According to a study presented at the AHA meeting, having a snack and a non-alcoholic beverage before you get on a plane can help you feel better and might even prevent an in-flight heart attack.

The rate of medical emergencies during flights is about one person for every 800 to 1900 flights; more than half of these involve a heart attack or loss of consciousness. People with high blood pressure or vascular disorders are especially at risk for cardiac emergencies during flights, when the body’s oxygen level might drop. That happens when low cabin pressure at higher altitudes causes the blood vessels to expand as the body tries to increase its oxygen supply. But if the amount of blood pumped by the heart remains the same, the body cannot get more oxygen.

Having a snack and increasing fluid intake are good ways to increase your circulating blood volume - thereby helping you feel better because you're not lacking oxygen and also helping to prevent fainting or a heart attack due to reduced oxygen.

Source:  Food in flight fights fainting spells and heart attacks, Report to AHA, Nov. 15, 2000.