Are all calories the same? No, and here's why
Common sense would suggest that a calorie is a calorie is a calorie, right? Unfortunately, there is no law that human nutrition must follow common sense. The truth is that the body treats different kinds of calories differently and that fat calories tend to be "fatter" than those from proteins and carbohydrates. There are 4 nutrients that provide energy: Fat, Carbohydrate, Protein and Alcohol. As a rule of thumb, fat provides 9 calories per gram, carbohydrates and protein provide 4 calories per gram and alcohol delivers 7 calories per gram.
In actual practice, however, these numbers are somewhat misleading because your body is more efficient at storing energy from fat than it is in storing energy from protein. For example, 97 percent of the calories provided by fat are actually available to the body. In contrast, your body can only use about 75 percent of the calories that protein provides. The metabolic reasons for the differences in efficiency between different fuels is not well understood, but may have something to do with differences in the amount of energy needed for digestion, absorption and delivery of each type of fuel. At 97 percent efficiency, about 3 percent of the energy from fat is "lost",usually as heat. This means that for every 100 calories of fat consumed, 3 calories are lost as heat and 97 calories are available for energy or storage.
Fuel Efficiency of Energetic Nutrients (per 100 calories):
In addition to the differences in fuel efficiency between the energetic nutrients, there is also a certain amount of variation in fuel selection between different people. Some people have a tendency to burn more fat, while others prefer to burn more carbohydrates.
Carbohydrate storage is carefully regulated in the body, while fat storage is not. The body has an almost unlimited capacity to store calories as fat (unfortunately), but, only about 4,000 calories (at most) can be stored as carbohydrate in the form of liver and muscle glycogen.
Experiments have shown that the body will burn carbohydrate calories first, turning only to fat calories when additional energy is needed. An excess intake of carbohydrates, while not directly promoting an increase in fat storage, will provide a ready source of calories for energy-requiring processes, preventing the body from accessing stored fat energy.
It is not well understood why some people are better fat burners or why others are better at storing fat. It may have to do with concentrations of fat-burning enzymes, number of blood vessels in adipose tissue, or the presence or absence of various genetic variables. What is clear, however, is that those individuals identified as "high fat burners" tend to burn more of the fat that they consume in their diets and therefore have less to store.
Conversely, "high carbohydrate burners" tend to store more of the fat that they eat and are more than twice as likely to become overweight as the high fat burners.
You can enhance your body's ability to burn fat through endurance exercises like running, walking, cycling and hiking. Over time, your body develops enhanced blood supplies to tissues, produces higher levels of enzymes involved in fat metabolism and "learns" to use fat more often as an energy source.
What are Calories
A calorie is a measurement of energy. We tend to associate calories with food, but any sort of energy can be measured in calories. The official definition of a calorie is the amount of energy needed to raise the temperature of a gram of water by 1 degree C. A kilocalorie is 1,000 calories. Just to make life confusing, the "calorie" that you see on packages of food is really a "kilocalorie" in the scientific sense.
Specifically, a calorie is the amount of energy, or heat, it takes to raise the temperature of 1 gram of water 1 degree Celsius (1.8 degrees Fahrenheit). One calorie is equal to 4.184 joules, a common unit of energy used in the physical sciences.
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