Patricia J. Neafsey, Ph.D.
Associate Professor, University of Connecticut, School of Nursing, Box U-26, Storrs, CT 06269
Tel: (860) 486-0508, Fax: (860) 486-0512
E-mail: pneafsey@uconnvm.uconn.edu
The contention that caloric restriction (CR) is a hormetic agent is untenable when one examines mortality functions of laboratory animals subjected to CR versus those chronically exposed to hormetic agents. The use of Gompertz age-specific mortality rate analyses can be used in phenomenological mathematical models to separate out the contributions of CR and hormesis on the lifetime mortality of a group of animals.
The effect of CR is to decrease the
slope of the linear Gompertz function (Fig. 1).
Fig. 1: The effect of CR is to decrease the slope of the linear Gompertz function.
Gompertz plots of Napierian logarithms of age-specific mortality rates versus time for control male F344 rats fed ad libitum (Group 1) and food-restricted rats (Group 2). Food-restricted rats were fed 60% of the mean caloric intake of Group 1 rats until 18 months of age and maintained at that level until death. Original data were obtained from Yu et al., 1985. Time on the abscissa refers to that period following the start of the experiment (0 time = 6 weeks of age). Redrawn from Fig. 1 of Neafsey et al., 1989a. Permission granted by Marcel Dekker, Inc.
Animals subjected to CR typically have lower body weights than controls. At very low doses, hormetic agents displace the linear Gompertz function downward in a parallel fashion (Fig. 2) and animals typically exhibit higher body weights than controls. Chronic exposure to toxic agents that do not induce hormesis increase the slope of the linear Gompertz function (Fig. 3).
Fig. 2: At very low doses, hormetic agents displace the linear Gompertz function downward in a parallel fashion.
Gompertz plots of Napierian logarithms of age-specific mortality rates (estimated Gompertzians signified by ln Wx) versus time for control and methylene chloride-treated female hamsters. Methylene chloride vapor exposure (3500 ppm, 6 h/day, 5 days/week) was begun at 8 weeks of age and continued an additional 2 years. Time on the abscissa refers to that period following initiation of exposure. Original survival data were obtained from Burek et al. (1984). Reproduced from Fig. 7 of Neafsey et al. (1988). Permission granted by Marcel Dekker, Inc.
Fig. 3: Chronic exposure to toxic agents that do not induce hormesis increase the slope of the linear Gompertz function.
Gompertz plots of Napierian logarithms of age-specific mortality rates (estimated Gompertzians signified by ln Wx) versus time for control and methylene chloride-treated female SD rats. Methylene chloride vapor exposure (3500 ppm, 6 h/day, 5 days/week) was begun at 8 weeks of age and continued an additional 2 years. Time on the abscissa refers to that period following initiation of exposure. Original survival data were obtained from Burek et al. (1984). Reproduced from Fig. 8 of Neafsey et al. (1988). Permission granted by Marcel Dekker, Inc.
Agents that produce hormesis cause toxicity at some dose. The effects of hormesis combined with the effects of chronic toxicity produce a characteristic "silver spoon" perturbation to the linear Gompertz function (Fig. 4).
Fig. 4: The effects of hormesis combined with the effects of chronic toxicity produce a characteristic "silver spoon" perturbation to the linear Gompertz function.
Gompertz plots of Napierian logarithms of age-specific mortality rates (estimated Gompertzians signified by ln Wx) versus time for control and ethyl acrylate vapor-exposed male F344 rats. Exposure (75 ppm, 6 h/day, 5 days/week) was begun at 7-9 weeks of age and continued for 27 months. Time on the abscissa refers to that period following initiation of exposure. Original survival data were obtained from Miller et al. (1985). Reproduced from Fig. 6 of Neafsey et al. (1988). Permission granted by Marcel Dekker, Inc.
How do the separate effects of CR, hormesis, and toxicity impact the mortality experience of a group of laboratory animals and potentially confound the interpretation of chronic toxicity studies?
Since CR and chronic toxicity displace the linear Gompertz function in opposite directions (CR decreases the slope; chronic toxicity increases the slope), laboratory animals experiencing inanition from chronic exposure to a toxic substance may benefit from the health effects of the CR thus clouding the interpretation of the study. The effect of depressed food intake has long been appreciated as a confounding variable in animal nutrition studies. Well-designed animal nutrition studies employ a pair-fed control group, i.e. one that is fed the control diet in the amount consumed by the nutrient depleted group. The use of pair-fed controls in bioassay studies would permit effects from depressed food intake to be factored out from those survival effects due to longevity hormesis and cumulative toxicity (Fig. 5).
Fig. 5: The use of pair-fed controls in bioassay studies would permit effects from depressed food intake to be factored out from those survival effects due to longevity hormesis and cumulative toxicity.
