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Integrative Physiology of Fasting

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ABSTRACT

Extended bouts of fasting are ingrained in the ecology of many organisms, characterizing aspects of reproduction, development, hibernation, estivation, migration, and infrequent feeding habits. The challenge of long fasting episodes is the need to maintain physiological homeostasis while relying solely on endogenous resources. To meet that challenge, animals utilize an integrated repertoire of behavioral, physiological, and biochemical responses that reduce metabolic rates, maintain tissue structure and function, and thus enhance survival. We have synthesized in this review the integrative physiological, morphological, and biochemical responses, and their stages, that characterize natural fasting bouts. Underlying the capacity to survive extended fasts are behaviors and mechanisms that reduce metabolic expenditure and shift the dependency to lipid utilization. Hormonal regulation and immune capacity are altered by fasting; hormones that trigger digestion, elevate metabolism, and support immune performance become depressed, whereas hormones that enhance the utilization of endogenous substrates are elevated. The negative energy budget that accompanies fasting leads to the loss of body mass as fat stores are depleted and tissues undergo atrophy (i.e., loss of mass). Absolute rates of body mass loss scale allometrically among vertebrates. Tissues and organs vary in the degree of atrophy and downregulation of function, depending on the degree to which they are used during the fast. Fasting affects the population dynamics and activities of the gut microbiota, an interplay that impacts the host's fasting biology. Fasting‐induced gene expression programs underlie the broad spectrum of integrated physiological mechanisms responsible for an animal's ability to survive long episodes of natural fasting. © 2016 American Physiological Society. Compr Physiol 6:773‐825, 2016.

