Macronutrients

Abstract

Carbohydrates are aldehyde or ketone derivatives of polyhydroxy alcohols. Some carbohydrates contain nitrogen, phosphorus, and sulfur. Carbohydrates consist of simple or free sugars (monosaccharides and disaccharides) and complex carbohydrates (oligosaccharides and polysaccharides). Dextran, inulin, digitalis glycosides, and cellulose are clinically important carbohydrates. Lipids are heterogeneous compounds characterized by relative insolubility in water, but free solubility in organic solvents like ether, chloroform, acetone, etc. Dietary lipids mainly consist of triglycerides (about 90%). The remainder dietary lipids consist of cholesterol, cholesteryl esters, phospholipids, and free fatty acids. Lipids are transported by plasma as lipoproteins. Lipoproteins are important constituents of plasma membrane and mitochondrial membrane. Physio-biochemical important lipids are steroids, prostaglandins, lipid-soluble vitamins, carotenoids, etc. Lipids are the major source of energy fuel. About 80% of energy fuel of the body is derived from lipids. Proteins are polymers of amino acids. Proteins differ from carbohydrates and fats in containing nitrogen, sulfur, and occasionally phosphorus. Amino acids take part in the formation of neurotransmitters, hormones, and digestive enzymes and maintain the contents of enzyme systems of every cell. Essential amino acids cannot be synthesized by humans and must be supplied in diet. Whereas nonessential amino acids are synthesized in vivo by metabolic pathways.