Just juice and water for a few days? Some argue the food trend will do your body good and help you shed some extra weight. We asked health experts what exactly juice diets do and how useful they are.

Who doesn't want to wake up in the morning full of energy and feeling great?

This is what makers of juice cleanses - or juice fasts - promise you: a "reset" for your body. Sounds enticing.

There are pre-packaged juice plans to buy with everything already mixed and instructions on when to drink what. Or, if you're willing to put in the effort, you can save some money by using your own juicer if you've got one.

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But are juice cleanses really beneficial to your health? Two experts fielded some burning questions on the fad below.

A simple juice cleanse - not to be confused with therapeutic fasting or so-called "detox products" - generally lasts between two and five days, and no longer than a week.

"During this period you only drink fruit and vegetable juices," said Niklas Schwarz, an instructor at the German University of Applied Sciences for Prevention and Health Management (DHfPG). "It additionally can be tea, water and vegetable broth. You eat no solid foods."

You drink from three to six servings of juice per day, each a mixture of various types of fruits and vegetables. "The total amount ranges from 1 to 1 1/2 liters," Schwarz explained.

You can start immediately. Some people indulge in their favorite foods the day before, while others avoid foods such as meat and white flour products.

People with a pre-existing medical condition should consult their doctor first, said Antje Gahl, spokesperson for the German Nutrition Society (DGE). Those who should proceed with caution include people with high blood pressure, disorders of the liver, kidneys or thyroid gland, and also cancer patients and diabetics.

"Women who are pregnant or nursing shouldn't fast either," Gahl said. "Nor is it suitable for the elderly and children, because they have a high nutrient requirement."

A juice cleanse is unproblematic for healthy people. Both Gahl and Schwarz said it can be the start of an overhaul of your eating habits.

"Then it's a launch pad that makes changing your diet easier," remarked Schwarz.

"The juices provide the body with many vitamins and secondary plant metabolites along with minerals and fiber," said Gahl, adding that the cleanses also lighten your bowels' workload thanks to the juices' easy digestibility.

As to whether they truly make you feel better, she said it's purely subjective: "Establishing a direct connection is difficult."

On the negative side, juice cleanses deprive your body of many nutrients, for example proteins and essential fatty acids. "So in some respects it's an unbalanced diet," she warned.

This is why it's important not to do juice cleanses for longer than a week, and to spread the servings over the day rather than drinking everything at once. "Otherwise," she cautioned, "your blood sugar levels will rise relatively high and then quickly fall."

This can cause an energy slump, not an energy surge.

A one-week juice cleanse usually results in a weight loss of between 2 and 3 pounds, and even as much as 6 to 10, depending on your baseline, according to Schwarz.

"It's not only fat though, but to a large extent the water that you always lose when you start a diet," he explained. "The drop in digestive tract content reduces your weight too, and the lack of protein intake causes your muscles to break down some protein."

So the key to losing weight is to overhaul your diet after you've finished the juice cleanse.

"If you then resume your old eating habits, you'll regain the weight in a week or two," Schwarz says.