Disposable soma theory of aging

British biologist Thomas Kirkwood first proposed the disposable soma theory of aging in a 1977 Nature review article. The theory was inspired by Leslie Orgel's Error Catastrophe Theory of Aging, which was published fourteen years earlier, in 1963. Orgel believed that the process of aging arose due to mutations acquired during the replication process, and Kirkwood developed the disposable soma theory in order to mediate Orgel's work with evolutionary genetics.

The disposable soma theory of aging posits that there is a trade-off in resource allocation between somatic maintenance and reproductive investment. Too low an investment in self-repair would be evolutionarily unsound, as the organism would likely die before reproductive age. However, too high an investment in self-repair would also be evolutionarily unsound due to the fact that one's offspring would likely die before reproductive age. Therefore, there is a compromise and resources are partitioned accordingly. However, this compromise is thought to damage somatic repair systems, which can lead to progressive cellular damage and senescence.{{cite journal |author=Drenos F |author2=Kirkwood, TB | year = 2005| title = Modelling the disposable soma theory of ageing | journal = Mech Ageing Dev. | volume = 126 | issue = 1 | pages = 99–103| doi=10.1016/j.mad.2004.09.026|pmid=15610767 |s2cid=2256807 }} Repair costs can be categorized into three groups: (1) the costs of increased durability of nonrenewable parts; (2) the costs of maintenance involving cell renewal, and (3) the costs of intracellular maintenance.{{cite journal |author=Kirkwood, TB |author2=Rose MR | year = 1991| title = Evolution of senescence: late survival sacrificed for reproduction.| journal = Philos Trans R Soc Lond B Biol Sci | volume = 332 | issue = 1262 | pages = 15–24| doi=10.1098/rstb.1991.0028 | pmid = 1677205}} In a nutshell, aging and decline is essentially a trade-off for increased reproductive robustness in youth.{{cn|date=November 2024}}

File:IGF-1Pathway.png, which represses FOXO, thus preventing gene expression of longevity-inducing proteins]]

{{main|Mechanistic target of rapamycin#Aging}}

Much research has been done on the antagonistic effects of increased growth on lifespan. Specifically, the hormone insulin-like growth factor 1 (IGF-1), binds to a cell receptor, leading to a phosphorylation cascade. This cascade results in kinases phosphorylating forkhead transcription factor (FOXO), deactivating it. Deactivation of FOXO results in an inability to express genes involved in responding to oxidative stress response, such as antioxidants, chaperones, and heat-shock proteins.{{cite journal |author=O'Neill BT |author2=Lee KY |author3=Klaus K |author4=Softic S|display-authors=et al | year = 2016 | title = Insulin and IGF-1 receptors regulate FoxO-mediated signaling in muscle proteostasis | journal = J Clin Invest | volume = 126 | issue = 9 | pages = 3433–3346 | doi=10.1172/JCI86522| pmc = 5004956 | pmid=27525440}}

Additionally, uptake of IGF-1 stimulates the mTOR pathway, which activates protein synthesis (and therefore growth) through upregulation of the translation-promoting S6K1, and also inhibits autophagy, a process necessary for recycling of damaged cellular products.{{cite journal |vauthors=Johnson SC, Rabinovitch PS, Kaeberlein M| year = 2013 | title = mTOR is a key modulator of ageing and age-related disease| journal = Nature | volume = 493 | issue = 7432| pages = 338–345 | doi=10.1038/nature11861| bibcode = 2013Natur.493..338J| pmc = 3687363 | pmid=23325216}} Decline of autophagy causes neurodegeneration, protein aggregation and premature aging.{{cite journal |vauthors=Komatsu M, Waguri S, Chiba T, Murata S|display-authors=et al | year = 2006 | title = Loss of autophagy in the central nervous system causes neurodegeneration in mice | journal = Nature | volume = 441 | issue = 7095| pages = 880–884 | doi=10.1038/nature04723| bibcode = 2006Natur.441..880K | pmid = 16625205 |s2cid=4407360 }} Lastly, studies have also indicated that the mTOR pathway also alters immune responses and stimulates cyclin-dependent kinase (CDK) inhibitors such as p16 and p21. This leads to alteration of the stem-cell niche and results in stem cell exhaustion, another theorized mechanism of aging.{{cite journal |vauthors=Castilho RM, Squarize CH, Chodosh LA, Williams BO|display-authors=et al | year = 2009| title = mTOR mediates Wnt-induced epidermal stem cell exhaustion and aging.| journal = Cell Stem Cell | volume = 5 | issue = 3 | pages = 279–289 | doi=10.1016/j.stem.2009.06.017| pmc = 2939833 | pmid = 19733540 }}

