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Basal Metabolism of Rats in Relation to Old Age and Exercise during Old Age

Three Figures

Nutrition Laboratory, Carnegie Institution of Washington, Boston, and Department of Chemistry, Columbia University, New York City

Previous investigations of the basal metabolism of the rat have here been extended by observations upon about 100 over 1 year of age. In most cases periodic measurements of the waking basal metabolism at thermic neutrality (28°C.) and after 24 hours of fasting were begun in the period of middle age and continued throughout the latter half of the normal life cycle. Measurements in the moribund state or immediately premortal physiologic breakdown are not included in the data discussed in this paper, but the normal old-age measurements usually continued until within 2 weeks of the natural death of the animal, and necropsies showed no predominance of any one type of lesion or cause of death. Presumably these rats lived out their natural life cycle.

Groups of rats of widely different ages and essentially equal average weight showed a somewhat higher total basal metabolism per 24 hours in old age than in middle age. The trend is clearest in the comparison of the groups that were most numerous and in which the comparison is probably most valid and conclusive. This is probably the closest approach to the isolation of a pure age factor that has yet been accomplished under satisfactory experimental control.

With our rat population as a whole, of rather widely differing weights as well as ages, studied throughout the latter half of the life cycle, we have found a tendency similar to that noted by Benedict and Meyer with elderly women, namely, to a relatively constant basal metabolism for the elderly individual. For these rats this average waking basal heat output per individual per 24 hours is: for males of about 250 to 270 gm., about 26 calories; for females of about 210 to 230 gm., about 22 calories.

Expressed with reference to size, the average becomes, for both sexes, about 100 calories per kilogram, or about 700 calories per square meter, for the basal waking metabolism per 24 hours. These averages may be regarded as sufficiently established for purposes of comparison with other animal species.

Although the relative constancy in total 24-hour basal metabolism is a significant feature of our data, a closer scrutiny reveals that the actual basal heat output per individual decreased slightly with advancing age and that at the same time there was (in the majority of individuals and in the general average) a relatively larger, though also small, decrease in body weight. Hence with rats in advancing age we find a slight decrease of per capita basal metabolism and at the same time a slight increase per unit of weight or per unit of surface area (computed from weight). The latter finding confirms that of Benedict and MacLeod.

The present data show also a tendency to decrease of body temperature in old age. This is usually apparent only after 800 days of age, but in very advanced age may amount to about 2°C.

When middle-aged rats, not previously exercised, were given vigorous enforced exercise daily (except Sundays and the days devoted to the basal metabolism measurement with its preliminary fasting and resting at regulated environmental temperature), the males failed to adjust themselves to such strenuous exercise begun so late in life, lost weight rapidly, and died. The females, on the contrary, were evidently benefited by the exercise, and its permanent effect was shown in a distinct tendency to a lower basal metabolism.

As a possible explanation of the fact that the basal metabolism, as computed per unit of weight or surface area, shows a slightly upward trend with age in the non-exercised but not in the exercised rats, we suggest that the organism is freed by muscular exercise from something in the nature of a middle age restlessness or chronic useless tenseness and so is enabled to relax better in rest periods. The exercised aging rat thus becomes comparable to the wholesome, cooperative, able-to-relax person who constitutes a satisfactory subject and from whom, therefore, the accepted data for basal metabolism of elderly people are chiefly obtained. The nonexercised rat corresponds to a less-able-to-relax person, whose lowest attainable waking metabolism is a little above the truer basal of the more placid subject. The fact that the aging rats studied by Benedict and MacLeod and by ourselves were (with the exceptions here separately recorded) unexercised, and the further fact that they showed some indications of a diminished efficiency of temperature regulation (not supplemented by clothing, bedding, or warmer rooms as with elderly people) may together explain what would superficially appear to be a slight species difference in the trend of the energy metabolism with advancing age.