Energy Provision in the Body

Energy provision in the body is the set of metabolic pathways by which carbohydrates and fats are converted into ATP, aerobically or anaerobically.

Definition

“Energy provision in the body” is the totality of metabolic pathways through which the organism obtains energy for movement and basic functions. The end product is always ATP; the sources are mainly the macronutrients carbohydrates and fats, with a small contribution from protein.

Three systems work in parallel, in proportions that depend on intensity and duration. This entry covers the fundamentals; the entries “Energy Provision at Rest” and “Energy Provision under Load” treat the two states in detail.

System	Substrate	Oxygen	Power	Capacity	Dominant in
Phosphagen (ATP/creatine phosphate)	Creatine phosphate	No	Very high	approx. 10 s	Sprints, attacks
Anaerobic glycolysis	Glycogen	No	High	1–3 min	Intervals above VT2
Aerobic metabolism	Fats and carbohydrates	Yes	Moderate	Hours	Base, long distance

Why it matters

Which system dominates when decides pace, fatigue, and nutrition in training and racing. Riding at an intensity where the body burns mostly glycogen empties the stores after 90 to 120 minutes, depending on training status.

The systems do not switch over sharply; they overlap. Even at low intensity part of the energy comes from carbohydrate, and at high intensity fat metabolism keeps running, just with a smaller share. Where exactly the transitions lie is individual and cannot be derived from heart rate or FTP.

How obseed measures it

obseed relies on spiroergometry. From oxygen uptake (VO2) and carbon dioxide output (VCO2) the respiratory quotient RQ = VCO2 / VO2 is computed, which indicates the substrate mix:

RQ	Fat share	Carbohydrate share
0.70	100 %	0 %
0.80	67 %	33 %
0.85	50 %	50 %
0.90	33 %	67 %
1.00	0 %	100 %

From the course of the RQ across the step test come the fat oxidation curve with its maximum (FatMax), the first ventilatory threshold (VT1), from which the carbohydrate share rises steeply, and the second ventilatory threshold (VT2), from which anaerobic glycolysis dominates. A field test yields only the power, not the metabolism behind it.

Example

A cyclist, 75 kg, is measured in a step test. At 180 W his RQ is 0.82 and his oxygen uptake 2.6 l/min.

Energy expenditure: 2.6 l/min × 4.85 kcal/l ≈ 12.6 kcal/min. At RQ 0.82 about 60 % comes from fat and 40 % from carbohydrate.

Measure	Calculation	Result
Fat oxidation	12.6 × 0.60 / 9.4 kcal/g	0.80 g/min
Carbohydrate oxidation	12.6 × 0.40 / 4.1 kcal/g	1.23 g/min
Carbohydrate per hour	1.23 × 60	74 g/h

At 180 W he uses 74 g of carbohydrate per hour. With a glycogen store of roughly 400 g and no intake, that lasts a good five hours; at 240 W with RQ 0.95, consumption rises above 150 g/h and the stores are empty after two and a half hours.

References

Jeukendrup, A. E. & Wallis, G. A. (2005): Measurement of substrate oxidation during exercise by means of gas exchange measurements.