While reproduction inhibits lifespan with regard to multicellular organisms, the precise mechanism responsible for this effect remains unclear. Although many models do illustrate an inverse relationship, and the theory makes sense from an evolutionary perspective, the cellular mechanisms have yet to be explored. However, with regards to cellular replication, the progressive shortening of telomeres is a mechanism which limits the amount of generations of a single cell may undergo.{{cite journal |vauthors=Shay JW, Wright WE | year = 2011 | title = Role of telomeres and telomerase in cancer | journal = Semin Cancer Biol. | volume = 21 | issue = 6 | pages = 349–353 | doi=10.1016/j.semcancer.2011.10.001| pmc = 3370415 | pmid=22015685}} Furthermore, in unicellular organisms like Saccharomyces cerevisiae, the formation of extrachromosomal rDNA circles (ERCs) in mother cells (but not daughter cells) upon every subsequent division is an identifiable type of DNA damage that is associated with replication. These ERCs accumulate over time and eventually trigger replicative senescence and death of the mother cell.{{cite journal |vauthors=Kaeberlein M, McVey M, Guarente L | year = 1999 | title = The SIR2/3/4 complex and SIR2 alone promote longevity in Saccharomyces cerevisiae by two different mechanisms| journal = Genes Dev. | volume = 13 | issue = 19| pages = 2570–2580 | doi=10.1101/gad.13.19.2570 | pmid = 10521401 | pmc = 317077}}

There is a large body of evidence indicating the negative effects of growth on longevity across many species. As a general rule, individuals of a smaller size generally live longer than larger individuals of the same species.{{cn|date=November 2024}}

In dwarf models of mice, such Snell or Ames mice, mutations have arisen, either rendering them incapable of producing IGF-1 or unable to have adequate receptors for IGF-1 uptake. Furthermore, mice injected with growth hormone have been shown to have progressive weight loss, roughing of the coat, curvature of the spine, enlargement of the organs, kidney lesions and increased cancer risk.{{cite journal |vauthors=Bartke A, Brown-Borg H | year = 2004 | title = Life extension in the dwarf mouse | journal = Curr Top Dev Biol | volume = 63 | issue = 6 | pages = 189–225| doi=10.1016/j.semcancer.2011.10.001| pmc = 3370415 | pmid=22015685}} This effect is also seen in different breeds of dogs, where smaller breeds of dogs typically live significantly longer compared to their larger counterparts. Selectively bred for their small size, smaller dog breeds like the Chihuahua (average lifespan of 15–20 years) have the B/B genotype for the IGF-1 haplotype, reducing the amount of IGF-1 produced. Conversely, large dogs like the Great Dane (average lifespan of 6–8 years) are homozygous for the IGF-1 I allele, which increases the amount of IGF-1 production.{{cite journal |vauthors=Sutter NB, Bustamante CD, Chase K, Gray MM | year = 2007| title =A Single IGF1 Allele Is a Major Determinant of Small Size in Dogs| journal = Science | volume = 316 | issue = 5821 | pages = 112–115 | doi=10.1126/science.1137045| bibcode =2007Sci...316..112S| pmc = 2789551 | pmid=17412960}}

