You've heard it so many times it's become cliché: Breakfast is the most important meal of the day. Still skipping it? Here's why you shouldn't: Your brain runs on glucose, the energy source that allows you to perform everything from simple mental tasks like answering e-mail to physical challenges like tackling a five-mile run or getting through your favorite yoga class. When you start the day off without this critical energy source you're effectively starting at a deficit, causing your body to work extra hard to convert stored carbohydrates or protein to energy, and compromising its ability to burn fat. The result is a listless mind, sluggish body and, most likely, a compromised mood.

      Studies show that skipping breakfast may also lead to weight gain, and as a result, greater risk for heart disease and stroke. Depriving yourself in the morning (consider your body has been bereft of energy for 10 hours or more) can lead to overeating later in the day, and over time can slow your metabolic rate, putting you at greater risk for a steady weight creep as you age.

     In essence, breakfast can mean the difference between being your ideal weight or overweight, being alert or feeling flighty and forgetful, fighting infection or succumbing to infections like colds and flu viruses.

      So what should you eat? And what are some quick choices when you're crunched for time? In one meal you can meet much of your daily requirements for carbohydrates, proteins, fats, vitamins, minerals, fiber and phytochemicals to keep you performing at your best. Smoothies, protein shakes, energy bars or even a glass of fortified juice are quick and easy ways to start your day on the right foot. Include a complex carbohydrate such as a whole grain bagel, piece of fresh fruit or a few veggies in an omelet, along with a low-fat protein or calcium source such as a yogurt or two hard-boiled eggs, and you'll be more energized and more alert throughout the day.

      Research shows that adding a high-quality protein source, such as egg whites or other forms of dairy, along with citrus fruit to your diet might even help you burn extra calories throughout the day.

      But steer clear of unnecessary fat and carbohydrate calories. A cinnamon raisin bagel with cream cheese could stick you with as much as 500 calories and 12 grams of fat. Make the better choice of a whole wheat bagel topped with two tablespoons of jam for 100 fewer calories and only about one gram of fat. And a 170-calorie nutrient-dense energy bar compared to a sugar- and fat-packed cranberry orange muffin at 450 calories is a no-brainer.

      The carbohydrate calories will restore muscle glycogen stores, giving you enough energy to get through the day and to fit a workout in to boot. What's more, you'll enjoy a natural high: the brain chemical serotonin is also produced after eating a carbohydrate-rich meal, providing a calm, satiated feeling—a great way to start any day of work or play.