An interesting article by Roberto H. Baglione, RD. Departament of Nutrition, National Sport High Performance Center (CeNARD), Buenos Aires, Argentina on the nutritional practices of Grandmasters can be found HERE.
Investigations carried out by the German Grandmaster Helmut Pfleger, MD, et al. have demonstrated that the elite chess players have comparable physiological parameters in competition as those who practice sports such as shooting, car racing and golf; consequently, chess could be classified as the same category of sport as those according to these criteria. As a result, it will be necessary not to limit preparation to hours of study on the board, but to extend it to a more complete program involving physical training and an adequate nutritional plan for the demands, among others.
Regardless of what you are doing many investigations have shown that mental concentration can be affected when doing intellectual activities in the morning without having had breakfast previously. The most preferred solid food by Grandmasters included chocolate (80.5 %), fruits (14.6 %) and cereal bars (9.8 %). Regarding types of fluid, main preferences were water (72.1 %), coffee (42.6 %), tea (29.5 %) and fruit juice (23.6 %).