Initially, it was believed that growth hormone actually prolonged lifespan due to a 1990 study that indicated that injection of growth hormone to men over 60 years of age appeared to reverse various biomarkers implicated in aging, such as decreased muscle mass, bone density, skin thickness, and increased adipose tissue.{{cite journal |vauthors=Rudman D, Feller AG, Nagraj HS, Gergans GA|display-authors=et al | year = 1990| title =Effects of human growth hormone in men over 60 years old| journal = N Engl J Med | volume = 323 | issue = 1 | pages = 1–6 | doi=10.1056/NEJM199007053230101 | pmid = 2355952| doi-access = free }} However, a 1999 study found that administering growth hormone also significantly increased mortality rate.{{cite journal |vauthors=Takala J, Ruokonen E, Webster NR, Nielsen MS, Zandstra DF|display-authors=et al | year = 1999| title =Increased mortality associated with growth hormone treatment in critically ill adults| journal = N Engl J Med | volume = 341 | issue = 11 | pages = 785–792 | doi=10.1056/NEJM199909093411102 | pmid = 10477776| doi-access = free }} Recent genomic studies have confirmed that the genes involved in growth hormone uptake and signaling are largely conserved across a plethora of species, such as yeast, nematodes, fruit flies, mice and humans.{{cite journal |vauthors=Longo VD, Finch CE | year = 2003| title =Evolutionary medicine: from dwarf model systems to healthy centenarians?| journal = Science | volume = 299 | issue = 5611 | pages = 1342–1346 | doi=10.1126/science.1077991| pmid = 12610293| s2cid = 14848603}} These studies have also shown that individuals with Laron syndrome, an autosomal recessive disorder resulting in dwarfism due to defects in growth hormone receptors, have increased lifespan. Additionally, these individuals have much lower incidences of age-related diseases such as type 2 diabetes and cancer.{{cite journal |vauthors=Guevara-Aguirre J, Balasubramanian P, Guevara-Aguirre M, Wei M|display-authors=et al | year = 2011| title = Growth hormone receptor deficiency is associated with a major reduction in pro-aging signaling, cancer, and diabetes in humans | journal = Sci. Transl. Med. | volume = 3 | issue = 70 | pages = 70–83 | doi=10.1126/scitranslmed.3001845| pmc = 3357623 | pmid=21325617}} Lastly, human centenarians around the world are disproportionately of short stature, and have low levels of IGF-1.{{cite journal |vauthors=Govindaraju D, Atzmon G, Barzilai N | year = 2015| title =Genetics, lifestyle and longevity: Lessons from centenarians.| journal = Appl Transl Genom. | volume = 4| pages = 23–32 | doi=10.1016/j.atg.2015.01.001| pmid = 26937346| pmc = 4745363| doi-access =free}}

Numerous studies have found that lifespan is inversely correlated with both the total amount of offspring birthed, as well as the age at which females first gives birth, also known as primiparity.{{cite journal |vauthors=Descamps S, Boutin S, Berteaux D, Gaillard JM | year = 2006| title =Best squirrels trade a long life for an early reproduction| journal = Proc Biol Sci | volume = 273| issue = 1599 | pages = 2369–2374 | doi= 10.1098/rspb.2006.3588| pmc = 1636082 | pmid=16928640}} Additionally, it has been found that reproduction is a costly mechanism that alters the metabolism of fat. Lipids invested in reproduction would be unable to be allocated to support mechanisms involved in somatic maintenance.{{cite journal |vauthors=Hansen M, Flatt T, Aguilaniu H | year = 2013| title =Reproduction, fat metabolism, and life span: what is the connection?| journal = Cell Metab.| volume = 17| issue = 1 | pages = 10–19 | doi= 10.1016/j.cmet.2012.12.003| pmid = 23312280| pmc = 3567776| doi-access = free}}

