[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Macronutrients and the energy value of food [Biochemistry](https://durgical.com/en/biochemistry/) · Topic # 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. ## In detail - 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 ## Clinical notes - 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. ## Related conditions - [Obesity](https://durgical.com/en/conditions/obesity/) - [Type 2 diabetes mellitus](https://durgical.com/en/conditions/type-2-diabetes-mellitus/) - [Metabolic syndrome](https://durgical.com/en/conditions/metabolic-syndrome/) ## Related - [Energy requirements and basal metabolic rate (BMR)](https://durgical.com/en/biochemistry/topics/energy-requirements-and-basal-metabolic-rate/) - [Specific dynamic action (thermic effect of food)](https://durgical.com/en/biochemistry/topics/specific-dynamic-action/) - [Dietary fibre](https://durgical.com/en/biochemistry/topics/dietary-fibre/) - [Protein quality and essential amino acids](https://durgical.com/en/biochemistry/topics/protein-quality-and-essential-amino-acids/) - [Balanced diet](https://durgical.com/en/biochemistry/topics/balanced-diet/) - [Glucose](https://durgical.com/en/biochemistry/molecules/glucose/) - [Palmitate (palmitic acid)](https://durgical.com/en/biochemistry/molecules/palmitate/) - [Ethanol](https://durgical.com/en/biochemistry/molecules/ethanol/) - [Carbohydrate chemistry](https://durgical.com/en/biochemistry/topics/carbohydrate-chemistry/) - [Lipid chemistry](https://durgical.com/en/biochemistry/topics/lipid-chemistry/) ## Test yourself 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 %. [Add to my cards (5)](https://durgical.app/estudiar/?add=/en/biochemistry/topics/macronutrients-and-the-energy-value-of-food/) Educational information, not medical advice. For any symptom, see a health professional. Sources: [Carbohydrates: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002469.htm) (MedlinePlus, US National Library of Medicine, 2026); [Dietary Fats: MedlinePlus](https://medlineplus.gov/dietaryfats.html) (MedlinePlus, US National Library of Medicine, 2026); [Protein in diet: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002467.htm) (MedlinePlus, US National Library of Medicine, 2026); [Healthy diet](https://www.who.int/news-room/fact-sheets/detail/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). [← All of biochemistry](https://durgical.com/en/biochemistry/) ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. 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