Energy balance- the game has changed. Tschop et al Nature methods Dec 2012
30g 30g of energy out 31g of food in 1 tonne of energy out 1 tonne and 1g Of food in 1. ENERGY BALANCE Energy balance describes whether an organism will lose or gain weight. The size Of the organism is actually irrelevent for considering energy balance 31g 1 tonne 1 tonne + 1g
30g of energy out 31g of food in 1 tonne of energy out 1 tonne and 1g Of food in 1. ENERGY BALANCE and rate of weight gain
30g 30g 5g of energy out 7g of energy out 5g of food in 7g of food in 2 Metabolic rate With metabolic rate then the size of organism is very important, as we care about The rate of energy expenditure per g of the organism
How do we measure food intake, energy balance and metabolic rate?
To assess metabolic rate and energy balance we need • 3 pieces of information • The body weight of the animal • The amount of food it has consumed* • How much energy it has expended *This correctly should be the amount it has assimilated. See point at the end.
How to measure energy expenditure Direct Methods: Calorimetry Indirect methods: Indirect calorimetry using gas exchange Doubly labelled water
Direct Calorimetry Advantage – Most accurate method – does exactly what it says on the tin. Disadvantage – Sealed unit so (very) limited time scale for experiments.
Indirect calorimetry 2 – Gas exchange indirect calorimetry.
What is Energy expenditure and how does it relate to VO2 and VCO2? EEJ = 15.818xV02 + 5.176*VCO2
Indirect calorimetery Oxygen consumption gives an indication of how much energy an animal is using as an animal will produce 4.8 KJ of energy per litre of oxygen with an error of around 6%. Carbon dioxide production is not a 1 to 1 ratio with oxygen consumption and the amount of CO2 produced varies dependenton the source of energy (Carbohydrate, Fat or Protein) Combining the amount of O2 consumed and the amount of CO2produced gives us a value called the RQ which provides some information about fuel usage. RQ varies from 0.7-1 dependent on metabolic status of the animal
What is Energy expenditure and how does it relate to VO2 and VCO2? EEJ = 15.818xV02 + 5.176*VCO2 Varies in ratio to oxygen by about 30% Therefore about 75% of the EE equation is driven by VO2 25% is driven by VCO2, which varies from equalling V02 to -30%. Therefore 25% of 30 ~6% - the error based on using 02 alone
H C O H n H O C O C H H H n O A bit more on RQ RQ is VC02/VO2 Burning carbohydrate has an RQ of 1 because…. O Therefore 1 molecule of C02 Requires 1 molecule of O2 O C H H O Burning fat has an RQ of 0.7 because…. Therefore 1 molecule of C02 Requires 1.5 molecules of O2 (1/1.5 =0.7)
What does RQ mean? -Gives information about substrate utilisation -Mice fed a constant diet, so in mice probably gives more information about Energy balance -Can exceed 1 during periods of lipogenesis as lipogenesis has an RQ of about 5
Important point 1 A larger mouse WILL expend more energy than a smaller mouse. P=0.0004
Important point 2 A larger mouse WILL eat more energy than a smaller mouse.
Important point 3 When you study your mouse is very important. Bodyweight Time
If a larger animal expends more energy, and my genotype of interest is obese and getting more obese relative to a control when I measure it, will it have higher EE? Probably yes Therefore what I need to know is if my animal has a DISPROPORTIONATELY Low EE for its body weight.
Traditionally people have used different comparators to straight body weight. A particularly common comparator is BW0.75 However this, along with BW0.72 and BW0.66 were developed for comparing across species, ie elephants and mice, rather than within species. Evidence suggests that these are not valid comparators for comparing within species, as we do. So then describing oxygen consumption as Ml/min/kg0.75 is not a good idea Is there a better method? ANCOVA
How ANCOVA works A group with increased oxygen consumption per gram of animal Oxygen consumption Body weight
A group with the same oxygen consumption per gram of animal Oxygen consumption Body weight
When regression lines are not equal Idealised regression Oxygen consumption Body weight
When regression lines are not equal and cross in the area of interest Idealised regression Oxygen consumption Body weight
Oxygen consumption rates from 7 month old male mice fed a standard laboratory chow diet. Data collected from free living mice with ad libitum access to food over a period of 48 hours. N=8 per group. Chow diet. Oxygen consumption expressed as adjusted means based on a normalised mouse weight of 33.36g determined using ANCOVA. P<0.05
Other types of data Delta Body weight: weight out - weight inImportant for understanding RQ and energy balance. Mice in negative energybalance will have lower RQ values (they are oxidising fat) and potentiallydisproportionately low EE (as they try to maintain fat reserves) ActivityA very poorly understood variable and would take about another hour to discuss fully. I would recommend entirely ignoring this for the time being. Water intake May give very useful information regarding diabetic mice/kidney function. In general not a major variable in energy balance studies, however a dramatic loss in weight may be attributable to a failure to drink. Important to check this. Gut assimilation efficiencyNot all food that is consumed will be absorbed into the body. Mice general operate in The mid-80% range for assimilation efficiency. Assimilation efficiency can be assessed using a bomb calorimeter to measure fecal energy content.
Conclusions Energy balance describes the processes by which animals get fat. Metabolic rate describes the unitary mass energy expenditure. Assessing these variables accurately is essential for determining why your mouse is obese, lean or has altered metabolic function.
Practical session • Testing that your data is valid • How to analyse your data.
An important point about how a calorimeter measures CO2 and O2 CO2 = Air out – air in to give VCO2 O2 = Air in – Air out to give VO2 This is not a trivial difference!!! Lets imagine that we have a lot of water in our cage…
The water dilutes the gases in the cage, reducing the CO2 and O2 measurements Coming out of the chamber 02out = 90% of normalCO2out = 90% of normal 02in = 100% of normalCO2in = 100% of normal
These errors do not cancel because: CO2 = Air out – air in to give VCO2 O2 = Air in – Air out to give VO2 CO2 = 90 -100% of correct values so error is negative O2 = 100 – 90% of correct values so error is positive Therefore excessive moisture will drive down RQ as RQ = VCO2/VO2 Unusually low RQ values are indicative of an issue with moisture or drying… If all cages drift down, as well as the room air value, it may suggest the Air drying on your system is not working. If one animal has a drift in RER it may be a problem with a specific cage
Exercise 1 – checking the data On your computer you will find the EXCEL sheet ‘training analysis’. Please open this file and proceed to tab ‘training data 1’ On this data set please graph (scatter plot) the O2% for room air Also place on a separate graph the RQ values for each mouse. You can compare this to training data 2, which does not have any problems Can we see any problems?
Exercise 2 Look at training data 1 Please graph the EE column for each mouse Can we see a problem? What might have caused it?