When it comes to keeping hunger at bay, exactly which foods you choose to eat — and not eat — are a key part of controlling the urge to eat.

Science shows that it's not always enough to simply eat when you're hungry — we've all experienced the sensation of feeling hungry when it seems like we just ate a meal.

Indeed, researchers say, the foods you choose have an impact on how long you'll continue to feel full after you eat. Here are the best — and most healthful ways — to fill up, according to the experts.

"Low-calorie-density foods" may sound like a mouthful (and, well, that's the point!), but the important thing is that these are foods that take up a lot of space in your digestive system but don't pack a lot of calories into each bite.

"Low-calorie-density foods are what you want to encourage to help people fill up on fewer calories," said Barbara Rolls, chair of nutritional sciences at Pennsylvania State University and author of "The Ultimate Volumetrics Diet" (William Morrow Cookbooks, 2013).

These foods are the opposite of, for example, high-fat foods, which are very calorie-dense, meaning they pack a lot of calories into each bite, Rolls told Live Science.

Low-calorie-density foods include water-rich foods, such as fruits and vegetables. They have fewer calories per bite, but a lot of volume, she said. Other good examples of low-calorie-density foods include broth-based soups, and salads, Rolls said.

One of the reasons these low-calorie-density foods help fill you up is your perception of how much you are eating.

Perception is a big part of satiety, or the feeling of fullness, Rolls said. People who eat low-calorie-density foods perceive that they are eating more food than people who consume the same number of calories from high-calorie foods. Therefore, eating low-calorie-density foods can lead to increased feelings of fullness, she said.

Melinda Manore, a professor of nutrition at Oregon State University, agreed that low-calorie-density foods are a good way to fill up.

These foods take up space in the gut — so the body registers that it's being fed, and you get a sense of fullness without as many calories, Manore told Live Science.

Fiber-rich foods are also filling, but for different reasons than the low-calorie-density foods. It turns out that feeling full is just one part of not feeling hungry.

"Fiber is thought to have other effects" that help keep hunger away, Rolls said. It not only keeps you feeling full but also adds texture and body to foods as you eat them, changing the way food feels in your mouth, which can also prevent you from feeling hungry right away, she said.

In addition, fiber moves slowly through the digestive tract, so it slows down the digestive process, Rolls said. It also may have more potent effects on some of the satiety hormones in the body, Rolls said. In other words, fiber may have a stronger effect on the hormones that signal fullness.

And what about protein, which some experts say is the most satiating type of food?

Researchers have known for a long time that eating protein can suppress appetite a bit and, as a result, cause people to eat less, Manore said.

But there are still some questions about protein's filling effects.

Some research suggests that protein keeps people feeling full longer than other nutrients do, but other research suggests that isn't the case, Rolls said. But in general, experts do think that eating protein can help with satiety, she said. (There's still debate over what types of protein are best and how much a person should eat, she added.)

Part of protein's filling effects could be psychological, Rolls said. People are used to thinking of protein as the center of a meal, and if the meal doesn't have any protein, a person may not perceive it as satisfying, she said.

Downing a glass of water before a meal certainly seems like it would fill you up, but experts still don't agree on the answer to this one.

Some studies have shown that drinking water before a meal may affect satiety a bit, but it's possible that there's a psychological effect, too, Rolls said. For example, if people believe that drinking water before a meal will lead them to eat less, that may well be the case, she said.

On the other hand, drinking water does distend your stomach, which can make you feel full, Manore said. But the body doesn't register liquids the same way it does solid foods, she said. For example, if a person drinks a martini with 300 calories before a meal, that person wouldn't necessarily eat 300 fewer calories at dinner, she said.

In one interesting study by Rolls and her team, people were given either a glass of water and a vegetable stew, or a soup made from the same amount of water and stew, mixed together. The researchers found that when people were given the soup, they ate fewer calories overall than when they were given the glass of water and the stew.