No evidence that competition for food underlies lifespan shortening by dietary restriction in multiply housed mice: response to commentary

Department of Integrative Physiology, University of Colorado,Boulder, CO 80309, USAOur recent report (Liao et al., 2010) that dietary restriction(DR) shortened lifespan in many strains of recombinant inbred(RI) mice was unexpected in the context of a long history ofresearch on the life-extending action of DR (Weindruch W Masoro, 2005). It is important to recognize,however,thatourreportisnotthe ﬁrst(see Liaoet al.,2010,fora number of earlier reports in which DR had no effect onlifespan, or even led to shorter lifespan). Although we hadhypothesized that there would be strain variation in the extentof lifespan extension by DR, we too had overlooked dissonantdata, which in retrospect would have led us to expand ourhypothesis to include possible negative effects of DR. Thegenetic variation we have found in the response to DR, particu-larly given its magnitude, provides a valuable tool for elucidatingunderlying genes and pathways that mediate the lifespan-mod-ulatingactionofDR–theultimategoalofresearch inthisarea.We are pleased to respond to the commentary by Mattson(2010) and the opportunity it offers to provide additional infor-mation bearing on mechanisms by which DR can shorten as wellas lengthen lifespan. The commentary offers some interestingsuggestions about these mechanisms. Here, we provide datathat largely refute these suggestions. We also respond to othercomments and speculations raised in the commentary thatdeserveclariﬁcation. The author’s primary explanation for the lifespan shorteningby DR of some strains is based on postulated effects of multiplehousing. The author assumes that multiple housing results incompetition for limited food, thereby leading to dominancebehaviour and unequal distribution of food among the individu-als. The commentary speculates that the dominant mousewouldeat adisproportionate shareof the food,andthus receiveless than the normal 40% restriction, and that subordinate micein the cage would be restricted more than 40% leading to earlydeath, i.e., shortened lifespan in relation to ad libitum (AL) fedcontrols of the same strain. Our published data, as well as newdata presented here, do not support this idea. First, exclusionfrom the analysis of early deaths, which in Mattson’s modelwould represent subordinate mice with exceptionally low foodintake, has little impact on the frequency or distribution of RIstrains with shortened lifespan (supporting Fig. S3, Liao et al.,2010). Second, taking body weight as an indicator of feedingdominance, there is no evidence that such dominance led toshortened life of subordinates. Inmales, arguedby theauthor tobe more prone to feeding dominance, mean lifespan of themost subordinate (i.e. smallest) mouse was no shorter than thatof the most dominant (i.e., largest) mouse, and the same is truefor females, both for the strains that had signiﬁcantly shortenedlifespans under DR (Fig. 1), as well as for strains with shortenedlifespans that did not reach statistical signiﬁcance (data notshown). Finally, although we were unable to measure the foodconsumption of individual mice housed multiply, using bodyweight as a measure of food consumption provides little evi-dence formarkedinequality offood intakeamongcagemates.Iffood were unequally distributed among cagemates because ofcompetition, the range and variance of body weight in DR cageswould be greater than that in AL fed cages. Neither range (datanot shown) nor coefﬁcient of variation of body weight was sig-niﬁcantly greater in the DR than in the AL cages (Table 1). Theseﬁndings are consistent with our behavioural observations of themice during feeding in this study as well as previously (Ikenoet al., 2005). Although we are in agreement with the commen-tary that DR mice consume virtually all of their allotment within1–2 h of feeding, we see no ﬁghting during this period. Whenfood is available, each DR mouse spends its time obtainingpellets of food from the hopper and consuming them. In somecases, a mouse will take the pellet from another mouse butthat mouse will obtain another pellet from the hopper andboth will have food to eat (C.-Y. Liao, V. Diaz, J. F. Nelson;unpublished observations). The food hoppers are large enoughtoallowallmicetoaccessfoodsimultaneously.The most compelling evidence that competition for food doesnot underlie the DR shortening of lifespan is the ﬁnding that DR