Animal Energetics

Learning Objectives You should be able to understand the importance of budgets and to apply the idea of budgets to homeostatic systems. Understand the differences between the three major groups of macronutrients as energy sources. Use data from oxygen consumption and carbon dioxide production to estimate energy use and to guesstimate the nutrient type used by an animal. Understand current understanding of body weight regulation in humans.

ANIMAL ENERGETICS -Energy Budgets -Measuring how much energy animals use -Respirometry -Respiratory Quotient (RQ) -Energy Homeostasis and the Obesity Epidemic

Maintaining a relatively constant internal milieu (environment) requires paying a cost. Animals require energy, if only to maintain themselves in the same state. They also need energy to move, grow, and reproduce. A very important theme in physiology is how animals use energy. This sub-discipline is called energetics or bioenergetics.

One can construct energy budgets for animals: Energy Assimilated = energy ingested -energy defecated Energy Assimilated = Maintenance of Homeostasis+ Movement+ Growth and Reproduction+ Urinary Energy+ Heat Loss.

Energy Budgets Ingested Energy Assimilated energy Fecal energy Synthesis (S) (storage Growth And reproduction) Maintenance (M) (Thermoregulation) Movement (W) (physical activity) Excretion (E) (detoxification metabolic waste,…etc.) Total Energy = S + M + W + E M+W+E = energy spent BROAD PRINCIPLE

Organisms are always "balancing budgets". We can see homeostasis as the process of balancing biological budgets: Energy Budget Protein/Nitrogen Budget Salt/Minerals/Electrolyte Budgets Water Budget Homeostasis is a lot more than just balancing budgets, but... balancing budgets is an important component.

The units of energy We measure energy in Joules (J, in the SI system), but some physiologists continue using the calorie (cal). It is very useful to be able to convert from one unit to another 1 J = cal 1 cal = J The unit of work in the SI is the Watt = J/sec. A Joule is not a lot of energy, so we will often use kJ =1000 J

An adult, non-growing, deer mouse assimilates 10 kcal/day. Of these, 40% are used for temperature regulation, 25% are used to fuel basal metabolism, and 15% are used for activity. How many kcal per day remain for storage of lipids and reproduction? Assume that the cost of excretion is negligible. a) 8 b) 20 c) 2 d) 4 e) 3

How do we measure the rate of energy use (expenditure): 1)Calorimetry 2)Doubly labeled water 3)Respirometry (indirect calorimetry). Antoine Laurent Lavoisier ( ) (Chemist and revolutionary) -Law of conservation of mass -Respiration as a combustion -Laws of gases -Chemical nomenclature “The Republic has no need of scientists” Jean Paul Marat (and then they chopped his head off)

Calorimetry (we measure energy use by estimating the amount of heat produced by an organisms directly) Respirometry (we rely on the analogy of respiration as a combustion that consumes oxygen and produces carbon dioxide)

Once again, we owe the notion of a respiration as a combustion to Lavoisier!

Respirometry? C 6 H 12 O 6 +6O > 6CO 2 + 6H 2 O kJ/mole This is an example of “catabolism” (breakdown). ?? glucose

Burning Carbs 1 gram of carbs requires 840 ml of O 2 1 gram of carbs produces 840 ml of CO 2 1 gram of carbs generates 15.9 kJ of energy

Burning Lipids 1 gram of lipids requires 2000 ml of O 2 1 gram of lipidsproduces 1400 ml of CO 2 1 gram of lipids generates 39.2 kJ of energy

Comparing Lipids and Carbs 1 kJ = kCal. RQ = respiratory quotient RQ = rate of CO 2 production/rate of O 2 consumption RQ (Carb) = 840/840 = 1 RQ (Lipid) = 840/1400 = 0.7 mlO 2 /g mlCO 2 /g kJ/g RQ Carbohydrate Lipids

Comparing Lipids and Carbs mlO 2 /g mlCO 2 /g kJ/g RQ Carbohydrate Lipids Messages: 1)Per unit gram, fat has more than twice the amount of energy. 2)If RQ = 1 the animal is burning (catabolizing) carbs. 3) If RQ = 0.7 the animal is burning (catabolizing) fats.

Burning Protein 1 gram of protein requires 960 ml of O 2 1 gram of protein produces 776 ml of CO 2 1 gram of protein generates 20.1 kJ of energy

Respiration is an enzymatically controlled combustion mlO 2 /g mlCO 2 /g kJ/gRQ Carbohydrate Protein Lipids kJ = kCal. RQ=respiratory quotient = rate of CO 2 production/rate of O 2 consumption

Messages -lipids are very good energy sources and great molecules to store energy (why?? two reasons..) i)Lipids are super energy rich ii)Lipids can be stored “neat” (without water, why?) -If we know RQ, we can guess the type of “fuel” burned by an animal. -If we measure the rate of oxygen consumption (or the rate of CO 2 production) we can estimate “metabolic rate” mlO 2 /g mlCO 2 /g kJ/gRQ Carbohydrate Protein Lipids BROAD PRINCIPLE

mlO 2 /g kJ/mlO 2 kJ/gRQ Carbohydrate Protein Lipids Note that we get more or less the same amount of energy per ml of oxygen consumed independently of the fuel that we are burning. If we need to know how much energy the animal is using, we simply multiply the rate of energy consumption bythe amount of energy liberated by the consumption of a certain volume of oxygen.