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Figure 1. Figure 1. Animals that exemplify diverse forms of extended fasting have been valuable for exploring the ecology and physiology of fasting. For hibernation (A and B), illustrated are the 13‐lined ground squirrel (Ictidomys tridecemlineatus) and black bear (Ursus americanus); (). For aestivation (C and D), illustrated are the African lungfish (Protopterus annectens) and green‐striped burrowing frog (Cyclorana alboguttata) in cocooned state (); For migration (E and F), illustrated are the green sea turtle (Chelonia mydas) and the red knot (Calidris canutus) (). For molting (G and H), illustrated are the king penguin (Aptenodytes patagonica) and the northern elephant seal (Mirounga angustirostris) (). For reproduction (I and J), illustrated are the northern elephant seal and emperor penguin (Aptenodytes forsteri) (). For development (K and L), illustrated are the juvenile king penguin (Aptenodytes patagonica) and grey seal (Halichoerus grypus) (). For natural long bouts of fasting (M and N), illustrated are the raccoon dog (Nyctereutes procyonoides) and the Burmese python (Python molurus) (). Photo Credit: (A) Lesa Hollen; (B) Lynn Rogers, bear.org; (C) http://en.wikipedia.org/wiki/Protopterus#mediaviewer/File:G%C5%91tehal‐2.jpg (D) E.A. Meyer; (E) Evan D'Alessandro; (F) Theunis Piersma; (G) Katie O'Reilly; (H) Dan Costa; (I) Dan Costa; (J) http://joshsjungle.com/wp‐content/uploads/2011/06/emperor‐penguin‐and‐chick.jpg; (K) http://www.factzoo.com/sites/all/img/birds/king‐penguin‐chicks.jpg; (L) Andreas Trepte; (M) http://www.factzoo.com/sites/all/img/mammals/raccoon‐dog‐nice‐coat.jpg; (N) Stephen Secor.
Figure 2. Figure 2. Circannual body temperature cycle in a 13‐lined ground squirrel (Ictidomys tridecemlineatus). The animal was housed conventionally (T a ∼ 20°C, 12:12 LD with food/water), then moved in September to a 4°C cold room in constant darkness with no food/water. Note spontaneous interbout arousals to normothermia that interrupt torpor bouts during the hibernation season. Figure courtesy of Sandy Martin, University of Colorado School of Medicine.
Figure 3. Figure 3. Energy expenditure (kJ/h) as a function of days postfeeding for the (A) common carp Cyprinus carpio, (B) Burmese python Python molurus, (C) Adelie penguin Pygoscelis adeliae, and (D) laboratory rat Rattus norvegicus. These postprandial metabolic plots illustrate the variation in metabolic rate during meal digestion and the significant decline in metabolic rate that occurs upon the completion of digestion. Figures were drawn from data presented in ().
Figure 4. Figure 4. Characteristic profiles of metabolic variables during the three phases of fasting, including plasma concentrations of glucose, β‐hydroxybutyrate, urea, protein utilization, mass‐specific body mass loss, metabolic rate, and tissue concentrations of glycogen. Placement of individual profiles with respect to the Y‐axis is only illustrative and not quantitative. Figure adapted, with permission, from Figure 1 in ().
Figure 5. Figure 5. Variation in glycogen depletion with fasting is illustrated with the relative change in liver glycogen as a function of days postfeeding for the mudskipper Boleophthalmus boddaerti, sturgeon Acipenser naccarii, bats Artibeus lituratus and A. jamaicensis (combined data), and vole Clethrionomys rufocanus. Figure drawn, with permission, from data provided in ().
Figure 6. Figure 6. Plasma concentrations of glucose as a function of days of fasting for (A) subantarctic fur seal pups (Arctocephalus tropicalis) which experiences no significant change in blood glucose, (B) the Atlantic salmon (Salmo salar) which experiences an initial drop in blood glucose, (C) northern elephant seal pups (Mirounga angustirostris) which experiences a steady decline in blood glucose with fasting, and (D) greater snow geese (Chen caerulescens atlantica) whose blood glucose is maintained elevated until the entrance into phase III before declining. Figure drawn, with permission, from data presented in ().
Figure 7. Figure 7. Plasma concentration of fatty acids as a function of days of fasting for (A) subantarctic fur seal pups (Arctocephalus tropicalis) and (B) molting king penguins (Aptenodytes patagonica). Fur seal pups experience a doubling of fatty acid levels with the onset of Phase II (day 4) and both animals experienced a rise in fatty acids toward the end of Phase II (day 32). Figures drawn, with permission, from data presented in ().
Figure 8. Figure 8. Relative differences (in percent compared to fed active animals) in the concentration of individual fatty acids for (A) the scapular adipose tissue of the raccoon dog (Nyctereutes procyonoides) following an 8‐week fast, (B) the plasma of diamondback rattlesnakes (Crotalus atrox) fasted for 180 days, and (C) the plasma of female nonlactating black bear (Ursus americanus) following several months of hibernation. Illustrated is the dynamic nature of circulating fatty acids in response to fasting and the variation of that response among animals. Figure drawn, with permission, from data presented in ().
Figure 9. Figure 9. Plasma concentrations of β‐hydroxybutyrate (β‐OHB) as a function of days of fasting for (A) subantarctic fur seal pups (Arctocephalus tropicalis), (B) greater snow geese (Chen caerulescens atlantica), and (C) breeding male king penguins (Aptenodytes patagonica). For these species, β‐OHB steadily increased with fasting however peaked midway through Phase II fasting for the snow geese and king penguins before declining and returning to initial levels during Phase III. Figures were drawn, with permission, from data presented in ().
Figure 10. Figure 10. Plasma concentrations of triglycerides and cholesterol as a function of days of fasting for the (A) trout Oncorhynchus mykiss, (B) mink Mustela vison, and (C) Burmese python Python molurus. For the Burmese python, the start of the fast is noted at 3 days postfeeding when plasma triglycerides peaked. Both the trout and python experienced an initial decrease in plasma triglycerides that was not experienced by the mink. Python possess nondetectable levels of triglycerides in their plasma once they have completed digestion. Fasting resulted in a modest decrease in plasma cholesterol for the trout, but remained unchanged for both the python and mink. Figures were drawn, with permission, from data presented in () and (S. Secor, unpublished data).
Figure 11. Figure 11. (A) Plasma concentration of proteins as a function of days of fasting for the greater snow geese (Chen caerulescens atlantica). Protein levels remain relatively stable through phase I and II, before declining with the onset of Phase III (day 30). (B) The relative change in plasma concentrations of amino acids following 8 weeks of fasting for the raccoon dog (Nyctereutes procyonoides). While the majority of amino acids decreased in concentration, two amino acids (glutamine and serine) increased. Figures drawn, with permission, from data presented in ().
Figure 12. Figure 12. Plasma concentrations of urea and uric acid as a function of days of fasting for (A) breeding male king penguin (Aptenodytes patagonica) and (B) the common buzzard (Buteo buteo) For the king penguin plasma urea and uric acid concentrations slowly increased during phase II fasting and rose more steeply with transition to Phase III (∼day 35). For the buzzard, both urea and uric acid peak midway through the fast. Figures were drawn, with permission, from data presented ().
Figure 13. Figure 13. Plasma urea (stippled) and total osmolality (solid) of active and fed (red) and aestivated and fasted (blue) greater siren (Siren lacertina), northern burrowing frog (Neobatrachus aquilonius), African bullfrog (Pxyicephalus adspersus), western spotted frog (Heleioporus albopunctatus), Kunapalari frog (Neobatrachus kunapalari), humming frog (Neobatrachus pelobatoides), and Couch's spadefoot toad (Scaphiopus couchii). Duration of aestivation range from 2 to 8.5 months. During aestivation, amphibians accumulate urea in their blood which contributes in some cases to a doubling of plasma osmolality. Figure drawn, with permission, from data presented in ().
Figure 14. Figure 14. Plasma concentrations of glucagon, insulin, corticosterone or cortisol, triiodothyronine (T 3), and thyroxin (T 4) as a function of days fasting for breeding male king penguins (Aptenodytes patagonicus) and raccoon dogs (Nyctereutes procyonoides). With the exception of glucagon, which increased with time fasting for both species, plasma concentrations of the other five hormones experienced contrasting fasting profiles. Figures drawn, with permission, from data presented in ().
Figure 15. Figure 15. Plasma concentration of (A) immunoglobulin (IgY), (B) natural antibodies (NABs), and (C) corticosterone for fasted and refed female mallard ducks (Anas platyhyrhychos). Sampling treatments included fed, 48‐h fasted, phase II of fasting (PII), phase III of fasting (PIII), fasted to PIII and refed for 1 day (RI), fasted to PIII and refed for 3 days (R3), and fasted to PIII and refed to recovery of initial body mass (Rt). Note the fasting decline in immune function and rapid increase in corticosterone, both reversed with refeeding. Figures drawn, with permission, from data presented in ().
