Build a carbs, fluid, and sodium plan for any ride or run.

Bouncing your stomach for hours is harder than rolling it. Running gait creates mechanical stress on the GI tract that limits carbohydrate absorption - the same gut that handles 90g/hr on the bike often refuses past 60g/hr on the run. Trained ultrarunners can push past 80-90g/hr with practice, but on a first-time marathon you should plan conservatively or pay the price around 30km.

Yes, but you have to train your gut for it. Mix glucose (or maltodextrin) with fructose at roughly a 2:1 ratio to push past the 60g/hr single-transporter ceiling - glucose uses SGLT1, fructose uses GLUT5, so the two together absorb roughly additively. Practice this exact mix on long rides for a few months before race day. Dropping cold turkey to 90g/hr on race day will give you GI cramps and possibly worse.

Highly individual. 300-700mg per litre of fluid covers most athletes in temperate conditions. Heavy sweaters (you can see white salt stains on your kit after a hot ride) lean toward 700-1000mg/L. Cramping after long efforts is a hint you are on the low end. Hyponatremia (sodium too low for the fluid volume) is the real risk for ultra-endurance and slower marathoners drinking too much plain water - sodium is what prevents it.

Carb targets carry through to ultras, but real food (rice cakes, sandwiches, boiled potatoes, peanut butter pretzels) becomes important past about four hours. Pure sugar feeding gets nauseating, and your gut wants some protein and fat for variety. The tool shows a "real food" callout at the four-hour mark for that reason. Past eight hours, plan to drop carb targets slightly (60-70g/hr) and supplement with whatever calories you can actually keep down.

For temperate conditions and moderate intensity, 500-750ml/hr is typical. In hot conditions, 750-1000ml/hr. Hot and humid pushes it higher. Sweat rate is highly individual - you can measure yours by weighing yourself nude before and after a known-duration ride or run, subtracting fluid intake, and dividing by hours. 1kg of body-weight loss equals roughly 1L of sweat plus a bit.

Caffeine improves endurance performance by 2-4% on average across the literature - genuinely meaningful. 3-6mg per kg of body weight, taken 30-60 minutes before the effort, peaks at the 1-2 hour mark. For longer efforts, a smaller top-up dose every 2-3 hours extends the effect. Avoid first-time caffeine experiments on race day - some people handle it badly.

Most marathoners do best on gels and chews - solid food is slow to digest at marathon pace and the stomach is already stressed. For sub-3-hour marathoners, all-liquid plus gels usually works. For 3-4 hour finishers, gels plus some chews. For 4+ hour finishers, a banana or half a bar at the mid-point can be a relief. Practice in long training runs first.

Yes. At endurance pace (Zone 2), you burn fat for a large fraction of fuel and only need 30-60g/hr of carbs to keep glycogen topped up. At threshold and above, the body switches almost entirely to carb burning and the cost climbs to 80-120g/hr of intake to keep up. Going hard on too little carb is what bonking feels like - the body did not run out of energy, it ran out of the right kind.