What are calories and how meaningful is it to count them?
A calorie is a superceded unit of heat energy, defined as the amount of heat required to increase the temperature of 1g of water 1 degree (from 15.5C to 16.5C). A Calorie (capitalised) is the conventional unit of dietary energy; however, it is not the same as a calorie — it is equivalent to 1,000 calories. The prefix ‘kilo’ (as in kcal) is used in some parts of the world. To confuse things more, the capitalisation is not strictly adhered to and often dropped. If you see kCal then you know something is seriously amiss.
The modern unit for energy is the joule and 1,000 joules is sensibly referrred to as a kJ (note the J is a capital). Nutritional labels sometimes include the superceded measure (Cal. or kcal) and the contemporary measure (kJ). In this post, I will reluctantly adhere to conventional confusion by ignoring kJ and writing about calories when I might mean Calories or kcal.
How are calories measured?
Originally, by incineration in a device known as a ‘bomb calorimeter’.
A measured weight of food is sealed in a chamber filled with oxygen. This chamber is placed in a waterbath. The oxygen is ignited (by an electrical spark) and the food incinerated. The heat given off warms the water and the increase in temperature measured. From this, the total calorie content of the food can be deduced. There are also other types of calorimeters that measure these gross food calories.
How is the total calorie content on a nutritional label determined? — By calculation.
In the late 19th/early 20th century, Wilbur O. Atwater came up with the system still in use today. He used the gross calories of the food, adjusted for faecal and urinary elimination, to calculate the average calories in fat, protein and carbohydrate. He determined that per gram, fat had 9 Calories (kcal), alcohol 7, protein 4 and carbohydrates 4.
Nutritional labels use these values to calculate total calories from the fat, protein and carbohydrate content of the food (i.e. as specified on the label), rather than measuring its caloric content directly. Thus if a nutritional label stated that a standard serving of the food contained 10g of fat, 5g of protein and 20g of carbohydrates, then it would also say it contained 190 Calories (10×9 + 5×4 + 20×4). It’s as rough as that.
What are the problems with the Atwater Scale?
There are some significant ones.
Your body is not an incinerator for a start. While there are a total number of calories in the food that is ingested, it does not mean that all of these will be digested, even when allowing for waste elimination. Some other factors:
Dietary fibre is a carbohydrate that we do not directly digest. In the Atwater system the amount of dietary fibre is subtracted from total carbohydrates before the calorie calculation is made. However, our gut bacteria ferment dietary fibre for us and we obtain nutrients and energy from their activity. There has been some suggestion that gut bacteria may be over-active in people who are obese.
Cooking breaks down cell walls and releases nutrients. It makes the food considerably more digestable and we absorb more energy and nutrients from cooked food than from raw food.
It has been argued that cooking our food enabled us to evolve our energy-demanding brain while keeping our digestive system relatively small. It was the topic of my very first post. Strict raw-food diets cannot sustain us and are unhealthy (it is serious enough to cease menstruation in women).
Despite all this, the calculation of calorie content ignores whether the food is raw or cooked.
Another form of pre-digestion (like cooking).
It might be a peanut, but is it peanut paste or a whole peanut? How much did you chew it?
How food is prepared alters how available its nutrients are to us. Pasting, pounding, pureeing etc will break apart cells and increase calorie availability. Likewise, there will be a difference between food that is diced or eaten whole. None of this figures in the calculation.
Is there an alternative?
It is not the potential number of calories in food that matter but rather how available they are to our digestive system. These calculations have been estimated for a large range of food and food preparations and have been known for quite some time (see here for a review). While this system also has its problems, they are nothing like the problems of the Atwater method.
However, the Atwater method is easy. It is considered rough enough to get away with. If that is the reason, then it is a feeble one. I have not come across any other explanation.
Do we really need to count calories at all?
It is a tedious business, slightly neurotic if taken to extremes (and inaccurate anyway) but, most important, I think it reduces the pleasure of eating by replacing deliciousness with arithmetic.
By concentrating on the delight of good food well-prepared and served with care, we may find a contentment in eating that solves the probem of us overdoing it. Mindful eating.