Chapter4

Carbohydrates: chemical composition

Carbohydrates contain carbon (C), oxygen (O), and hydrogen (H). Carbon forms four chemical bonds, oxygen forms two, and hydrogen forms one.

Monosaccharides

The simplest carbohydrates (single sugars). The three important monosaccharides are glucose, fructose, and galactose. All have the formula C6H12O6 but different structures.

Disaccharides

Carbohydrates made of two monosaccharides joined together. The three common disaccharides are maltose, sucrose, and lactose.

Maltose, sucrose, and lactose

Maltose = glucose + glucose; sucrose = glucose + fructose; lactose = glucose + galactose.

Polysaccharides

Long chains of monosaccharides. They include glycogen, starches, and dietary fibres.

Condensation reaction

A reaction that joins two monosaccharides to form a disaccharide and water.

Hydrolysis reaction

A reaction that uses water to split a disaccharide into its two monosaccharides.

Glycogen vs. starch

Both are glucose storage forms that provide energy. Glycogen stores glucose in the body; starch stores glucose in plants.

Starches: amylose and amylopectin

The two forms of starch identified in the carbohydrate family.

Dietary fibres

Fibres contain glucose and other monosaccharides, but human digestive enzymes cannot break their bonds. They provide little, if any, energy.

Soluble and insoluble fibre

The two categories of dietary fibre.

Carbohydrate digestion and absorption

Starches are digested into maltose. Maltose, lactose, and sucrose are broken down into monosaccharides, which are absorbed.

Lactase and lactose intolerance

Lactase breaks lactose into glucose and galactose. Lactose intolerance occurs when there is insufficient lactase to digest lactose in milk and milk products, causing gastrointestinal distress.

Amylase, maltase, sucrase, and lactase

Enzymes involved in carbohydrate digestion. Amylase digests starch; maltase breaks maltose down; sucrase breaks sucrose down; lactase breaks lactose down.

Monosaccharides in the liver

Monosaccharides arriving at the liver are converted mostly to glucose.

Fibre during digestion

Fibre helps regulate the passage of food through the gastrointestinal system and slows glucose absorption.

Fates of glucose in the body

Glucose can provide energy to cells, be stored as glycogen in the liver and muscles, or be converted into fat when intake exceeds needs.

Cells that depend on glucose

All body cells depend on glucose. Cells of the central nervous system and red blood cells (RBCs) are especially dependent on it.

What happens when glucose is insufficient

The body breaks down protein tissues to make glucose and changes energy metabolism to produce ketone bodies from fats.

Insulin vs. glucagon

Insulin moves glucose from the blood into muscle and adipose tissue cells when blood glucose is high. Glucagon releases glucose from liver glycogen stores into the blood when blood glucose is low.

Glycogen

The storage form of glucose in the body, mainly in the liver and muscles.

Health effects of excessive sugar intake

May increase dental caries, displace needed nutrients and fibre, increase plasma triglycerides, and contribute to obesity when energy intake exceeds needs.

Added sugars vs. naturally occurring sugars

Concentrated sweets with added sugars are often low in nutrients and high in calories. Sugars naturally found in fruits, vegetables, and milk occur in more nutrient-dense foods.

Alternative sweeteners

Artificial sweeteners, herbal products, and sugar alcohols that can help limit calories and added sugar intake.

Why use alternative sweeteners?

They may help control weight gain, blood glucose, and dental caries by reducing sugar and calorie intake.

Benefits of adequate fibre intake

Fibre supports weight management, lowers blood cholesterol, and may help prevent colon cancer, diabetes, hemorrhoids, appendicitis, and diverticulosis.

Risks of excessive fibre intake

May displace energy- and nutrient-dense foods, cause intestinal discomfort and distention, and interfere with mineral absorption.

Recommended carbohydrate intake

Carbohydrates should provide 45–65% of daily energy intake.

Foods recommended for starch and fibre

Whole grains, vegetables, legumes, and fruits. These foods help support weight management and may help prevent heart disease, cancer, diabetes, and gastrointestinal disorders.

Foods that do not contain carbohydrates

Meat is the correct answer among milk, meat, bread, and fruit.

Carbohydrate recommendations

Choose plenty of whole grains, vegetables, legumes, and fruits, with carbohydrates providing 45–65% of daily energy intake.