Many athletes use all kinds of supplements despite there is little or no evidence that these supplements actually help.
Road cyclists often have long seasons and cover huge distances. The last big races are in October and the first races in the new season start for the professionals in January and the amateurs in March.
During exercise you mainly lose fluids by sweating and breathing. If you fail to replete this deficiency by drinking, it can have a negative impact on your performance.
The potential upper limit of all sports performance is remarkably high, but is not achieved by most athletes. A power meter can make the difference here.
The high mileage of professional cyclists, they require a high-energy diet with adequate amounts of protein, vitamins, minerals and of course many carbohydrates as fuel for the muscles.