Fuelling for health and performance practice questions.

A distance runner races 10 km at about 85 % of maximal oxygen uptake (VO2max). Which fuel supplies the greatest proportion of the ATP resynthesised?

• Free fatty acids released from adipose tissue
• Glycogen stored in the working muscle
• Triglyceride stored inside the muscle fibres
• Amino acids released from muscle protein

Answer: Glycogen stored in the working muscle

Carbohydrate can be broken down fast enough to meet a high rate of ATP demand, whereas the mobilisation and oxidation of fat is comparatively slow. Fat still contributes at this intensity, but its share falls as intensity rises.

A rower increases work rate from 40 % to 80 % of VO2max on the ergometer. Which change in substrate use occurs?

• The proportion of energy from fat rises while carbohydrate use falls steadily
• Both fat and carbohydrate use fall as protein becomes the main fuel
• The proportion of energy from carbohydrate rises and that from fat falls
• Fat oxidation supplies most of the energy at both work rates

Answer: The proportion of energy from carbohydrate rises and that from fat falls

As intensity rises, more fast-twitch fibres are recruited and glycolytic flux increases, so carbohydrate takes an ever larger share of the fuel mix. The absolute rate of fat oxidation can still be substantial, which is why the question asks about proportions.

Carbohydrate loading is used by endurance athletes in the days before a long event. What is the physiological principle behind it?

• Muscle glycogen is stored above its usual concentration
• Fat oxidation is increased throughout the whole event
• Blood glucose concentration is raised well above normal at the start line
• Less water is stored alongside glycogen in the muscle

Answer: Muscle glycogen is stored above its usual concentration

The strategy exploits the muscle's capacity to supercompensate its glycogen store, so the point at which glycogen runs low is delayed. It does not alter the fuel mix by making fat oxidation faster, and the extra glycogen is stored with additional water, not less.

The recovery meal provided to a rowing squad is made up of foods with a high glycaemic index. What does a high glycaemic index indicate?

• The food contains a large amount of dietary fibre
• The food yields more energy per gram than fat does
• Insulin secretion is suppressed after the food is eaten
• Blood glucose rises quickly after the food is eaten

Answer: Blood glucose rises quickly after the food is eaten

The glycaemic index ranks foods by how quickly they raise blood glucose after ingestion, which in turn drives insulin release rather than suppressing it. It says nothing about the energy density of the food, which depends mainly on its fat content.

A cyclist rides for 3 h at 60 % of VO2max, drawing a large share of the energy from fat. Which statement about fat as a fuel is correct?

• Fat can be broken down to resynthesise ATP without oxygen
• Fat oxidation needs more oxygen per unit of ATP than carbohydrate
• Fat stored in adipose tissue is depleted after about two hours
• Fat supplies almost no energy at exercise intensities of this kind

Answer: Fat oxidation needs more oxygen per unit of ATP than carbohydrate

Fatty acids are more reduced than glucose, so their complete oxidation consumes more oxygen for each molecule of ATP formed, which is one reason the fuel mix shifts towards carbohydrate when oxygen supply limits the work rate. Fat cannot be used anaerobically at all, and the adipose store is far too large to be emptied in a single ride.

A middle-distance runner trains indoors through a northern winter and her serum vitamin D concentration is found to be low. Which consequence follows most directly?

• Calcium is absorbed less well from the gut, which puts bone mineral density at risk
• Oxygen carriage falls, because vitamin D forms part of the haem group of haemoglobin
• Muscle glycogen is resynthesised more slowly in the hours after each training session
• Sodium is lost in larger amounts in sweat, because vitamin D governs the sweat gland duct

Answer: Calcium is absorbed less well from the gut, which puts bone mineral density at risk

Vitamin D is needed for calcium absorption in the small intestine and for normal bone remodelling, so a runner who sees little sunlight carries a raised stress fracture risk. It is not part of haemoglobin, and neither glycogen resynthesis nor the sodium content of sweat is under its control.