Eating too many GI carbohydrates will cause a rapid release of insulin, which could cause excess carbohydrates to be turned into fat and deposited in fat cells. So stick to low GI carbs in balanced amounts alongside protein and fat because carbs are important especially when you train a lot, it's just a case of getting the right kinds and in the right amounts.
Eating carbohydrates can actually increase your metabolic rate. 10-15% of your calories from carbohydrates are expended as heat (thermogenesis). The rest of the carbohydrate is converted into glycogen. This causes a rise in blood glucose (blood sugar levels). The higher the GI of the carb the sharper the rise in blood glucose, which causes your body to release insulin at a faster rate and the more insulin in your bloodstream, from eating higher GI carbohydrates, the more likely that excess carbohydrate will be turned into fat and deposited in your fat cells.
The key to keeping insulin levels low is to eat low GI meals (complex carbs) in balanced amounts, which do not cause a sharp rise in insulin release, alongside balanced amounts of protein and fats.
If you are trying to lose weight, don't cut out carbohydrates completely. Rather, drop your amounts by 15%, proportional to your drop in calories.