Metabolic profiling without a lab

Key points

• A metabolic profile describes how an athlete produces energy: aerobic capacity (VO2max), glycolytic capacity (VLamax), and the thresholds and fuel use that follow.
• Lab and blood-lactate tests are accurate but expensive and hard to repeat.
• Two field efforts, a 20-second sprint and a 5-minute all-out, plus body composition give a usable estimate without blood samples.
• FuelingSense runs this test for cycling and running and turns it into zones and a race fuelling plan.

What a metabolic profile tells a coach

Two athletes with the same threshold power can be built very differently. One has a high VO2max and a low VLamax: a diesel that burns fat well and holds pace for hours. Another has a high VLamax: punchy, but burning more carbohydrate at the same power.

That difference changes how you train them and how they should fuel. It is why two riders on the same plan can respond in opposite ways.

Why not just go to a lab?

Lab testing with gas exchange and blood lactate is the reference standard. It is also expensive, needs specialist staff, and is hard to repeat often enough to track a training block. I used to sell INSCYD tests through this site for that reason, at around £150 to £250 each.

Most athletes would benefit from a profile they can repeat every few months for free.

The field test

FuelingSense uses two maximal efforts and body composition:

1. 20-second all-out sprint (P20). Mainly reflects glycolytic power, and so VLamax relative to fat-free mass.
2. 5-minute all-out effort (P300). Mainly reflects aerobic power, and so VO2max.
3. Body mass and body fat, to work out fat-free mass.

From these the model estimates VO2max and VLamax, then models lactate production and clearance across intensities. That gives LT1, LT2, training zones, and how much fat and carbohydrate the athlete burns at each intensity.

You can try the VO2max part alone with the VO2 max calculator, which compares this model with the published equations.

The equations were calibrated against INSCYD profiles, and the outputs are checked against them in a technical validation report.

From profile to fuelling

The same model answers the race-day question: how many grams of carbohydrate an hour does this athlete burn at target race intensity? FuelingSense turns that into a race fuelling plan, including the glucose and fructose mix.

How it connects to Endurance Signals

Endurance Signals uses the same engine to keep each athlete's profile current from their training data, auto-filling peak 20-second and 5-minute power from the last 90 days. Daily Fuel, now in development, will use it to set daily carbohydrate targets from planned training.

Common questions

What is VLamax?

VLamax is the maximal rate at which the body can produce lactate through glycolysis. A high VLamax supports sprinting and surges; a low VLamax favours fat use and long steady efforts.

Can you estimate VLamax without blood lactate?

Yes, approximately. A short all-out sprint relative to fat-free mass tracks glycolytic capacity, and FuelingSense estimates VLamax from a 20-second sprint without blood samples.

How often should I retest?

Every 8 to 12 weeks, or at the start and end of a training block, is a practical rhythm for tracking change.

Do I need a power meter?

For cycling, yes. FuelingSense also has a running profiler based on running efforts.