Carbohydrates caloric yield

Triacylglycerols are highly concentrated stores of metaholic energy because they are reduced and anhydrous. The yield from the complete oxidation of fatty acids is about 9 kcal g" (38 kJ g- ), in contrast with about 4 kcal g l (17 kJ g-i) for carbohydrates and proteins. The basis of this large difference in caloric yield is that fatty acids are much more reduced. Furthermore, triacylglycerols are nonpolar, and so they are stored in a nearly anhydrous form, whereas much more polar proteins and carbohydrates are more highly hydrated. In fact, 1 g of dry glycogen binds about 2 g of water. 

Those with a normal diet take in food in the forms of carbohydrates, fats and proteins. Because it has a low water content and produces so many ATP molecules, fat yields 9.3 Calories per gram while carbohydrates and proteins yield less than half as much (4.1 and 4.3 calorics per gi-am respectively). Thus, we get a huge number of calories from a small quantity of fat eaten. The average person in the United States has a diet with 50 percent of the calories in the form of carbohydrates, 35 percent in the form of fat and 15 percent in the form of protein. We need about 1 gram of protein per kilogram of body weight per day to replace body proteins that are broken down. A 70 kg person... 

Food is "burned" in the body to yield H20, C02, and energy, just as natural gas or coal is burned in a furnace to yield the same products. In fact, the "caloric value" of a food is just the heat of reaction, AH0, for complete combustion of the food (minus a small correction factor). The value is the same whether the food is burned in the body or in a laboratory calorimeter. One gram of protein releases 17 kj (4 kcal), 1 g of carbohydrate releases 17 kj, and 1 g of fat releases 38 kj. As shown in Table 8.4, which gives the caloric value of some common foods, data are usually given in Calories (note the capital C), where 1 Cal = 1000 cal = 1 kcal = 4.184 kj. 

Many people used to believe that alcohol (ethanol, in the context of the diet) has no caloric content. In fact, ethanol (CH3CH2OH) is oxidized to CO2 and H2O in the body and yields approximately 7 kcal/g— that is, more than carbohydrate but less than fat. 

In formulation of diets to meet energy requirements, proteins and carbohydrates are considered to yield 4 cal. per gram, and fats, 9 cal. per gram. The specific dynamic action of foodstuffs must also be considered and is usually calculated as 6 % of the total caloric value of the food over a 24-hr. period.2 ... 

The caloric contents of diets are usually calculated by the use of Atwater caloric conversion factors which were derived for the mixed diets consumed by Americans around the turn of the century. These factors are based upon the assumptions that each gram of carbohydrate, fat, and protein in the diet will yield 4, 9, and 4 Calories (kcal), respectively. However, the Atwater factors were not intended to be used for single foods or for mixed diets that differed markedly in composition from those for which they were derived. Today, there are a wide variety of diets that have been drastically modified from the average American dietary pattern. Therefore, it is important that dietary planners understand the basic principles of food calorimetry. 

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