There are three essential food groups known as macronutrients: carbohydrates, fats and proteins.

Carbohydrates are all composed of carbon, hydrogen and oxygen atoms. Many different varieties fall into five basic categories.

>> Monosaccharides like glucose, fructose and galactose are simple sugars. They are rings of carbon and oxygen with hydrogen, hydroxyl, or methyl groups attached. Glucose is the energy molecule used by cells. They form the basis for disaccharides and polysaccharides.

>> Disaccharides like sucrose and lactose are two monosaccharide molecules connected by an oxygen atom. The liver or digestive enzymes break them apart into monosaccharides.

>> Polysaccharides are polymers of simple sugars. The most common are starch and fiber, which are strings of glucose molecules connected in long linear chains. Dietary fiber is indigestible polysaccharides.

>> Oligosaccharides also are composed of repeating units of simple sugars contained in legumes. Because of their unique bonds, the liver and enzymes cannot break them down into their simple sugars and so the small intestine cannot absorb them. Instead, bacteria metabolize them in the large intestine to form gaseous byproducts.

>> Nucleotides serve as the subunits of the nucleic acids DNA and RNA. They are composed of a nitrogenous base, a five-carbon sugar (ribose or deoxyribose), and at least one phosphate group.

Most fats (lipids) are triglycerides, composed of three fatty acids attached to a glycerin base. Each fatty acid is a chain of carbon atoms with a carboxyl group at the free end. Each chain, which may be saturated or unsaturated, consists of six to more than 20 carbon atoms.

Unsaturated chains contain at least one double bond between two adjacent carbon atoms. The only differences between one fat and another are the length of each chain and the number of double bonds.

The most complex and perhaps the most important macronutrients are the proteins. Proteins are structural, as in the case of muscles, organs, bones, teeth, hair and red blood cells. They are enzymes that catalyze metabolic reactions. They also assist in DNA replication, respond to stimuli and transport molecules from one place to another.

All proteins are composed of long chains of amino acids. The sequences in the chain, which are dictated by their genes, distinguish one protein from another.

For most proteins, the shape of the molecule is of equal importance to its composition. After a gene segment of DNA assembles the amino acids, the protein molecule folds into a unique structure that fits like a key into a lock to catalyze a reaction.

Proteins perform the primary actions within the cell, following the instructions specified by their genetic information. With few exceptions, other biological molecules are relatively inert elements upon which proteins act.

Proteins also may bond with other macronutrient molecules. One example is the lipoproteins, of which HDL and LDL are the most common. Lipoproteins are complex molecules composed of multiple proteins, typically 80 to 100 proteins per particle. They transfer all lipid molecules — predominantly cholesterol, phospholipids and triglycerides — around the body, making them available to the cells.