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"A calorie is a calorie" is a tautology used to convey the concept that sources of dietary calories are interchangeable. This concept has been subject to debate since its emergence in the early 19th century.
In 1883, German nutritionist Max Rubner published what he called the "isodynamic law". [1] The law claims that the basis of nutrition is the exchange of energy. [2] In the early 1900s, Carl von Noorden applied isodynamic law to the study of obesity. Von Noorden had two theories about what caused people to develop obesity. The first simply avowed Rubner's notion that "a calorie is a calorie". The second theorized that obesity development depends on how the body partitions calories for either use or storage. These competing theories continue to be a source of dispute within nutrition and diet communities. [3]
A kilocalorie is the equivalent of 1000 calories or one dietary Calorie, which contains 4184 joules of energy. The notion that "a calorie is a calorie" is related to the idea that weight maintenance is a result of equalizing calorie input and calorie output, [4] [5] and may lead to the practice of calorie restriction.
Calorie amounts found on food labels are based on the Atwater system. [6] The accuracy of the system is disputed. For example, a 2012 study by a USDA scientist concluded that the measured energy content of a sample of almonds was 32% lower than the estimated Atwater value. [3]
The driving mechanism behind caloric intake is absorption, which occurs largely in the small intestine and distributes nutrients to the circulatory and lymphatic capillaries by means of osmosis, diffusion and active transport. Fat, in particular is emulsified by bile produced by the liver and stored in the gallbladder where it is released to the small intestine via the bile duct. A relatively lesser amount of absorption—composed primarily of water—occurs in the large intestine.
Protein has a higher thermic effect (TEF) than carbohydrates and fats, meaning that more energy is expended as heat during its digestion and metabolism. Although protein provides roughly 4 calories per gram, about 20–30% of these calories are used to process the protein, resulting in a lower net energy yield compared to carbohydrates and fats. [7]
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