Impact of different macronutrient definitions and energy conversion factors on energy supply estimations

Abstract

The magnitude of differences in energy supply using different definitions for carbohydrates and protein as well as different energy conversion factors was investigated. Food supply data for 1999–2001 from FAOSTAT were used for nine countries with different types of diets. Nutrient values were derived from USDA and the British food composition tables for three definitions of carbohydrate (total, available by difference, available as monosaccharide equivalents), three protein definitions (nitrogen (N)×Jones factors, N×6.25, sum of amino acids), fat, and two dietary fibre definitions (AOAC, non-starch polysaccharide). Then three sets of energy conversion factors were applied (Merrill & Watt, general Atwater with/without energy value for fibre, and gross energy—GE). Using the same nutrient definitions, differences between general and specific Atwater factors accounted for 50–320 kJ/capita/day (10–75 kcal/capita/day) and for 290–1500 kJ/capita/day (70–360 kcal/capita/day) between GE and metabolizable energy supply calculations. Protein definitions have a minor impact on per capita energy supply values. They generate differences of less than 1%, or 4–105 kJ (1–25 kcal), with N×6.25 values providing the highest values, followed by Jones factors and the sum of amino acids. The largest differences observed in per capita energy supply calculations are due to carbohydrate definitions. Differences of 3.5–8% or 330–780 kJ/capita/day (80–190 kcal/capita/day) are observed between total and available carbohydrates as monosaccharide equivalents within the general Atwater system. Differences in energy supply between total and available carbohydrates could be minimized by applying an energy factor of 8 kJ/g (2 kcal/g) for dietary fibre, resulting in a higher energy supply of 100–250 kJ/capita/day (25–60 kcal/capita/day) or 1–2%. Differences in energy supply are less influenced by the energy factors as such than by the nutrient definition used, especially for carbohydrates. Differences in energy supply of up to 780 kJ/capita/day (160 kcal/capita/day) or 8% may be statistically relevant and might change research results, estimates of the dietary energy supply and consequently the estimation of the prevalence of undernourishment which may affect nutrition program and policies. Global harmonization of macronutrient definitions and energy factors is important to achieve unambiguous and comparable macronutrient and energy values among countries.