Dietary restriction in rats and mice: a meta-analysis and review of the evidence for genotype-dependent effects on lifespan

Abstract

Laboratory survival experiments have shown that dietary restriction (DR) can increase median and maximum lifespan. This paper provides a meta-analysis of laboratory experiments that have evaluated the effects of DR on lifespan in rats and mice (1934-present). In rats, DR increased median lifespan by 14-45% in half of all experiments, but in mice the effects of DR have been much weaker (4-27%). The least favorable effects of DR on lifespan have been observed among inbred rather than non-inbred mouse strains. In fact, some inbred mouse strains do not necessarily live longer with DR, including DBA/2 male mice and several strains from the ILSXISS recombinant inbred panel. Shortening of lifespan with DR has also been observed and confirmed for ILSXISS strain 114. Importantly, all rodent studies may be biased by the effects of laboratory breeding, since one study has shown that median lifespan is not improved by DR in wild-derived mice. These findings suggest that the set of genetic backgrounds studied in rodent DR experiments should be diversified. This will broaden the scope of genotypes studied in aging research, but may also be critical for translation of findings from rodents to historically outbred and genetically heterogeneous primate species.

Figure 1. Stronger increases in median and maximum lifespan in DR-fed rats compared to DR-fed mice (1934 – present; excluding ILSXISS strains)

A literature review identified 53 and 72 laboratory experiments that evaluated the effects of DR-type diets on survival in rats and mice, respectively (Supplemental Tables 1 and 2). For each experiment, the percent change in median and maximum lifespan with DR was estimated. The height of each bar corresponds to the average effect of DR on median or maximum lifespan among n experiments for which the necessary information was available (rats, grey bars; mice, white bars). The average effect and associated standard error were calculated by assigning greater weight to studies that used larger sample sizes. The asterisk symbol is used to denote cases in which the average effect of DR on rat lifespan is significantly different from the average effect of DR on mouse lifespan (P < 0.05; two-sample t-test and Wilcoxon rank sum test).

Figure 2. DR has more favorable effects on the lifespan of non-inbred mice compared to inbred mice (1934 – present; excluding ILSXISS strains)

A literature review identified 72 laboratory experiments that evaluated the effects of DR-type diets on survival in mice (Supplemental Table 2). Of these 72 experiments, 24 were performed using non-inbred mouse genotypes, including F₁ offspring derived from inbred line crosses, F₂ offspring derived from a four-way cross of inbred lines, or wild-derived genetically heterogeneous mice. The height of each bar corresponds to the average effect of DR on median or maximum lifespan among n experiments for which the necessary information was available (non-inbred strains, grey bars; inbred strains, white bars). The average effect and associated standard error were calculated by assigning greater weight to studies that used larger sample sizes. For a given category, asterisk symbols are used to denote cases in which the average effect of DR on the lifespan of inbred strains is significantly different from the average effect of DR on the lifespan of non-inbred strains (P < 0.05; two-sample t-test and Wilcoxon rank sum test).

Figure 3. Funnel plot analysis of publication bias in studies that have evaluated the effects of DR on rodent lifespan (excluding ILSXISS strains)

A literature review was carried out to identify survivorship experiments that have evaluated effects of DR on lifespan in rodents (Supplemental Tables 1 and 2). A total of 53 rat experiments (A and B) and 72 mouse experiments (C and D) were identified, where each experiment included a control cohort (ad lib access to food or fixed high-calorie ration) paired with a DR cohort (restricted in caloric content). Survivorship experiments using the ILSXISS recombinant inbred strains were excluded from this analysis (Liao et al., 2010a; Rikke et al., 2010). The effect of DR on median or maximum lifespan was estimated in each experiment (horizontal axis; note log₂ scale), and this effect was plotted against the size of each experiment (vertical axis). The size of each experiment is defined as min(n₁, n₂), where n₁ is the number of animals assigned to the DR cohort while n₂ is the number of animals assigned to the control cohort. Experiments for which DR decreased median or maximum lifespan are plotted within the grey region in each figure. The dotted vertical line denotes the average effect size among all experiments (i.e., average treatment effect weighted by study size). In the absence of publication bias, it is expected that effect sizes from individual experiments will be symmetrically distributed to the left and right of the average effect size (dotted line). In (A) and (B), the arrow denotes an outlying effect size estimate derived from the study of Ross (1961) (Supplemental Table 1). Analyses were performed with and without this outlying effect size estimate (see text).

