Only carbohydrates (including fiber), fats, proteins, organic acids, polyols, and ethanol contain calories. All foods are made up of a combination of these five nutrients. Everything else in food is non-caloric, including (but not limited to) water, vitamins, minerals, antioxidants, caffeine, spices and natural flavors, and enzymes. Tea and coffee also have no calories without sugar or cream added. Nutritionists usually talk about the number of calories in a gram of a nutrient. Fats and ethanol have the most calories per gram, 9 and 7 kcal/g (38 and 30 kJ/g), respectively. Proteins and most carbohydrates have about 4 kcal/g (17 kJ/g). Carbohydrates that are not easily absorbed, such as fiber or lactose in lactose-intolerant individuals, contribute fewer calories. Polyols (including sugar alcohols) and organic acids have fewer than 4 kcal/g.
Each food item has a specific metabolizable energy intake (MEI). Normally this value is obtained by multiplying the total amount of energy contained in a food item by 85%, which is the typical amount of energy actually obtained by a human after the digestive processes have been completed.
The particular food being measured must be burned in a calorimeter, so that the heat released from the food can be accurately measured. This amount is used to ascertain the G.E.V. of the specified food. This number is then multiplied by, usually, 85%; which represents the loss happening during human digestion. Foodstore is a chemical energy
|food component||energy density|
|polyols (sugar alcohols, sweeteners)||2.4||10|
Mathematically, it is much easier to lose weight than to gain weight. Energy is even utilized to digest new food and convert body fat into energy. The equations above assume that all the weight gained and lost is in the form of fat. In reality, this is a mixture of protein, carbohydrates, etc. (in muscle tissue, organs, etc.).
The conversion efficiency of food energy into physical power depends on the form of energy source (type of food) and on the type of physical energy usage (e.g. which muscles are used, whether the muscle is used aerobically or anaerobically). In general, the efficiency of muscles is rather low, and roughly speaking, only about 15% of the food energy is actually converted into mechanical energy. For example, when calculating food energy burnt per unit time gym equipment manufacturers multiply the value of physical power by a factor of eight (assuming 12.5% efficiency). Thus if an exercise bike registers a 150-watt physical power output it might display 17 kcal/min as the rate of food energy burnt per unit time (since 150 W × 8 = 1200 W ≈ 17 kcal/min).
Swings in body temperature - either hotter or cooler - increase the metabolic rate burning more energy, Prolonged exposure to extremely warm or very cold environments increases the BMR. People who live in these type of settings often have BMR's that are 5-20% higher than those in other climates. Physical activity also significantly increases body temperature which in turn burns more calories.
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