Figure 16. Figure 16. Relative change in body mass and individual organs and tissues as a function of time fasting for (A) the fish Clarius lazera, (B) the snake Nerodia rhombifer, and (C) the rat Rattus norvegicus. Compared to the rate of body mass loss, individual organs and tissue lose mass at a lower or greater rate with time fasting. Figures for Clarius lazera and Rattus norvegicus were drawn, with permission, respectively, from data presented in () and (). Figure for Nerodia rhombifer was drawn, with permission, from unpublished data of M. Larkin and S. Secor.
Figure 17. Figure 17. Absolute (g day−1) and mass specific (g kg−1 day−1) loss of body mass plotted against body mass for birds, mammals, and ectotherms. Data were generated from studies that provided body mass before and after a known period of fasting. Note the near identical allometric scaling exponents for birds, mammals, and ectotherms.
Figure 18. Figure 18. Mass specific daily loss of body mass illustrated for the three phases of fasting for (A) common barn owl (Tyto alba) and (B) breeding king penguin (Aptenodytes patagonica). Birds experienced greater specific mass loss during phases I and III of fasting compared to phase II. Figures drawn, with permission, from data presented in ().
Figure 19. Figure 19. (A) Micrographs of intraperitoneal fat bodies from king penguin chicks (Aptenodytes patagonicus) following control feeding (NCA), three weeks of laboratory fasting (STF), and several months of a natural fast (LTF) (). Mass of regional fat deposits of fed (closed bars) and fasted (open bars) (B) raccoon dog (Nyctereutes procyonoides) and (C) American marten (Martes americana). Raccoon dogs and martens had been fasted for 56 and 2 days, respectively. Illustrated is the characteristic depletion of fat stores during fasting. Figures B and C were drawn, with permission, from data presented in ().
Figure 20. Figure 20. Electron micrographs of (A) gastrocnemius and (B) pectoralis muscle sampled from king penguin chicks (Aptenodytes patagonicus) following control feeding (NCA), 3 weeks of laboratory fasting (STF), and several months of a natural fast (LTF) as presented in (). Below each set of micrographs is the relative mass (% of body mass) of these muscles for each treatment. Note the loss of sarcomere integrity following the natural fast (LTF) for both muscles, and the increase in relative mass of the gastrocnemius but decrease in relative mass of the pectoralis with duration of the fast. Figures illustrating relative muscle masses are drawn, with permission, from data presented in ().
Figure 21. Figure 21. (A) mean cross‐sectional area (blue) and density (red) of fast‐ and slow‐twitch muscle fibers in the gastrocnemius and biceps femoris of black bears (Ursus americanus) prior to hibernation (fall, open bars) and immediately following hibernation (spring, closed bars). (B) Citrate synthase (CS) activity (blue) of gastrocnemius and biceps femoris muscles and contraction force (red) of the tibialis anterior muscle determined from bears sampled in the fall or early in hibernation (open bars) and sampled in the spring or late in hibernation (closed bars). (C) Twitch parameters of the tibialis anterior muscle for black bears measured early and late during hibernation. Illustrated is the lack of change in muscle structure and function for black bears during hibernation. Figures drawn, with permission, from data presented in ().
Figure 22. Figure 22. (A) Muscle dry mass, (B) muscle fiber density, (C) maximum stress twitch, and (D) peak power output for the gastrocnemius, iliofibularis, and sartorius muscles for control (fed and active) Cyclorana alboguttata and following 9 months of aestivation. This frog did not experience any loss in muscle mass or performance even after 9 months of aestivation. Figures drawn, with permission, from data presented in ().
Figure 23. Figure 23. Gastric pH profile for the (A) dog (Canis familiaris), (B) leopard shark (Triakis semifasciata), (C) white sea bream (Sparus aurata), and (D) Burmese python (Python molurus). The downturn arrows note the time of feeding and the upturn arrows note the time of gastric emptying. For the dog and leopard shark, the stomach remains acidic and experiences an increase in pH with feeding (due to the buffering effects of the food). In contrast, the white sea bream and python maintain a near neutral gastric pH while fasting, experience a rapid drop in pH during digestion, and then curtail acid production with the emptying of the stomach. Figures are drawn, with permission, from data presented in ().
Figure 24. Figure 24. Fasting‐induced changes (relative to fed state) in (A) small intestinal mass, (B) mucosal thickness, and (C) enterocyte volume for Micropterus salmoides, Cyclorana alboguttata, Pyxicephalus adspersus, Chelydra serpentina, Heloderma suspectum, Python molurus, Nerodia rhombifer, Passer domesticus, and Ictidomys tridecemlineatus. Noted is the condition and duration of the fast for each species. All species experience a fasting reduction in small intestinal mass, mucosal thickness, and enterocyte volume, however there is considerable variation in the extent of the reduction. Figures drawn, with permission, from data presented in () and (S. Secor and D. Crossley, unpublished data).
Figure 25. Figure 25. Electron micrographs of intestinal microvilli of fed and fasted 13‐lined ground squirrel Ictidomys tridecemlineatus diamondback water snake Nerodia rhombifer, channel catfish Ictalurus punctatus, vermiculated sailfin catfish Pterygoplichthys disjunctivus, Gila monster Heloderma suspectum, South American horn frog Ceratophrys ornata, African bullfrog Pyxicephalus adspersus, and Burmese python Python molurus (). Fasting conditions are respectively; 6‐week hibernation, 30‐day fast, 5‐day fast, 150‐day fast, 30‐day fast, 1‐month aestivation, 1‐month aestivation, and 30‐day fast. Below each set of micrographs is illustrated the relative change in microvillus length with fasting.
Figure 26. Figure 26. Fasting generated changes (relative to fed state) in small intestinal (A) aminopeptidase activity, (B) maltase or sucrase activity, (C) D‐glucose uptake, and (D) L‐proline uptake for Ctenopharyngodon idella, Ceratophrys ornata, Amphiuma tridactylum, Chelydra serpentina, Heloderma suspectum, Python molurus, Nerodia rhombifer, Passer domesticus, and Ictidomys tridecemlineatus. Noted is the condition and duration of the fast for each species. Among these species, there is considerable variation in magnitude by which intestinal function is regulated with fasting, ranging from sever downregulation, to no change, to significant increases in mass‐specific function. Figures drawn, with permission, from data presented in () and (S. Secor and D. Crossley, unpublished data; S. Secor and M. Smith, unpublished data).
Figure 27. Figure 27. Correspondence between temporal changes in microvillus surface area (MVSA, black) and intestinal function (blue) with fasting for (A) the Burmese python (Python molurus) and (B) the southern catfish (Silurus meridionalis). In this figure, MVSA is compared with L‐proline uptake rates for P. molurus and with aminopeptidase activity for S. meridionalis. Electron micrographs of intestinal microvilli at each time point demonstrate the fasting‐related shortening of the microvilli and hence decrease in MVSA. Micrographs and the data used to make figures originate, with permission, from ().
Figure 28. Figure 28. Fasting related change in wet mass of the (A) heart, (b) liver, (C) gall bladder, (D) pancreas, and (E) kidneys for the Burmese python (Python molurus). Time zero (0) represents the time point postfeeding (2 or 3 days) that each organ was at its maximum (heart, liver, pancreas, and kidneys) or minimum (gall bladder) mass. The insert for (D) illustrates the decline with fasting of pancreas' capacity for amylase activity. Figures drawn, with permission, from data presented in () and (S. Secor, unpublished data).
Figure 29. Figure 29. (A) Postprandial separation of the relative percentages of the microbiota phylums Bacteroidetes and Firmicutes within the large intestine for the Burmese python (Python molurus). Within hours of feeding, Firmicutes come to dominate the microbiota community, while the contribution of Bacteroidetes is greatly depressed. With the clearing of the intestine and through fasting, both phyla then contribute equally to the microbiota community. (B). The relative abundance of microbiota phyla in cecal contents of 13‐line ground squirrels (Ictidomys tridecemlineatus) during summer activity, early and late winter hibernation, and spring activity 2 weeks postfeeding. Note the shifts with hibernation of the phyla Bacteroidetes, Firmicutes, and Verrucomicrobia. Figures drawn, with permission, from data presented in ().
Figure 30. Figure 30. (A) Representation of the up‐ and downregulation of cellular function by the differential expression of genes within the gastrocnemius muscle of green‐striped burrowing frogs (Cyclorana alboguttata) following 4 months of aestivation. Plots represent the −log of the P value stemming from Ingenuity Pathway Analysis. (B) Up and downregulation of genes following a 1 month fast (compared to 1‐day fed) for the heart, kidneys, liver, and small intestine of the Burmese python (Python molurus). Genes include those involved in transcription (DDX21), translation (Rmb4), development (GPR180), and metabolism (PDK4). Plots illustrate log2 fold change in expression. (C) Factorial changes in mRNA abundance of genes of the heart, muscle, and white adipose between summer (August) active and hibernating torpor 13‐lined ground squirrels (Ictidomys tridecemlineatus). Figures drawn, with permission, from data presented in ().


