3,500 calories to lose one pound?

There is a generalized statement floating around mainstream fitness enthusiasts regarding how many calories it takes to burn off one pound of bodyweight. Many are led to believe a loss of 3,500 calories is required to lose one pound of weight. According to recent research, it’s possible to lose much more at the same energy deficit. Although haste makes waste when it comes to muscle preservation.

In Matrix, a 1999 science-fiction action movie, humans were separated between a tangible reality and a computer-generated dreamscape. The world known as reality was actually being produced by a sophisticated supercomputer. People were unconsciously living their lives in the Matrix, unaware of any deception. In the real world, their body heat and bioelectricity was used to generate energy for the machines that kept them contained. Human bodies, grown in pods and artificially controlled, were exploited as batteries.

Energy is needed by the body to stay alive, warm and active. Loosely defined, energy is the power used to do work, produce heat or emit light. It can not be created or destroyed – only changed from one form to another. When the human body’s core temperature increases, energy is being converted to heat. We obtain energy contained within food and drinks; such as carbohydrates, proteins and fats. We oxidize dietary macro nutrients to produce energy for automatic and voluntary bodily functions and movements. Energy is also used to build new tissues for growth and repair.

To maintain bodyweight, input must match output. To lose bodyweight, a person must consume less energy (food calories) than he/she expends (activity plus basal metabolic rate). Different people require different amounts of energy. Some activities and physical environments require more energy than others.

Energy is measured in joules (J) and calories (cal). A joule is defined as the energy needed to move one kilogram across one meter with the force of one Newton. A calorie is defined as the energy required for raising the temperature of one gram of water from 14.5 to 15.5 degrees Celsius. One calorie is equal to 4.184 J.

Since people use a lot of energy, nutritionists use larger units to measure consumption. For instance, 1,000 calories are equal to one kilocalorie (kcal), commonly known as a food calorie. Nutrition facts on food labels display “calories” when they are actually referring to kilocalories.

One megajoule (MJ) is equivalent to 1,000 kJ; or 1 million joules. To measure energy intake and output in the human body, one megajoule equals 239 kcal. One kilocalorie is a mere 4.184 kJ. Using this information, a 3,500 calorie deficit is equivalent to 14,644 kJ.

On Sept. 11, the International Journal of Obesity published a meticulous look into how human metabolism affects how the body burns substrates for fuel. The research from the National Institutes of Health, in Bethesda, Md., explained how glycogen, stored carbohydrate, and protein have similar energy densities with comparable water in body tissues. According to their research, the metabolizable energy densities of body glycogen, protein and fat are 17.6, 19.7 and 39.5 MJ per kilogram, respectively.

To calculate the loss of pure muscle (protein), 19.7 MJ could be converted to 19,700 kJ; which further breaks down to 4,708.41 kcal. A person who burns 3,500 kcal (14,644 kJ) could chop 740 grams (.74 kg), or 1.6 pounds of pure muscle off the body. This is much more than a one-pound weight loss.

What about pure fat loss? First, convert 39.5 MJ into 39,500 kJ. This is equivalent to 9,440.73 kcal. Burning 3,500 calories would chop 370 grams (.37 kg), or around 13 ounces (.814 pounds) of lard – not even a pound!

The underlining issue is: the body doesn’t burn pure fat or muscle – and stored glycogen is readily available and routinely used to feed the metabolic furnace. A sustained deficit can result in an increased rate of weight loss; if metabolized tissue comes less from fat and more from less energy dense lean body mass. Resistance training and high-protein diets are routinely used to preserve muscle while losing fat.

If weight loss slows over time, either energy expenditure is decreasing or dietary energy intake is increasing. The later is a likely event since less body mass requires less energy for maintenance. Weight loss should be a slow process – if maximum fat loss and muscle preservation is the goal. While maintaining the same caloric deficit, considerable falls in bodyweight may be eluding to an increase in metabolized lean body mass.

Manipulations in diet (input) and physical activities (output) can lead the body toward a preference, but a combination of substrates will always fuel the human battery. Burning a pound of body weight is much more complicated than a simple 3,500-calorie deficit.

KD Hall, What is the required energy deficit per unit weight loss? Int J Obes, 2007.