Your metabolism is responsible for processing energy and burning calories you have stored in your body. But how does that energy get stored in the first place?
Unless you're eating for emotional reasons—such as when you use food to quell boredom or sadness, or even celebrate something good happening—you eat because you're hungry. This sensation is your body's way of telling you it needs more fuel. A series of complex chemical reactions govern how your body asks for and receives food for fuel, but the ones that are particularly important for breakfast-skippers have to do with blood sugar levels.
Blood sugar levels play an important role in governing how your body uses and stores energy. You may have heard about the blood sugar spike that happens when you eat something with a lot of processed sugar like a candy bar, or something made of refined white flour like a bagel. Take the bagel as an example: It's made almost entirely of white flour. White flour is manufactured by using a machine to strip away the fibrous shell that would normally encase the sugary kernel interior in a grain of wheat, then the remaining kernel is ground up.
When you eat something made from white flour, your body doesn't have to do much work at all to access the sugars (energy) that the food provides. As soon as it hits your stomach and intestines, the sugar molecules are absorbed across the skin membrane as glucose into the blood, leading to a huge blood sugar spike. Because it's experienced a flood of blood sugar, your body goes into overdrive, releasing huge amounts of insulin. The hormone insulin converts the glucose into glycogen, which can be stored in the muscle and fat tissues for use the next time you need energy.
Because the body released so much insulin, however, the blood sugar levels are quickly depleted. Your body's natural response is to ask you to send it more sugar in the form of simple carbohydrates to replenish blood glucose levels. This is what we know as the vicious cycle of carb-begets-carb. If you start your day with a simple carbohydrate meal, not only will you send your body's internal regulating mechanisms into a tizzy, you will be hungry again within hours and will crave another sugar fix.
However, in complex carbohydrates the external fiber is still intact, making it more difficult for your body to access the sugar on the inside. Your body will actually have to expend energy to get energy, and it will absorb a little bit of sugar at a time as it chips away at the fiber shell on every grain. This slow and steady flow of sugar into your bloodstream through your stomach and intestinal walls during digestion means you won't experience the blood sugar spike a bagel would have caused. Your body only releases a little bit of insulin and converts glucose to glycogen more slowly. It won't ask you to eat something more to replenish blood sugar levels as soon, and the fiber content itself will help you to feel full for longer.
Eating breakfast is a crucial part of regulating your blood sugar levels because it is the first chance your body has to replenish the energy stores that have been depleted over night to help fuel its reparative work. Since whatever you eat for breakfast will dictate how stabilized your blood sugar is for the rest of the day, you want to make sure to have something with fiber and protein, as well as some complex carbohydrates. A scrambled egg on whole grain toast is an excellent combination, as is a thin spread of almond butter on whole grain toast, or yogurt with muesli. Each of these combinations will offer you fiber and protein to stay full, as well as complex carbohydrates that will provide a slow-release sugar to your blood stream. You'll end the sugar spikes and inevitable sugar lows that send you running for the nearest vending machine fix.
Drop that doughnut!