HYPOTHESES ON LONGEVITY HORMESIS 29Figure 1.19. Gompertz plots for control and chloroform-treated male Osborne-Mendel
rats. The curves illustrated are for three groups of animals: (1) control
animals provided food and water ad libitum— no chloroform; (2) animals
exposed to 1800 ppm of chloroform (drinking water) beginning at 7 weeks of
age and continuing an additional 104 weeks; and (3) pair-fed controls
attempting to match the reduced food and water intake of the
chloroform-treated group, but without chloroform. Time on the abscissa
refers to the period following initiation of exposure. For the first group, a
linear Gompertz function was used to characterize mortality experience. For
the third group (reduced food and water— no chloroform), a linear Gompertz
function was also used, but with a reduced value for a (consistent with
caloric restriction). For the second (chloroform) group, which had reduced
caloric consumption and apparent longevity hormesis in concert with
irreversible toxicity, the linear Gompertz function (with reduced a) was used
together with terms for longevity hormesis and irreversible toxicity. All
functions were fit simultaneously employing weighted least-squares
analysis. The original data came from Jorgenson et a l." Reprinted from
Neafsey et al.,24 p. 395, by permission of Marcel Dekker, Inc.
While this model is most certainly non-unique, it is consistent and does
demonstrate the potential of the generalized Gompertz model approach
advocated herein.
Figures 1.20-1.22 illustrate Gompertz plots of the effects of hexachloro-
benzene (Figure 1.20) and DDT (Figures 1.21 and 1.22) on rodent mortal
ity.25100101 In all three cases, the results are consistent with the longevity
hormesis-irreversible toxicity model, although visual inspection suggests
alternative models would also suffice.
PROBLEMS IN ASSESSING LONGEVITY HORMESIS IN HUMANS
As concluded by Neafsey,21 evidence for the existence of longevity horme
sis in humans is fragmentary at best. However, focusing on mortality statis
tics, there have been a few reports that suggest the possibility and/or poten
tial of its occurrence in humans. Matanoski et al.102 studied the mortality
experience of radiologists (radiation exposure) relative to groups of three