Figure 1. Animals that exemplify diverse forms of extended fasting have been valuable for exploring the ecology and physiology of fasting. For hibernation (A and B), illustrated are the 13‐lined ground squirrel (Ictidomys tridecemlineatus) and black bear (Ursus americanus); (). For aestivation (C and D), illustrated are the African lungfish (Protopterus annectens) and green‐striped burrowing frog (Cyclorana alboguttata) in cocooned state (); For migration (E and F), illustrated are the green sea turtle (Chelonia mydas) and the red knot (Calidris canutus) (). For molting (G and H), illustrated are the king penguin (Aptenodytes patagonica) and the northern elephant seal (Mirounga angustirostris) (). For reproduction (I and J), illustrated are the northern elephant seal and emperor penguin (Aptenodytes forsteri) (). For development (K and L), illustrated are the juvenile king penguin (Aptenodytes patagonica) and grey seal (Halichoerus grypus) (). For natural long bouts of fasting (M and N), illustrated are the raccoon dog (Nyctereutes procyonoides) and the Burmese python (Python molurus) (). Photo Credit: (A) Lesa Hollen; (B) Lynn Rogers, bear.org; (C) http://en.wikipedia.org/wiki/Protopterus#mediaviewer/File:G%C5%91tehal‐2.jpg (D) E.A. Meyer; (E) Evan D'Alessandro; (F) Theunis Piersma; (G) Katie O'Reilly; (H) Dan Costa; (I) Dan Costa; (J) http://joshsjungle.com/wp‐content/uploads/2011/06/emperor‐penguin‐and‐chick.jpg; (K) http://www.factzoo.com/sites/all/img/birds/king‐penguin‐chicks.jpg; (L) Andreas Trepte; (M) http://www.factzoo.com/sites/all/img/mammals/raccoon‐dog‐nice‐coat.jpg; (N) Stephen Secor.


