Breakfast and your health.

Your mother was right (again): It is important to start your day with a good breakfast. But the hearty feast of bacon and eggs that you may remember from your youth is hardly a good start by today's standards, and the doughnut and coffee that have replaced it in today's fast-paced world is no better. Why is breakfast so important, and how can you plan a meal that is enjoyable, convenient, and healthful?

Morning metabolism

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 socked 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 or converted into glucose. 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 …