Rate of energy consumption = (ml O 2 /time)x(0.02 kJ/ml O 2 )

Metabolic rate = rate of energy use (energy/time = Watts) DEFINITION Watt = J/sec

What have we learned so far? One can construct energy budgets for animals. One can measure the rate of energy use (we will call it metabolic rate) by measuring rate of O 2 consumption and/or the rate of CO 2 production. If you measure both, the you can use RQ to find out what the animal is burning.

A flying hummingbird consumes 125 ml of oxygen per hour and produces 124 ml/h of carbon dioxide. What kind of organic compound is this bird using to fuel its metabolism for one hour? a) Lipid b) Protein c) Carbohydrates d) A mixture of lipid and protein

What are the major factors that determine an animal’s metabolic rate? 1)Temperature regulation (but not all animals regulate a constant body temperature). 2)Activity. 3)Production (growth and reproduction) 4)Body size

Observations (for consideration later on): 1) relative costs of thermoregulation (mouse > penguin > woman) 2) Costs/mass WomanPenguinMousePython 13,333 kCal/Kg85,000160,

Why is it that per unit body mass, the mouse spends ≈ 12 times more energy than the woman? How come the 4 kg python spends (per unit gram) only about 1% of the energy used by the mouse? WomanPenguinMousePython 13,333 kCal/Kg 85,000160, (THE RATE OF ENERGY USE PER UNIT MASS IS GREATER IN SMALL THAN IN LARGE ANIMALS) (THE RATE OF ENERGY USE PER UNIT MASS IS GREATER IN ENDOTHERMS (WARM BLOODED ANIMALS) THAN IN ECTOTHERMS (COLD BLOODED ANIMALS) OF THE SAME SIZE)

Energy homeostasis Marmot before hibernation Swainson’s thrush before migration

34% of all American adults are overweight and ≈ 30% are obese (this makes 64% of the adult population). The prevalence of overweight children and teenagers has more than doubled since J. Clin. Invest. (2003)111: Increases the risk of morbidity (disease) from: Type II diabetes Hypertension Coronary Heart Disease Congestive Heart Failure Stroke Gall-Bladder Disease … etc.

Percent of Obese (BMI > 30) in U.S. Adults

How do we know if someone is overweight? Body Mass Index BMI =[weight (kg)]/[height(cm)] 2

Body weight is homeostatically regulated… We just have not figured out how exactly.

Energy Budgets Ingested Energy Assimilated energy =A Fecal energy Synthesis (S) (storage Growth And reproduction) Maintenance (M) (Thermoregulation) Movement (W) (physical activity) Excretion (E) (detoxification metabolic waste,…etc.) Total Energy = S + M + W + E M+W+E = energy spent WHAT HAPPENS IF A > M+W+E ???

Balancing a budget (or not...) Assimilated energy < Spent energy (weight loss) Assimilated energy = Spent energy (weight maintenance) Asimilated energy > Spent energy (weight gain) Energy Assimilated = S + M + W + E M+W+E = Energy spent Energy Assimilated = Synthesis + Energy spent Synthesis =Energy assimilated – Energy spent

Energy Assimilated = S + M + W + E M+W+E = Energy spent Energy Assimilated = Synthesis + Energy spent Synthesis =Energy assimilated – Energy spent The homeostatic regulation of body weight must involve a) how much you eat (energy assimilated) b) how much energy you use (your metabolic rate) HOW DO WE DO IT???

We know a few of the “characters”, we just don’t have the full story…

Leptin -Discovered in Secreted by adipocytes (fat cells) -Acts in the hypothalamus -Animals that do not secrete it become obese parasympathetic internal organs (rest and digest), sympathetic (muscles, flight or fight) autonomic/unconscious ns

Mice without the gene for leptin are really obese. Connecting leptin-free mice with mice with leptin (or providing) exogenous leptin makes the mice loose weight.

Unfortunately, many (most) obese humans are leptin resistant. Humans are not mice….. There is a rare condition in which the leptin gene is defective (congenital leptin deficiency). In this case leptin therapy is very effective, but this is not the most common cause of obesity. Please read Leptin’s legacy (at web site) congenital leptin defficiency Treated with leptin

-Discovered in Secreted by stomach (more during fasting) -Acts in the hypothalamus, where it enhances appetite and food intake -It also acts on the pituitary where it stimulates growth hormone. Ghrelin

HormoneWhere is it secreted? What is the stimulus that elicits secretion Where (in what organ or cell) does it act? What is its effect? What is its molecular mode of action Leptin Ghrelin HOMEWORK

Construct a “decision making” diagram for body mass regulation (include leptin and ghrelin)

To Remember We still have much to learn about how we control body weight and energy homeostasis. Leptin is synthesized by adipocytes and in some organisms (including humans) it inhibits food intake. Ghrelin is synthesized by the stomach and stimulates food intake. Leptin and ghrelin have antagonic (opposite) effects, but their effects may take place at different time scales. The food intake center is in the hypothalamus. Both leptin and ghrelin act there (through other peptide hormones) There are a bunch of other related hormones that influence energy homeostasis (including insulin), but the pathways remains to be completely elucidated.

Oh well, science goes on and we will figure it out one day….

Review Questions 1)Write an equation with the elements of an animal’s energy budget. 2)How did Antoine Lavoisier die? 3)Why do bears store lipids rather than carbohydrates before they enter hibernation? 4)Why can we use the rate of oxygen consumption to estimate the rate at which organisms use energy? 5)What is RQ and why it is useful? 6)Where is leptin secreted? What is the main determinant of how much leptin is secreted? 7)Where is ghrelin secreted? What is the main determinant of how much ghrelin is secreted?