Calories and Energy

Beginner · 10 min

A calorie is a unit of measurement

A calorie is a unit that measures energy. What we call a calorie in everyday speech is actually a kilocalorie (kcal): the amount of energy needed to raise the temperature of one kilogram of water by one degree. On labels you will also see kilojoules (kJ) alongside kcal; 1 kcal equals about 4.2 kJ. A calorie itself is neither good nor bad; what a ruler is to a product's length in centimetres, the label is to its calories.

Remembering the unit's function also calms the good-versus-bad debates about calories: nobody holds the metre responsible for length. The question to ask is not about the unit itself, but which foods the energy comes from and what the total means for you.

Where food energy comes from

The energy in food comes from four components: carbohydrate carries about 4 kcal per gram, protein about 4 kcal, fat about 9 kcal, and alcohol about 7 kcal. Water, minerals and vitamins carry no energy. The energy value on a food's label is found by multiplying its macronutrients by these coefficients and adding them up. Fat's density shows up here too: a small amount of fat takes up a large share of the calculation.

Note already that label values, too, are averages based on laboratory measurements and carry legal rounding and tolerance margins; this detail will meet you again in the programme's final lesson.

Energy balance: in and out

The body takes energy from food and spends it on staying alive, moving and digesting. The relationship between energy taken in and energy spent is called energy balance: over the long run, if intake exceeds expenditure the body stores the surplus; if expenditure exceeds intake, the stores are drawn on. The principle is solid; but because neither side can be measured exactly in daily life, in practice everything runs on estimates. The rest of this programme explains precisely how those estimates are made.

The key word in the balance concept is long run: a single meal, even a single day, is not decisive in this balance. The body is a flexible system that can buffer short-term fluctuations; what carries meaning is the pattern spread across weeks.

The three main items of expenditure

Daily energy expenditure consists of roughly three items. The largest is basal metabolism (BMR): the energy the body spends at complete rest on vital functions alone; in most people it makes up more than half of the total. The second is physical activity: every movement from sport to climbing the stairs. The third is the thermic effect of food: the share spent on digestion and processing, roughly ten percent of the total. In the next lesson we take on the largest of these items, the BMR estimate.

The proportions of these three items vary from person to person: the activity share of someone in physical work and someone at a desk differ greatly. Knowing the items makes it easier to understand what the formulas in the coming lessons represent.