A calorie is not a calorie
Cutting back calories can help you maintain a healthy weight. But not all calories are equal in the energy equation, explains Dr Karl.
We people in the wealthier countries are getting fatter. In the USA, about two-thirds of the population are overweight, and about one-fifth are obese. This has been happening despite an enormous increase in weight-loss diets.
The latest research seems to show two main results.
First, the actual diet doesn't matter so long as you actually reduce your calorie intake and do more exercise. And second, while reducing calorie intake is an important part of achieving and maintaining a healthy weight, a calorie that you eat may not add up to a calorie available to your body.
A lot of diets tend to focus on the constituents of our food: fats, proteins and carbohydrates. It turns out that you can make a lot of money by writing a book demonising any two of these constituents — say the carbohydrates and the fats — and then claiming that only the remaining constituent — say the protein — is worth eating. Or they may claim that carbohydrates and protein are evil, but you can eat all the fats you like.
However, research did find that the differing proportions of carbohydrate, protein and fat didn't really matter in terms of weight loss. All you had to do was to reduce overall calorie intake in a way that was healthy to your heart. That is, low levels of saturated fats and cholesterol, and high levels of dietary fibre. The individual macro nutrient choice — fat, protein, carbohydrate — didn't matter; just the reduced calories with a bit of exercise.
Now for the second, and rather confusing, claim that "a calorie is not a calorie". Now, in the land of dietetics, a calorie is the amount of energy needed to heat up a kilogram of water by 1°C. (Strictly speaking I know we should talk in kilojoules where one calorie is about 4.2 kilojoules, but calories are so well-known.)
The number of calories assigned to each food is based on a system set up by Wilbur Olin Atwater around the year 1900. He simply 'burnt' each food type — fat, protein and carbohydrate -— in a machine called a bomb calorimeter and measured how much energy it generated. From that, he worked out how much energy was in each foodstuff depending upon its relative proportions of fat, protein or carbohydrate. And this is the overly simple system that we still use today.
Of course, the real energy calculation is much more complicated.
For one thing, it costs your body different amounts of energy to digest different foods. So fats need only two or three per cent of their inherent energy to digest them; while carbohydrates need five to 10 per cent; and proteins need a massive 25 per cent to unravel the tightly wound amino acids that make them up.
So if you eat 100 calories of fat or 100 calories of protein, your body will have access to 98 calories for the fat, but only 75 calories for the proteins.
Second, the Atwater method deals with the total potential amount of energy in the food. But, for example, in the case of almonds, a significant amount of energy — about one-third — does not get extracted but goes straight through your gut into your faeces.
Food preparation is the third factor in the 'calorie is not a calorie' story. Depending on the food, you will make a lot more energy available to your body if you first process it by grinding or pounding it to increase its surface area; or by cooking it (boiling it, baking, grilling, sautéing etc); or fermenting it. For example, mice lose weight when they only eat raw sweet potato, but gain weight eating cooked sweet potato. And they gain a little weight when eating raw meat, but gain more weight with eating cooked meat.
A fourth factor is that people differ from one another. Consider the length of the gut. Back in the early 1900s some European scientists decided to measure the length of the intestine. They found that some specific Russian populations had an extra half metre of bowels as compared to some Polish populations. The longer gut meant that the Russians could extract more energy from their diet than could the Poles.
And of course, we all have different populations of bacteria (and the like) in our digestive tracts that can both extract more energy from our food, and then leave for us greater or lesser amounts of energy.
And finally, it costs us more energy to process whole foods than processed foods.
So the weighty question is: how do we make a simple message about eating fewer calories easier to digest …?
Published 12 November 2013
© 2014 Karl S. Kruszelnicki Pty Ltd