HRV, Body Battery, Training Load: How to Actually Fuel Off Your Garmin Data

In Summary

HRV tells you how carbohydrate-sensitive you are today — suppressed HRV means add 10–25% to your carb intake.
Body Battery trend (not daily number) reveals whether your weekly fueling is keeping pace with training load.
Training Load has a near-linear relationship to carb needs: 3–5 g/kg in taper, up to 10–12 g/kg in peak blocks.
Read all three together each morning and adjust once — don't overcomplicate it.

Every morning you open your watch and see a wall of numbers. HRV: 48, down from 54. Body Battery: 32. Training Load: high.

Then you close the app and go eat whatever you were going to eat anyway.

That is the problem Garmin, Apple Watch, and Whoop have not solved. They tell you your body isn't fully recovered. They do not tell you what to do about it — especially at the table and the bottle.

Why your watch tells you everything and nothing

Modern watches capture more physiology than the labs of 20 years ago. What none of them do well is close the loop. The watch sees your HRV has dropped. It doesn't know whether that's from yesterday's 30 km, four hours of sleep, two glasses of wine, or a cold brewing. And even if it did know, it doesn't know what's in your kitchen or how many carbs you fuelled with on yesterday's long run.

So it gives you a number and hopes you figure it out. This article is the manual.

Metric 1: HRV — the signal you're being asked to ignore

Heart rate variability is the millisecond-level fluctuation between heartbeats. Higher variability at rest means your parasympathetic (rest and recover) system is in charge. Lower means your sympathetic (stress) system is dominant.

That matters because your sympathetic nervous system runs up for two different reasons — training stress and life stress — and your body cannot tell the difference. Hard intervals, late flight, an argument, a calorie deficit: all drop HRV.

What HRV should change about your fueling

HRV correlates strongly with carbohydrate sensitivity. Low HRV often means elevated morning cortisol, lower insulin sensitivity, and a slightly elevated resting heart rate. On a low-HRV day, underfuelling is more punishing because glycogen will refill more slowly and perceived effort for the same pace will be higher.

Practical adjustments:

HRV within normal range (Balanced): fuel as planned.
HRV down 5–10% after an easy day: add 10–20 g of carbs to your pre-workout meal.
HRV down more than 10%, or flagged Low/Poor: add 15–25% to carb intake across the day; add 300–500 mg of extra sodium for sessions over 75 minutes. If the drop persists three days, it's a recovery problem — not a fueling one.
HRV sharply up during taper: this is normal and good. Don't suddenly overfuel thinking you've unlocked a big day.

Metric 2: Body Battery — the most misunderstood number

Body Battery expresses your overall energy reserve on a 0–100 scale using HRV, sleep quality, daytime stress, and activity. It rises during rest, falls during exercise and stress.

Useful benchmarks: 76–100 = high reserve; 51–75 = medium; 26–50 = low; 5–25 = very low.

What Body Battery should change about your fueling

Two patterns come up repeatedly:

Ceiling stuck below 70 for three or more days: you're probably in a mild energy deficit. Most endurance athletes under-eat carbs on rest days because "I didn't train hard today." Rest days are when glycogen refills. Keep carbs near training-day levels until Body Battery normalises.
Morning Body Battery under 30 consistently: add a small carbohydrate + electrolyte drink within 30 minutes of waking. This tends to stabilise morning Body Battery within a week.

Body Battery won't tell you whether to take a gel at km 12. Use it for weekly fueling audits, not in-race decisions.

Metric 3: Training Load — the metric that drives carb needs

Training Load is a weighted rolling sum of physiological stress from recent workouts. Unlike HRV and Body Battery, it has a near-linear relationship to fueling needs. If your load doubles, your glycogen turnover roughly doubles.

Practical carbohydrate bands by Training Load:

Low (taper, recovery, off-season): 3–5 g/kg/day. A 70 kg runner: 210–350 g/day.
Moderate (base building): 5–7 g/kg/day. Same runner: 350–490 g/day.
High (peak marathon block): 7–10 g/kg/day. Same runner: 490–700 g/day.
Very high (back-to-back threshold weeks, Ironman peak): 10–12 g/kg/day.

When your 7-day Training Load spikes and you leave intake unchanged, you're effectively on a forced diet. That's why week six of a marathon block often feels inexplicably heavy — you're not out of shape, you're under-fuelled relative to load.

During taper, Training Load drops 30–50%. Keep intake identical to peak-week and you'll feel sluggish. The sweet spot: scale carbs to roughly 80% of peak during taper week one, and 100% of peak in taper week two as carb loading begins.

The morning decision: putting three metrics together

Every morning, your watch gives you three numbers that answer one question: how should today's fueling differ from default?

Check Training Load — sets your baseline carb intake for the day.
Check HRV — normal means fuel to baseline; suppressed means add 10–25% carbs and top up sodium.
Check Body Battery as a sanity check — if it's low three days running, the problem is usually total energy or sleep, not today's session.

Then fuel the session: 30–60 g of carbs per hour for anything over 75 minutes, 60–90 g/hr for race-pace efforts, plus 500–800 mg of sodium per hour depending on sweat rate.

Race week: the one HRV rule that matters

HRV at or above your 60-day baseline: taper is working. Keep carbs high, hydrate to pale urine, hold nerve.
HRV down 10%+ for two consecutive days: something is off. Add 1 g/kg extra carbs, 500 ml extra fluid, 500 mg extra sodium, and an extra hour of sleep before adding training to the diagnosis.
HRV drops sharply 48 hours before race: don't panic. Taper HRV routinely wobbles as fatigue unloads. Stable sleep and full carb load are the priorities.

Stop relying on Body Battery in race week. Taper boredom, travel, and pre-race nerves send it all over the map. HRV trend and subjective leg-heaviness are better signals.

Why Sweatr closes the loop

Reading HRV, Training Load, and Body Battery then cross-referencing a carb-per-kg table then translating that into "have a second bagel and add half a scoop of electrolytes" is genuinely hard to sustain day after day.

Sweatr reads your HRV, Body Battery, Training Load, and Estimated Sweat Loss from Apple Watch, Garmin, or Strava, and converts them into a fueling plan for the day and for the race — in plain English, every morning.

The takeaway

HRV tells you how hard the session will cost you today. Fuel more on suppressed days.
Body Battery tells you whether your weekly fueling is keeping up with your weekly load. Audit the trend, not the daily number.
Training Load tells you how many grams of carbs you need per kilo of body weight, this week, right now. Scale with load.

Don't overthink any single metric. Read the three together, adjust once per day, and stop letting your watch's best data die in the notifications tab.