There are three key principles that make up a healthy diet; variety, balance, and moderation. First off, a healthy diet must have an adequate amount of variety. This means the food you are eating needs to come from multiple sources. Many people get stuck in a routine of eating the same foods over and over again, and while they may be healthy foods, you could be missing out on key nutrients. For example, eating a certain vegetable every day is great except you are neglecting your body of key nutrients which may not be found in that vegetable but are in another. Also, by making your diet more diverse you will continue to be interested in eating and will be more likely to continue healthy eating habits.
The second key principle of a healthy diet is balance. Balance means that you are receiving the proper nutrients from all the major food groups. This ensures that the individual is getting the proper amount of nutrition they need which is found uniquely in each food source.
The third and final key principle of a healthy diet is moderation. No matter how perfect a diet is, it can still be harmful and cause weight gain if it is eaten in too large of a quantity. And on the flip side, no matter how great a diet is, if the person is not eating enough then that person will not benefit fully from the diet. Also, moderation means that each meal must contain the proper amount of food from each food group. You should not eat meals that are super high in protein but lack an adequate amount of carbohydrates, for example. Also, by eating in moderation, you are allowing yourself to not have to completely eliminate any one type of food, meaning you can still eat foods in which you enjoy.