An average adult consumes nearly a million calories a year. Despite this huge number, some healthy people are able to maintain a relatively stable body weight over years and decades without much conscious effort. When “calories in” equals “calories out,” a state of energy balance is achieved and body weight remains constant.
Metabolism is the process of converting food to fuel for the body. Metabolic rate is the number of calories used to fuel the body. Therefore, your resting metabolism is the amount of energy your body uses at rest.
The energy balance equation is driven by common laws of physics and thermodynamics:
- Energy cannot be created or destroyed.
- Energy is either used or stored. When “calories in” are less than “calories out,” a state of negative energy balance occurs and body weight decreases.
In contrast, when “calories in” are greater than “calories out,” a state of positive energy balance occurs and body weight increases.
Being overweight is a result of being in a positive energy balance where “calories in” is greater than “calories out”. This can be caused by:
- eating too much
- not enough exercise or activity
- having a low metabolic rate
Food provides calories (fuel) for the body in the form of carbohydrates, proteins, and fats. Any calories from food that are not used immediately for energy production are stored. Each source of calories is unique in the way it is used and stored by the body.
Carbohydrates are usually the main source of energy for the body. Carbohydrates are broken down into glucose (sugar) and stored in individual muscle cells in the form of glycogen. The body can only store limited amounts of carbohydrates as glycogen. Carbohydrates contains four calories per gram.
Proteins are used by the body to build and maintain body tissues and is rarely used by the body for energy. Like carbohydrates, the body can only store limited amounts of protein. Protein also contains four calories per gram.
Fat is the most calorie-dense of the nutrients. Unlike carbohydrate and protein, the body has an almost unlimited capacity to store fat and body fat stores act as the long term fuel reserves to prevent against starvation. A common misunderstanding is that you can only increase body fat by eating too much fat but studies show that an abundance of fuels, especially carbohydrates, also leads to increased body fat storage. Fat contains nine calories per gram.
Watching the number of calories in the food you eat helps manage your “calories in,” but to achieve energy balance you also need to know your “calories out.” The body uses (burns) calories in three ways: Physical Activity, Digestion of Food, and your Resting Metabolism.