Figure 2. Circannual body temperature cycle in a 13‐lined ground squirrel (Ictidomys tridecemlineatus). The animal was housed conventionally (T a ∼ 20°C, 12:12 LD with food/water), then moved in September to a 4°C cold room in constant darkness with no food/water. Note spontaneous interbout arousals to normothermia that interrupt torpor bouts during the hibernation season. Figure courtesy of Sandy Martin, University of Colorado School of Medicine.


Figure 3. Energy expenditure (kJ/h) as a function of days postfeeding for the (A) common carp Cyprinus carpio, (B) Burmese python Python molurus, (C) Adelie penguin Pygoscelis adeliae, and (D) laboratory rat Rattus norvegicus. These postprandial metabolic plots illustrate the variation in metabolic rate during meal digestion and the significant decline in metabolic rate that occurs upon the completion of digestion. Figures were drawn from data presented in ().


Figure 4. Characteristic profiles of metabolic variables during the three phases of fasting, including plasma concentrations of glucose, β‐hydroxybutyrate, urea, protein utilization, mass‐specific body mass loss, metabolic rate, and tissue concentrations of glycogen. Placement of individual profiles with respect to the Y‐axis is only illustrative and not quantitative. Figure adapted, with permission, from Figure 1 in ().


Figure 5. Variation in glycogen depletion with fasting is illustrated with the relative change in liver glycogen as a function of days postfeeding for the mudskipper Boleophthalmus boddaerti, sturgeon Acipenser naccarii, bats Artibeus lituratus and A. jamaicensis (combined data), and vole Clethrionomys rufocanus. Figure drawn, with permission, from data provided in ().


Figure 6. Plasma concentrations of glucose as a function of days of fasting for (A) subantarctic fur seal pups (Arctocephalus tropicalis) which experiences no significant change in blood glucose, (B) the Atlantic salmon (Salmo salar) which experiences an initial drop in blood glucose, (C) northern elephant seal pups (Mirounga angustirostris) which experiences a steady decline in blood glucose with fasting, and (D) greater snow geese (Chen caerulescens atlantica) whose blood glucose is maintained elevated until the entrance into phase III before declining. Figure drawn, with permission, from data presented in ().


