Several anthropometric indices have been proffered for the evaluation of undernutrition in older adults. These include:
• body weight less than 80% of the ideal body weight for height and age;
• weight loss exceeding 10% of baseline weight in the preceding 6 months; or
Erroneously, the normative references for most of these criteria are younger subjects.
Within the older population the usefulness of this index is hampered by the lack of age-adjusted reference values. Reference values applicable in younger adults are not suitable for use in older persons as sarcopenia, age-related skin changes, and vertebral osteoporosis with height loss confound such norms. Within the older population, intentional weight loss resulting from dietary restriction should not discourage comprehensive nutritional assessment, as recent evidence indicates that both voluntary and involuntary weight loss in older persons portend similar adverse health outcomes.
Calculation of the body mass index (BMI) is considered to be one of the most objective anthropo-metric indices, as it permits correction of body weight for height. The BMI, calculated by dividing the weight in kilograms by the height in meters squared, is based on the proven premise that weight in the younger adult increases proportionately with height. However, this concept is false in older persons as height is significantly affected by age-related changes. Loss of height with aging occurs secondary to shortening of the axial skeleton due to age-related osteoporosis, degenerative disc changes, vertebral thinning, and kyphoscoliosis. Furthermore, using height as an anthropometric index is impractical in nonambulant and bed-bound persons. Nevertheless, clinical use of the BMI in the older population has been preserved by the development of adapted nomograms. Such nomograms are based on the determination of BMI using surrogate parameters of height adapted from the appendicular skeleton, which is relatively unaffected by age-related osseous changes. These parameters include total arm length, arm span, erect forearm length, and knee to floor height.
Skin fold thickness measurements are also used as anthropometric indices of total body fat in younger adults. However, the precise relationship between skin fold thickness and total body fat is unpredictable, as is the response of subcutaneous fat to under-nutrition. Furthermore, in the older adult, the accuracy of this technique is confounded by age-related qualitative and quantitative changes in body fat. Altered compressibility of body fat has also been shown to occur with aging, rendering skin fold thickness measurements unreliable for use in older adults. Measurement of mid-arm circumference is another frequently used anthropometric index. However, several factors influence muscle bulk including exercise, disease, and genetic factors. In the older person this index is of doubtful clinical utility.
Several factors confound the use of anthropo-metric indices, underscoring the importance of serial measurements. These allow for quantification of response to intervention and also enhance accuracy of data interpretation by utilizing intrasubject comparison. More accurate methods of body composition analysis are available but are unlikely to be suitable for routine clinical use. These include computerized tomography, bioelectrical impedance, nuclear magnetic resonance imaging, in vivo neutron activation analysis, dual energy X-ray absorptiometry (DEXA) and direct photon absorptiometry (DPA). Because of alterations in body water with aging, the value of bioelectrical impedance is questionable. The DEXA technique is excellent but the migration of body fat to the abdomen with aging may result in an underestimation of body fat in older persons. DPA is based on analysis of tissue attenuation of photons transmitted at two different energy levels. This technique permits measurement of different tissue compartments. Both fat mass and fat-free mass can be measured using this technique. Currently, these methods are used almost exclusively for research purposes. Most of these emerging techniques are very expensive and have not been validated for use in clinical settings. Therefore, for practical clinical purposes, the most cost-effective nutritional parameter of proven clinical utility in older adults remains serial body weight measurements.
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