First of all, let’s define calories. Calories, or more precisely kilocalories, are simply units (such as pounds, grams, or miles) that are used to measure the amount of energy released during food breakdown or the energy required for metabolism[1].
Most foods and beverages contain caloric nutrients (carbohydrates, proteins, and fats) that are broken down into small units during digestion. These units then enter the body cells where they are converted into energy. The amount of energy obtained from foodstuff is known as energy input. The human body uses caloric nutrients to obtain energy to perform basic physiological processes such as heartbeat, breathing, digestion, and kidney function, among others. Voluntary activities such as exercise also require energy. The total amount of energy expended by the body is called energy output. Both energy input and energy output are measured in calories.
When the energy output is greater than the energy input, there is a negative energy balance. This means that the body has to find ways to obtain extra energy to match the expenditure and balance the equation. This energy comes from body storages, mainly glycogen[2] and fat. Don’t get too excited, though, because burning fat as fuel is not as easy as it seems. Before mobilizing fat, the body uses glucose stored in the liver because this is the fastest way to obtain energy. However, the glycogen storage is very limited, and if the energy shortage persists, the body has to come up with a different solution.
You must be thinking, “Now we’ll be using fat, right?” Wrong! The problem is that fat can only be used for energy in a few body tissues; our nervous system, for instance, is not one of them. The brain can only use glucose as fuel. Since fat cannot be converted into glucose, the body starts breaking down protein for energy, protein that comes from your hard-working muscles and other lean tissues. In addition, as you consume your own body protein, large amounts of water are lost in the process. That’s how people obtain rapid weight-loss through very low calorie diets. In reality, they are not burning fat at all – just muscle and water.
If the low calorie intake persists, the body’s last alternative is a process called ketosis. Ketosis is the partial breakdown of fat which creates compounds known as ketone bodies. The upside is that half of the brain cells can use these compounds for energy, momentarily sparing muscles and other lean tissues from being consumed. However, ketosis upsets the acid-base balance of the blood which can harm the body. In addition, the body ultimately shifts into a conservation mode, slowing down the metabolism in order to save energy. This makes you feel sluggish and fatigued.
Furthermore, don’t think that your body will allow you to cut back calories without a fight. The moment you start restricting your diet, it will respond by increasing the hunger signal. If you think you can outsmart your body with willpower, think again. Hunger is our primary physiological drive to eat and is controlled by internal mechanisms only. This means you can’t win this battle. Your body will make you crave the foods you are trying to avoid, especially sugar- and fat-laden ones. Also, the lack of calories will have a negative impact on your mood, making your life (and the life of your family members) a living hell.
The bottom line is that if your goal is weight-loss, you should strive to improve your diet (eating nutrient-rich foods) rather than jumping into the fad-diet bandwagon. According to specialists, a modest decrease in caloric intake is advisable. Reduce daily caloric intake by 250 calories, while increasing energy expenditure (through physical activity) by 250 calories. This will create an energy deficit of 500 calories per day, which will result in a loss of one pound per week without stressing body systems. This seems to be a more reasonable approach that is also easier to maintain in the long run.
References
Boyle, M. and Long, S. Personal Nutrition. Belmont, CA: Wadsworth Cengage Learning. 2010.
Bryant, C., et al, eds. ACE Lifestyle & Weight Management Consultant Manual. The Ultimate Resource for Fitness Professionals. San Diego: American Council on Exercise. 2007.
Wardlaw, G. and Smith, A. Contemporary Nutrition. New York: McGraw-Hill. 2009.