S1, L3: Energy Expenditure

What are reasons for measuring energy expenditure? (6)

• Assess metabolic needs

• Fuel utilisation

• Thermic effect of foods

• Nutritional interventions for performance or recovery

• Assessment of economy

• Insight into the demands of exercise

What is a calorie?

1 calorie is the energy required to increase the temperature of 1 g of water by 1°C

What is the process of direct calorimetry?

• A subject exercises in a contained room with a carbon dioxide absorber, oxygen supply and water cooling circuit

• As the subject exercises, the heat they give off is used to heat water to get a direct value for energy expenditure

What are the strengths of direct calorimetry? (2)

• Direct measure of energy expenditure

• Accurate as a steady state measure

What are the weaknesses of direct calorimetry? (4)

• Expensive

• Slow to generate results - results are more of an average over time rather than an energy expenditure over different intensities

• Few chambers exist

• Possibly not accurate for exercise energy expenditure - changes in intensity are not reflected and machines or ergometers give off heat

What is the process of indirect calorimetry?

Expired gas samples are collected and analysed to calculate energy expenditure and predict substrate use

What is the equation for RER?

RER = VCO₂/VO₂

What are the strengths of indirect calorimetry? (5)

• Can detect changes during exercise with breath by breath systems

• Not affected by the heat of equipment

• Easy and cheap to administer

• Fairly accurate for aerobic measures

• Direct assessment of gas exchange

What are the weaknesses of indirect calorimetry? (6)

• Assumes that the body’s oxygen content is constant, but the body has oxygen stores not directly reflected by pulmonary measures

• Assumes carbon dioxide exchange in the lungs is proportional to release from cells’ aerobic processes

• Assumes little contribution from protein during exercise, but protein contributes up to 5% of total energy in prolonged exercise

• RER values greater than 1 won’t provide a valid estimation of energy expenditure, even values approaching 1 could be inaccurate

• Gluconeogenesis from catabolism of fat and amino acids gives an RER of less than 0.7

• RER is not an appropriate estimate of energy expenditure for values outside of a range of 0.7 to 1.0

How does basal metabolic rate (BMR) change over life?

On average, there is a 2-3% reduction in BMR per decade of life

What implications does a reduced BMR have (2) and what affects the reduction in BMR throughout life (2)?

• A reduced BMR causes a decrease in fat-free mass and depression of metabolic activity of lean tissues

• A change in body composition (increase in fat-free mass) and physical activity can slow down the reduction in BMR independently of each other

What is resting metabolic rate?

Energy expenditure in resting conditions

Why could resting metabolic rate be more applicable than basal metabolic rate?

Resting metabolic rate is less tightly controlled than basal metabolic rate and there is a low chance of a subject accurately being in the tight window for basal metabolic rate

What are METs?

• Metabolic equivalents of tasks

• The MET is a multiple of the resting metabolic rate

• 1 MET = 3.5 mL/kg/min

What is fat max?

The exercise intensity at which maximal fat oxidation is observed

Describe where fat max is generally achieved and how fat oxidation declines after this point

• Fat oxidation rate peaks around 65% of VO_{2 max} at 0.6 g/min

• The rate of fat oxidation gradually decreases to around 0.5 g/min at 75% of VO_{2 max} before a rapid decline to no fat oxidation at around 90% of VO_{2 max}

What explains the lower rate of fat oxidation at percentages of VO_{2 max} above the fat max? (2)

• Lower availability of plasma free fatty acids at a high percentage of VO_{2 max}

• Reduced entry of fatty acids into the mitochondria at a high percentage of VO_{2 max}

What is the difference in rate of fat oxidation between trained and untrained individuals?

Trained individuals are able to oxidise fat at a faster rate than untrained individuals at all intensities other than the lowest intensities, with the largest difference being around 60% of VO_{2 max}

How does substrate use differ between levels of marathon runners?

• Slower recreational runners (3h 45) run at 60-65% of VO_{2 max}, giving an RER of 0.9 and 68% carbohydrate use

• Faster athletes (2h 45) run at 70-75% of VO_{2 max}, giving an RER of 0.95 and 84% carbohydrate use

• Elite athletes (2h 02 - 2h 10) run at 80-90% of VO_{2 max}, so it’s possible that they could complete the marathon with an RER of 1.0, using only carbohydrates

What is the equation for mechanical efficiency?

ME = external work accomplished/energy expenditure

How does mechanical efficiency vary between sports?

• Average values for cycling, running and walking are 20-25%

• Swimming can be less than 20% due to more drag

What is the importance of mechanical efficiency for running?

• A relationship between running mechanical efficiency exists

• VO_{2 max} can be similar between elite athletes but mechanical efficiency varies up to 30%

• Across 11 years, a female marathon runner’s mechanical efficiency improved to the extent that her VO_{2 max} was around 40 mL/kg/min less at 16 km/h after 11 years of training

• VO_{2 max} remained relatively stable but running speed at VO_{2 max} increased by 3 km/h

Describe the EPOC graph

• Oxygen deficit - represents a lag in oxygen uptake response until steady state occurs

• EPOC - a delay in the recovery of oxygen consumption

• Fast portion of EPOC - rapid decline in oxygen uptake during the first 2-3 minutes following exercise

• Slow portion of EPOC - occurs after 2-3 minutes and continues for up to an hour after exercise ends

• Ultraslow portion of EPOC - can persist for several hours before basal levels return

What happens in the fast portion of EPOC? (3)

• Resynthesis of ATP and phosphocreatine

• Oxygen levels restored in myoglobin and haemoglobin

• Thermogenic effects of hormones

What happens in the slow portion of EPOC? (3)

• Heart rate and minute ventilation remain elevated for several minutes after exercise

• Resynthesis of lactate to glycogen

• Thermogenic effects of elevated core temperature