Keep the blues at bay by eating enough iron.
Iron transports oxygen, supports energy levels, and boosts muscle strength. For those same reasons, not consuming enough iron (which can result in a syndrome called anemia) can result in lethargy, fatigue, apathy, and depression. Keep iron levels (and your mood) up by chowing down on plenty of oatmeal, soybeans, lentils, and meat.
Learn how iron (and other vitamins and minerals) can affect your physical and mental health.
Vegetarians can be just as successful as meat-eaters in endurance sports, as long as they pay attention to nutrition.