Energy Expenditure

Semester 1, week 3

Why measure energy expenditure?

• Fuel utilisation

• Assess metabolic needs

• Insight into demands of exercise

• Assessment of economy

• Nutritional interventions for performance/recovery

• Thermic effects of foods

What is a calorie?

The amount of energy required to raise the temeperature of 1g of water by 1C

What is a bomb calorimeter?

A device used to measure the heat combustion of a sample

How can we estimate EE?

• Measurement of expired gases using a Douglas Bag (indirect calorimetry)

• Measure heat given off to the environment directly (direct calorimetry)

What are the pros and cons of direct calorimetry?

• Pros: direct measure of heat, accurate for steady state measures

• Cons: expensive, slow to generate results, few in operation, not accurate for exercise

What are the benefits of indirect calorimetry?

• Can detect changes during exercise with breath by breath systems

• No longer affected by the heat of equipment etc.

• Easy to administer etc.

• Fairly accurate for aerobic measures

• Direct assessment of gas exchange

What are the limitations of indirect calorimetry?

• Assumes body's O2 content constant, CO2 exchange in lung is proportional to release from cells aerobic processes

• BUT, CO2 released in the lung may not be representative of CO2 released by working cells, and body has O2 stores not directly reflected in pulmonary measures

• Assumes little contribution from protein during exercise (non protein RER)

• But protein contributed up to 5% of total energy in prolonged exercise

• RER values >1 won't provide a valid estimation of energy expenditure

• Even values approaching 1 may be inaccurate in assessing energy expenditure

• Gluconeogenesis from catabolism of fat and amino acid produces RER <0.7

• RER not appropriate for EE estimations outside range of 0.7-1

What are the RER values for glucose and palmitic acid?

What are the caloric equivalents for oxygen?

Fat is the fuel substrate that gives the most energy per gram

How does BMR change as age increases?

• 2-3% decrease in metabolic rate per decade

• Decrease in fat free mass

• Depression of metabolic activity of lean tissues

• Altered change in body composition (increase FFM)

• Altered by physical activity (independent of change in body composition)

How is resting metabolic rate different to BMR?

• Less tightly controlled vs BMR

• More accessible/applicable

What is MET?

• The MET = multiples of RMR (resting metabolic rate)

• 1 MET = 3.5ml/kg/min

• VO2 ~200ml/min for average sized woman (57kg x 3.5ml/kg/min)

• VO2 ~250ml/min for average sized man (71kg x 3.5ml/kg/min)

• An activity at 7ml/kg/min would be a 2 MET activity

What is fat max?

• Facilitation of fat metabolism important for health and performance

• Exercise intensity at which maximal fat oxidation is observed "fatmax"

• Explained by: lower availability of plasma FFA, reduced entry of fatty acids into mitochondria

What determines energy expenditure and carbohydrate reliance in marathon performance?

EE for Performance:

• Slower recreational runners (3h 45m) run at 60-65% VO2max RER = 0.9 - CHO 68%

• Faster athletes (2h 45m) run at 70-75% VO2max RER = 0.95 - CHO 84%

• Elite runners run at 80-90% of VO2max for 2.02 to 2.10

• Possible that these athletes could compete marathon using only CHO as fuel (RER=1)

What is mechanical efficiency?

• = (external work accomplished)/(energy expenditure)

• Average value for cycling, running, walking: 20-25%

Eg.

• Cycling: 160 Watts: ~2.29 kcal/min

• Oxygen uptake at 160 Watts: ~2.0L/min

• 2.0L/min equivalent to 10kcal/min (as ~5kcal per litre O2)

• Mechanical efficiency: 2.29/10=0.229 22.9%

• Swimming (more drag): <20%

What is the importance of efficiency?

• Relationship between VO2max and performance not evident in homogenous group of runners

• But there is a relationship between running economy and performance

• Although VO2max similar between elite RE varies up to 30%

What is EPOC?

• = excess post-exercise oxygen consumption

• Fast: resynthesis of ATP and PCr

• Fast: oxygen levels restored to myoglobin and haemoglobin

• Fast: thermogenic effects of hormones

• Slow: HR and VE remain elevated for several minutes after exercise

• Slow: resynthesis of lactate to glycogen

• Slow: thermogenic effects of elevated core temperature

What is a summary of energy expenditure in exercise?

• Respiratory exchange ration (RER) describes the ratio between CO2 produced and O2 consumed. This ratio depends on the relative contributions of fat and glucose in total energy production

• 1 MET describes the energy required to cover resting metabolic rate, and depends on body mass (1 MET = 3.5ml/kg/min), activities can be classed according to METs

• Excess post exercise oxygen consumption (EPOC) occurs to refuel the cells (eg. Replenish high-energy phosphates and resynthesise lactate to glycogen). Increased body temperature and hormones can also increase EPOC