Metabolic Physiology

• Importance of maintaining body temperature

  • Optimise enzyme activity
  • Maintain immunity
  • Disruptions to osmosis and oxygen dissociation
  • Heat exhaustion/Heat stroke
  • Hypothermia
• Internal core temperature


  Homeostatically maintained at 100°F (37.8°C)
• Sites for monitoring body temperature

  • Oral
  • Axillary
  • Rectal
  • Eardrum
  • Temporal
• Basal Metabolic Rate


  A measurement of the number of calories needed to perform your body's most basic (basal) functions
• Calorie


  Basic unit of heat energy. Amount of heat required to raise the temperature of 1g of H2O by 1°C
• Kilocalorie or Calorie


  Used when discussing the human body. Equivalent to 1000 calories
• Direct vs Indirect Calorimetry

  • Direct Calorimetry: Extremely accurate but expensive, difficult to operate and not suitable for large groups
  • Indirect Calorimetry: Much cheaper & easy to carry out but not as accurate as direct calorimetry
• Metabolic rate


  The rate at which the body burns calories to produce energy
• Factors influencing basal metabolic rate

  • Body size: weight, height, and surface area increase
  • Body composition: Fat vs Muscle
  • Gender
  • Age
  • Climate and body temperature
  • Hormonal levels Thyroxine (T4), and adrenalin (epinephrine)
  • Health: Fever, illness, or injury may increase resting metabolic rate two-fold
• Energy balance


  Energy consumed from food = external work + internal heat ± stored energy production
• Adipose stores


  Provide information about the body's energy status
• Feeding status


  Provides information about the body's energy status
• Food intake control


  Determined by the integration of many inputs that provide information about the body energy status
• Hypothalamus


  • Central role in long-term control of energy balance and body weight and short-term control of food intake from meal to meal
  • Arcuate nucleus: central role in long-term control of energy balance and body weight and short-term control of food intake from meal to meal
• Feeding/appetite signals


  Give rise to the sensation of hunger
• Satiety


  The feeling of being full
• Neuropeptide Y (NPY)


  Potent appetite stimulator
• Melanocortins


  Suppress appetite, thus leading to reduced food intake and weight loss
• Leptin


  Essential for normal body-weight regulation
• Insulin


  Important role in long-term control of body weight
• Hypothalamic areas beyond the arcuate nucleus

  • Lateral hypothalamic area (LHA): Orexins - potent stimulators of food intake
  • Paraventricular nucleus (PVN): Corticotropin-releasing hormone - inhibitors of food intake
• Ghrelin


  Potent appetite stimulator
• PYY 3-36


  Inhibits appetite-stimulating NPY secreting neurons
• Nucleus tractus solitarius (NTS)

  Processes signals for the feeling of being full and thus contributes to short-term control of meals
• Cholecystokinin (CCK)


  Satiety signal for regulating meal size
• Gastrointestinal hormones


  Released from the duodenal mucosa during digestion, act as satiety signals
• Nutrient stores (Average 70 kg male)

  • CHO (0.5 - 1 kg glycogen, rapidly mobilize)
  • Fat (~13 kg triglyceride, slowly mobilized)
  • Protein (~6 kg muscle protein that can be slowly converted to glucose during fasting)
• Glucose


  Main short-term energy source. Brain uses ~5g/h of glucose, brain uses ketones during starvation. Other organs/tissues are generally more flexible and can modulate and adapt between CHO and fat according to supply and demand.
• Routes of storage


  1. Glycogenesis (Glucose to Glycogen)

  2. Lipogenesis (Fatty acids and glucose to Triglyceride)
• Routes of mobilization


  1. Glycogenolysis (Glycogen to Glucose)

  2. Lipolysis (Triglyceride to Fatty acids)

  3. Gluconeogenesis* (Protein and lactate to Glucose)
• Nutrient Homeostasis Day profiles

  • Plasma insulin (uU/mL)
  • Plasma glucose (mmol/L)
  • Plasma FFA (mmol/L)
• Cycles That Facilitate Nutrient Selection

  1. Storage - anabolism

  2. Mobilization - catabolism

  3. Glucose - Lactate - Gluconeogenesis - Anaerobic metabolism - Cori cycle

  4. FFA - Glucose-fatty acid cycle (Randle cycle)
• FFAs are used when glucose is scarce
• Daily Fluctuations in Nutrient Homeostasis

  • Day: Mainly nutrient anabolism (> catabolism)
  • Glucose intake: replacement of glycogen stores and fuel processes
  • Glucose and FFA stored as triglyceride in adipose tissue
• Insulin


  Key regulator of fuel selection and balance of anabolism to catabolism. Stimulates glucose and protein anabolism, reduces lipid catabolism. Stimulates glycogenesis, lipogenesis, amino acid uptake and conversion to protein. Inhibits lipolysis, gluconeogenesis and protein catabolism.
• Glucose concentrations rise after feeding: this stimulates insulin release. When glucose concentrations fall during the night, insulin declines, and catabolic processes take over.
• Insulin


  A protein made of two short polypeptide chains linked by disulphide bridges. Synthesised at the ribosome on Rough ER) as a single polypeptide (proinsulin) and later activated by enzymatic cleavage into the two chains. Released from beta-cells in islets.
• Glucagon


  A small protein made up of 29 amino acids. It is made by the alpha cells in the Islets. It is also made in an inactive form proglucagon before it is released.
• Action of Insulin


  • Increases the entry of glucose into the body cells
  • Inhibits glycogenolysis in liver and muscle
  • Inhibits lipolysis in liver and adipose tissue
  • Increases the uptake of amino acids by cells and increases the rate of protein synthesis
• Action of Glucagon


  • Glucagon binds to receptor on liver cells, stimulating glycogenolysis
  • Increases lipolysis (although its influence is small)
  • Stimulates the formation of glucose from amino acids in the liver (gluconeogenesis)