In one sentence
The carbohydrate, protein and fat percentages on the activity sheet estimate which fuel a session ran on, from its intensity, because the body burns mostly fat at easy effort and more carbohydrate as effort rises.
The physiology
Brooks and Mercier called it the crossover concept: at low intensities fat is the main fuel, and as intensity rises the muscles draw increasingly on carbohydrate, through the recruitment of faster fibers, the breakdown of muscle glycogen and the rise of the sympathetic nervous system, until carbohydrate predominates; endurance training shifts the crossover point to the right, so a trained person burns fat at higher intensities than an untrained one (Brooks & Mercier 1994). Measured by calorimetry in moderately trained cyclists, fat oxidation peaked at about 64 percent of VO₂ max, which was about 74 percent of maximum heart rate, stayed within 10 percent of its peak between about 55 and 72 percent of VO₂ max, and became negligible above about 89 percent of VO₂ max (Achten 2002).
What the sheet shows
VT reads each heart-rate reading of a session against your heart-rate reserve, from your resting rate to your maximum (the one you recorded, or 220 minus your age), applies the crossover curve, and weights each reading by the energy it represents to estimate the split for the session as a whole. Protein shows 0 percent for sessions up to 90 minutes and rises to 5 percent by three hours. The founder's indoor cycle of 28 October split about two-thirds carbohydrate and one-third fat, which is what a session spent largely in the upper zones looks like. The calorie figure beside the split is the device's own when it recorded one, with the device's error, and VT's estimate, labeled as such, when it did not.
What to make of it
A rough guide to why an easy session before breakfast feels different from intervals, and to why a long easy ride leaves your glucose alone while a hard one can raise it; not a nutrition prescription, and not a reason to train slowly for the fat-burning zone, since total energy and fitness, not the fuel at the moment, are what a plan is built on.
What moves it
Intensity, most of all; your fitness, which moves the crossover when your device supplies your thresholds; the maximum heart rate behind it, recorded or estimated from your age; and what you ate before the session, which the estimate does not see.
How Vitality Tracker uses it
Displayed on the activity sheet per session; not an input to any engine.
How far to trust the number
Estimate, from a population curve applied to your heart rate; the direction is right (easier sessions burn more fat) and the percentages are approximate.
Sources
- Brooks GA, Mercier J. Balance of carbohydrate and lipid utilization during exercise: the "crossover" concept. J Appl Physiol 1994;76:2253–61. doi:10.1152/jappl.1994.76.6.2253
- Achten J, Gleeson M, Jeukendrup AE. Determination of the exercise intensity that elicits maximal fat oxidation. Med Sci Sports Exerc 2002;34:92–7. doi:10.1097/00005768-200201000-00015.