NUTRIENTS AND ENERGY
Diets are often described in terms of their energy content, despite the fact that they need to supply materials for needs other than just their energy provision (Figure 10.1). It is possible for a diet to supply an adequate amount of energy but be deficient in some essential raw materials, such as vitamins and minerals needed by the individual. A balanced diet is one that supplies adequate energy distributed appropriately between carbohydrates, lipids, and proteins and contains the necessary amounts of vitamins, minerals, trace elements, water and nondigestible fiber.
Nutritional energy is usually measured in joules or kilojoules. The number of joules required by any one person needs to be matched to their energy output (Table 10.1). The term recommended daily amount (RDA) was originally used by the UK Department of Health in 1979 to define the amounts of certain nutrients needed by different groups within the population.
Unfortunately, RDAs were often used inappropriately to assess the dietary needs of individuals. Accordingly, in 1991, they were replaced with dietaryreference values (DRVs) that are guidance values as to the amounts of energyand nutrients that should be ingested, rather than exact recommendations. Dietary reference values indicate the amount of energy or amount of an individual nutrient required by a group of a certain age range and sex for good health. The value of any DRV is given as a daily intake. However, in practice DRVs must be determined from the energy and nutrient intakes averaged from several days eating, because food intake and appetite vary from day to day. Groups for which DRVs have been set include seven groups of infants of both sexes from birth to age 10 years, four groups each for males and females between the ages of 11 and 50-plus, with extra groups for women who are pregnant or are lactating. The value of a DRV applies to healthy people, since patients who are unwell for whatever reason may have differing nutritional requirements. Dietary terms that are related to DRVs are the estimated averagerequirement (EAR), reference nutrient intake (RNI) and lower reference nutrient intake (LRNI). The EAR is the mean amount of energy or nutrientneeded by a population, while the RNI is an amount of a nutrient that is greater than the dietary needs of 97.5% of that group. The LRNI is the amount of a nutrient sufficient to meet the requirements of 2.5% of a population. Note that most people will need more than the LRNI. Figure 10.2 shows how EAR, RNI and LRNI are related. Finally, the safe intake is the amount judged to be sufficient for the needs of most people and anything below this level could be undesirable. This arbitrary value is given when there is simply insufficient evidence to establish reliable values for EAR, RNI or LRNI.