Describe the structure, food sources and functions of carbohydrates, proteins and lipids, and explain how each contributes to the body's energy balance

What this dot point is asking

You need to identify the three macronutrients, describe what each is made of, list common food sources, and explain the role each plays in supplying energy and structural material to the body.

Carbohydrates

Carbohydrates are made from the elements carbon, hydrogen and oxygen. Their basic units are simple sugars (monosaccharides such as glucose). Sugars join to form disaccharides (such as sucrose) and long chains called polysaccharides (starch and the dietary fibre cellulose).

Carbohydrates are the body's preferred and most readily available energy source. Each gram supplies about 17 kilojoules. Glucose is the fuel that cells break down in respiration, and the brain relies on a steady glucose supply.

Food sources include bread, rice, pasta, potatoes, fruit and legumes. Wholegrain foods also supply dietary fibre, which is not digested but supports healthy bowel function.

Proteins

Proteins are made of carbon, hydrogen, oxygen and nitrogen. Their building blocks are amino acids, joined by peptide bonds into long chains that fold into specific shapes.

There are about twenty amino acids. Essential amino acids cannot be made by the body and must come from food, while non-essential ones can be synthesised internally. Animal foods (meat, eggs, dairy, fish) usually supply all essential amino acids and are called complete proteins; many plant foods are incomplete, so vegetarians combine foods (such as legumes with grains) to obtain the full set.

Protein's main role is growth and repair: building muscle, enzymes, antibodies and hormones. It supplies about 17 kilojoules per gram, but the body uses it for energy only when carbohydrate and fat are scarce.

Lipids

Lipids (fats and oils) are made of carbon, hydrogen and oxygen, built from glycerol and fatty acids (triglycerides). Saturated fatty acids have no double bonds and are usually solid at room temperature; unsaturated fatty acids contain double bonds and are usually liquid.

Lipids are the most energy-dense macronutrient at about 37 kilojoules per gram, roughly double carbohydrate or protein. They store energy, insulate the body, cushion organs and carry the fat-soluble vitamins A, D, E and K.

Sources include oils, butter, nuts, oily fish and avocado.

Energy balance

Energy balance compares energy taken in as food with energy used by the body. The body uses energy for its basal metabolic rate (keeping organs working at rest), physical activity and digesting food.

• Balance: energy in equals energy out, so body mass stays stable.
• Positive balance: energy in exceeds energy out, and the surplus is stored mainly as body fat.
• Negative balance: energy out exceeds energy in, so stored fat is used and body mass falls.

Because fat is so energy-dense, diets high in fatty and sugary foods make a positive energy balance more likely, which links to the overweight and diet-related disease content in Topic 2.

In short, the three macronutrients each supply energy but differ in their main role: carbohydrates fuel the body, proteins build it and lipids store concentrated energy. Their balance against energy expenditure determines whether body mass rises, falls or stays steady.