Name: CHO/FAT Metabolic
Author: irazuzta

Fuel smarter, not by guesswork.

CHO/FAT Metabolic is a Data Field that estimates, second by second, how many grams of carbohydrate and fat your body is burning during exercise — using only your heart rate. No extra sensors, no manual logging.

What you see on screen
- CHO (g) — total carbohydrate burned so far this session.
- CHO avg (g/h) — a smoothed, real-time carbohydrate burn rate, color-coded as a fueling guide: green, orange, or red depending on how your current burn rate compares to standard sports-nutrition intake guidelines for that point in your workout.
- FAT (g) — total fat burned so far this session.
- A small zone indicator (Z1-Z5), colored the same way Garmin Connect colors your heart rate zones, so you can see effort and fueling status at a glance.

Built on established exercise science

The underlying model is not a gimmick — it combines well-known methods from exercise physiology:
- Heart-rate-based VO2 estimation (heart rate reserve method).
- The classic Frayn equations for calculating carbohydrate and fat oxidation from gas-exchange variables.
- Duration-aware carbohydrate fueling bands based on published sports-nutrition guidelines for endurance exercise.

Works out of the box

The app automatically reads your weight, resting heart rate, and heart rate zones from your Garmin profile to personalize the calculation. If you know your own physiological numbers (max heart rate, lactate thresholds, VO2max), you can enter them manually in the app settings for even more accurate results.

A note on accuracy

This app provides an estimate based on established exercise science models, calibrated for endurance athletes. It is a training aid, not a medical or diagnostic device, and should not replace professional medical or nutritional advice.

Tip: for best accuracy, set up your Garmin heart rate zones using the Lactate Threshold method instead of %HRmax — modern Garmin watches estimate your lactate threshold automatically during running/cycling activities (or you can enter it manually). This app estimates your anaerobic threshold from your zone 4 ceiling, so zones based on lactate threshold line up much more closely with the real value.

Tip: fat burning naturally takes a few minutes to ramp up at the start of exercise, so readings in the first 10-20 minutes will lean more toward carbs than later in the session — a short warm-up beforehand can reduce this delay.

Why your anaerobic threshold (LT2) matters most — especially for cyclists

Of all the numbers the app uses, your anaerobic/lactate threshold (LT2) has by far the biggest impact on accuracy. The app's whole carb/fat curve is anchored to it: your aerobic threshold and the point where carb burning starts ramping up are both calculated as a percentage of LT2, not set independently.

This matters even more if you ride with an Edge: Garmin auto-detects lactate threshold for running, but not for cycling — the automatic cycling equivalent is FTP, a power number, not a heart rate. So if you're a cyclist, it's worth setting LT2 manually: use the heart rate from your FTP test (the steady effort, not the wattage itself), or your cycling Lactate Threshold Heart Rate from Garmin Connect's Physiological Metrics if you've set one.

Want the details? Full documentation — User guide plus the underlying science and references — is available at https://irazuzta.github.io/ChoFatMetabolic/