Unless you plan to run a marathon, carbo-loading for energy is out.  Instead, eat protein to increase mental alertness and energy.  Protein contains tyrosine, an amino acid that elevates the brain chemicals dopamine and norepinephrine.  It increases satiety, too.  And when you feel fuller, you’re not apt to overdo the breads and sweets that induce roller coaster highs and lows.  Eat plan and animal based protein throughout the day–an egg or high-protein cereal for breakfast, 10 almonds midmorning, a cup of low-sugar yogurt in the afternoon–and your stamina should stabilize.