Macronutrients

Nutrients are substances needed for growth, energy provision and other body functions. Macronutrients are those nutrients required in large amounts that provide the energy needed to maintain body functions and carry out the activities of daily life. There are 3 macronutrients – carbohydrates, proteins and fats.

Macronutrients give us energy

Although each of these macronutrients supplies the energy needed to run body functions, the amount of energy that each provides varies.

Carbohydrates and proteins each provide 17kJ/g whereas fats provide 37kJ/g. 1 kilojoule (kJ) = 1000 joules.

4.2 joules is the energy needed to raise the temperature of 1g of water by 1°C.

Nutritional research evidence shows that the relative proportion of energy-giving foods in the diet can increase or decrease the likelihood of problems such as heart disease. A balance of energy-giving nutrients is suggested.

For example, if an active teenager’s energy requirements are around 12,000kJ per day, an intake for energy purposes of about 388g of carbohydrate along with some protein (110g) and fat (97g) would meet this need. These values equate approximately to 55% of energy needed from carbohydrate, 30% from fats and 15% from protein.

Why do we need carbohydrates?

Carbohydrates, in the form of starches and sugars, are the macronutrients required in the largest amounts. When eaten and broken down, carbohydrates provide the major source of energy to fuel our daily activities. It is recommended that carbohydrates should supply 45–65% of our total daily energy needs.

Some of the carbohydrate we consume is converted into a type of starch known as glycogen, which is stored in the liver and muscles for later use as an energy source.

Not all of the carbohydrates found in foods are digestible. For example, cellulose is a non-digestible carbohydrate present in fruits and vegetables. Although unable to be used as an energy source, this type of carbohydrate plays a very important role in maintaining the health of the large intestine and assisting with the removal of body waste. It is often referred to as ‘dietary fibre'.

Why do we need proteins?

The proteins we consume as part of our diet are broken down in the gut to amino acids. The body can then use these amino acids in 3 main ways:

• As ‘building blocks’ in the production of ‘new’ proteins needed for growth and repair of tissues, making essential hormones and enzymes and supporting immune function.
• As an energy source.
• As starting materials in the production of other compounds needed by the body.

All the proteins in the body are made up of arrangements of up to 20 different amino acids. Eight of these amino acids are described as ‘essential’, which means that the food we eat must contain proteins capable of supplying them. The other amino acids can be synthesised by the liver if not provided by the diet.

Protein in the diet that comes from animal sources contains all of the essential amino acids needed, whereas plant sources of protein do not. However, by eating a variety of plant sources, the essential amino acids can be supplied.

Why do we need fats?

Although fats have received a bad reputation in relation to heart disease and weight gain, some fat in the diet is essential for health and wellbeing.

It is recommended that 20–35% of our daily energy requirement should be supplied through the consumption of fats and oils. In addition to supplying energy, fats are needed to:

• supply fatty acids that the body needs but cannot make (such as omega-3)
• assist with absorption of the fat-soluble vitamins A, D, E and K and carotenoids
• provide foods with flavour and texture.

Dietary fats are of 3 main types:

• Saturated fat – found in foods like meat, butter and cream (animal sources).
• Unsaturated fat – found in foods like olive oil, avocados, nuts and canola oil (plant sources)
• Trans fats – found in commercially produced baked goods, snack foods, fast foods and some margarines.

Replacing saturated fats and trans fats in the diet with unsaturated fats has been shown to decrease the risk of developing heart disease.