Thursday, February 15, 2007

Do you know what a calorie is?

Do you know what a calorie is? I mean, think about it. What is it and how does your body use it? This afternoon, I asked myself this question and was shocked to find that I only had a vague notion that it was some form of potential energy in food. But, I didn't know how it was measured nor how the body "burns" calories. We've all grown up with "experts" preaching the correct number of calories for consumption. Calorie counting is indoctrinated in our culture to the extent that people take it for granted. It is high time we start asking some questions...

The USDA's Agriculture Research Service (ARS) website says:

In the U.S., energy in foods is expressed in kilocalories (kcal). The scientific definition of a kilocalorie is the amount of energy needed to raise the temperature of 1 kilogram of water one degree Celsius from 15° to 16° at one atmosphere. The true calorie, sometimes referred to as a "small calorie," is the amount of energy needed to raise the temperature of one gram of water one degree Celsius from 15° to 16° at one atmosphere. A kilocalorie is equal to 1000 calories. While the term "calorie" technically applies to the "small calorie," in common usage, such as in reference to food energy, the term "calorie" is actually a kilocalorie. Internationally, most countries express food energy in kilojoules (kJ). One kcal equals 4.184 kJ. The USDA Nutrient Database for Standard Reference contains values for both kilocalories and kilojoules.

Wikapedia says:

The amount of food energy in a particular food could be measured by completely burning the dried food in a bomb calorimeter, a method known as direct calorimetry [1]. However, the values given on food labels are not determined this way, because it overestimates the amount of energy that the human digestive system can extract, by also burning dietary fiber. Instead, standardized chemical tests and an analysis of the recipe are used to estimate the product's digestible constituents (protein, carbohydrate, fat, etc.). These results are then converted into an equivalent energy value based on a standardized table of energy densities:

Other substances found in food (water, non-digestible fibre, minerals, vitamins) do not contribute to this calculated energy density.

hmmm... so calories are measured by literally burning the digestible portions of our foods. This is interesting. I wonder how they measure the non-digestible fibre, minerals, and other substances that need to be excluded from the calculation. I am beginning to wonder if this really an accurate measure of food's potential energy. Is it, as most doctors and dietitians claim really a good metric? If it is, how does the body "burn" these calories?

Recommended daily energy intake values for young adults are: 2500 kcal/d (10 MJ/d, 120 W) for men and 2000 kcal/d (8 MJ/d, 100 W) for women. Children, sedentary and older people require less energy, physically active people more. Many dietitians commonly state that you will put on one pound for every additional 3500 (kilo)calories that you consume.

Based on so-called evidence for risk of heart disease and obesity, the Institute of Medicine recommends that American and Canadian adults get between 40-65% ofdietary energy from carbohydrates. The Food and Agriculture Organization and World Health Organization jointly recommend that national dietary guidelines set a goal of 55-75% of total energy from carbohydrates.

Now do the math. I'm a male with moderate activity level. So, I need to consume 2500 kcal/day. 55% to 75% of this would be 1375 kcal to 1875 kcal/day from digestable carbs. According to our chart above, there are 4 kilocalories per digestable carb. So, this means that I should, according to the "experts" consume 344 to 469 carbohydrates per day to maintain weight. To lose weight, suppose I cut back to 1900 (kilo)calories per day, I'd be expected to consume 261 carbs (ie. 1900 x 0.55 / 4). This assumes first of all that the "calorie" is a good metric. On my diet, I am only eating 30 to 45 net carbs per day. The levels recommended based on this calculation seem absurdly high.

I am not arguing that reducing what we call calories can in some cases result in weight loss. But, I have seen nothing to convince me of the accuracy of this unit of measure. And, I am not convinced that the effect of actually burning food to see the impact on water temperature has much to do with how our body utilizes the energy potential in food. Perhaps it's time to drop this archaic measurement all together?

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