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The successful validation of the doubly labeled water technique (Schoeller, 1988; Roberts, 1989) has provided an improved means for determining TEE and hence energy requirements in different population groups. Currently available data from 74 subjects (mean ages 64-74 years) studied by four research groups indicate
that the energy requirements of older individuals may be higher than indicated in the current recommended energy requirements (FAO/WHO/UNU, 1985), with mean PAL values for both men and women being 1.641.68. This observation should not be taken as a recommendation for increasing energy intake, but rather as an acceptance of the fact that both energy intake and energy expenditure may be higher than anticipated previously in this age group. Data from our own laboratory also indicate that aTEE/pREE ratios for older adults are lower than those for young men and women by approximately 0.2 PAL on average. Based on reported information on physical activity, it appears likely that these high values for TEE/REE reflect a previous underestimation of energy requirements by the factorial approach rather than unusually high levels of physical activity in the subjects studied. It is important to note that these observations are based on studies in a very limited number of older subjects and many more doubly labeled water measurements are required to provide the information needed for a detailed assessment of changes in energy requirements with age. Future studies are needed to address the following issues:
1. Concerning the methodology of determining energy requirements, more detailed calculations of changes in REE with age after 50 years are required in order to normalize measurements of TEE, considering indices of height as well as weight. It is important to use predicted rather than :measured values for REE when determining TEE/REE ratios because otherwise the TEE/PEE ratio can be substantially influenced by between-laboratory differences in measurement of REE as indicated in Tables 3 and 4. In addition, further standardization of doubly labeled water calculation procedures and more detailed between-laboratory standardization of the doubly labeled water method is needed.
2. Individuals over the age of 50 years are not a homogeneous group with regard to energy requirements. Currently available data are restricted to a relatively narrow age range within the younger section of the older population (mean ages 64-74 years), and much more information is required on changes of TEE with age in individuals in each age decade within the range 50-90 years. Both men and women should be investigated in these studies, as well as young control subjects, so that the range of changes in energy requirements with age can be assessed accurately.
3. There is currently no general consensus over the desirable range of physical activity, and hence energy requirements, for long-term health in older individuals. However, it is clear that a sedentary lifestyle is associated with numerous adverse health consequences and should not be recommended. Although further information on desirable levels of physical activity will necessarily come from investigations with much larger numbers of subjects than are feasible for doubly labeled water measurements, additional studies are needed to define the relationship between TEE and simple measures of physical activity that can be used in larger population studies to predict the energy requirements of representative populations.
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Definition and classification of 'older' individuals
During the discussion, several attempts were made to define what is meant by 'older' individuals or the 'elderly'. It was recognized that the use of descriptive terms like these can vary enormously from country to country and in different contexts, and that it is therefore preferable to specify the age range when describing such a population. It was further agreed that one could not make general recommendations for adults ranging in age from 60 or 65 to over 100 years, and that this whole range needed to be subdivided into narrower age bands.
The discussion on age ranges and age limits was, however, eclipsed by the realization that, in a discussion of energy and protein requirements, health and life style were of greater importance than age, and that, from that point of view, the elderly were an extremely heterogeneous group: on the one hand they are freed from the constraints imposed by a professional activity and can choose life styles implying widely varying physical activity levels; on the other hand a considerable proportion of them are likely to suffer from various disabilities that restrict their activity or even make them bedridden. This implies that recommendations have to cover a wide range, and that mean values may not offer valid information. Stratifying the elderly by age subgroups and activity level may be the most useful approach.
Formulating energy requirements for the elderly is particularly important since a relatively large proportion of this age group is institutionalized and fed meals planned by dietitians relying on such recommendations. A recent survey in the UK revealed that over 50% of institutionalized elderly subjects ate all the food provided to them and still lost LBM, a clear indication that their dietary requirements were not met (Prentice).
