Macronutrients and the energy value of food !

Carbohydrates, fats and proteins supply energy: about 4, 9 and 4 kcal per gram. Alcohol gives 7 kcal per gram with no nutrients.

Updated 2026-11-01

Key facts

Physiological fuel values (Atwater factors): carbohydrate 4 kcal/g, protein 4 kcal/g, fat 9 kcal/g, ethanol 7 kcal/g (1 kcal = 4.184 kJ).
Burned in a bomb calorimeter, protein gives more (about 5.4 kcal/g) than in the body, because the body excretes its nitrogen as urea instead of oxidising it.
Respiratory quotient (RQ = CO₂ produced ÷ O₂ used): carbohydrate 1.0, protein about 0.8, fat about 0.7, mixed diet about 0.8; an RQ above 1 means net fat synthesis from excess carbohydrate.
WHO adult guidance: carbohydrate about 45–75 % of energy (free sugars under 10 %, ideally under 5 %), fat 15–30 % (saturated under 10 %, trans under 1 %), protein about 10–15 %.
Essential parts: linoleic (ω-6) and α-linolenic (ω-3) fatty acids and the essential amino acids; no carbohydrate is strictly essential, but the brain prefers glucose.

1. Example: a meal with 50 g carbohydrate, 20 g protein and 10 g fat gives 200 + 80 + 90 = 370 kcal.
2. Carbohydrates: the main fuel of brain and red blood cells; complex starches and fibre-rich foods are preferred over free sugars.
3. Fats: the densest energy store; they carry the fat-soluble vitamins (A, D, E, K) and supply essential fatty acids for membranes and eicosanoids.
4. Proteins: supply amino acids to build body proteins, enzymes and hormones; they are burned for energy when eaten in excess or during fasting.
5. Water, vitamins and minerals give no energy; vitamins and minerals are the micronutrients.
6. Fat has the lowest RQ because it contains little oxygen: oxidising it needs more O₂ for each CO₂ produced.

Also called: Calorific value of foods, Atwater factors, Respiratory quotient

In indirect calorimetry of ventilated patients, an RQ above 1 points to carbohydrate overfeeding, which raises CO₂ production and can make weaning harder.
Alcohol supplies 'empty' calories: heavy drinkers may get enough energy yet lack protein, thiamine and folate.
Essential fatty acid deficiency (for example in long fat-free parenteral nutrition) causes scaly dermatitis, hair loss and poor wound healing.

Think of the answer, then tap the question to see it.

How many kilocalories does a gram of carbohydrate, protein, fat and ethanol give in the body?

About 4, 4, 9 and 7 kcal per gram respectively.

Why does protein give more energy in a bomb calorimeter than in the body?

In the calorimeter its nitrogen is burned too; in the body the nitrogen leaves as urea, which still holds energy.

What does a respiratory quotient above 1 indicate?

Net lipogenesis: excess carbohydrate is being turned into fat, which releases CO₂ without using matching O₂.

Which two fatty acids are essential in the diet?

Linoleic acid (ω-6) and α-linolenic acid (ω-3); humans cannot put double bonds beyond carbon 9 of a fatty acid.

What upper limits does WHO give for free sugars and saturated fat?

Free sugars under 10 % of energy (ideally under 5 %) and saturated fat under 10 %; trans fat under 1 %.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: Carbohydrates: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Dietary Fats: MedlinePlus (MedlinePlus, US National Library of Medicine, 2026); Protein in diet: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Healthy diet (World Health Organization, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.