Figure 7. Plasma concentration of fatty acids as a function of days of fasting for (A) subantarctic fur seal pups (Arctocephalus tropicalis) and (B) molting king penguins (Aptenodytes patagonica). Fur seal pups experience a doubling of fatty acid levels with the onset of Phase II (day 4) and both animals experienced a rise in fatty acids toward the end of Phase II (day 32). Figures drawn, with permission, from data presented in ().


Figure 8. Relative differences (in percent compared to fed active animals) in the concentration of individual fatty acids for (A) the scapular adipose tissue of the raccoon dog (Nyctereutes procyonoides) following an 8‐week fast, (B) the plasma of diamondback rattlesnakes (Crotalus atrox) fasted for 180 days, and (C) the plasma of female nonlactating black bear (Ursus americanus) following several months of hibernation. Illustrated is the dynamic nature of circulating fatty acids in response to fasting and the variation of that response among animals. Figure drawn, with permission, from data presented in ().


Figure 9. Plasma concentrations of β‐hydroxybutyrate (β‐OHB) as a function of days of fasting for (A) subantarctic fur seal pups (Arctocephalus tropicalis), (B) greater snow geese (Chen caerulescens atlantica), and (C) breeding male king penguins (Aptenodytes patagonica). For these species, β‐OHB steadily increased with fasting however peaked midway through Phase II fasting for the snow geese and king penguins before declining and returning to initial levels during Phase III. Figures were drawn, with permission, from data presented in ().


Figure 10. Plasma concentrations of triglycerides and cholesterol as a function of days of fasting for the (A) trout Oncorhynchus mykiss, (B) mink Mustela vison, and (C) Burmese python Python molurus. For the Burmese python, the start of the fast is noted at 3 days postfeeding when plasma triglycerides peaked. Both the trout and python experienced an initial decrease in plasma triglycerides that was not experienced by the mink. Python possess nondetectable levels of triglycerides in their plasma once they have completed digestion. Fasting resulted in a modest decrease in plasma cholesterol for the trout, but remained unchanged for both the python and mink. Figures were drawn, with permission, from data presented in () and (S. Secor, unpublished data).


Figure 11. (A) Plasma concentration of proteins as a function of days of fasting for the greater snow geese (Chen caerulescens atlantica). Protein levels remain relatively stable through phase I and II, before declining with the onset of Phase III (day 30). (B) The relative change in plasma concentrations of amino acids following 8 weeks of fasting for the raccoon dog (Nyctereutes procyonoides). While the majority of amino acids decreased in concentration, two amino acids (glutamine and serine) increased. Figures drawn, with permission, from data presented in ().


Figure 12. Plasma concentrations of urea and uric acid as a function of days of fasting for (A) breeding male king penguin (Aptenodytes patagonica) and (B) the common buzzard (Buteo buteo) For the king penguin plasma urea and uric acid concentrations slowly increased during phase II fasting and rose more steeply with transition to Phase III (∼day 35). For the buzzard, both urea and uric acid peak midway through the fast. Figures were drawn, with permission, from data presented ().


Figure 13. Plasma urea (stippled) and total osmolality (solid) of active and fed (red) and aestivated and fasted (blue) greater siren (Siren lacertina), northern burrowing frog (Neobatrachus aquilonius), African bullfrog (Pxyicephalus adspersus), western spotted frog (Heleioporus albopunctatus), Kunapalari frog (Neobatrachus kunapalari), humming frog (Neobatrachus pelobatoides), and Couch's spadefoot toad (Scaphiopus couchii). Duration of aestivation range from 2 to 8.5 months. During aestivation, amphibians accumulate urea in their blood which contributes in some cases to a doubling of plasma osmolality. Figure drawn, with permission, from data presented in ().


Figure 14. Plasma concentrations of glucagon, insulin, corticosterone or cortisol, triiodothyronine (T 3), and thyroxin (T 4) as a function of days fasting for breeding male king penguins (Aptenodytes patagonicus) and raccoon dogs (Nyctereutes procyonoides). With the exception of glucagon, which increased with time fasting for both species, plasma concentrations of the other five hormones experienced contrasting fasting profiles. Figures drawn, with permission, from data presented in ().


