How fat are Humans? - Humans have been described as being among the fattest of all mammals

Size: px

Start display at page:

Download "How fat are Humans? - Humans have been described as being among the fattest of all mammals"

Asher Tyler
5 years ago
Views:

1 The evlutin f human fatness and susceptibility t besity: an ethlgical apprach Jnathan C. K. Wells Abstract - This article cnsiders the prximate causes, ntgeny, fitness value and evlutinary histry f human fat depsitin Prximate causes include diet cmpsitin, physical activity level, feeding behaviur, endcrine and genetic factrs, psychlgical traits, and expsure t brader envirnmental factrs Fat depsitin peaks during late gestatin and early infancy, and again during adlescence in females Human fat stres nt nly buffer malnutritin, but als regulate reprductin and immune functin, and are subject t sexual selectin - Early hminid evlutin was characterized by adaptatin t a mre seasnal envirnment, when selectin wuld have favured general thriftiness The evlutin f the large expensive brain in the genus Hm then favured increased energy stres in the reprducing female, and in the ffspring in early life Intrductin f agriculture has had three significant effects: Expsure t regular famine Adaptatin t a variety f lcal niches favuring ppulatin-specific adaptatins The develpment f scial hierarchies which predispse t differential expsure t envirnmental pressures Thus, humans have persistently encuntered greater energy stress - The capacity t accumulate fat has therefre been a majr adaptive feature f ur species, but is nw increasingly maladaptive in the mdern envirnment - Alteratins t the besgenic envirnment are predicted t play a key rle in reducing the prevalence f besity Intrductin - Mammalian bdy mass can be divided int fat and fat free cmpnents - Traditinally, besity has been regarded as a physilgical trait, and has been interpreted using bichemical r physilgical mdels T the evlutinary bilgist, hwever, energy stres represent a strategy, and cmprise an imprtant cmpnent f the way in which rganisms adapt themselves and their descendants t the envirnment Life-histry thery: cnsiders hw factrs such as size and grwth rate cntribute t variability in the pace f life and reprductive schedule f rganisms - There s difficulty f adpting evlutinary appraches t the phenmena f besity that are nt directly preserved in the fssil recrd Hw fat are Humans? - Humans have been described as being amng the fattest f all mammals

2 - Prlnged energy stress is ptentially cmmn t all kinds f animals, yet besity in the animal kingdm is rarely encuntered - Mst fat mammals inhabit very different envirnments t humans Prximate Causes - The energy balance equatin: Fat depsitin can be attributed t psitive energy balance, n the basis f the principles f thermdynamics But has limited ability t explain the causatin f fat depsitin Unless weight gain is extremely rapid, the level f psitive energy balance required fr fat depsitin is negligible in prprtin t daily energy turnver Thus making it impssible t identify which f the tw cmpnents is respnsible It is nly by measuring aspects f energy intake and expenditure ther than their calric quantity that the causes f weight gain may be identified - Genetic factrs Obesity is the result f interplay between multiple genetic and envirnmental factrs Genes cannt explain secular trends in bdy mass within a ppulatin Paternal genes enhance ffspring grwth whereas maternal genes suppress In accrdance with parent-ffspring cnflict thery which predicts a cnflict f interest between parental genmes ver maternal investment in each ffspring - Diet cmpsitin Prtein intake is generally assumed nt t act as a prximal cause f weight gain in mst age grups, but in early life higher prtein intakes have been assciated with higher grwth rates that in turn have been linked t subsequent besity High fat intakes have been linked t greater bdy mass and fatness A high-carbhydrate diet is assumed t be beneficial as it implies a reductin in dietary fat cntent. Hwever, the frm in which carbhydrates are ingested may influence mass status Carbhydrate-cntaining fds can be categrized in terms f glycemic index and glycemic lad Referring t variability in bld-glucse levels fllwing ingestin, and the cnsequent prductin f insulin Dietary cmpsitin therefre influences weight gain thrugh ne f tw pathways: stimulating appetite, r inducing alteratins in metablism Each f which can lead t ver-cnsumptin f energy relative t requirements - Physical activity Physical activity culd be cnsidered merely a prxy fr energy expenditure

