Energy metabolism: how the body spends energy
Food gives us energy, measured in kilocalories (kcal) or kilojoules (kJ). 1 kcal = 4.18 kJ. Metabolism is all the chemical work the body does to use this energy.
Energy is spent in three ways:
- Basal metabolic rate (BMR): energy for breathing, heartbeat, brain and keeping warm while resting. It is the biggest part, about 60 to 70 percent. A rough rule is 1 kcal per kg of body weight per hour, so about 24 x weight in kg per day.
- Physical activity: walking, sport, chores. This part changes the most from person to person.
- Diet-induced thermogenesis: a little heat made when we digest food, about 10 percent of intake.
Total need = BMR x physical activity level (PAL). PAL is about 1.4 to 1.5 for a light day and 1.9 or more for a very active day.
Where food energy comes from
Only three nutrient groups give energy. Per gram: carbohydrate 4 kcal, protein 4 kcal, fat 9 kcal. (Alcohol gives 7 kcal, but it is not a nutrient.) Vitamins, minerals and water give none.
Energy balance: if intake is more than use, the extra is stored as fat; if it is less, the body uses its stores. About 7,700 kcal of surplus makes 1 kg of body fat, as a rule of thumb.
Dietary reference intakes (DRIs)
A dietary reference intake is a set of numbers for healthy people that tells us how much energy and each nutrient to take in. Countries publish them, for example the Japanese DRIs. The main values are:
- EAR (estimated average requirement): the amount that meets the need of half the people in a group.
- RDA (recommended dietary allowance): a little higher than EAR, enough for almost everyone (about 97 percent). Plan diets with this.
- AI (adequate intake): used when there is not enough proof to set an EAR. It is a safe level seen in healthy people.
- UL (tolerable upper intake level): the highest daily amount that is unlikely to cause harm. Above it, risk goes up.
- DG (tentative dietary goal): used in Japan for preventing lifestyle diseases, for example a target for salt or fibre.
- EER (estimated energy requirement): for energy we aim for the amount that keeps weight steady, not "more is safer".
The numbers change with age, sex, pregnancy and activity. Example (adult calcium, rounded): EAR 650 mg, RDA 800 mg, UL 2,500 mg.
Assessing nutritional status
Nutritional status means how well the body is fed. We check it with four groups of methods, easy to remember as ABCD:
- A: Anthropometric (body measures): height, weight, BMI = weight (kg) / height (m)2, waist size, skin-fold, upper-arm size. BMI below 18.5 is low, 18.5 to under 25 is normal, 25 or more is high (adult cut-offs used in Japan; WHO also uses 30 for obese).
- B: Biochemical (blood and urine tests): haemoglobin for iron, albumin for protein, blood sugar, cholesterol.
- C: Clinical: signs seen on the body such as pale eyelids, bleeding gums, swollen feet, dry hair.
- D: Dietary: what the person eats, by a 24-hour recall or a food diary, then compared with the DRIs.
BMI is simple, but it does not tell fat from muscle, so we combine it with the other methods. Children use growth charts, not adult BMI cut-offs.
Try it: your own check at home
1) Write down everything you ate yesterday and guess the energy. 2) Measure your weight and height and work out BMI. 3) In the 3D, set the sliders to your numbers and compare food and energy bars. Do not use this to diet. Share the result with a parent or doctor.
Key formulas and definitions
- BMR (rough) = 24 x weight in kg (kcal/day)
- Energy need = BMR x PAL
- Energy from food = 4 x carbohydrate g + 4 x protein g + 9 x fat g
- BMI = weight (kg) / height (m)^2
- EAR < RDA < UL (for most nutrients)
- 1 kcal = 4.18 kJ; about 7,700 kcal = 1 kg body fat
Worked examples
1. Find the rough BMR of a 50 kg person.
BMR = 24 x 50 = 1,200 kcal per day.
2. A meal has 50 g carbohydrate, 20 g protein and 10 g fat. How much energy does it give?
Carbohydrate 50 x 4 = 200. Protein 20 x 4 = 80. Fat 10 x 9 = 90. Total = 370 kcal.
3. A person has BMR 1,500 kcal and PAL 1.5. What is the daily energy need?
Need = 1,500 x 1.5 = 2,250 kcal per day.
4. Find the BMI of a person who weighs 54 kg and is 1.5 m tall. Which group?
BMI = 54 / (1.5 x 1.5) = 54 / 2.25 = 24. This is in the normal range (18.5 to under 25).
5. A person needs 2,000 kcal but eats 2,500 kcal every day. Estimate weight gain in a week.
Surplus = 500 kcal per day, so 3,500 kcal in 7 days. 3,500 / 7,700 = about 0.45 kg of fat per week.
6. A diet of 2,200 kcal should have 25 percent of energy from fat. How many grams of fat is that?
Fat energy = 0.25 x 2,200 = 550 kcal. Grams = 550 / 9 = about 61 g.
Common mistakes
- Thinking vitamins and minerals give energy. Only carbohydrate, protein and fat do.
- Treating RDA as the minimum. The average need is the EAR; RDA is set higher to cover almost everyone.
- Believing that more is always better. Above the UL, risk rises (for example too much calcium or vitamin A).
- Judging health by BMI alone. BMI cannot tell muscle from fat, so use other checks too.