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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 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]
Since lipids are hydrophobic molecules, they need to be solubilized before their metabolism can begin. Lipid metabolism often begins with hydrolysis, [7] which occurs with the help of various enzymes in the digestive system. [2] Lipid metabolism also occurs in plants, though the processes differ in some ways when compared to animals. [8]
The energy yield from a gram of fatty acids is approximately 9 kcal (37 kJ), much higher than the 4 kcal (17 kJ) for carbohydrates. Since the hydrocarbon portion of fatty acids is hydrophobic , these molecules can be stored in a relatively anhydrous (water-free) environment.
In an aqueous system, the polar heads of lipids align towards the polar, aqueous environment, while the hydrophobic tails minimize their contact with water and tend to cluster together, forming a vesicle; depending on the concentration of the lipid, this biophysical interaction may result in the formation of micelles, liposomes, or lipid bilayers.
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. The differing energy density of foods (fat, alcohols, carbohydrates and proteins) lies mainly in their varying proportions of carbon, hydrogen, and oxygen atoms.
This affects first the gross energy that is assigned to carbohydrate—sucrose has a heat of combustion of 3.95 kcal/g (16.53 kJ/g) and starch 4.15 kcal/g (17.36 kJ/g). Secondly it does not provide for the fact that sugars and starch are virtually completely digested and absorbed, and thus provide metabolisable energy equivalent to their heat ...
In humans and many animals, fats serve both as energy sources and as stores for energy in excess of what the body needs immediately. Each gram of fat when burned or metabolized releases about nine food calories (37 kJ = 8.8 kcal). [5] Fats are also sources of essential fatty acids, an important dietary requirement.