The disposable soma theory has been consistent with the majority of animal models. It was found in numerous animal studies that castration or genetic deformities of reproduction organs was correlated with increased lifespan.{{cite journal |vauthors=Drori D, Folman Y | year = 1976| title =Environmental effects on longevity in the male rat: exercise, mating, castration and restricted feeding| journal = Exp. Gerontol.| volume = 11| issue = 1 | pages = 25–32 | doi=10.1016/0531-5565(76)90007-3| pmid = 1278267| s2cid = 37672329}}{{cite journal |author=Flatt T| year = 2011| title =Survival costs of reproduction in Drosophila | journal = Exp. Gerontol.| volume = 46| issue = 5 | pages = 369–375 | doi= 10.1016/j.exger.2010.10.008 | pmid = 20970491| s2cid = 107465469| url = http://doc.rero.ch/record/324277/files/2011_scr.pdf}}{{cite journal |vauthors=Hsin H, Kenyon C| year = 1999| title =Signals from the reproductive system regulate the lifespan of C. elegans| journal =Nature| volume =399| issue = 6734 | pages = 362–366 | doi= 10.1038/20694| pmid = 10360574| bibcode =1999Natur.399..362H| s2cid = 4358840}} Moreover, in red squirrels, it was found that females with an early primiparity achieved the highest immediate and lifetime reproductive success. However, it was also found that these same individuals had a decreased median and maximum lifespan. Specifically squirrels who mated earlier had a 22.4% rate of mortality until two years of age compared to a 16.5% rate of mortality in late breeders. In addition, these squirrels had an average maximum lifespan of 1035 days compared to an average maximum lifespan of 1245 days for squirrels that bred later.

In another study, researchers selectively bred fruit flies over three years to develop two different strains, an early-reproducing strain and a late-reproducing strain. The late-reproducing line had a significantly longer lifespan than the early-reproducing line. Even more telling was that when the researchers introduced a mutation in the ovarian-associated gene ovoD1, resulting in defective oogenesis, the differences in lifespan between the two lines disappeared. The researchers in this case concluded that "aging has evolved primarily because of the damaging effects of reproduction earlier in life".{{cite journal |vauthors=Sgrò CM, Partridge L| year = 1999| title = A delayed wave of death from reproduction in Drosophila.| journal =Science| volume =286| issue =5449 | pages = 2521–2524 | doi = 10.1126/science.286.5449.2521| pmid = 10617470}}

Prominent aging researcher Steven Austad also performed a large-scale ecological study on the coast of Georgia in 1993. Austad isolated two opossum populations, one from the predator-infested mainland and one from the predator-absent nearby island of Sapelo. According to the disposable soma theory, a genetically isolated population subject to low environmentally-induced mortality would evolve delayed reproduction and aging. This is because without the pressure of predation, it would be evolutionarily advantageous to allocate more resources to somatic maintenance than reproduction, as early offspring mortality would be low. As predicted, even after controlling for predation, the isolated population had a longer lifespan, delayed primiparity, and reduced aging biomarkers such as tail collagen cross-linking.{{cite journal |author=Austad, SN| year = 1993| title =Retarded senescence in an insular population of Virginia opossums (Didelphis virginiana)| journal = Journal of Zoology| volume = 229| issue = 4 | pages = 695–708 | doi= 10.1111/j.1469-7998.1993.tb02665.x}}

