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Proteins and most carbohydrates have about 17 kJ/g (4 kcal/g), though there are differences between different kinds. For example, the values for glucose, sucrose, and starch are 15.57, 16.48 and 17.48 kilojoules per gram (3.72, 3.94 and 4.18 kcal/g) respectively.
Fat has a food energy content of 38 kilojoules per gram (9 kilocalories per gram) proteins and carbohydrates 17 kJ/g (4 kcal/g). [2] Water makes up a large proportion of the total mass ingested as part of a normal diet but it does not provide any nutritional value.
The physiological caloric value of glucose, depending on the source, is 16.2 kilojoules per gram [106] or 15.7 kJ/g (3.74 kcal/g). [107] The high availability of carbohydrates from plant biomass has led to a variety of methods during evolution, especially in microorganisms, to utilize glucose for energy and carbon storage.
The unit is most commonly used to express food energy, namely the specific energy (energy per mass) of metabolizing different types of food. For example, fat (triglyceride lipids) contains 9 kilocalories per gram (kcal/g), while carbohydrates (sugar and starch) and protein contain approximately 4 kcal/g. [29] Alcohol in food contains 7 kcal/g. [30]
All of our energy comes from the calories we eat, and all calories come from three categories: protein, fat and carbohydrates. These are called “macronutrients.”
Carbohydrates and proteins provide 17 kJ approximately (4 kcal) of energy per gram, while fats provide 37 kJ (9 kcal) per gram. [8] However, the net energy derived from the macronutrients depends on such factors as absorption and digestive effort, which vary substantially from instance to instance.
The tables below include tabular lists for selected basic foods, compiled from United States Dept. of Agriculture sources.Included for each food is its weight in grams, its calories, and (also in grams,) the amount of protein, carbohydrates, dietary fiber, fat, and saturated fat. [1]
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