Figure 4. Effects of DR on survivorship of white rats in early laboratory studies from McCay and colleagues

Survivorship curves were generated based upon experiments reported by McCay et al. (1935; . Grey regions in (A) – (D) outline the survival curve of AL-fed rats, while the solid line represents the survival curve of DR-fed rats. Dotted lines denote one standard error above and below the estimated survival curve of DR-fed rats. P-values shown in (A) – (D) were generated from a log-rank test of the difference between the AL-fed and DR-fed survival curves. (A) Female rats were restricted in caloric intake at the time of weaning (n = 23 DR-fed; n = 22 AL-fed). At 25 months of age (arrow), half of the restricted rats were placed on the control diet, while the other half remained on the restricted diet. (B) Female rats were restricted in caloric intake two weeks after weaning (n = 19 DR-fed; n = 22 AL-fed). At 25 months of age (arrow), half of the restricted rats were placed on the control diet, while the other half remained on the restricted diet. (C) Male rats were provided a DR diet starting at an early age and maintained on this diet for at least 10 months (n = 35 DR-fed; n = 17 AL-fed). Prior to 10 months of age (arrow), temperature declines in the laboratory caused deaths in approximately half of the DR-fed rats. (D) Female rats were provided a DR diet starting at an early age and maintained on this diet for at least 10 months (n = 37 DR-fed; n = 17 AL-fed). Prior to 10 months of age (arrow), temperature declines in the laboratory caused deaths in approximately half of the DR-fed rats.

Figure 5. Accelerated failure time model-based estimates of DR effects on survivorship in 44 ILSXISS strains

Effects of DR on survivorship across ILSXISS recombinant inbred mouse strains were evaluated based upon survivorship experiments performed at the University of Texas Health Science Center at San Antonio (UTHSCSA) or University of Colorado at Boulder (UCB) (Liao et al., 2010a; Rikke et al., 2010). For each strain, the percent increase in survivorship due to DR was estimated by fitting an accelerated failure time model to survival data (Swindell, 2009). Calculations were performed using UTHSCSA data pooled from both sexes (A), UTHSCSA data from males only (B), UTHSCSA data from females only (C) or UCB female data (D). Filled symbols represent the estimated percent increase in survivorship due to DR and error bars span the 95% confidence interval associated with this estimate. Shaded regions denote significant effects of DR (P < 0.05), including significantly elevated survivorship due to DR (up-triangles) or significantly decreased survivorship due to DR (down-triangles). Strain labels in the left margin have an asterisk symbol (*) if the estimated effect of DR on lifespan remained significant following FDR adjustment of p-values for multiple testing among all ILSXISS strains.

Figure 6. Funnel plot analysis of publication bias in studies that have evaluated the effects of DR on mouse lifespan (including ILSXISS strains)

The funnel plot analyses shown in Figures 3C and 3D were repeated using survival data from 72 mouse experiments in combination with 121 experiments that had evaluated ILSXISS recombinant inbred mouse strains (193 experiments total; Supplemental Table 2) (Liao et al., 2010a; Rikke et al., 2010). Experiments for which DR decreased median or maximum lifespan are plotted within the grey region in each figure. The dotted vertical line denotes the average effect size among all experiments (i.e., average treatment effect weighted by study size). In the absence of publication bias, it is expected that effect sizes from individual experiments will be symmetrically distributed to the left and right of the average effect size (dotted line).

Figure 7. Female ILSXISS lifespan estimates are correlated between independent studies under the AL condition but not the DR condition

Female lifespan of the ILSXISS recombinant inbred mouse strains was measured at University of Texas Health Science Center at San Antonio (UTHSCSA) and independently measured at the University of Colorado (UCB) (Liao et al., 2010a; Rikke et al., 2010). Figures (A) and (B) show the association between median lifespan estimates from the two studies. In part (C), a comparison is made between ratios (DR / AL) of the median lifespan estimates derived from each study. Figures (D) and (E) show the association between maximum lifespan estimates from the two studies, where maximum lifespan is calculated by averaging the two longest survival times for each strain and each study. In part (F), a comparison is made between ratios (DR / AL) of the maximum lifespan estimates derived from each study. The Spearman rank correlation is shown in the lower-right of each figure, and an asterisk symbol is used to denote correlation estimates that are significant (P < 0.05).

Figure 8. Effects of DR on median, early and late survival in ILSXISS recombinant inbred mouse strains

Effects of DR on median, early and late survivorship were evaluated across ILSXISS strains based upon experiments performed at University of Texas Health Science Center at San Antonio (A – I) and University of Colorado at Boulder (J – L) (Liao et al., 2010a; Rikke et al., 2010). In the left column (A, D, G and J), the median lifespan is plotted for each strain under the AL and DR conditions. In the middle column (B, E, H and K), the lowest two survival times for each strain were averaged and plotted for the AL and DR conditions. In the right column (C, F, I and L), the highest two survival times for each strain were averaged and plotted for the AL and DR conditions. In each plot, the percentage of strains with a larger survival estimate in the DR condition (i.e., above-diagonal points) is indicated in the upper left (dark grey region). The percentage of strains with a larger survival estimate in the AL condition (i.e., below-diagonal points) is indicated in the lower right (light grey region). In one case, survival estimates were significantly more likely to be lower under the DR condition in comparison to the AL condition (see Figure 8K; P = 0.02; sign test with n = 42 strains).