3 Physical activity may nt prmte weight lss but restricts weight gain - Feeding behaviur Human feeding behaviur is characterized by cmplex regulatry mechanisms and their interactin with the fds being cnsumed Psitive drives t initiate r cntinue feeding, based n sensual cues such as the sight, smell r palatability f fd, interact with negative feedback signals derived frm learned assciatins and gastrintestinal r metablic signals - Endcrine factrs A wide variety f hrmnes influence, and are influenced by, bdy fat, perating at different levels Individually such hrmnes have little lng-term influence n fat stres Satiety signals, chlecystkinin and ghrelin, are prduced by the gut in respnse t fd ingestin Cmbines with nnhrmnal factrs, gastric distensin, t terminate eating Adipsity signals which are transprted int the brain These include leptin, secreted directly by adipse tissue, and insulin and amylin, secreted by b cells in the pancreas Insulin is the main hrmne cntrlling bld glucse levels, and its secretin by the pancreas is partly determined by ambient glucse cncentratin Fatter individuals secrete prprtinally mre insulin fr a given increase in bld glucse levels Amylin, linked als t gastric emptying, plays similar rle, such that insulin and amylin c-signal adipsity t the brain Leptin functin as a starvatin signal, as many energyrequiring physilgical prcesses d nt perate in its absence When plasma insulin cncentratin is increased, it causes a rapid decrease in bld glucse levels, and stimulates rather than reduces appetite Diabetics treated with insulin therefre tend t gain weight including fat Obese individuals are characterized by alteratins in insulin sensitivity, and by insulin resistance, where abnrmally high levels f insulin are required t cntrl bld glucse levels Insulin resistance is assciated with hyperinsulinaemia (increased insulin prductin) - Inflammatin Adipse tissue, in particular visceral adipse tissue, secretes a range f inflammatry factrs Fat plays an imprtant rle in prviding energy fr immune functin The inflammatry respnse t infectin is mediated by certain cytkines, knwn t induce fat synthesis in the liver

4 - Psychlgical factrs Insulin resistance als cntributes t inflammatry respnse by influencing the availability f energetic substrates Changes in scial psychlgy, fr example in attitudes t infant behaviur, may alter the dynamics f signaling systems and hence change the level at which signals f hunger are rewarded with fd - Scial and Envirnmental factrs The western life-style (energy-dense diets, reduced physical activity) have shwn substantial increases in mean bdy weight and the prevalence f besity Txic envirnment has been used t refer t the unhealthy eating and activity patterns that have been linked t ppulatin weight gain Highest prevalence f besity have tended t be thse f higher sci-ecnmic status Ontgeny - Levels f fatness are strikingly high in early life but tend t decline and then increase again prir t adulthd - Fetal fat depsitin Fetal mass increases relatively steadily thrughut pregnancy, but cmprises nly lean mass fr the first tw trimesters Fetal fat depsitin is minimal until the third trimester, when it becmes substantial Infants brn preterm fail t gain the levels f fat achieved by term infants At birth, the average female is bth smaller and fatter than the average male - Infant fat depsitin Infant weight gain is initially rapid befre slwing ver the secnd six mnths In the first few mnths substantial fat is depsited then fat declines as grwth is disprprtinately in lean mass Males have similar fat but greater lean mass - Childhd fat depsitin Childhd grwth is under the cntrl f grwth hrmne, rather than nutritin Variatin in energy balance influences bdy cmpsitin rather than grwth rate Adipsity rebund: develpmental pattern f fatness, with the increase after the lwest pint Fatness declines mre marked in bys than in girls - Adlescent fat depsitin Increase in gain is lean mass - Adult fat depsitin Fat can be differentiated int essential and strage fat Essential fat, lcated in bne marrw, the heart, lungs, liver, kidneys, intestines, muscles and central nervus system Cmprises bth triglycerides and ther cmpunds such as phsphlipids, and is cnsidered higher in wmen due t fat lcated in the breast and pelvic regin

