When you fast you deprive your body of energy. One reason people can go 12 hours or so in the evening without eating is because most of that time they are in low-energy mode, sleeping or doing very sedentary activity. Cells are always active, even during deep sleep, so the body still uses energy. But it can rely on body-fat stores and carbs from the liver.
In the morning, after a night of not eating, the liver glycogen (carbs) is low. Breakfast replenishes these.
The longer you fast, the sooner you need energy, and the more energy you may need—especially if you lead an active life (waking up and walking the dog, exercising before work, running around at your job or after the kids).
Even when you’re not fasting, if you are using your body in a way that requires more energy (long, hard exercise, for example), you need fuel to meet those excess energy demands.
Many people can skip breakfast without feeling hungry. Depleted glycogen stores in the liver produce a state of ketosis, the early stages of the body’s starvation response. One of the side-effects of ketosis is a lack of hunger. If the body was truly starving, this is thought to be a way to preserve precious energy that might be wasted looking for unavailable food.
But not feeling hungry does not mean that you don’t need breakfast, or won’t benefit from eating it.
How long you can go all depends on when you last ate and how much you ate, and how active you are in the morning. Basically, the longer your evening fast lasted, or the more energetic you are in the morning, the sooner you should get some calories in your body.
A smoothie with fruit and/or yogurt and/or juice is a good start to the day. If you don’t have time to make one, prepare something fast like peanut butter on whole-grain toast. Or keep quick, easy snacks on hand to nibble on in the car (nuts, a prepackaged energy bar or shake, fruit).
