Why doesn't this use the calories from my watch?
Because watch calorie estimates are not good enough to eat by. A systematic review found errors above 50 % even under controlled conditions, and in the real world the error exceeds 10 % about 82 % of the time. Recent devices have been measured at 14.9 % to 47.8 % mean error, approaching 100 % for resistance work. On top of that, burning 500 calories does not raise your needs by 500 — your body quietly compensates elsewhere, and by a different amount in each person. Duration, intensity and temperature are cruder inputs, and they are more honest ones.
How many carbohydrates per hour do I actually need?
Under an hour, essentially none if you started fed. One to two hours, 30–60 g. Beyond two hours, 60–90 g, and trained athletes can push to 100–120. The jump above 60 g only works if the drink or gel mixes glucose and fructose, because they are absorbed by separate pathways and one alone saturates.
Is 120 grams an hour realistic, or is that just for pros?
It's real, but it's the ceiling and it has to be earned. In one 2020 study seven of nine trained runners handled 120 g an hour without stomach trouble. The ones who can do it have trained for it. Start where you are and build.
How do I train my gut?
Four to six weeks, one or two hard sessions a week, adding roughly 10 g per hour every one to two weeks — and always with the exact products you plan to race with. A gut trained on one brand is not automatically trained on another.
How much should I drink?
Sweat rates run from 400 to over 2,500 ml an hour, so any single number is a guess. As a starting range, 300–500 ml an hour below 15 °C and 600–800 above 25 °C. Then drink to thirst rather than to a schedule, and don't exceed the sweat rate you have actually measured.
How do I measure my own sweat rate?
Weigh yourself naked. Run an hour at your normal pace, keeping track of everything you drink. Towel off and weigh again. Sweat rate = weight lost + fluid drunk. Do it once in the cold and once in the heat — the two numbers will not look alike.
Do I really need salt tablets?
Under an hour, no. Between 60 and 90 minutes they're insurance. Past 90 minutes, 300–600 mg of sodium an hour starts to matter, and more if you're a heavy or salty sweater or it's hot. Sweat carries somewhere between 500 and 1,500 mg of sodium per litre depending on the person.
Can I get this badly wrong in the dangerous direction?
Yes, and it's usually by drinking too much rather than too little. Exercise-associated hyponatremia has killed marathon runners, and the risk is highest for people out for four hours or more, smaller athletes, and anyone following a rigid drinking schedule instead of thirst. If a number here looks high to you, trust your instinct and go lower.
Does it work for cycling or hiking?
The carbohydrate and sodium guidance carries over — it's driven by duration and intensity, not by which sport. Cycling usually lets you take on more, because the gut is less shaken and you can carry more bottles. Treat the numbers as a floor rather than a ceiling on the bike.