Feel full on fewer calories? It might sound like another gimmick for weight loss, but it's not. The concept of energy density really can help with weight loss.

In fact, well-planned weight-loss diets, such as the Mayo Clinic Diet, use the concept of energy density to help you lose weight and keep it off long term.

Weight loss with more food, fewer calories

Simply put, energy density is the number of calories (energy) in a specific amount of food. High energy density means that there are a lot of calories in a little food. Low energy density means there are few calories in a lot of food.

When you're striving for weight loss, the goal is to eat low-energy-dense foods. That is, you want to eat a greater volume of food that's lower in calories. This helps you feel fuller on fewer calories.

Here's a quick example with raisins and grapes. Raisins have a high energy density — 1 cup of raisins has about 434 calories. Grapes have a low energy density — 1 cup of grapes has about 82 calories.

The keys to energy density and weight loss

Three main factors play a role in what makes food high or low in energy density:

• Water. Fruits and vegetables generally have high water and fiber content, which provide volume and weight but not calories. That's why they're low-energy-dense foods. Grapefruit, for example, is about 90 percent water. Half a grapefruit has just 37 calories. Raw, fresh carrots are about 88 percent water. A medium carrot has only about 25 calories.
• Fiber. High-fiber foods not only provide volume but also take longer to digest, making you feel full longer on fewer calories. Vegetables, fruits and whole grains all contain fiber. Popcorn is a good example of a high-volume, low-calorie whole grain. One cup of air-popped popcorn has about 30 calories.
• Fat. Fat is high in energy density. One pat of butter, for example, contains almost the same number of calories as 2 cups of raw broccoli. Foods that contain fat naturally, such as dairy products and various meats, or foods with added fats are higher in calories than are their leaner or lower fat counterparts.

Energy density and the food pyramid

Changing lifestyle habits is never easy, and creating an eating plan using the energy-density concept is no exception. The first step is knowing which foods are better options when it comes to energy density.

Here's a look at energy density by the categories in the Mayo Clinic Healthy Weight Pyramid.

Vegetables

Most vegetables are very low in calories but high in volume or weight. Most vegetables contain water, which provides weight without calories. Examples include salad greens, asparagus, green beans, broccoli and zucchini.

To add more vegetables to your diet, top your pasta with sauteed vegetables instead of meat or cheese sauce. Decrease the meat portion on your plate and increase the serving of vegetables. Add vegetables to your sandwiches. Snack on raw vegetables.

Fruits

Practically all types of fruit fit into a healthy diet. But some fruits are lower calorie choices than others are. Whole fresh, frozen and canned fruits without syrup are good options. In contrast, fruit juices and dried fruits are concentrated sources of natural sugar and therefore have a high energy density — more calories — and they don't fill you up as much.

To fit more fruits into your diet, add blueberries to your cereal in the morning. Try mango or peach slices on whole-wheat toast with a little peanut butter and honey. Or toss some mandarin orange and peach slices into a salad. Keep whole fruit in a bowl within easy sight or in the fridge and eat it anytime you like.

Carbohydrates

Many carbohydrates are either grains or made from grains, such as cereal, rice, bread and pasta. Whole grains are the best option because they're higher in fiber and other important nutrients.

Emphasize whole grains by simply choosing whole-wheat bread, whole-wheat pasta, oatmeal, brown rice and whole-grain cereal instead of refined grains. Because many carbohydrates are higher in energy density, keep an eye on portion sizes.

Protein and dairy

These include food from both plant and animal sources. The healthiest lower energy-dense choices are foods that are high in protein but low in fat and calories, such as legumes (beans, peas and lentils, which are also good sources of fiber), fish, skinless white-meat poultry, fat-free dairy products and egg whites.

Fats

While fats are high-energy-dense foods, some fats are healthier than others. Include small amounts of healthy monounsaturated and polyunsaturated fats in your diet. Nuts, seeds and oils, such as olive, flaxseed and safflower oils, contain healthy fats.

Sweets

Like fats, sweets are typically high in energy density. Good options for sweets include those that are low in added fat and contain healthy ingredients, such as fruits, whole grains and low-fat dairy. Examples include fresh fruit topped with low-fat yogurt, a cookie made with whole-wheat flour or a scoop of low-fat ice cream.

The keys to sweets are to keep the serving size small and the ingredients healthy. Even a small piece of dark chocolate can fit into a weight-loss plan.

Making energy density work for you

When you stick to the concept of energy density, you don't have to feel hungry or deprived. By including plenty of fresh fruits and vegetables and whole grains in your diet, you can feel full on fewer calories. You may even have room in your diet for a sweet on occasion.