Retaining the patterns that served our hunter-gatherer ancestors so well in the Stone Age, your metabolism maintains a nearly steady supply of energy whether food is on board or not. It's a good thing since your brain can't store any energy on its own, depending instead on a constant infusion of glucose (sugar) from the bloodstream. After a meal, food is digested in your stomach and intestines; carbohydrates, proteins, and fats are broken down into smaller fragments that are absorbed into your bloodstream. As your blood sugar level rises, your pancreas secretes insulin, the hormone that allows glucose to enter cells, where it powers the body's metabolism. Glucose that's not needed on the spot is converted into glycogen, which is stored in the liver and muscles to meet future energy needs. But the body can store only a small amount of glycogen; most of your energy reserves are in the fatty acids that are locked away in your body's fat deposits. In contrast to the treatment of other nutrients, your body does not store excess protein; it converts it into fat.
When it's not getting new supplies of food, your metabolism goes into reverse, thanks to hormones such as glucagon, adrenaline, and cortisol. Your liver converts glycogen back into glucose and produces additional glucose to keep your blood levels nearly steady. If you need even more energy, your body releases fatty acids that can be burned for energy. But since all your proteins are serving important functions, they stay put during short periods of energy deprivation. In times of real famine, however, the body cannibalizes itself, burning protein for energy it can't get any other way.
A lapse of 10 or 12 hours between dinner and breakfast is hardly a famine, but it's enough to put your metabolism into a fasting, energy-mobilizing mode. Your first meal of the day will help flip the switch back to energy storage, so it's important to do it right.
Your diet should be prudent but not punitive or even boring. Choose a variety of foods and experiment with new ones. Keep your fat consumption low to moderate (20%–30% of your daily calories) by reducing your consumption of saturated fat from meat and whole dairy products and trans fatty acids from stick margarine, fried foods, and snack foods. Favor omega-3 fats from fish and nuts and monounsaturated fats from olive oil. Eat only a small amount of simple sugars and other rapidly absorbed carbohydrates, but get enough complex, slowly absorbed carbohydrates to bring your total carbohydrate intake to 50%–65% of your daily calories. Eat foods that provide at least 25 grams of dietary fiber a day. Don't neglect protein, but don't feel you have to push your ration above 15% of your daily calories.
Credits: Harvard Medical School Publications