Why sweat sodium differs so much between people
The concentration of sodium in sweat is strikingly individual and can vary roughly fivefold from one person to another. It is influenced by genetics, heat acclimatization, sweat rate, and diet, and reabsorption of sodium in the sweat gland is part of what sets the final concentration. This is why two athletes losing the same volume of sweat can lose very different amounts of sodium.
So-called salty sweaters, who leave visible salt residue and crusting on skin or clothing, sit at the high end and need proportionally more sodium replacement during long efforts.
Turning variability into a personal plan
Because the range is so wide, generic electrolyte advice often misses for the individuals at the extremes. Sweat testing or careful trial during training helps an athlete estimate their own losses rather than guessing from population averages, which the diagram emphasizes with its spread.
This is general educational information, not medical advice. Both inadequate and excessive sodium intake carry risks, so athletes with blood-pressure, kidney, or cardiac concerns should set replacement strategies with a qualified professional.