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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.
Macronutrients are defined as a class of chemical compounds which humans consume in relatively large quantities compared to vitamins and minerals which provide humans with energy. 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]
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]
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.
The complete oxidation of fatty acids releases about 38 kJ/g (9 kcal/g), compared with only 17 kJ/g (4 kcal/g) for the oxidative breakdown of carbohydrates and proteins. The adipocyte , or fat cell, is designed for continuous synthesis and breakdown of triglycerides in animals, with breakdown controlled mainly by the activation of hormone ...
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.
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 ...
Typically, the complete breakdown of one molecule of glucose by aerobic respiration (i.e. involving glycolysis, the citric-acid cycle and oxidative phosphorylation, the last providing the most energy) is usually about 30–32 molecules of ATP. [16] Oxidation of one gram of carbohydrate yields approximately 4 kcal of energy. [3]