Nutrition at a Glance
Nutrition at a Glance
Introduction to the nutrients
To show how nutrients are classified and discuss their main roles
To describe how nutrients may interact to fulfil similar roles
Food is composed of a large variety of chemical substances, some of which are recognised as nutrients. Over the past century or so, scientists have identified the roles of these nutrients in the body and the consequences of insufficient intakes. Many other substances are present in foods of plant origin that help promote the plant's growth, protect it against predators or contribute to its appearance or smell to attract animals that will spread its seeds. Although these substances (phytochemicals) are not recognised as nutrients, some may be biologically active in humans and could have either beneficial or harmful effects.
Classification of nutrients
Traditionally, the major nutrients have been classified according to the amounts in which they are required by the body, their chemical nature and their functions. The principal distinction is between macronutrients and micronutrients :
Macronutrients are required in relatively large amounts, usually expressed in terms of grams per day.
Micronutrients are required in small amounts, usually expressed in terms of milligrams or micrograms per day.
Some classifications also include ultratrace nutrients . These are found in the diet in very small amounts (typically <1Âµg/g of dry food). For many of these substances, their roles are, as yet, uncertain.
Water is an essential component of the diet, as an adequate intake of fluid is vital to sustain life.
This category comprises carbohydrates, fats and proteins:
Carbohydrates and fats are the major providers of energy , although proteins can also be used to provide energy.
They all have a structural role in the body, the most important in this respect being proteins.
All contain carbon , hydrogen and oxygen . In addition, all proteins contain nitrogen, while some amino acids (cysteine and methionine) that are found in proteins contain sulphur. Carbohydrates
Carbohydrates are the most important source of food energy in the world. Carbohydrates occur in the diet in various degrees of complexity, ranging from simple sugars (mono- and disaccharides) to larger units such as oligosaccharides and polysaccharides. Simple sugars include the monosaccharides glucose, fructose and galactose and the disaccharides sucrose and lactose. Oligosaccharides include maltodextrins and fructo-oligosaccharides. Important polysaccharides include starch and glycogen.
The main function of carbohydrates is to act as a source of energy, in the form of glucose. However, some carbohydrates resist digestion and are termed 'non-glycaemic' (see Chapter 8 ). They comprise the non-starch polysaccharides (NSP), which are part of the category known as 'dietary fibre'. These carbohydrates play an important role in bowel function.
Fats are a diverse group of lipid-soluble substances, the majority of which are triacylglycerols (TAGs). Other lipid-soluble substances including phospholipids and sterols (e.g. cholesterol) are also included in this group.
TAGs are broken down to yield energy and are the body's richest source of energy, having over twice the caloric content of carbohydrates and proteins. They are also the body's major energy reserve, stored in the adipose tissue. Specific fatty acids found in TAGs (called essential fatty acids ) are important for cell membrane structure and function. Since the body lacks the ability to manufacture essential fatty acids, they must be supplied in t