Figure 15. Plasma concentration of (A) immunoglobulin (IgY), (B) natural antibodies (NABs), and (C) corticosterone for fasted and refed female mallard ducks (Anas platyhyrhychos). Sampling treatments included fed, 48‐h fasted, phase II of fasting (PII), phase III of fasting (PIII), fasted to PIII and refed for 1 day (RI), fasted to PIII and refed for 3 days (R3), and fasted to PIII and refed to recovery of initial body mass (Rt). Note the fasting decline in immune function and rapid increase in corticosterone, both reversed with refeeding. Figures drawn, with permission, from data presented in ().


Figure 16. Relative change in body mass and individual organs and tissues as a function of time fasting for (A) the fish Clarius lazera, (B) the snake Nerodia rhombifer, and (C) the rat Rattus norvegicus. Compared to the rate of body mass loss, individual organs and tissue lose mass at a lower or greater rate with time fasting. Figures for Clarius lazera and Rattus norvegicus were drawn, with permission, respectively, from data presented in () and (). Figure for Nerodia rhombifer was drawn, with permission, from unpublished data of M. Larkin and S. Secor.


Figure 17. Absolute (g day−1) and mass specific (g kg−1 day−1) loss of body mass plotted against body mass for birds, mammals, and ectotherms. Data were generated from studies that provided body mass before and after a known period of fasting. Note the near identical allometric scaling exponents for birds, mammals, and ectotherms.


Figure 18. Mass specific daily loss of body mass illustrated for the three phases of fasting for (A) common barn owl (Tyto alba) and (B) breeding king penguin (Aptenodytes patagonica). Birds experienced greater specific mass loss during phases I and III of fasting compared to phase II. Figures drawn, with permission, from data presented in ().


Figure 19. (A) Micrographs of intraperitoneal fat bodies from king penguin chicks (Aptenodytes patagonicus) following control feeding (NCA), three weeks of laboratory fasting (STF), and several months of a natural fast (LTF) (). Mass of regional fat deposits of fed (closed bars) and fasted (open bars) (B) raccoon dog (Nyctereutes procyonoides) and (C) American marten (Martes americana). Raccoon dogs and martens had been fasted for 56 and 2 days, respectively. Illustrated is the characteristic depletion of fat stores during fasting. Figures B and C were drawn, with permission, from data presented in ().


Figure 20. Electron micrographs of (A) gastrocnemius and (B) pectoralis muscle sampled from king penguin chicks (Aptenodytes patagonicus) following control feeding (NCA), 3 weeks of laboratory fasting (STF), and several months of a natural fast (LTF) as presented in (). Below each set of micrographs is the relative mass (% of body mass) of these muscles for each treatment. Note the loss of sarcomere integrity following the natural fast (LTF) for both muscles, and the increase in relative mass of the gastrocnemius but decrease in relative mass of the pectoralis with duration of the fast. Figures illustrating relative muscle masses are drawn, with permission, from data presented in ().


Figure 21. (A) mean cross‐sectional area (blue) and density (red) of fast‐ and slow‐twitch muscle fibers in the gastrocnemius and biceps femoris of black bears (Ursus americanus) prior to hibernation (fall, open bars) and immediately following hibernation (spring, closed bars). (B) Citrate synthase (CS) activity (blue) of gastrocnemius and biceps femoris muscles and contraction force (red) of the tibialis anterior muscle determined from bears sampled in the fall or early in hibernation (open bars) and sampled in the spring or late in hibernation (closed bars). (C) Twitch parameters of the tibialis anterior muscle for black bears measured early and late during hibernation. Illustrated is the lack of change in muscle structure and function for black bears during hibernation. Figures drawn, with permission, from data presented in ().


Figure 22. (A) Muscle dry mass, (B) muscle fiber density, (C) maximum stress twitch, and (D) peak power output for the gastrocnemius, iliofibularis, and sartorius muscles for control (fed and active) Cyclorana alboguttata and following 9 months of aestivation. This frog did not experience any loss in muscle mass or performance even after 9 months of aestivation. Figures drawn, with permission, from data presented in ().


Figure 23. Gastric pH profile for the (A) dog (Canis familiaris), (B) leopard shark (Triakis semifasciata), (C) white sea bream (Sparus aurata), and (D) Burmese python (Python molurus). The downturn arrows note the time of feeding and the upturn arrows note the time of gastric emptying. For the dog and leopard shark, the stomach remains acidic and experiences an increase in pH with feeding (due to the buffering effects of the food). In contrast, the white sea bream and python maintain a near neutral gastric pH while fasting, experience a rapid drop in pH during digestion, and then curtail acid production with the emptying of the stomach. Figures are drawn, with permission, from data presented in ().