In general, only a few studies exist in human models. It was found that castrated men live longer than their fertile counterparts.{{cite journal |vauthors=Hamilton JB, Mestler GE| year = 1969| title =Mortality and survival: comparison of eunuchs with intact men and women in a mentally retarded population)| journal = J. Gerontol.| volume = 24| issue = 4 | pages = 395–411| doi=10.1093/geronj/24.4.395| pmid = 5362349}} Further studies found that in British women, primiparity was earliest in women who died early and latest in women who died at the oldest ages. Furthermore, increased number of children birthed was associated with a decreased lifespan.{{cite journal |vauthors=Westendorp RG, Kirkwood TB| year = 1998| title =Human longevity at the cost of reproductive success |journal = Nature| volume = 396| issue = 6713 | pages = 743–746 | doi= 10.1038/25519| pmid = 9874369| s2cid = 151331241}} A final study found that female centenarians were more likely to have children in later life compared to the average, especially past the age of 40. The researchers discovered that 19.2% of female centenarians had their first child after the age of 40, compared to 5.5% of the rest of the female population.{{cite journal |vauthors=Perls TT, Alpert L, Fretts RC| year = 1997| title =Middle-aged mothers live longer |journal = Nature| volume = 389| issue = 6647 | pages = 133 | doi= 10.1038/38148| pmid = 9296486| bibcode = 1997Natur.389..133P| s2cid = 4401775| doi-access = free}}

{{main|Free radical theory}}{{main|DNA damage theory of aging}}

File:Naked Mole Rat Eating.jpg has a disproportionately long life of 30 years through efficient cellular repair mechanisms.]]

There are numerous studies that support cellular damage, often due to a lack of somatic maintenance mechanisms, as a primary determinant for aging, and these studies have given rise to the free radical theory of aging and the DNA damage theory of aging. One study found that the cells of short-living rodents in vitro show much greater mutation rates and a general lack of genome surveillance compared to human cells and are far more susceptible to oxidative stress.{{cite journal |vauthors=Lorenzini A, Stamato T, Sell C | year = 2011| title = The disposable soma theory revisited: time as a resource in the theories of aging | journal = Cell Cycle| volume = 10 | issue = 22 | pages = 3853–3856 | doi=10.4161/cc.10.22.18302 | pmid = 22071624| doi-access = free }}

Other studies have been conducted on the naked mole rat, a rodent species with remarkable longevity (30 years), capable of outliving the brown rat (3 years) by ten-fold. Additionally, almost no incidence of cancer has ever been detected in naked mole rats. Nearly all of the differences found between these two organisms, which are otherwise rather genetically similar, was in somatic maintenance. Naked mole rats were found to have higher levels of superoxide dismutase, a reactive oxygen species clearing antioxidant. In addition, naked mole rats had higher levels of base excision repair, DNA damage response signaling, homologous recombination repair, mismatch repair, nucleotide excision repair, and non-homologous end joining. In fact, many of these processes were near or exceeded human levels. Proteins from naked mole rats were also more resistant to oxidation, misfolding, ubiquitination, and had increased translational fidelity.{{cite journal |vauthors=Azpurua J, Ke Z, Chen IX, Zhang Q, Ermolenko DN|display-authors=et al | year = 2013| title = Naked mole-rat has increased translational fidelity compared with the mouse, as well as a unique 28S ribosomal RNA cleavage | journal = Proc Natl Acad Sci U S A| volume = 110 | issue = 43 | pages =17350–17355 | doi=10.1073/pnas.1313473110| bibcode =2013PNAS..11017350A | pmid = 24082110 | pmc = 3808608|doi-access=free }}

