It is the end of September 2026. In Year 3 of the hotel and catering vocational school (Italian alberghiero, the food service and hospitality track), Food Science and Culture restarts from where the first two years left it: the nutrients, which I covered in Nutrients in Year 1 of hotel school. It now opens the last three years with bioenergetics: how much energy the body uses and how to calculate it. The first test falls four to seven weeks away, so between mid-October and mid-November, and in this topic the part that decides the grade is the calculations with the formulas, not the definitions.
The very same calculation of basal metabolic rate appears, under different names, in Physical Education and in the applied sciences liceo (Italian academic high school) with a biology focus. If you got here from another course, the method below still applies; only the context in which they ask you changes.
This block of bioenergetics is also the first in the last three years where the subject stops being only “knowing what a food contains” and becomes “doing the sums with those numbers”. It is a change of pace that catches out students coming from a more descriptive first two years: here the formulas count as much as the definitions, and a wrong calculation costs you in a test as much as a wrong definition, if not more.
What energy expenditure is made of
Before calculating anything, you need to know what you are adding up. A person’s total energy expenditure in a day breaks down into three parts:
- Basal metabolic rate (BMR): the energy the body uses at complete rest, awake but still, just to keep the vital organs working: heart, breathing, body temperature. It is the biggest slice, usually 60-70% of the total.
- Diet-induced thermogenesis (DIT): the energy spent digesting, absorbing and metabolising the food itself. It is estimated at around 10% of the total requirement. It is a small share, but the textbook often asks you to know it by heart.
- Physical activity: the energy spent moving, from walking to sport. It is the component that varies most from person to person.
The total energy requirement (TER) is the sum of these three, but in school calculations you almost never add them up separately. You estimate the basal metabolic rate and multiply it by a coefficient that accounts for both DIT and physical activity together. This is the PAL, and I will come to it in a moment.
How to estimate basal metabolic rate
Two formulas circulate in textbooks, and it is worth knowing both because not every teacher uses the same one.
Simplified estimate (quick, used as a first approximation): you assume a consumption of about 1 kcal per kg of body weight per hour for men, and about 0.9 kcal per kg per hour for women. For a daily basal metabolic rate you multiply by 24 hours:
This is a school estimate, handy for a quick calculation but not very precise because it ignores height and age. Two people with the same weight but very different heights (one stocky, one slender) actually have different basal metabolic rates, which this formula cannot tell apart.
Harris-Benedict equation (the “official” one, more precise): it uses weight, height and age together, with different coefficients for men and women. Watch the units: weight in kg, height in centimetres, age in years.
The result is in kcal per day. Note that age has a negative coefficient in both: for the same weight and height, basal metabolic rate falls with age, because lean mass tends to decrease.
From basal metabolic rate to total energy requirement: the PAL
Basal metabolic rate alone is not enough: a sedentary person and an athlete with the same weight and height have the same BMR but very different energy requirements. The total energy requirement is obtained by multiplying the basal metabolic rate by the PAL, the physical activity level:
The LARN (the Italian reference intake levels for nutrients and energy) give indicative PAL values of about:
- ≈ 1.45 for a sedentary lifestyle;
- ≈ 1.6 for moderate activity;
- ≈ 1.75 for an active lifestyle.
One important warning here: your textbook may give a PAL table that differs slightly from this one, with more bands or slightly shifted values. This is normal and depends on the edition and the source. In a test, always use the values from the table your teacher gave you, not those of another book.
The reason the PAL exists as a single coefficient, instead of adding diet-induced thermogenesis and physical activity separately, is practical. At school you are not asked to estimate precisely how many kcal a person spends running or sitting in class, but to place them in a lifestyle band. It is a declared simplification, not an inaccuracy to be corrected.
Body Mass Index (BMI)
BMI (IMC in Italian) is an indicator separate from energy requirement, but it is almost always asked together with it because it is used to judge whether a person’s weight is appropriate for their height:
Watch the unit of measurement: here the height goes in metres, not in centimetres as in Harris-Benedict. It is by far the most frequent mistake, and I come back to it just below. The reference WHO bands are:
| BMI | Interpretation |
|---|---|
| below 18.5 | underweight |
| 18.5 – 25 | normal weight |
| 25 – 30 | overweight |
| above 30 | obese |
The height is squared, not simply multiplied, because BMI tries to approximate how much weight “corresponds” to a given stature. Without the square, two people of the same build but different height would end up with indices too far apart, whereas squaring makes the ratio more stable over a wide range of statures. You do not need to prove it in a test, but knowing that it is not a square “at random” helps you remember the formula.
Splitting nutrients in kcal
The last piece of the programme links energy requirement to how much of each nutrient you should eat. The recommended percentages of total daily kcal are:
- carbohydrates: 45-60%;
- lipids (fats): 20-35%;
- proteins: about 15%.
To convert the kcal assigned to each nutrient into grams, you need to know how many kcal one gram of each provides:
- carbohydrates: 4 kcal/g
- proteins: 4 kcal/g
- lipids: 9 kcal/g
The typical mistake
Two mistakes always come back in marked tests, and both come from mixing up the formulas:
- Using the height in the wrong unit. In Harris-Benedict the height goes in centimetres, in BMI it goes in metres. If you calculate the BMR and then the BMI in one go, without stopping to check the unit, you often leave the height in centimetres in the BMI too. The result comes out absurdly small (a number close to zero) and should set off an alarm bell.
- Applying the PAL twice. This happens when you mix up “I have already used the simplified formula that accounts for light activity” and then multiply again by the PAL: the final requirement comes out inflated. The PAL is applied only once, always and only to the basal metabolic rate.
Three worked exercises
Exercise 1: BMR and requirement of a 17-year-old boy. Marco is 17, weighs 68 kg and is 176 cm tall. He plays sport three times a week (moderate activity, PAL ≈ 1.6). Calculate his basal metabolic rate with Harris-Benedict and his total energy requirement.
Term by term: ; ; .
Total energy requirement:
Comparison with the simplified formula: kcal/day, about 135 kcal less than Harris-Benedict. The difference is normal, because the simplified estimate ignores height and Marco is taller than average for his age. It also shows why, in an important calculation, it is better to use Harris-Benedict, keeping the simplified formula only as an order-of-magnitude check (“does the result make sense?”).
Exercise 2: BMI and interpretation. A girl weighs 58 kg and is 1.64 m tall. Calculate her BMI and interpret the result.
21.56 lies between 18.5 and 25: the girl is normal weight.
Exercise 3: from kcal to grams of each nutrient. A requirement of 2,400 kcal a day is to be split as follows: 50% carbohydrates, 30% lipids, 20% proteins. How many grams of each nutrient does this correspond to?
Kcal per nutrient: carbohydrates kcal; lipids kcal; proteins kcal.
Conversion to grams:
Consistency check: g of nutrients in total, and kcal, which matches the starting figure.
There is not yet a dedicated practice test on this topic on the site: you can find all those available, on other subjects, in /simulazioni/. For food hygiene, the next step is HACCP and food contamination: a guide to the test; for prevention in a broader sense, primary, secondary and tertiary prevention in hygiene.
If these calculations are creating more confusion than clarity, or you need to go over them together before the test, you can find all the subjects we cover at /materie/scienze/. The first introductory call of 30 minutes is free, and you can find it at /contatti/.
