A major challenge is to identify the specific mechanism or mechanisms that initiate the multitude of deleterious changes characteristic of aging and to determine how caloric restriction affects these processes. Caloric restriction depresses the rate of energy metabolism, which decreases body temperature. When housed at room temperature (20 to 22°C), mice with restricted caloric intake have body temperatures that cycle from about 37°C to 23°C to 27°C daily.23
Body temperature also drops in rats with restricted caloric intake, although to a lesser extent,24
and in feral rodents when food is scarce.25
A depression in body temperature indicates a reduction in the rate of oxygen consumption.26
The hypometabolic state in animals with restricted caloric intake is reflected by the approximately 50 percent decrease in serum triiodothyronine concentrations.3–5
Additional factors, such as brown-fat metabolism and the activity of the sympathetic nervous system, are also likely to be involved in the decreased metabolic rate associated with caloric restriction in animals, although the role of these influences has not yet been specifically demonstrated.
Early in this century, Rubner27
postulated an inverse relation between metabolic rate and life span on the basis of a comparison among mammalian species. Later studies in cold-blooded animals, in which the rate of metabolism can be manipulated by altering the ambient temperature, confirmed the inverse relation between metabolic rate and longevity. These findings were the basis of Pearl’s “rate of living” theory of aging.28
More recently, the rate of living (or metabolic rate) has been linked to the rate of production of partially reduced oxygen species,7,29
which are normal byproducts of oxygen metabolism.30
but not a third33
found that the metabolic rate, usually expressed as oxygen consumption per unit of lean body mass, was depressed in rats and nonhuman primates after long-term caloric restriction. It is important to note that body composition and organ weights in animals with restricted caloric intake differ from those in control animals. Rats with restricted caloric intake generally have 70 percent less body fat than controls and a proportionally smaller heart, liver, kidney, prostate, spleen, and mass of skeletal muscle,3
whereas the weights of the brain and testes are similar to those in controls (). Caloric restriction also greatly reduces weights and cell numbers in lymphoid tissues.3
Because rates of oxygen consumption differ among organs, metabolic rates of control and caloric restriction based on total lean body mass may obscure organ-specific variations. Differences in the metabolic fuels used by control animals and those with restricted caloric intake also affect the amount of oxygen needed for energy metabolism. For example, a shift toward protein catabolism in mice with restricted caloric intake is indicated by the fivefold increase in the activity of hepatic carbamyl phosphate synthetase I.34
This mitochondrial enzyme, which has a rate-limiting role in the biosynthesis of urea, catalyzes the condensation of ammonia and bicarbonate, produced during protein catabolism, to carbamyl phosphate.
Effect of Caloric Restriction on Body Composition and Organ Weights in Rats
The link between oxygen consumption and aging is now widely believed to involve reactive oxygen metabolites, the byproducts of oxygen metabolism (). Approximately 2 to 3 percent of oxygen used by cells is chemically reduced by additions of single electrons, which sequentially generate superoxide anion (
) and hydrogen peroxide. The latter readily permeates cellular membranes and can enter virtually all cellular compartments.30
The scission of hydrogen peroxide, catalyzed by free or loosely bound ferrous cation (Fe2+
) or another reduced transition metal, forms one of the most reactive molecules known, the hydroxyl free radical (·OH). This moiety is believed to be the predominant agent of damage to macromolecules such as proteins, DNA, and lipids.7,35,36
Oxidative damage to these molecules appears to be site-specific and dependent on hydrogen peroxide and a reduced transition metal. Hence, macromolecules containing transition metals, such as aconitase, may be prime targets of oxidative damage.37
Although cells contain an elaborate network of antioxidative defenses, reactive oxygen metabolites are detectable in them under normal conditions. Indeed, cells are probably under continuous oxidative stress because of an innate imbalance between the generation and inactivation of reactive oxygen metabolites.
Cumulative Oxidative Damage during Aging
Reactive oxygen metabolites can cause enzymes to lose activity, induce mutations, and damage cell membranes.7,35,36
However, they also have useful functions, such as modulation of the cellular redox state, signal transduction, activation of gene-transcription factors, and apoptosis.38,39
Shifts in the level of oxidative stress probably affect all these cellular functions.
It is widely postulated that damage to macromolecules from reactive oxygen metabolites is a major cause of senescence.7,37,40
The irreversible decline of all organisms in the latter part of life may reflect the accumulation of oxidative stress, and the magnitude of the stress may account for variations in life span among species and among individuals of the same species. The extension of the life span through caloric restriction, hibernation, and in cold-blooded animals, lower body temperature may be attributable to the attenuation of oxidative stress.
In a variety of species, concentrations of oxidatively damaged proteins, DNA, and lipids in tissues increase with age.7,29,37
Oxidative damage to proteins (indicated by carbonylation, loss of sulfhydryl groups, and selective decreases in enzymatic activity) increases with age in both mammalian and insect tissues. Oxidative damage to DNA, often determined by the measurement of 8-hydroxydeoxyguanosine, also increases with age. The primary reasons for the age-associated increase in oxidative damage appear to be increased rates of production of superoxide anion and hydrogen peroxide in mitochondria and, to a lesser extent, decreased efficiency of defenses against antioxidative damage.7,29
A direct link between oxidative stress and aging was found in studies with transgenic strains of the fruit fly Drosophila melanogaster
that overexpress copper-zinc superoxide dismutase and catalase.41
These enzymes eliminate superoxide anion and hydrogen peroxide, and thus provide the first line of defense against oxidative damage. The transgenic flies had a lower rate of oxidative damage to DNA and proteins and lived up to 34 percent longer than control flies.
Comparisons among mammals and insects with wide variations in longevity indicate that the species with longer life spans generate superoxide anion and hydrogen peroxide at lower rates,29,42
accrue less oxidative damage,43,44
and resist experimentally induced oxidative stress.45
Birds, which have high metabolic rates but are nonetheless quite long-lived, generate mitochondrial superoxide anion and hydrogen peroxide at low rates and have relatively high activities of superoxide dismutase and glutathione peroxidase.46
Caloric restriction decreases the steady-state concentrations of the products of oxidative damage to proteins, DNA, and lipids.47,48
This effect may involve decreased production of mitochondrial superoxide anion and hydrogen peroxide47
() and increased antioxidative defenses.49
In mice, most of the age-associated increase in oxidative damage to DNA occurs in postmitotic tissues such as brain, heart, and skeletal muscle, and most of the attenuation of the damage by caloric restriction also occurs in these tissues.48
The functional consequences of oxidative damage to protein were demonstrated by a study in which the severity of oxidative damage in different regions of the brain in mice was correlated with age-related losses in cognitive and motor functions.50
Caloric restriction retarded these losses and lowered the level of oxidative damage to proteins in the pertinent brain regions.16
Effects of Age and Caloric Restriction on Rates of Mitochondrial Oxygen Consumption, Mitochondrial Superoxide Anion and Hydrogen Peroxide Production, and Concentrations of Protein Carbonyls in the Brain in Mice