Further studies have been conducted on patients with Hutchinson-Gilford Progeria Syndrome (HGPS), a condition that leads to premature aging. Patients with HGPS typically age about seven times faster than average and usually succumb to the disease in their early teens. Patients with HGPS have cellular defects, specifically in the lamin proteins, which regulate the organization of the lamina and nuclear envelope for mitosis.{{cite journal |vauthors=Pollex RL, Hegele RA | year = 2004| title = Hutchinson-Gilford progeria syndrome | journal = Clin. Genet.| volume = 66| issue = 5 | pages =375–381 | doi=10.1111/j.1399-0004.2004.00315.x| pmid = 15479179| s2cid = 7529899}} A-type lamins promote genetic stability by maintaining levels of proteins that have key roles in the repair processes of non-homologous end joining and homologous recombination.Redwood, Abena B.; Perkins, Stephanie M.; Vanderwaal, Robert P.; Feng, Zhihui; Biehl, Kenneth J.; Gonzalez-Suarez, Ignacio; Morgado-Palacin, Lucia; Shi, Wei; Sage, Julien; Roti-Roti, Joseph L.; Stewart, Colin L.; Zhang, Junran; Gonzalo, Susana (27 October 2014). "A dual role for A-type lamins in DNA double-strand break repair". Cell Cycle. 10 (15): 2549–2560. doi:10.4161/cc.10.15.16531. PMC 3180193. {{PMID|21701264}} Mouse cells deficient for maturation of prelamin A show increased DNA damage and chromosome aberrations and have increased sensitivity to DNA damaging agents.Liu, Baohua; Wang, Jianming; Chan, Kui Ming; Tjia, Wai Mui; Deng, Wen; Guan, Xinyuan; Huang, Jian-dong; Li, Kai Man; Chau, Pui Yin; Chen, David J; Pei, Duanqing; Pendas, Alberto M; Cadiñanos, Juan; López-Otín, Carlos; Tse, Hung Fat; Hutchison, Chris; Chen, Junjie; Cao, Yihai; Cheah, Kathryn S E; Tryggvason, Karl; Zhou, Zhongjun (26 June 2005). "Genomic instability in laminopathy-based premature aging". Nature Medicine. 11 (7): 780–785. doi:10.1038/nm1266. {{PMID|15980864}}. S2CID 11798376

Lastly, as mentioned previously, it has been found that the suppression of autophagy is associated with reduced lifespan, while stimulation is associated with extended lifespan. Activated in times of caloric restriction, autophagy is a process that prevents cellular damage through clearance and recycling of damaged proteins and organelles.{{cite journal |vauthors=Glick D, Barth S, Macleod KF | year = 2010| title = Autophagy: cellular and molecular mechanisms| journal = J. Pathol. | volume = 221 | issue = 1 | pages =3–12| doi=10.1002/path.2697| pmc = 2990190 | pmid=20225336}}

One of the main weaknesses of the disposable soma theory is that it does not postulate any specific cellular mechanisms to which an organism shifts energy to somatic repair over reproduction. Instead, it only offers an evolutionary perspective on why aging may occur due to reproduction. Therefore, parts of it are rather limited outside of the field of evolutionary biology.

{{main|Calorie restriction#Sirtuin-mediated mechanism}}

File:Soma Disposable.png during caloric restriction]]

Critics have pointed out the supposed inconsistencies of the disposable soma theory due to the observed effects of caloric restriction, which is correlated with increased lifespan.{{cite journal |author=Blagosklonny, MV| year = 2010| title = Solving puzzles of aging: From disposable soma to signal-transduction pathways| journal = Russ J Gen Chem | volume = 80 | issue = 7| pages =1407–1414 | doi=10.1134/s1070363210070364| s2cid = 95075587}} Although it activates autophagy, according to classical disposable soma principles, with less caloric intake, there would less total energy to be distributed towards somatic maintenance, and decreased lifespan would be observed (or at least the positive autophagic effects would be balanced out). However, Kirkwood, alongside his collaborator Darryl P. Shanley, assert that caloric restriction triggers an adaptive mechanism which causes the organism to shift a higher proportion of resources to somatic maintenance, away from reproduction.{{cite journal |vauthors=Shanley DP, Kirkwood TB | year = 2000| title = Calorie restriction and aging: a life-history analysis.| journal = Evolution | volume = 54 | issue =3 | pages =740–750|doi=10.1111/j.0014-3820.2000.tb00076.x | pmid = 10937249| s2cid = 26293762}} This theory is supported by multiple studies, which show that caloric restriction typically results in impaired fertility, but leave an otherwise healthy organism.{{cite journal |author=Holliday R.| year = 2000| title = Food, reproduction and longevity: is the extended lifespan of calorie-restricted animals an evolutionary adaptation?| journal = BioEssays | volume = 10 | issue =4 | pages =125–127|doi=10.1002/bies.950100408| pmid = 2730632}}{{cite journal |vauthors=Masoro EJ, Austad SN | year = 1996| title = The evolution of the antiaging action of dietary restriction: a hypothesis.| journal = J Gerontol A Biol Sci Med Sci | volume = 51 | issue =6 | pages =387–391| doi = 10.1093/gerona/51a.6.b387| pmid = 8914486| doi-access = free}} Evolutionarily, an organism would want to delay reproduction to when resources were more plentiful. During a resource-barren period, it would evolutionarily unwise to invest resources in progeny that would be unlikely to survive in famine. Mechanistically, the NAD-dependent deacetylase Sirtuin 1 (SIRT-1) is upregulated during low-nutrient periods. SIRT-1 increases insulin sensitivity, decreases the amount of inflammatory cytokines, stimulates autophagy, and activates FOXO, the aforementioned protein involved in activating stress response genes. SIRT-1 is also found to result in decreased fertility.{{cite journal |vauthors=Cantó C, Auwerx J | year = 2009| title = Caloric restriction, SIRT1 and longevity.| journal =Trends Endocrinol Metab| volume = 20 | issue =7 | pages =325–331|doi=10.1016/j.tem.2009.03.008| pmc =3627124 | pmid=19713122}}

