Metabolism and Body Weight
Understanding the energy content of food, metabolic rates and the law of thermodynamics.
Calories are a way to measure energy. Just as watts measure electrical energy, calories measure heat energy.1
By definition, a calorie (technically called a kilocalorie or kcal) is the amount of heat required to raise 1 kilogram of water 1 degree Centigrade. Calories are the typical method of energy measurement in many parts of the world to quantify the fuel energy that is available from food and to measure the heat output that comes from metabolism, which is the number of calories the body burns.
Metabolism has three major components:
- resting metabolic rate;
- physical activity; and
- calories used to process food.
Resting metabolism is the calories used to keep all systems going day in and day out; it is the calories burned by the brain, heart, kidneys and all organs and cells in the body. About two-thirds to three-quarters of the calories we burn every day are accounted for in resting metabolism.
The second component of metabolism is the calories burned in activities such as walking, stair climbing, picking up children and planned exercise. Calories burned in physical activity are the most variable part of metabolism and also the component over which you have the most control.
Finally, a relatively smaller percentage of calories are used to convert food calories into energy the body can use and this varies by macronutrient. e.g., protein and fiber-rich carbohydrates require more calories to process than non-fiber containing carbohydrates and fat.
The Law of Thermodynamics
The only scientifically proven method for losing weight involves burning more calories than are taken in.2 This fact, often called the "law of thermodynamics," has been shown time and time again in decades of rigorous scientific studies. One example of the hundreds of studies that exist was done in Switzerland. Fifty-four obese people had their calories restricted to 1,100 per day. Different combinations of foods and meal timing were tested. There was no difference in weight losses; it came down to the number of calories, not how they were provided, that accounted for the weight loss that the participants experienced.3
At the end of the day, the only true way to lose weight is to eat fewer calories in food and/or burn more calories. Yet only about 1/3 of Americans trying to lose weight try to do so by using the recommended method of eating less and exercising more; the fundamental foundation of weight loss.4
Other Science Library Topics:
Elert, Glenn. Energy
. Copyright 1998-2004.
22005 Dietary Guidelines Advisory Committee Report
Golay A, Allaz AF, Ybarra J, Bianchi P, Saraiva S, Mensi N, Gomis R, de Tonnac N. Similar weight loss with low-energy food combining or balanced diets
. Int J Obes Relat Metab Disord. 24 (4):492-6, 2000.
4Yaemsiri S, Slining MM, Agarwal SK. Perceived weight status, overweight diagnosis, and weight control among US adults: the NHANES 2003-2008 Study
. Int J Obes (Lond). 2011 Aug;35(8):1063-70.