Figure 24. Fasting‐induced changes (relative to fed state) in (A) small intestinal mass, (B) mucosal thickness, and (C) enterocyte volume for Micropterus salmoides, Cyclorana alboguttata, Pyxicephalus adspersus, Chelydra serpentina, Heloderma suspectum, Python molurus, Nerodia rhombifer, Passer domesticus, and Ictidomys tridecemlineatus. Noted is the condition and duration of the fast for each species. All species experience a fasting reduction in small intestinal mass, mucosal thickness, and enterocyte volume, however there is considerable variation in the extent of the reduction. Figures drawn, with permission, from data presented in () and (S. Secor and D. Crossley, unpublished data).


Figure 25. Electron micrographs of intestinal microvilli of fed and fasted 13‐lined ground squirrel Ictidomys tridecemlineatus diamondback water snake Nerodia rhombifer, channel catfish Ictalurus punctatus, vermiculated sailfin catfish Pterygoplichthys disjunctivus, Gila monster Heloderma suspectum, South American horn frog Ceratophrys ornata, African bullfrog Pyxicephalus adspersus, and Burmese python Python molurus (). Fasting conditions are respectively; 6‐week hibernation, 30‐day fast, 5‐day fast, 150‐day fast, 30‐day fast, 1‐month aestivation, 1‐month aestivation, and 30‐day fast. Below each set of micrographs is illustrated the relative change in microvillus length with fasting.


Figure 26. Fasting generated changes (relative to fed state) in small intestinal (A) aminopeptidase activity, (B) maltase or sucrase activity, (C) D‐glucose uptake, and (D) L‐proline uptake for Ctenopharyngodon idella, Ceratophrys ornata, Amphiuma tridactylum, Chelydra serpentina, Heloderma suspectum, Python molurus, Nerodia rhombifer, Passer domesticus, and Ictidomys tridecemlineatus. Noted is the condition and duration of the fast for each species. Among these species, there is considerable variation in magnitude by which intestinal function is regulated with fasting, ranging from sever downregulation, to no change, to significant increases in mass‐specific function. Figures drawn, with permission, from data presented in () and (S. Secor and D. Crossley, unpublished data; S. Secor and M. Smith, unpublished data).


Figure 27. Correspondence between temporal changes in microvillus surface area (MVSA, black) and intestinal function (blue) with fasting for (A) the Burmese python (Python molurus) and (B) the southern catfish (Silurus meridionalis). In this figure, MVSA is compared with L‐proline uptake rates for P. molurus and with aminopeptidase activity for S. meridionalis. Electron micrographs of intestinal microvilli at each time point demonstrate the fasting‐related shortening of the microvilli and hence decrease in MVSA. Micrographs and the data used to make figures originate, with permission, from ().


Figure 28. Fasting related change in wet mass of the (A) heart, (b) liver, (C) gall bladder, (D) pancreas, and (E) kidneys for the Burmese python (Python molurus). Time zero (0) represents the time point postfeeding (2 or 3 days) that each organ was at its maximum (heart, liver, pancreas, and kidneys) or minimum (gall bladder) mass. The insert for (D) illustrates the decline with fasting of pancreas' capacity for amylase activity. Figures drawn, with permission, from data presented in () and (S. Secor, unpublished data).


Figure 29. (A) Postprandial separation of the relative percentages of the microbiota phylums Bacteroidetes and Firmicutes within the large intestine for the Burmese python (Python molurus). Within hours of feeding, Firmicutes come to dominate the microbiota community, while the contribution of Bacteroidetes is greatly depressed. With the clearing of the intestine and through fasting, both phyla then contribute equally to the microbiota community. (B). The relative abundance of microbiota phyla in cecal contents of 13‐line ground squirrels (Ictidomys tridecemlineatus) during summer activity, early and late winter hibernation, and spring activity 2 weeks postfeeding. Note the shifts with hibernation of the phyla Bacteroidetes, Firmicutes, and Verrucomicrobia. Figures drawn, with permission, from data presented in ().


Figure 30. (A) Representation of the up‐ and downregulation of cellular function by the differential expression of genes within the gastrocnemius muscle of green‐striped burrowing frogs (Cyclorana alboguttata) following 4 months of aestivation. Plots represent the −log of the P value stemming from Ingenuity Pathway Analysis. (B) Up and downregulation of genes following a 1 month fast (compared to 1‐day fed) for the heart, kidneys, liver, and small intestine of the Burmese python (Python molurus). Genes include those involved in transcription (DDX21), translation (Rmb4), development (GPR180), and metabolism (PDK4). Plots illustrate log2 fold change in expression. (C) Factorial changes in mRNA abundance of genes of the heart, muscle, and white adipose between summer (August) active and hibernating torpor 13‐lined ground squirrels (Ictidomys tridecemlineatus). Figures drawn, with permission, from data presented in ().
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Stephen M. Secor, Hannah V. Carey. Integrative Physiology of Fasting. Compr Physiol 2016, 6: 773-825. doi: 10.1002/cphy.c150013