Total energy expenditure (TEE)
Roberts draws her conclusions from 74 measurements of TEE made by DLW in healthy and relatively active and lean individuals from industrialized countries, in an age range from 64 to 74 years; this limits the external validity (generalizability) of her conclusions considerably.
Most discussants find that DLW provides an interesting new approach, but they emphasize that this should not be taken to mean that older data used to infer energy requirements of the elderly are invalid and ought no longer be considered.
Currently available TEE data, obtained by the DLW method and compiled by Prentice, suggest that, throughout the whole life span, energy intakes should be slightly higher than those recommended in the 1985 report. Roberts attributes this to the fact that the 1985 recommendations relied heavily on the factorial method which, according to her, leaves certain activities unaccounted for and therefore leads to an underestimation of TEE. Other participants (e.g. Shetty and Ferro-Luzzi) believe that the differences are more likely to be attributable to a bias in the DLW data base.
Ferro-Luzzi has BMR and energy expenditure data (obtained by observation of activities and multiplication by their energy cost) of some 600 free-living people in Italy. She and others found that mean values of TEE decline with age, due to a decline of both BMR and physical activity.
Basal metabolic rate (BMR)
Throughout her paper, Roberts used the term resting energy expenditure (REE), because she was not sure if the metabolic rate measurements in the studies from which she obtained her data were all carried out under the strict conditions required of BMR measurements. Such precaution may be appropriate in this specific instance, but several discussants re-emphasized that REE is not a well defined concept, and that measuring and using BMR is preferable.
Schofield et al's data base contained only very few BMR values of individuals over 60, and their predictive equations therefore rest on an insecure basis. Henry and Durnin's ongoing re-analysis will include more data points, but their number remains inadequate; in particular, there are only very few data from this age group in less developed countries.
Ferro-Luzzi compared measured with predicted BMR values and found that, in the 60 to 75-year age group, predicted values exceed measured ones by about 10%.
A biologically interesting question is, whether changes in body mass and body composition in the elderly fully explain changes in BMR, as they do in younger adults, or if the aging process involves other metabolic changes that have to be taken into consideration. It is difficult to answer this question, because of the scarcity of body composition data from elderly people. Fat is more internal than subcutaneous, so that skinfolds become poor indicators of fat mass, and for practical reasons densitometric measurements have not been frequently made in this age group. Furthermore, aging results in changes within the fat-free mass that can have energy-metabolic effects, but on which sufficient information is not yet available. BMR per kg LBM seems to decrease slowly with age. This appears somewhat surprising. Since the elderly lose more muscle (with a relatively low metabolic rate at rest) than organ tissue (with a relatively high metabolic rate), one could expect BMR to increase in relation to body weight or LBM, but this phenomenon has actually only been observed in very undernourished, terminal patients.
Practically, the issue of body composition changes may be of lesser importance, since only few investigators will be able to make body composition measurements, and body composition changes that are typical of the elderly will be reflected in the predictive equations for BMR of this age group.
Inter-individual variation in BMR in the elderly is similar to that in older adults: a little over 10% (Ferro-Luzzi).
Physical activity level (PAL)
Roberts took adjusted values of total energy expenditure (TEE) (obtained by DLH method) and divided them by predicted BMR to obtain PAL values, which were higher than the average PAL value used to derive energy recommendations in the 1985 report. Several discussants argued that it would have been better to divide TEE by measured rather than predicted BMR.
Even though the PAL of certain people increases after retirement (Prentice), on average it decreases with age. Roberts reported a drop from 1.85 in 20-year-olds to 1.65 in 65-year-olds; Prentice estimated the mean drop of PAL in the elderly at about 0.1. In certain situations, e.g. in institutionalized people or elderly in poorer countries, energy deficiency should always be taken into consideration as a possible cause of low PALs.
Does the energy cost of activities change with increasing age? According to Durnin and Hautvast increases in energy cost have been observed in activities involving the whole body, like walking on a treadmill, where elderly people show an increasing number of extraneous movements.