This is my take on this.. Not a Medic, but am staying in a Holiday Inn Express
I'm looking at it as we are athletes. Hydration is just as important as food intake before and after exercise. Two hours before exercise, athletes should consume 16 ounces of water or a sports drink to help hydrate them ahead of time. Thirty minutes before exercise, athletes should intake another eight ounces to prepare themselves for activity.
During activity, fluids should be available for athletes at all times. Because athletes are sweating out important fluids, they must replenish them by drinking eight ounces every 20 minutes. If players are engaging in short
activity, of 30 seconds or less, they are at a high risk for dehydration because of the intensity of the work.
Long-term activity of 30 minutes or more requires periodic rehydration, such as the eight ounces every 20 minutes just suggested by BearW.
If an activity lasts more than 40 minutes, water is not sufficient to rehydrate the body. The nutrient loss through sweat requires a sports drink to replenish electrolytes.
Many athletes will prefer not to drink during activity or will feel ill directly after intense exercise. All athletes must drink adequate liquids before, during, and after activity to avoid dehydration, which can lead to nausea, dizziness, and fatigue. After activity, athletes should continue to intake fluids.
At this point, fluids can be the normal amount the athlete would consume with a meal and through the rest of the day.
A total of 64 ounces of fluid is a minimum for athletes, though more is suggested. A good test of proper hydration is a urine test. If you don't know this, ask your medic. One rule is total body weight devided by two; equals you intake in ounces. Most athletes that is over a gallon a day. Just drinking, not training. Training you most intake more before, during and after training event.
I like to encourage people to pay attention to their own needs, as all athletes will have slightly different needs. If you feels uncomfortable, light-headed, or otherwise abnormal, you should seek medical help (Attention). Wait us SF never go to sick call. Medic!!!
As fluid intake levels will change based on environmental effects, pay attention to the outside influences affecting fluid needs. Weather, Season, combat, Temp of the day, Elevation training at and will be doing your event at (higher?? Training lower??), terrain.
Meal Replacement Drinks.. during endurance events stay away from them. Weight training, ok. I think eating meat is best. But your take. During endurance events you should not intake Meal Replacement drinks. They pull blood away from your muscle that need the blood there to feed the muscles. Also if your using them before a big endurance event, then mix as you normally do, but top off your shaker bottle with water. Warmer the water the better too. It will be absorbed into your system faster.