In additional to differential partitioning of energy allocation during caloric restriction, less caloric intake would result in less metabolic waste in the forms of free radicals like hydrogen peroxide, superoxide and hydroxyl radicals, which damage important cellular components, particularly mitochondria. Elevated levels of free radicals in mice has been correlated with neurodegeneration, myocardial injury, severe anemia, and premature death.{{cite journal |vauthors=Pérez VI, Bokov A, Van Remmen H, Mele J|display-authors=et al| year = 2009| title = Is the oxidative stress theory of aging dead?| journal =Biochim Biophys Acta| volume = 1790 | issue =10 | pages =1005–1014|doi=10.1016/j.bbagen.2009.06.003| pmc =2789432 | pmid=19524016}}

No changes were observed in the spontaneous chromosomal mutation frequency of dietary restricted mice (aged 6 and 12 months) compared to ad libitum fed control mice.Stuart GR, Oda Y, Boer JG, Glickman BW. No change in spontaneous mutation frequency or specificity in dietary restricted mice. Carcinogenesis. 2000 Feb;21(2):317-9. doi: 10.1093/carcin/21.2.317. PMID 10657975 Thus dietary restriction appears to have no appreciable effect on spontaneous mutation in chromosomal DNA, and the increased longevity of dietary restricted mice apparently is not attributable to reduced chromosomal mutation frequency.{{cn|date=November 2024}}

{{main|Menopause#Evolutionary rationale}}

Another primary criticism of the disposable soma theory is that it fails to account for why women tend to live longer than their male counterparts.{{cite journal |vauthors=Ginter E, Simko V | year = 2013| title = Women live longer than men| journal = Bratisl Lek Listy | volume = 114 | issue = 2 | pages =45–49 | doi=10.4149/bll_2013_011| pmid = 23331196| doi-access = free}} After all, females invest significantly more resources into reproduction and according to the classical disposable soma principles, this would compromise energy diverted to somatic maintenance. However, this can be reconciled with the grandmother hypothesis. The Grandmother Hypothesis states that menopause comes about into older women in order to restrict the time of reproduction as a protective mechanism. This would allow women to live longer and increase the amount of care they could provide to their grandchildren, increasing their evolutionary fitness.{{cite journal |author=Hawkes K | year = 2003| title = Grandmothers and the evolution of human longevity| journal = Am. J. Hum. Biol.| volume =15 | issue =3 | pages =380–400| doi= 10.1002/ajhb.10156 | pmid = 12704714| s2cid = 6132801}} And so, although women do invest a greater proportion of resources into reproduction during their fertile years, their overall reproductive period is significantly shorter than men, who are able of reproduction during and even beyond middle age.{{cite journal |vauthors=Vinicius L, Mace R, Migliano A| year = 2014| title = Variation in Male Reproductive Longevity across Traditional Societies| journal = PLOS ONE| volume =9 | issue =11 | pages =e112236| doi=10.1371/journal.pone.0112236| pmid = 25405763| pmc = 4236073| bibcode =2014PLoSO...9k2236V| doi-access =free}} Additionally, males invest more resources into growth compare to females, which is correlated with decreased lifespan. Other variables such as increased testosterone levels in males are not accounted for. Increased testosterone is often associated with reckless behaviour, which may lead to a high accidental death rate.{{cite journal |vauthors=Celec P, Ostatníková D, Hodosy J | year = 2015| title = On the effects of testosterone on brain behavioral functions| journal = Front. Neurosci.| volume =9 | pages =12| doi=10.3389/fnins.2015.00012 | pmid = 25741229 | pmc = 4330791| doi-access = free}}

