Metabolic response to exercise and factors influencing fuel choice
At the start of exercise an oxygen deficit forms; afterwards EPOC repays it. Intensity, duration, training, diet and sex decide whether muscle burns more carbohydrate or fat.
Oxygen uptake from rest to recovery
EPOC (excess post-exercise oxygen consumption) rebuilds PCr and oxygen stores, clears lactate and cools the body.
Metabolic and body responses during exercise
| System | Response |
|---|---|
| Hormones | Adrenaline, noradrenaline, glucagon, cortisol and growth hormone rise; insulin falls |
| Fuel release | Glycogen broken down in muscle and liver; fat broken down in adipose tissue (lipolysis); in long exercise the liver makes new glucose (gluconeogenesis) |
| Heart | Heart rate, stroke volume and cardiac output rise |
| Lungs | Breathing becomes faster and deeper |
| Blood flow | Shifted to working muscles and skin, away from gut and kidneys |
| Temperature | Core temperature rises; sweating starts |
| Blood lactate | Steady at easy paces; rises sharply above the lactate threshold |
Factors influencing the choice of fuel
| Factor | Effect on fuel use |
|---|---|
| Intensity | Crossover concept: at about 25% of VO2max mostly fat; at about 65% a mix with the most fat burned; above about 75-85% mostly carbohydrate |
| Duration | As exercise goes on and glycogen falls, the share of fat rises |
| Training status | Trained muscle burns more fat at the same pace, sparing glycogen |
| Diet | A high-carbohydrate diet or a carbohydrate meal before exercise raises carbohydrate use; a high-fat diet raises fat use |
| Sex | Women burn relatively more fat at the same relative intensity, linked to oestrogen |
| Environment | Heat and high altitude increase carbohydrate use |
| Fibre type | Type I fibres favour fat; Type II fibres favour carbohydrate |
RQ vs RER
RQ of glucose and of palmitic acid
Find the RQ when glucose and when palmitic acid (a common fatty acid) are fully oxidised.
- Glucose: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O.
- RQ = CO2 ÷ O2 = 6 ÷ 6 = 1.0.
- Palmitic acid: C16H32O2 + 23 O2 → 16 CO2 + 16 H2O.
- Check the oxygen atoms: left 2 + 46 = 48; right 32 + 16 = 48. Balanced.
- RQ = 16 ÷ 23 = 0.696, rounded to 0.70.
Glucose RQ = 1.0; palmitic acid RQ ≈ 0.70 (rounded to two decimals). Fat needs more oxygen per CO2 produced, so its RQ is lower.
Hard efforts burn mostly carbohydrate, so hockey, kabaddi and middle-distance athletes need full glycogen stores; the slow 'fat-burning zone' does not match match play. A carbohydrate-rich meal before training raises insulin and cuts fat use, which suits performance but not an easy fat-loss session.
2 marks: “What is EPOC?” Excess post-exercise oxygen consumption: extra oxygen used after exercise to rebuild PCr and oxygen stores, clear lactate and cool the body.
5 marks: “Explain the crossover concept of fuel use.” As intensity rises, the share of carbohydrate rises and fat falls: mostly fat at about 25% VO2max, a mix at about 65%, mostly carbohydrate above 75-85%; reasons.
10 marks: “Describe the metabolic response to exercise and the factors influencing the choice of fuel.” Oxygen deficit, steady state and EPOC with a curve, hormone and body responses, factors in a table, RQ values, and a diet note.
Fuel choice: I Don't Train Daily, Sir, Ever? Intensity, Duration, Training, Diet, Sex, Environment.