialized activities. The required inorganic nutrients can be grouped into two classes, Calcium, phosphorus and magnesium are required in relatively large amount per day, whereas iron, iodine, zinc, copper and many others are needed in only milligram or microgram quantities. The inorganic elements have many functions as components of bones and teeth, as electrolytes in maintaining water balance in the vascular system and tissues and in the prosthetic groups of enzymes, among others.