Energy

The last decades’ debate on the determination of the content of protein, fat and carbohydrates in foods has called for more sophisticated analytical methods.

Based on papers and discussions at the Second International Food Data Base Conference in Lathi, Finland, 1995, a statement, the Memorandum on terms, definitions, and analytical procedures of protein, fat and carbohydrates in food for basic consumption data: issues and recommendations, was presented [1]. This Memorandum concludes ‘that the traditional composition measurements of protein, fat and carbohydrates are inadequate. It is also concluded that most of the data obtained by these measurements do not meet criteria of modern scientific knowledge, nor the needs for national and international application’. The Memorandum emphasises the needs for more comprehensive and detailed information on protein, fat and carbohydrates and gives recommendations for ‘new’ definitions of these basic components of foods. The Memorandum also points at the legal implications of these changes, and it mentions the sometimes rather conflicting use of definitions in legal documents (e.g. on nutritional labelling).

Regional and national legislation may require certain special procedures in the estimation of the energy content of foods, for example for nutrition labelling. Food composition data from tables or databases are frequently used as the basis for nutrition labelling. Therefore, the way in which the energy yielding components and the energy calculation are defined is important for the resulting energy value. Changing the definitions and measurements of protein, fat and carbohydrates has an important impact on the estimation/calculation of the foods’ energy content. As the dietary intake of energy plays an important role in dietary assessments, the impact of changes of definitions and analytical procedures will be discussed in the following.

The basis for today’s calculation of the energy content of foods was established more than a century ago (Rubner, 1883, 1885, 1901 [2,3,4,5]; Atwater and Bryant, 1899 [6]). The concepts of the energy calculations are still valid and in use today, and the concepts defined then are used in legal aspects concerning energy calculation and the definition of the terms like protein, fat and carbohydrate (e.g. in nutritional labelling). The Lathi Memorandum points out, that these definitions, and measurements carried out in accordance with the definitions, are ‘inadequate and do not meet criteria of modern scientific know-ledge, nor the needs for national and international application’.

The late Dr. E.M. Widdowson expressed in a Note on the Calculation of the Calorific Value of Foods and of Diets [7,8]: Atwater, working over 50 years ago, contributed more to our knowledge about the energy value of foods than any one else before and since his time. Although we have learned much since then, and the statement given about 50 years, it still holds.

The works of Rubner and Atwater and Bryant both defines food energy as the sum of energy from the basic constituents protein, fat, carbohydrate and alcohol.

In the following, only reference to Atwater and Bryant’s works will be given as it is this work that is the basis of today’s calculation of the energy content of food.

Merrill and Watt explain Prof. Atwater’s classic investigations in detail in [9]. It reviews the basis and derivation of the Atwater energy factors, which have been determined for basic food commodities. It should be emphasised already here that Prof Atwater’s original work takes into account the variability of the food matrix. Thus, it explains the food specific energy factors, the specific factors that take into account the physiological availability of the energy from these foods.

The more general rounded factors 4 kilocalories/g protein (17 kJ/g), 9 kilocalories/g fat (37 kJ/g), 4 kilocalories/g carbohydrate (17 kJ/g), were developed from the original specific energy factors.

According to the conclusion by Atwater and Bryant in [6], the general factors should only be used for the evaluation of energy content of the average food supply, not to individual foods. For individual foods, the specific factors should be applied.

 It is also concluded that ‘for food supplies of a totally different composition (from the American) the general factors might not be suitable’. Especially for diets with high intakes of foods of plant origin, the energy supply will be overestimated.