One of the earliest observations on the dietary modification of carcinogenesis was the effect of caloric restriction on tumor incidence. This relatively simple modification significantly de- creased tumor incidence, both experimentally induced by chemicals and spontaneously induced in animals and possibly in humans. An early example of this effect was the reduction of the incidence of skin tumors induced by dibenz[a,h]anthracene by more than one-half by stringent caloric restriction (cf. Tannenbaum, 1947). In addition, caloric restriction to about 50% of ad libitum markedly reduced the incidence of spontaneous mammary tumors (Sarkar et al., 1982) as well as of pulmonary neoplasms, hepatomas, and leukemias in mice. Tucker (1979) demon- strated that, under the conditions of caloric restriction described by Tannenbaum, mice lived longer and exhibited fewer hepatic neoplasms than mice fed ad libitum. The effect was much more marked in the males than in the females. Caloric restriction of 25% reduced both lung and lymphoid tumors, but the most dramatic decrease was seen in the incidence of hepatic neoplasms (Conybeare, 1980). On the other hand, caloric restriction had no effect on the pro- gression of hyperplastic nodules to hepatocellular carcinoma (M.S. Rao et al., 1983); this sug- gested that both the development and the stage of tumor progression are not sensitive to this dietary effect.
Concomitant with the decrease in spontaneous tumor incidence with caloric restriction is an extension of the life span of the animal (Weindruch, 1992; Masoro, 1996) as well as a greater maintenance of immune function (Fernandes et al., 1997), amelioration of chronic renal disease (Gumprecht et al., 1993), and age-related changes in the small intestine of the rat (Heller et al.,1990). In Figure 8.5 may be seen the relation of survival to age as well as tumor incidence in female mice (Weindruch, 1992). There was an increase of approximately 35% in life span in the group that ate about 60% of the calories of the normal group. Overall tumor incidence for the control group was 78%, and for the restricted group 38%. The caloric effect on neoplastic devel- opment, induced or spontaneous, is not limited to one organ system but rather can be noted in the development of a number of different histogenetic types of neoplasms (Albanes, 1987; Birt,1987). Furthermore, as expected, in both hepatocarcinogenesis in mice (Lagopoulos and Stalder,1987; Kolaja et al., 1996) and in pancreatic carcinogenesis in the rat (Roebuck et al., 1993), preneoplastic lesions also decreased in number and in size in calorically restricted animals. This indicates that caloric restriction exerts its effect very early in the stage of promotion, inhibiting this stage during its development. Keenan et al. (1996) showed that the incidence of spontaneous tumors in restricted and ad libitum fed rats was similar, indicating that caloric restriction does not affect the initiation stage. Furthermore, in concert with the earlier findings of M.S. Rao et al.
Figure 8.5 Influence of caloric restriction initiated at 3 weeks of age on the life span and tumor inci- dence of female mice of the long-lived C3B10RF1 hybrid strain. The circles depict the age at death for tumor-bearing mice on normal (closed circles) and restricted (open circles). (Adapted from Weindruch,
1992, with permission of author and publisher.)
(1983), Keenan et al. (1995) demonstrated that moderate dietary restriction does not affect the stage of progression in spontaneous neoplastic development.
In the diet, the most concentrated source of calories is from fat. Thus, there has been con- siderable discussion of the role of dietary fat vis-à-vis total dietary calories in effecting the alter- ations of caloric restriction and ad libitum feeding. Several studies have now indicated that it is total caloric intake rather than fat intake that governs the ultimate yield of neoplasms both in mouse skin carcinogenesis and rat mammary carcinogenesis (cf. Boutwell, 1992). This effect is noted in Table 8.5. However, Birt et al. (1992) demonstrated that restriction of fat calories re- sulted in a greater inhibition of papillomas than when carbohydrate was restricted in the diet.
Table 8.5 Restriction of Caloric Intake Modulates the Enhancing Effect of Fat on Mouse Skin Carcinogenesis and Rat Mammary Carcinogenesis
Adapted from Boutwell, 1992, with permission of the author and publisher.
Still, the majority of experimental evidence argues that the caloric content of the diet is para- mount in importance in producing the effects of caloric restriction. However, later in this chap- ter, we note that the composition of the fat can actually make significant differences in modifying the effects of carcinogenesis.
In addition to the dramatic effects of caloric restriction on spontaneous and chemically induced carcinogenesis, caloric restriction also alters carcinogenesis by radiation and viruses. Gross and Dreyfuss (1990), in a study with female rats, found that 89% of animals fed ad libitum developed neoplasms, predominantly leukemias and mammary carcinomas. When fed a restricted diet (about 50% of control calories), only 23% of irradiated animals developed neo- plasms. Of the controls, 48% of the ad libitum unirradiated females developed similar neoplastic lesions, while none of the unirradiated females on a restricted diet developed neoplasms. Males showed a similar effect but at lower incidences. In another study with mice bearing the MMTV/v-Ha-ras transgene, Fernandes et al. (1995) examined the effect of caloric restriction on the expression of the transgene in mammary tissue of transgenic mice. Using a diet containing
60% of the calories of the ad libitum control group, they found that, whereas the tumor incidence was 83% in the ad libitum animals, it was 27% in the calorie-restricted animals. Since the trans- gene construct utilized the MMTV long terminal repeat as a promoter of the v-Ha-ras oncogene, this study suggests that both virus-induced and transgene-induced carcinogenesis are subject to the inhibitory effect of caloric restriction. In contrast, while caloric restriction inhibited the in- duction of intestinal neoplasms in rats by methylazoxymethanol acetate, which requires meta- bolic activation to exert its carcinogenic effect, caloric restriction had no effect on the induction of these neoplasms in rats administered the direct-acting carcinogen N-methylnitrosourea (Pol- lard and Luckert, 1985). In this latter study the restricted animals were fed at 75% of the caloric level of the ad libitum animals. Thus, whether carcinogenesis by direct-acting carcinogens is not affected by caloric restriction or requires a higher degree of caloric restriction to show the effect has yet to be determined.
Unlike chronic caloric restriction, acute complete caloric restriction by fasting has not been intensively studied. Initiation by diethylnitrosamine in rats is quantitatively unaffected by a
48-hour fast (Schmitt et al., 1993). When the fasting period is followed by refeeding, a signifi- cant enhancement of the initiation of preneoplastic focal lesions occurred (Tessitore et al., 1996) as well as an enhancing effect on the growth of nodules after a selection protocol of hepatocar- cinogenesis (Laconi et al., 1995). It is likely that the enhancement of initiation is the result of an enhancement of DNA synthesis on refeeding, as was shown later by Hikita et al. (1997), with simultaneous enhancement of “fixation” of initiation. These last authors fasted animals for two
5-day periods interspersed by a 2-day feeding respite. At the end of the fasting periods, preneo- plastic lesions had essentially disappeared, but reappeared quantitatively within 1 week of re- feeding at levels identical to those of nonfasted animals. These authors demonstrated that the loss of preneoplastic cells was due to apoptosis with sparing of a stem cell(s) to allow rapid regrowth of preneoplastic lesions on refeeding. Thus, acute fasting and refeeding can modulate the stage of promotion in hepatocarcinogenesis and may enhance the efficiency of initiation through increased cell replication upon refeeding.