The science, shown not hidden
The forearm is the one site with a published equation linking a single patch to whole-body sodium loss, and it's easy to keep consistent. Testing the same spot every time is what makes two of your results comparable.
The one site with a published regression linking a single patch to whole-body sodium loss — and easy to place consistently, so two of your results compare.
Back, chest, and thigh sweat differ in saltiness and lack a validated single-patch correction — so they can’t be translated the same way.
Sweat from a single patch reads saltier than your body's average. We don't hand you the raw patch number — we run it through a published regression to estimate your whole-body concentration, and we show both figures so nothing is hidden.
Whole-body [Na⁺] = 0.57 × forearm + 11.05 (mmol/L)Baker et al. 2016 — forearm-to-whole-body sweat sodium regression.
The correction explains about 70% of the variation between athletes (R² ≈ 0.70), so your whole-body number carries a band of roughly ±300 mg/L. Treat it as a strong estimate to aim with and fine-tune by feel — not a fixed constant.
The published correction explains about 70% of the spread between athletes, so your number carries a band of roughly ±300 mg/L.Aim with it, then fine-tune by feel — it’s a strong estimate, not a fixed constant.
Your sweat rate shifts with heat and effort, so we measure that fresh on every session. Your sweat saltiness drifts more slowly — as you adapt to training in the heat, it can fall 30–60% over a couple of weeks. A test taken in the cool season can over-predict your summer sodium loss, which is exactly why we suggest retesting across conditions rather than guessing a correction.
Volume tracks heat and effort, so we re-measure it fresh on every workout rather than carrying an old figure forward.
As you acclimatize to training in the heat, sweat [Na⁺] can fall over a couple of weeks — so a cool-season test can over-predict your summer loss.
No black-box number
Mail-in. Measured. Both figures and the equation between them — the sodium number you fuel against, and a race-day plan that moves with the forecast.
Sources: Baker et al. 2009 / 2016 (GSSI SSE-161) · Buono 2007 · Baker et al. 2022. Example figures shown are illustrative; your results depend on your own measured sweat profile and conditions.