Update 25th Feb:
There have been several pieces in the news recently about Calories and whether we can trust the labels. The first was a piece in the New York Times where the journalist tested 5 foods and found that 4 out of the 5 were mislabelled, they actually contained more calories in total than listed. In fact the total was 500 calories more, the equivalent of eating a cheeseburger! (You can see the post by clicking here).
Secondly, research done at the U.S. Department of Agriculture’s Human Nutrition Research Center last year, found that almonds have 20% fewer calories than listed. Some foods could differ from their listed calorie value by as much as 50%. The difference is due to more complex and unprocessed foods being harder for the body to digest. The traditional method of calculating calories shown below is outdated and inaccurate.
We are constantly reading or being told about Calories and how many we need (or don’t need!) but when it comes to the labels of Calories in food, can we trust them? How are they calculated?
In order to answer this question, it helps to define a Calorie. A Calorie is a unit that is used to measure energy. The Calorie you see on a food package is actually a kilocalorie, or 1,000 calories. A Calorie (kcal) is the amount of energy needed to raise the temperature of 1 kilogram of water by 1 degree Celsius.
The original method used to calculate the number of Calories in a food was to measure the energy it produced. The food was placed in a sealed container surrounded by water–an apparatus known as a bomb calorimeter. The food was completely burned and the resulting rise in water temperature was measured. This method is not frequently used today.
Nowadays most food tables are based on an indirect Calorie estimation made using the so-called Atwater system. In this system, Calories are not determined directly by burning the foods. Instead, the total calorific value is calculated by adding up the Calories provided by the energy-containing nutrients: protein, carbohydrate, fat and alcohol. Because carbohydrates contain some fibre that is not digested and utilised by the body, the fibre component is usually subtracted from the total carbohydrate before calculating the calories.
The Atwater system uses the average values of 4 Kcal/g for protein, 4 Kcal/g for carbohydrate, and 9 Kcal/g for fat. Alcohol is calculated at 7 Kcal/g. (These numbers were originally determined by burning and then averaging.) Thus the label on an energy bar that contains 10 g of protein, 20 g of carbohydrate and 9 g of fat would read 201 kcals or Calories.
Does this give us a true calorific value? It still seems a bit hit and miss to me as the Atwater system assumes that all proteins, carbohydrates and fats are burnt in a similar way. For example, comparing lamb steak which is 159 Kcal per 100g, with almonds which are 645 Kcal per 100g, do the nuts really give that us that many more Calories or does it depend on how easy foods are to digest?
Are you confused yet?! Basically, Calories are not calculated for each food, they are based on averages, so you cannot trust the label on a food packet to be accurate.
My feeling is that this will also vary by individual too, so only use Calorific values on packets as a rough guide and try foods yourself to see whether you put weight on! As a rule of thumb, the more unprocessed the food, the fewer Calories you will get as the body has to work harder to digest it.
Do you have any thoughts on Calories listed on food labels?