SingaporeNutrition & Food ScienceSyllabus dot point

What are the macronutrients, what does each one do, and how much energy does each provide?

Identify the three macronutrients, state their main functions, and compare the energy each provides per gram

A focused answer on the three macronutrients - protein, carbohydrate and fat - their main functions in the body, and the energy each provides per gram, with a worked daily energy calculation.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

The syllabus wants you to know that food supplies three nutrients in large amounts, called the macronutrients - protein, carbohydrate and fat - and to be able to state the main job of each one and how much energy each releases per gram. The big idea is that all three can be used for energy, but each also has its own special role, and they do not all provide the same amount of energy.

The answer

What "macro" means

Macronutrients are the nutrients the body needs in large (macro) amounts, measured in grams per day. They are protein, carbohydrate and fat. Vitamins, minerals and water are the other nutrients; vitamins and minerals are needed in tiny amounts and are called micronutrients.

The three macronutrients and their main jobs

Protein is the body's building material. Its main function is the growth, repair and maintenance of body tissues such as muscle, skin and organs. It can also be used for energy if needed, and it is needed to make enzymes, hormones and antibodies.

Carbohydrate is the body's main and preferred source of energy. Sugars and starches are broken down to glucose, which cells use as fuel. Carbohydrate also supplies dietary fibre, which keeps the gut healthy.

Fat is a concentrated source of energy and a store of energy in the body. It also insulates the body to keep it warm, protects organs, and carries the fat-soluble vitamins A, D, E and K.

Energy values per gram

The three macronutrients do not release equal energy. The values you must know are:

\text{protein} = 17\ \text{kJ g}^{-1}, \quad \text{carbohydrate} = 17\ \text{kJ g}^{-1}, \quad \text{fat} = 37\ \text{kJ g}^{-1}

In kilocalories these are roughly $4\ \text{kcal g}^{-1}$ for protein and carbohydrate and $9\ \text{kcal g}^{-1}$ for fat. The key point for exam answers is that fat provides more than twice the energy of the same mass of protein or carbohydrate, which is why fatty foods are so energy-dense.

How they fit together

A balanced diet uses each macronutrient for its strength: carbohydrate for everyday energy, protein for building and repair, and fat for energy storage, insulation and vitamin transport. If you eat more energy than you use, the surplus is stored as body fat regardless of which macronutrient it came from.

Calculating the energy in a meal

A serving of chicken rice contains $25\ \text{g}$ protein, $70\ \text{g}$ carbohydrate and $18\ \text{g}$ fat. Find the total energy and the percentage of that energy that comes from fat.

Step 1: Energy from each macronutrient

Protein: $25 \times 17 = 425\ \text{kJ}$ .

Carbohydrate: $70 \times 17 = 1190\ \text{kJ}$ .

Fat: $18 \times 37 = 666\ \text{kJ}$ .

Step 2: Total energy

425 + 1190 + 666 = 2281\ \text{kJ}

Step 3: Percentage of energy from fat

\frac{666}{2281} \times 100 = 29\%

Step 4: Interpret the result

About $29\%$ of the energy in the serving comes from fat. This is within the general guideline that fat should supply no more than about a third of total energy, so the dish is reasonable but not low in fat.

Examples in context

Example 1. A plate of nasi lemak. The coconut rice and fried items are high in carbohydrate and fat, the egg and ikan bilis add protein, and the sambal adds little energy. Because the fried elements raise the fat content, a single plate can be very energy-dense, which is why portion size and how often it is eaten matter for energy balance.

Example 2. A tofu and vegetable stir-fry. Tofu supplies protein, the small amount of oil supplies fat, and any noodles or rice eaten with it supply carbohydrate. Swapping deep-fried tofu for soft tofu cuts the fat, and therefore the energy, of the dish without changing the protein much.

Try this

Cue. State the energy value, in $\text{kJ g}^{-1}$ , of protein, carbohydrate and fat, and say which is the most energy-dense. Recall $17$ , $17$ and $37$ , with fat the most energy-dense.
Cue. A snack contains $5\ \text{g}$ protein, $30\ \text{g}$ carbohydrate and $15\ \text{g}$ fat. Calculate its total energy. Work out $5 \times 17 + 30 \times 17 + 15 \times 37 = 85 + 510 + 555 = 1150\ \text{kJ}$ .
Cue. Explain why two foods of equal mass can provide very different amounts of energy. Link it to their different proportions of fat, since fat gives more than twice the energy per gram of protein or carbohydrate.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original6 marksName the three macronutrients. For each one, state one main function in the body and one good food source.

Show worked answer →

Protein: builds and repairs body tissues such as muscle; a good source is fish or eggs.

Carbohydrate: provides the body with energy; a good source is rice or noodles.

Fat: provides a concentrated source of energy and insulates the body; a good source is cooking oil or nuts.

What markers reward: exactly three named macronutrients (protein, carbohydrate, fat), a correct function paired with each, and a sensible food source for each. One function and one source per nutrient is enough; vague answers like "good for you" score nothing.

Original5 marksA bowl of laksa contains

20\ \text{g}

of protein,

60\ \text{g}

of carbohydrate and

25\ \text{g}

of fat. Using the values

17\ \text{kJ g}^{-1}

for protein and carbohydrate and

37\ \text{kJ g}^{-1}

for fat, calculate the total energy the bowl provides.

Show worked answer →

Energy from protein: $20 \times 17 = 340\ \text{kJ}$ .

Energy from carbohydrate: $60 \times 17 = 1020\ \text{kJ}$ .

Energy from fat: $25 \times 37 = 925\ \text{kJ}$ .

Total energy: $340 + 1020 + 925 = 2285\ \text{kJ}$ .

What markers reward: the correct energy value applied to each macronutrient (note fat is more than double the others per gram), correct multiplication, and a final total with the unit kJ. Forgetting that fat is $37\ \text{kJ g}^{-1}$ is the most common slip.