Carb loading is the practice of eating extra carbohydrates in the days before a long race to maximise glycogen stores in your muscles and liver. The goal is to start the race with a full fuel tank so you're less likely to run low on energy late in the race.
It's deliberately increasing carbohydrate intake for a few days before a marathon or long race to top up glycogen stores. More stored glycogen means more available fuel once the race gets underway.
Most runners begin 2 to 3 days before race day, alongside the taper when training volume is already reduced. Loading up too early or for too long mostly just adds extra weight without extra benefit.
Rice, pasta, bread, potatoes, oats, and fruit are all straightforward options. The idea is familiar, easily digestible carbohydrate sources, not an entirely new diet right before the race.
Yes, overeating, trying unfamiliar foods, or loading up on heavy, fibre-rich meals right before the race can cause stomach discomfort. It works best as a modest, planned increase, not a single massive meal the night before.