Energy Provision under Load

Energy provision under load is the shift of the substrate mix from fats to carbohydrates, and from aerobic to anaerobic, as exercise intensity rises.

Definition

“Energy provision under load” is the dynamic adaptation of metabolism to physical work. As intensity rises, the substrate mix shifts from fats to carbohydrates, and above a certain threshold anaerobic glycolysis joins in.

The underlying systems are described in the entry “Energy Provision in the Body”, the starting state in “Energy Provision at Rest”. The transitions between ranges are marked by the fat oxidation curve and the two ventilatory thresholds.

Range	Intensity	Dominant substrate	Anaerobic share
Below VT1	Low	Fats, FatMax in the upper part	Negligible
Between VT1 and VT2	Moderate	Increasingly carbohydrate	Beginning, compensated
Above VT2	High	Carbohydrate	Substantial, lactate accumulates

Why it matters

The textbook scheme is broadly right: at low intensity fatty acids dominate, at high intensity carbohydrates and anaerobic glycolysis. But where the individual transitions lie, and how high maximal fat oxidation is, varies enormously.

Depending on training status and focus, an athlete can obtain most of her energy from fat even at high intensity. In rare cases the fat oxidation maximum even lies above VT1; this is called a fully exploited metabolic potential. For long-distance events such as Ironman, marathon, or long road races, shifting this curve to the right is one of the most important training goals, because it lowers carbohydrate use per hour.

How obseed measures it

There is no reliable method to calculate metabolism under load from heart rate, power, or an FTP test. obseed therefore uses the spiroergometry step test, in which respiratory gases are measured on every stage.

This yields four individual values that obseed stores as zone boundaries:

FatMax: power and heart rate at maximal fat oxidation in g/min
VT1: first ventilatory threshold, start of the steep rise in carbohydrate oxidation
VT2: second ventilatory threshold, transition to dominant anaerobic glycolysis
Carbohydrate use in g/h per stage, as the basis for race nutrition

Repeat tests every three to four months show whether training has shifted the curve.

Example

A triathlete, 72 kg, is tested in November and April. Excerpt from the step tests:

Power	RQ November	Fat November	RQ April	Fat April
150 W	0.84	0.52 g/min	0.78	0.71 g/min
200 W	0.90	0.42 g/min	0.83	0.68 g/min
250 W	0.97	0.13 g/min	0.89	0.45 g/min
300 W	1.03	0.00 g/min	0.98	0.08 g/min

In November FatMax was at 150 W with 0.52 g/min, in April at 200 W with 0.68 g/min. VT1 rose from 175 to 215 W. At his planned Ironman bike power of 210 W he now burns about 95 g of carbohydrate per hour in April instead of 145 g in November. Over five hours of cycling that is 250 g less he has to take in through gels.

References

Jeukendrup, A. E. & Wallis, G. A. (2005): Measurement of substrate oxidation during exercise by means of gas exchange measurements.
Achten, J. & Jeukendrup, A. E.: work on maximal fat oxidation (FatMax).