BIOL 3542 Lecture Notes - Lecture 10: Neuropeptide Y, Adipose Tissue, Adipocyte
26 Jun 2018
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Human Physiology II
Chapter 22: Metabolism and Energy Balance
Appetite and Satiety
digestive system doesn’t regulate energy intake, we depend on behavioural mechanisms of
hunger and satiety
current model for behavioural regulation of food intake based on 2 hypothalamic centres:
1. Feeding Center: tonically active
2. Satiety Center: stops food intake by inhibiting feeding center
output signals from centers create sensations of hunger and fullness
higher brain centers including cerebral cortex, limbic system, provide input to hypothalamus
chemical signals influencing intake, satiety include: neuropeptides, “brain-gut” hormones
secreted by GI tract, adipocytokines secreted by adipose tissue
Glucostatic Theory: glucose metabolism by hypothalamic centers regulates food intake
when blood glucose concentrations decrease, satiety center is suppressed, feeding center
dominant
when glucose metabolism increases, satiety center inhibits feeding center
Lipostatic Theory: energy balance proposes that signal from body’s fat stores to brain modulates
eating behaviour so body maintains particular weight
if fat stores increase, eating decreases
in times of starvation, eating increases
obesity results from pathway disruption
Leptin: protein hormone synthesized in adipocytes that acts as negative feedback signal between
adipose tissue and brain
as fat stores increase, adipose cells secrete more leptin, food intake decreases
leptin synthesized in adipocytes under control of obese gene
Neuropeptide Y: brain neurotransmitter, stimulus for food intake
leptin inhibits neuropeptide Y in negative feedback pathway
other neuropeptides, hormones, adipocytokines influence neuropeptide Y secretion, leptin
release by adipocytes, hypothalamic centers controlling food intake
Ghrelin: peptide secreted by stomach that increases hunger
hormones CCK, GLP-1 released by gut during meal, help decrease hunger
many appetite-regulating peptides have other functions
ghrelin promotes release of growth hormone
brain peptides orexins play role in sleep, wakefulness
appetite, eating influenced by sensory input through nervous system
swallowing, chewing food helps create sensation of fullness
sight, smell, taste of food can stimulate or suppress appetite
psychological factors (ex. stress) play role in regulating food intake
Energy Balance
Energy Input Equals Energy Output
first law of thermodynamics = total amount of energy in universe is constant
all energy that goes into biological system can be accounted for
in body, most stored energy contained in chemical bonds
total body energy = energy stored + energy input – energy output
energy output = work + heat
at least half energy released in chemical reactions lost to environment as unregulated “waste”
heat
3 forms of work:
1. Transport work moves molecules from one side of a membrane to the other
transport processes bring materials into, out of body, transfer them between
compartments
2. Mechanical Work uses intracellular fibers, filaments to create movement
includes external (movement), internal work (movement of cytoplasmic vesicles,
pumping of heart)
3. Chemical work used for growth, maintenance, storage of info, energy
subdivided into synthesis, storage
storage includes short-term energy storage in high energy phosphate compounds (ex.
ATP) and long-term energy storage in chemical bonds of glycogen, fat
Oxygen Consumption Reflects Energy Use
caloric content of food can be calculated by multiplying # of grams or each component by its
metabolic energy content
metabolic rate calculated by multiplying oxygen consumption by # of kilocalories
metabolized per litre oxygen consumed
metabolic rate (kcal/day) = L O2 consumed/day X kcal/L O2
Many Factors Influence Metabolic Rate
metabolic rate can vary from person to person or from day to day in single individual
Basal Metabolic Rate (BMR): individual’s lowest metabolic rate
metabolic rate lowest during sleep
Resting Metabolic Rate (RMR): metabolic rate measured after 12-hour fast in person awake but
resting
other factors that affect metabolic rate include:
1. Age and Sex:
adult males BMR = 1kcal/hr/kg body weight
adult females BMR = 0.9kcal/hr/kg body weight
women have higher percentages of adipose tissue, less lean muscle
metabolic rates decline with age due to decreases in lean muscle mass
2. Amounts of Lean Muscle Mass: muscle has higher oxygen consumption than adipose
tissue
3. Activity Level: physical activity, muscle contraction increase metabolic rate
4. Diet: RMR increases after meal (diet-induced thermogenesis)
energetic cost to digestion, assimilation of food
fats cause little diet-induced thermogenesis
proteins increase heat production the most
5. Hormones: BMR increased by thyroid hormones, catecholamines (epinephrine,
norepinephrine), some peptides that regulate food intake
6. Genetics
Energy Is Stored in Fat and Glycogen
glucose polymer glycogen more compact form of energy than equal # of glucose molecules,
requires less water for hydration
our cells convert glucose to glycogen for storage
body keeps most of its energy reserves in compact, high-energy fat molecules
each lb of body fat stores 3500kcal
high caloric content of fat, histology of fat cells, with minimal cytosol and large central fat
droplet make adipose tissue very efficient at storing large amounts of energy in minimal
space
metabolically, fat harder to access, metabolism of fats slower than that of carbohydrates
Metabolism
Metabolism: sum of all chemical reactions in the body
reactions making up these pathways:
1. extract energy from nutrients
2. use energy for work
3. store excess energy so that it can be used later
Anabolic Pathways: metabolic pathways that synthesize large molecules from smaller ones
Catabolic Pathways: metabolic pathways that break large molecules into smaller ones
classification of pathway is its net result
metabolism divided into 2 states:
1. Fed/Absorptive State: period of time following a meal, when products of digestion are
being absorbed, used, stored
anabolic state in which energy of biomolecules is transferred to high-energy
compounds or stored in chemical bonds of other molecules
2. Fasted/Postabsorptive State: state after nutrients from recent meal are no longer in the
bloodstream and available for use by the tissues
body taps into stored reserves
catabolic state
Ingested Energy May Be Used or Stored
ingested biomolecules destined to meet 1 of 3 fates:
1. Energy: biomolecules can be metabolized immediately, with energy released from broken
chemical bonds trapped in ATP, phosphocreatine, other high-energy compounds
energy used to do mechanical work
2. Synthesis: biomolecules entering cells can be used to synthesize basic components
needed for growth, maintenance of cells, tissues
3. Storage: if amount of food ingested exceeds body’s requirements for energy, synthesis,
excess energy goes into storage in bonds of glycogen, fat
fate of absorbed molecules depends on whether it’s a carbohydrate, protein, or fat
Nutrient Pool: nutrients available for immediate use, located primarily in plasma
Document Summary
Glucostatic theory: glucose metabolism by hypothalamic centers regulates food intake. When blood glucose concentrations decrease, satiety center is suppressed, feeding center dominant. When glucose metabolism increases, satiety center inhibits feeding center. Lipostatic theory: energy balance proposes that signal from body"s fat stores to brain modulates eating behaviour so body maintains particular weight. Leptin: protein hormone synthesized in adipocytes that acts as negative feedback signal between adipose tissue and brain. As fat stores increase, adipose cells secrete more leptin, food intake decreases. Leptin synthesized in adipocytes under control of obese gene. Neuropeptide y: brain neurotransmitter, stimulus for food intake. Leptin inhibits neuropeptide y in negative feedback pathway other neuropeptides, hormones, adipocytokines influence neuropeptide y secretion, leptin release by adipocytes, hypothalamic centers controlling food intake. Ghrelin: peptide secreted by stomach that increases hunger hormones cck, glp-1 released by gut during meal, help decrease hunger. Brain peptides orexins play role in sleep, wakefulness appetite, eating influenced by sensory input through nervous system.