5 Males distribute fat predminantly n the trunk and abdmen, whereas in wmen it is depsited n the buttcks and thighs This difference in distributin is ften attributed t the sex hrmnes teststerne and estrgen, but may als be influenced by the enzyme lipprtein lipase While males are heavier than wmen, wmen are substantially fatter - Prgramming and tracking Prgramming: the lng-term phentypic effect f envirnmental factrs acting during critical r sensitive windws f time in early life Gestatin is an imprtant perid fr the prgramming f bdy cmpsitin Tracking: the stability f phentype nce such critical windws have clsed The existence f early-life prgramming perids has been prpsed t relate t parent-ffspring cnflict: Maternal phentype prgramming ffspring phentype during the perid when ffspring nutritin is derived directly frm the parent Accrding t this, maternal nutritinal status during gestatin and lactatin is hypthesized t prvide prxy infrmatin t the ffspring n the nature f the external envirnment Fitness value - Human fatness plays imprtant rles in reprductive fitness and sexual selectin which are cmpnents f evlutinary bilgy - Buffering malnutritin Fats mst basic functin is t buffer against fluctuatins in energy supply Lipid mbilizatin is slw in relatin t the immediate rate f energy utilizatin, hence fat stres are nt apprpriate fr rapid bursts f activity - Accmmdating fluctuatins in energy supply Ppulatins inhabiting seasnal envirnments demnstrate marked annual fluctuatins in bdy mass The flexibility f human energy stres suggests that evlutin has favured the ready strage f calries in respnse t seasnal dynamics - Regulatin f reprductin Leptin plays an imprtant rle signaling energy stres, fr example predicting the age f menarche Excess maternal fatness inhibits reprductin, reduces the prbability f cnceptin - Offspring grwth The need fr fat stres peaks at times when the brain requires a dependable energy supply in rder t ensure ptimum develpment During the last trimester f pregnancy, the fetus rapidly depsits fat s that by the time placental nutritin ceases, there is sufficient fat t accmmdate the nenatal perid during which breast-feeding at first prvides a very lw energy intake In the nenatal perid, much f the grss energy cntent f breast milk is nt utilized, as the prteins are used instead t prime the immune system As infancy prgresses, fat accumulatin accelerates, prpsed t supprt the energetic uncertainties f the weaning perid

6 The brain has inflexible energy requirements, and unlike ther rgans shws minimal capacity t shrink during starvatin The fact that brain metablism remains stable during fasting means that energy is necessarily diverted frm ther surces during negative energy balance During lactatin, fatty acids are the main means f energy transfer t the ffspring The breast milk is high in fat and lw in carbhydrate Immune functin Immune system requires substantial energy t maintain ptimal functining Leptin plays an imprtant rle in mediating the allcatin f energy t immune functin Evlutinary Histry Sft tissue cmpsitin is prly preserved in the fssil recrd, bliging mre indirect appraches t recnstructing the evlutin f human fatness Increased bdy size Increase in bdy size suggests an equivalent increase in abslute energy stres, an enhanced readiness t stre fat is nt an inevitable cnsequence f increasing size Increased brain size Increased energy requirements have been met by a variety f adaptatins, including (1) a reductin in expensive gut tissue (2) an increase in the energetic efficiency f lcmtin (3) an increase in the capacity t acquire energy thrugh increased fraging territry (4) changes in dietary cmpsitin thrugh shifts t higher return fds r mre efficient fraging methds (5) shifts in the energetics f reprductin and grwth (6) technlgical develpments, increasing efficiency Cnclusin - Fat depsitin is favured by selectin when reprductive fitness benefits mre frm buffering fluctuatins in energy supply than frm cnverting all energy intake directly int larger size r additinal ffspring - There has been intense selectin fr thriftiness within the last five millin years, in relatin t several diverse factrs seasnality, encephalisatin, and recent niche cnstructin, in particular agricultural systems - The critical rle f fat stres in human reprductive energetics ffers an explanatin fr the existence f sensitive perids in the life cycle fr the develpment f besity - Adding further weight t besity cannt be regarded as a simple famine-tackling adaptatin - Human bilgy is highly tailred t buffer regular fluctuatins in energy supply, and yet in the mdern western envirnment such fluctuatins n lnger ccur

7 - Mdern agricultural practices and imprvements in the capacity t stre fd have minimized the threat f famine, while mechanized technlgy allws us t maximize prductivity with minimal impact n energy expenditure - The speed with which mdern agricultural and industrial niches have been cnstructed that accunts fr the rapidity with which besity has emerged as a public health prblem - Energy balance equatin: weight gain can be attributed t the difference between energy intake and expenditure

Risk factors in health and disease

Risk factors in health and disease Risk factrs in health and disease Index 1 Intrductin 2 Types f risk factrs 2.1 Behaviural risk factrs 2.2 Psychlgical risk factrs 2.3 Demgraphic risk factrs 2.4 Envirnmental risk factrs 2.5 Genetic risk