Gompertz plots of Napierian logarithms of age-specific mortality rates (estimated Gompertzians signified by ln Wx) versus time. There were 3 groups of animals 1) controls fed ad libitum (0 ppm); 2) pair-fed controls; and 3) animals exposed to 1800 ppm chloroform beginning at 7 weeks of age and continuing for an additional 104 weeks. Time on the abscissa refers to that period following initiation of exposure. Original survival data were obtained from Jorgenson et al. (1985). Reproduced from Fig. 9 of Neafsey et al. (1988). Permission granted by Marcel Dekker, Inc.
Chronic studies are rarely conducted for the lifespan of the animal. A 2-yr rat study (assumed equivalent to 70 man-years) essentially ignores the cumulative impact of exposure on the aging animal. For agents that produce hormesis with toxicity, the hormetic effect can mask toxic effects at early ages and (usually) at low doses. Could the lack of apparent toxicity at early ages (signified by equivocal pathological results) be due to the confounding effects of hormesis? If the 2-year protocol were changed to lifetime studies, would late-life toxic effects be evident from exposures that produced no significant toxicity at earlier ages? If longevity hormesis does not actually occur in humans and is a phenomenon only observable in laboratory settings the implications are enormous.
On the other hand, if longevity hormesis is a phenomenon that occurs in humans, one can not assume hormetic agents have any benefit. With the plethora of mild environmental stresses that modern populations are exposed to (both chemical and physical, naturally occurring and man-made), would the separate hormetic contributions from each of these stresses be additive or would they saturate while and the same time the irreversible toxic effects of the stresses accumulate? The hormetic "machinery" in humans may already be "up-regulated" to maximal levels. Additional stresses from agents with combined toxic and hormetic effects might result in additional toxic injury without additional hormetic dissipation of injury.
The neoplastic and non-neoplastic pathologies seen in laboratory animals exposed to toxicants result from the interplay of effects from aging, depressed food intake, hormesis, and toxicity. Future toxicity studies should include pair-fed controls (e.g. NTP, 1997). Age-specific mortality rate analyses would better enable toxicologists to identify the separate contributions of aging, depressed food intake, longevity hormesis, and toxicity to mortality in both chronic and short-term exposure studies (Neafsey et al. 1989b, Neafsey and Lowrie 1993, 1994, 1995).
REFERENCES
1. Burek JD, Nitschke KD, Bell TJ, Wackerle DL, Childs RC, Beyer JE, Dittenber DA, Rampy LW, McKenna MJ. Methylene chloride: A two-year inhalation toxicity and oncogenicity study in rats and hamsters. Fundam. Appl. Toxicol. 4:30-47, 1984.
2. Jorgenson TA, Meierhenry EF, Rushbrook CJ, Bull RJ, Robinson M. Carcinogenicity of chloroform in drinking water to male Osborne-mendel rats and female B6C3F1 mice. Fundam. Appl. Toxicol. 5:760-769, 1985.
3. Miller RR, Young TJ, Kociba RJ, Keyes DG, Bodner KM, Calhoun LL, Ayres JA. Chronic toxicity and oncogenicity bioassay of inhaled ethyl acrylate in Fischer 344 rats and B6C3F1 mice. Drug Chem. Toxicol. 8:1-42, 1985.
4. NTP (National Toxicology Program) Effect of dietary restriction on toxicology and carcinogenesis studies in F344/N rats and B6C3F1 mice, NTP Technical Report (TR) 460, National Institute of Environmental Health Sciences, Research Triangle Park, NC, 1997.
5. Neafsey PJ, Boxenbaum H, Ciraulo A, Fournier DJ. A Gompertz age-specific mortality rate model of aging, hormesis and toxicity: Single dose studies. Drug Metab. Rev. 19(3,4):369-401, 1988.
6. Neafsey PJ, Boxenbaum H, Ciraulo A, Fournier DJ. Aging and the Gompertz function: Modification by food restriction in rats. Drug Metab. Rev. 21:217-231, 1989a.
7. Neafsey PJ, Boxenbaum H, Ciraulo DA, Fournier DJ. A Gompertz age-specific mortality rate model of aging, hormesis and toxicity: Dose-response studies. Drug Metab. Rev. 20:11-150, 1989b.
8. Neafsey PJ, Lowrie WB. A Gompertz age-specific mortality rate model of toxicity from short term neutron whole-body exposure to fission neutrons in rats. Radiat. Res. 133:234-244, 1993.
9. Neafsey PJ, Lowrie WB. A Mortality kinetics approach to characterizing the relative biological effectiveness of short-term exposure to fission neutrons. Rad. Res. 137:208-225, 1994.
10. Neafsey PJ, Lowrie WB, Cross FT. A mortality kinetics approach to characterizing the fractionated exposure-mortality response relationship of radon progeny. Mech. Ageing and Devel. 83:65-85, 1995.
11. Yu BP, Masoro EJ, McMahan CA. Nutritional influences on aging of Fischer 344 rats: I. Physical, metabolic and longevity characteristics. J. Gerontol. 40:657-670, 1985.