What are calories?

Calories are units of energy. The technical definition of a calorie is:

the amount of energy required to raise the temperature of 1g of water by one degree Celsius”

Obviously, that doesn’t mean much when we see it on a packet. So in food terms, the calorie content just means the amount of energy that food will give us. When you see ‘kcal’ written on the back of packets, this stands for kilocalorie. Because a calorie is such a small number, we express them in thousands. It’s just like the difference between meters and kilometres.


Where do calories come from?

There are two main determinants of the calorie content of foods; firstly, the water content and secondly, the macronutrient composition. Macronutrients are fat, protein and carbohydrate and all contribute to energy intake but by different amounts, as shown below. Alcohol is included in the list below because although it is not technically a nutrient, it still contributes to our energy intake.

  • Fat = 9 kcal/ gram
  • Alcohol = 7 kcal/ gram
  • Protein = 4 kcal/ gram
  • Digestible carbohydrate = 4 kcal/ gram

Practically, this means that foods that are higher in fat tend to be higher in calories when compared to foods that are higher in protein or carbohydrate.


How many do we need?

Our total daily energy expenditure, meaning the amount of calories we ‘burn’ in a day, is determined predominantly by our ‘fat free mass.’ This just means the part of our body which is not fat, including bones, muscles and organs. So someone very muscular will have a higher energy requirement than someone not muscular.

As a rough guide, men need approximately 2500 kcal/ day, whilst women need 2000 kcal/day; this is predominantly because men have a higher fat free mass. More accurate calculations are based on your weight, height and age. This is because as we age, our fat free mass declines. Lighter and shorter people will also have a lower fat free mass.


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