Pré-visualização50 páginas

of data values showing their frequency of occurrence throughout the range of the various possible values. One of the most common distributions is a \u201cnormal\u201d distribution, which is a symmetrical bell-shaped curve that has most of the values clustered in the center of the distribution and a few values falling out in the tails (see Figure 1). Important measures that describe a distribution are the mean, median, and standard deviation. \u2022 The mean is the average of the data values. It is calculated by adding all the data values and then dividing by the number of data values. \u2022 The median is the data value that occurs right in the middle of the distribution. It is the point at which half the data values are below and half the data values are above. The median is also referred to as the 50th percentile. In a symmetrical/normal distribution, the mean and median occur in the same place. \u2022 The standard deviation (SD) is a measure of how much, on average, each individual data value differs from the mean. The smaller the SD, the less each data value varies from the mean. The larger the spread of data values, the larger SD becomes. \u2022 The variance is another measure of how much individual data values differ from the mean. It is equivalent to the square of the standard de- viation (SD2). FIGURE 1 Schematic of a normal distribution. Copyright © National Academy of Sciences. All rights reserved. Dietary Reference Intakes: The Essential Guide to Nutrient Requirements http://www.nap.edu/catalog/11537.html 22 DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS FIGURE 2 Normal requirement distribution of hypothetical nutrient showing percentile rank and placement of EAR and RDA on the distribution. One important use of the normal distribution is the way it can be used to con- vert scores into percentile ranks, or probabilities. The \u201cz-score\u201d is a standard score that changes values into SD units (i.e., the score is now so many SDs above or below the mean). This score can be related directly to the normal distribution and the associated percentage probability of nutrient adequacy or inadequacy, as seen in Figure 2. By making use of this property of the normal distribution, the probability (or prevalence) of adequacy or inadequacy can be estimated. For example, a z- score of +1.50 is associated with a probability of adequacy of 93 percent. A z-score of \u20131.00 is associated with a probability of adequacy of 15 percent. Table 1 lists a selection of z-scores and their associated probabilities. It should be noted that not all data will form a normal distribution. For example, a \u201cskewed\u201d distribution is one where the curve has one tail longer than the other end. If the data do not form a normal distribution, then the properties of the normal distri- bution do not apply. 34% 13.5% 2.5% 2.5% 13.5% 34% 0 50 84 97.5162.5 +1 SD\u20131 SD\u20132 SD\u20133 SD +2 SD +3 SD 50%50% EAR Mean Median RDA Percentile Rank Copyright © National Academy of Sciences. All rights reserved. Dietary Reference Intakes: The Essential Guide to Nutrient Requirements http://www.nap.edu/catalog/11537.html PART I: APPLYING THE DIETARY REFERENCE INTAKES 23 Applying the DRIs Makes Use of Two Distributions In applying the DRIs, two distributions are used simultaneously. The first is the distribution of requirements. The second is the distribution of intakes. REQUIREMENT DISTRIBUTION The distribution of requirements is the distribution upon which the DRIs (spe- cifically the EAR and RDA) are based. This distribution reflects the variability in requirements between individuals. Variability exists because not all individuals have the same requirement for a nutrient. For nutrients where requirements are normally distributed, the EAR is located at the mean/median of the distribu- tion. The RDA is located at 2 standard deviations above the mean, the level at which 97.5 percent of requirements should be met. INTAKE DISTRIBUTION The distribution of intakes is obtained from observed or reported nutrient in- takes gathered through dietary assessment methods such as 24-hour recalls. A 24-hour recall is a detailed description of all foods and beverages consumed in the previous 24-hour period. Nutrient intake from supplements should also be collected. When more than one 24-hour recall is collected, intake data can reflect the day-to-day variability within an individual that occurs because dif- ferent foods are eaten on different days. TABLE 1 Probability of Adequacy for Selected Z-Scores z-score Probability of Adequacy 2.00 0.98 1.65 0.95 1.50 0.93 1.25 0.90 1.00 0.85 0.86 0.80 0.68 0.75 0.50 0.70 0.00 0.50 \u20130.50 0.30 \u20130.85 0.20 \u20131.00 0.15 Copyright © National Academy of Sciences. All rights reserved. Dietary Reference Intakes: The Essential Guide to Nutrient Requirements http://www.nap.edu/catalog/11537.html 24 DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS When working with individuals, this variability is taken into account in the formulas used for assessment. When working with groups, statistical proce- dures should be used to adjust the distribution of observed intakes by partially removing the day-to-day variability in individual intakes so that the adjusted distribution more closely reflects a usual intake distribution. Usual intake is an important concept in application of the DRIs. Usual intake is the average intake over a long period of time. It is seldom possible to accurately measure long-term usual intake due to day-to-day variation in in- takes as well as measurement errors. Therefore, mean observed intakes (over at least two non-consecutive days or three consecutive days) are used to estimate usual intake. Overlap of the Requirement Distribution and the Intake Distribution The requirement and intake distributions can overlap to varying degrees. In some cases, the two distributions will barely intersect, if at all (see Figure 3, Panel A), and in others there may be a lot of overlap between intakes and re- quirements (see Figure 3, Panel B). In applying the DRIs to assessment, the distribution of intakes is compared to the distribution of requirements and inferences are made about the degree of adequacy. In dietary planning, efforts are made to ensure that the distribution of intakes is adequate relative to the distribution of requirements. WORKING WITH INDIVIDUALS How to Assess the Nutrient Intakes of an Individual The goal of assessing an individual\u2019s nutrient intake is to determine if that in- take is meeting the person\u2019s nutrient requirements. Assessment of dietary ad- equacy for an individual is difficult because of the imprecision involved in esti- mating an individual\u2019s usual intake and the lack of knowledge of an individual\u2019s actual nutrient requirements. Interpreting nutrient intake data in relation to the DRIs can enhance the assessment of an individual\u2019s diet; however, the informa- tion obtained must be interpreted cautiously because an individual\u2019s true usual intake and true requirements must be estimated, and assessment of dietary ad- equacy is only one component of a nutritional status assessment. Ideally, intake data are combined with clinical, biochemical, or anthropometric information to provide a valid assessment of nutritional status. Recognizing the inherent limitations and variability in dietary intakes and requirements is a major step forward in nutrition. The reports on using the DRIs for assessment and planning have provided a method with which one can Copyright © National Academy of Sciences. All rights reserved. Dietary Reference Intakes: The Essential Guide to Nutrient Requirements http://www.nap.edu/catalog/11537.html PART I: APPLYING THE DIETARY REFERENCE INTAKES 25 0 1 2 3 4 5