Chemist's View of Carbohydrates

The Chemist's View of Carbohydrates

• Dietary sources include sugars, starch, and fiber
• Chemists classify sugars as monosaccharides (single sugars) and disaccharides (double sugars)
• Starch and fiber are polysaccharides, which are chains of monosaccharide units

Monosaccharides

• Three important monosaccharides are glucose, fructose, and galactose
• All have the same atoms but in different arrangements.

Glucose

• Most cells, especially brain and nervous system, rely on glucose for fuel.
• The body obtains glucose from carbohydrates and must maintain blood glucose within limits for optimal function

Fructose

• Fructose is the sweetest of sugars.
• Occurs naturally in fruit, honey, and is part of table sugar.
• Primarily consumed in beverages like soft drinks and sweetened products

Galactose

• It typically exists as part of lactose, a disaccharide known as milk sugar.
• During digestion, galactose is liberated as a single sugar.

Disaccharides

• Consist of pairs of monosaccharides linked together
• The disaccharides important for nutrition are maltose, sucrose, and lactose
• All contain glucose combined with either glucose (maltose), fructose (sucrose), or galactose (lactose).

Sucrose

• Sucrose, or table sugar, comes from refining sugarcane or sugar beets.
• Enzymes break it down into glucose and fructose.

Lactose

• Lactose is the main carbohydrate in milk.
• Infants possess enzymes to digest it into glucose and galactose, but some adults lose this ability, leading to lactose intolerance

Maltose

• Maltose consists of two glucose units and arises from starch breakdown.
• This happens in sprouting plants and carbohydrate digestion.

Polysaccharides

• Polysaccharides are primarily glucose-based, unlike sugars with glucose, fructose and galactose
• Three key polysaccharides are glycogen, starch, and fibers.

Glycogen

• Glycogen is a glucose storage form in humans and animals.
• It is not a significant dietary source.

Starch

• Starch is a glucose storage form in plants and is found in grains, root crops, tubers, and legumes.
• The body digests starch into glucose for energy.
• Grains are a major starch source globally, with rice, wheat, corn, and others serving as staple foods.
• Legumes and tubers are also significant starch sources.

Fibers

• They are structural components of plants, and found in vegetables, fruits, whole grains, and legumes
• They are polysaccharides with bonds indigestible by human enzymes.
• Fibers provide little or no energy.
• They include cellulose, hemicellulose, pectins, gums, mucilages, and lignins.

Resistant Starches

• A small number of starches are classified as resistant starches
• Resistant starches escape digestion and absorption in the small intestine.
• They are the starch molecules which are common in whole grains, legumes, raw potatoes, and unripe bananas
• Cooked and chilled potatoes, pasta, and rice also develop resistant starches

Fiber characteristics

• Soluble fibers dissolve in water, add consistency to foods, form gels, and are easily fermented by gut bacteria.
• Soluble fibers are found in barley, legumes, fruit, and oats, are associated with reduced risk of chronic diseases.
• Insoluble fibers do not dissolve in water, are less fermentable, and include cellulose and hemicelluloses
• They are in whole grains, celery strings, and seed hulls, aiding digestion by easing elimination.

Digestion and Absorption of Carbohydrates

• The purpose of digestion and absorption is to break down sugars and starches into small molecules for the body to absorb and use.
• Starch requires extensive breakdown, while disaccharides only need to be broken once.
• Monosaccharides do not require any breakdown.
• Most fiber passes through the small intestine and is digested by bacteria in the large intestine to produce short-chain fatty acids.

Mouth in Digestion and Absorption

• Salivary glands secrete saliva with amylase to start digesting starch.
• Mechanical action crushes and mixes fiber with saliva.

Stomach in Digestion and Absorption

• Stomach acid halts starch digestion.
• Fiber is not digested and delays gastric emptying.

Small Intestine in Digestion and Absorption

• The pancreas releases amylase to further digest starch.
• Fiber passes undigested, slowing nutrient absorption.
• Disaccharidase enzymes hydrolyze disaccharides into monosaccharides.

Large Intestine in Digestion and Absorption

• Bacterial enzymes digest soluble fibers into short-chain fatty acids.
• Insoluble fibers remain intact, bind substances, and aid in elimination.