Marathon fuelling and nutrition

It comes down to what you have trained. A sensible place to start is around 60 g of carbohydrate an hour, and trained up toward the elite end you can carry up to about 120 g. Build it with a coach or a sports dietitian rather than guessing. Fluid works the same way: there are rules of thumb, but it swings hard with temperature and conditions, so knowing your own sweat rate helps. As a default, keep taking fluid on across the whole course.

Practise it. Build it up across your long runs and workouts, a bit more on board each time, and find the gels or drinks that actually sit well for you. The gut is trainable, so the more you rehearse race fuelling, the more it will take.

A little personal, but high-carb and simple works well, something like white bread with honey. You want easy energy that sits well, not a big or rich meal.

Not strictly required, but it makes a real difference. Aim for about 8 to 12 g of carbohydrate per kg of body weight across the two days into a marathon. A smaller version, maybe a single day, ahead of your big key workouts helps too.

Health first, always. Being healthy matters more than any number on a scale. Weight and body composition rarely move things as much as people assume, and chasing them is a road that goes bad fast. Put your attention on fuelling and eating enough to stay healthy. If weight is turning into a worry or a fixation, take that as the signal to get proper support rather than to start cutting.

No, I would steer clear. For the marathon you want a gut used to taking on carbohydrate and a body used to running well-fuelled. Training fasted or low-carb teaches it the opposite.