Who doesn't want to wake up each morning brimming with energy and vitality? That's precisely the pledge made by advocates of juice cleanses, also known as juice fasts: a rejuvenating "reset" for your body. It's an alluring prospect, particularly as a fresh year begins.

There are pre-packaged juice plans 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 switching on your juicer if you've got one.

Are juice cleanses beneficial to health? Two experts field some questions.

Juice cleanse

Simple juice cleanses – not to be confused with therapeutic fasting or so-called detox products – generally last two to 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) in Saarbrücken, Germany.

"In addition, it 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. A serving contains between 250 and 500 milliliters of juice. "The total amount ranges from 1 to 1.5 liters," Schwarz says.

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.

According to Antje Gahl, spokesperson for the German Nutrition Society (DGE), people with a preexisting medical condition should consult their doctor first. They include those 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 say it can be the start of an overhaul of your eating habits. "Then it's a launch pad that makes changing your diet easier," Schwarz remarked.

Is it healthy?

"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.

She says whether they make you feel better is 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 said.

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 warned, "your blood sugar levels will rise relatively high and then quickly fall," which can cause an energy slump, not an energy surge.

Can a juice cleanse help you lose weight?

A one-week juice cleanse usually results in a weight loss of between 1 and 1 1/2 kilograms and even as much as 3 to 5, 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 overhauling your diet after the juice cleanse. "If you then resume your old eating habits, you'll regain the weight in a week or two," Schwarz said.