A few contradicting animal models weaken the validity of the disposable soma theory. This includes studies done on the aforementioned naked mole rats. In these studies, it was found that reproductive naked mole rats actually show significantly increased lifespans compared to non-reproductive individuals, which contradicts the principles of disposable soma. However, although these naked mole rats are mammalian, they are highly atypical in terms of aging studies and may not serve as the best model for humans. For example, naked mole rats have a disproportionately high longevity quotient and live in eusocial societies, where breeding is usually designated to a queen.{{cite journal |vauthors=Dammann P, Burda H | year = 2006| title =Sexual activity and reproduction delay ageing in a mammal | journal = Curr. Biol.| volume = 16 | issue = 4 | pages = 117–118 | doi=10.1016/j.cub.2006.02.012| pmid = 16488857| s2cid = 17842436| doi-access = free}}

The disposable soma theory is tested disproportionately on female organisms for the relationship between reproduction and aging, as females carry a greater burden in reproduction.{{cite journal |vauthors=Hammers M, Richardson DS, Burke T, Komdeur J | year = 2013| title = The impact of reproductive investment and early-life environmental conditions on senescence: support for the disposable soma hypothesis.| journal = J Evol Biol | volume = 26| issue = 9 | pages = 1999–2007| doi=10.1111/jeb.12204| pmid = 23961923| s2cid = 46466320| url = https://pure.rug.nl/ws/files/6799192/2013JEvolBiolHammers.pdf| doi-access = free}} Additionally, for the relationship between growth and aging, studies are disproportionately conducted on males, to minimize the hormonal fluctuations that occur with menstrual cycling.{{cite journal |vauthors=Wu JJ, Liu J, Chen EB, Wang JJ|display-authors=et al | year = 2013| title = Increased mammalian lifespan and a segmental and tissue-specific slowing of aging after genetic reduction of mTOR expression| journal = Cell Rep. | volume = 4| issue = 5 | pages = 913–920| doi=10.1016/j.celrep.2013.07.030 | pmid = 23994476 | pmc = 3784301}} Lastly, genetic and environmental factors, rather than reproductive patterns, may explain the variations in human lifespan. For example, studies have shown that poorer individuals, to whom nutritious food and medical care is less accessible, typically have higher birth rates and earlier primiparity.{{cite journal |author=Murray, S. | year = 2006| title = Poverty and health| journal = CMAJ | volume = 174| issue = 7 | pages = 923| doi=10.1503/cmaj.060235| pmid = 16567753| pmc = 1405857| doi-access = free}}

{{reflist}}

• [https://www.crsociety.org Calorie Restriction Society]

• [http://www.senescence.info/causes_of_aging.html Damage-Based Theories of Aging]—Includes a discussion of the free radical theory of aging

{{senescence}}

{{DEFAULTSORT:Free-Radical Theory}}

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Category:Evolution

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Category:Theories of biological ageing