The importance of good nutrition is nothing new. Back in 400 B.C., Hippocrates said, "Let food be your medicine and medicine be your food." Today, good nutrition is more important than ever. At least 4 of the 10 leading causes of death in the U.S. — heart disease, cancer, stroke and diabetes — are directly related to the way we eat; diet is also implicated in scores of other conditions. But while the wrong diet can be deadly, eating right is among the cornerstones of health.

Of course, food alone isn't the key to a longer and healthier life. Good nutrition should be part of an overall healthy lifestyle, which also includes regular exercise, not smoking or drinking alcohol excessively, stress management and limiting exposure to environmental hazards. And no matter how well you eat, your genes play a big part in your risk for certain health problems. But don't underestimate the influence of how and what you eat.

For example, atherosclerosis (hardening of the arteries) can begin in early childhood, but the process can be halted — even reversed — if you make healthy changes in your diet and lifestyle. The gradual bone thinning that results in osteoporosis may be slowed if you consume enough calcium, maintain adequate Vitamin D levels and participate in weight-bearing exercise. You may be genetically predisposed to diabetes, but keep your weight within a healthy range through diet and exercise and the disease may never strike you.

The keys to good nutrition are balance, variety and moderation. To stay healthy, your body needs the right balance of carbohydrates, fats, and protein — the three main components of nutrition.

You also need vitamins, minerals and other substances from many different foods, and while some foods are better than others, no single food or food group has it all — so eating a variety of different foods is essential.

Moderation means eating neither too much nor too little of any food or nutrient. Too much food can result in excess weight and even too much of certain nutrients, while eating too little can lead to numerous nutrient deficiencies and low body mass.