Atwaterfactor

Atwater factor, commonly referred to as Atwater general factors, is a set of energy conversion values used to estimate the metabolizable energy provided by macronutrients in foods. The system was developed by Wilbur Olin Atwater and colleagues in the late 19th and early 20th centuries, based on calorimetric measurements and observed digestion in humans and animals. The goal was to translate grams of nutrient intake into an estimate of available energy.

The most widely used general factors assign approximately 9 kilocalories per gram of fat, 4 kcal per gram of protein, 4 kcal per gram of available carbohydrate, and 7 kcal per gram of ethanol. In practice, nutrition labels and energy calculations often use the formula: energy = 9 × fat grams + 4 × protein grams + 4 × carbohydrate grams + 7 × alcohol grams. These values provide a practical approximation of the energy people can obtain from foods, rather than precise metabolizable energy for every individual.

Limitations of Atwater factors include variation in energy yield depending on food matrix, processing, digestibility, and individual metabolism. Dietary fiber, fermentable components, sugar alcohols, and other non-digestible elements are not consistently accounted for, and some jurisdictions employ modified or specific factors to reflect different digestibility or food types. Therefore, the Atwater system is a convenient standard for labeling and dietary planning, but it does not capture all nuances of actual energy absorption and use in every person or food.