You will eat far more calories than you realize when eating high fat and high sugar foods.

Telling people to “eat less” isn’t the whole story on weight loss because it’s not just large portion sizes that make you overweight. How many calories are in the food you eat matters just as much. It’s possible to eat less while eating more calories. And it’s also possible to eat more while eating fewer calories.

How many calories a particular amount of food has is called its energy density. Water, for example, has the lowest energy density because it has 0 calories for any weight of water. Fat has the highest energy density because it has the most calories.

Why does energy density matter? Dr. Barbara Rolls has found in her research that, over the course of a day or two, you will tend to eat about the same volume of food, regardless of its energy density or the number of calories in it.

This is probably related to the fact that your stomach needs a certain volume and weight of food before it will send the “I’m full” signal to your brain. You’ll most likely eat until your stomach has enough food in it to send this signal.

Volume simply means the amount of space the food takes. What Rolls is saying is pretty interesting: you’ll consistently try to eat the same volume of food. So picture how big a salad you need to make you full. Now picture how many candy bars you would need to eat to fill the same volume as the salad. Which has more calories, the salad or the candy bars?

This is the key point of the threat. You will have to eat a lot more of high energy density foods to reach a satisfying volume of food. And what are the high density foods? Junk foods. Foods chock full of fat and sugar.

If you eat foods that with a lot of calories in a little package, then you will eat far more calories than you realize. But if you limit yourself to smaller portions of high-calorie foods, you will end up feeling deprived and hungry and the chances are you’ll fall off your diet. It’s a no win situation.

The calorie bomb is when you eat large portions of junk foods. You’ll eat more to reach the necessary volume of food. The total number of calories you eat skyrockets.

In one study, students ate over 1600 calories in a single meal at a food court. That’s about 62 percent of the recommended daily intake. There was no way they realized how many calories they were eating. Combining large portions of high energy density foods is a sure diet killer.

In another study, women were served a casserole in three different sizes and two different energy density levels. You can easily lower the energy density of a casserole, for example by replacing cheese with more vegetables. This will keep the volume of the dish the same, but it will have fewer calories.

Amazingly, the women were just as full from the smaller low density casserole as they were from the larger high density casserole. Now get this, the large high density dish had 620 calories and the small low density dish had 398 calories. So you can save 220 calories by eating the smaller lower density meal and you will feel just as full!

Another variation on this theme is that by eating a large salad as the first course of a meal, you can eat significantly fewer calories in total. Eating a salad fills you up so you’ll eat fewer calories from later courses. You may have heard this advice before.  This works because you feel full by eating larger portions of a lower energy density food—the salad. You need to eat a certain volume of food and the salad helps you reach that volume for fewer calories. Eating the same volume in steak or pasta would be a lot more calories.

 In another study, Dr. Rolls found that increasing the amounts of fruits and vegetables in meals could reduce your total daily calorie intake by more than 400 calories a day. And you won’t feel hungry. That’s amazing stuff.

And the advantages just keep on coming. Researchers have also found that eating vegetables and fruits make you stay fuller for longer. You can keep hunger away by simply adding a few more veggies to your meals.

In another interesting finding by Dr. Rolls, men eating a low energy dense diet ate 425 less calories and women eating a low energy dense diet ate 275 fewer calories when compared to people eating a higher calorie density diet. People on the low energy density diet ate significantly more food too.

Dr. Rolls found people with normal weight had diets with lower energy densities than did the obese people. And the people who ate more fruit and vegetables had the lowest energy density diets and the lowest obesity rates.

Now, why might some people eat higher density diets than others? We talked about why some people are attracted to junk food in The Power of Food Threat and The Stress Threat. But many others simply haven’t heard of this amazing research.

You don’t need to eat small portions to lose weight. You can eat satisfying portions of lower energy density foods. You don’t have to starve yourself. You can change your diet, eat more, lose weight, and still feel full. What at deal! Knowing how your body works can teach you a lot about how you can have the best of all worlds.