As with all things diet there are pros and cons. Let's go through them so you can make the call for yourself.
PROMORE FRUITS AND VEGETABLES!
Putting a wide array of fruits and vegetables that you may not otherwise eat into a glass is obviously a pro and would provide concentrated vitamins and minerals probably better absorbed than in a supplement. I get stoked about anything that gets people to incorporate more plants into their diet.
While it is difficult to consume too many calories from whole fruits and vegetables, when we condense them into a juice glass and add that to our normal solid diet we are essentially adding unnecessary calories to our diet (unless you are underweight this is not ideal). Even these calories from your new healthy juice habit can make you gain unwanted fat. Calories, indeed are still calories.
For example this beet, carrot, apple, ginger juice (which tastes fabulous by the way!) delivers 169 calories, more than that glass of wine you'd probably rather drink, more than a serving of greek yogurt or a handful of almonds. It also has almost 29 grams of natural sugar-about 7 teaspoons!
Those extra daily calories may have you gaining over 17 pounds in a year!
You may think you will eat less food naturally but unfortunately we actually have found that humans eat just as much, if not more when they consume liquid calories before or during a meal.
If you are set on juicing keep this in mind: generally speaking, fruits have more natural sugar and therefore more calories than vegetables so to keep calories in check when juicing use mostly vegetables (beets and carrots are an exception since they are also high in sugars). Celery is particularly low in calories/sugar and could be added to juice to "dilute" the calories.
CONTOO LITTLE CALORIES?
On the other side of that spectrum if you take part in a juice fast or cleanse where all you are drinking is liquid juice, often the calories (and likely protein) are not substantial enough to sustain your metabolism or keep you from wanting to carjack the pizza delivery guy after a few days. Yes, you'll live to tell about it if you make it through but what will you have gained? An empty colon, low glycogen stores, and a great story about how miserable you were without chewing for a week. By the way, most of the weight you lost is water and unfortunately it will come back--quickly--when you start consuming an adequate diet again so don't get depressed about it-it's not your fault.
PRONO JUNK PLEASE
A juice fast, like any other very restrictive diet, not only provides structure (I am all about having a PLAN!) but by default it eliminates a lot of garbage in the diet which can't be bad. But, just perhaps it's not the juice that is making you feel so wonderful but the fact that you aren't eating crap while chugging coffee by day and wine by night? It's a possibility, yes?
Some juicers extract all the pulp/solid elements of the produce and leave only the water, sugar, and some nutrients. The fiber and other nutrients are discarded. I do not recommend any juicer that doesn't completely utilize the produce you put in it. But even if you have a juicer that includes the whole fruit or vegetable, do understand that the way your body will process these nutrients is quite different than if you eat these items intact.
Juicing is essentially "predigesting" these foods, in other words the juicer is doing the job that your mouth, teeth, and stomach would have done had you eaten the food. Because this "liquid" food is ready for your small intestine (where we absorb most all our nutrition) and contains virtually no protein or fat (unless you put nuts, seeds, or protein powders in it) it leaves your stomach faster. This means these juices will come with a very high glycemic index. (Learn about how the glycemic index is used manage weight and diabetes HERE). Thus, those on low-carb diets or with diabetes or metabolic syndrome would be advised to lay off the juice, even the vegetable juices.
I NEED A QUICK FIX!
I think even most juice advocates would agree that one cannot live on juice alone in the long term. But sometimes we need something to get us out of that slump and into that healthy lifestyle we are always dreaming we will have someday when we don't have work, or kids, or stress, or bills....
Juice fasts offer a plan. They don't require thinking, they eliminate all the stuff we tend to overeat (bread, meat, etc) so we don't have to do the hardest thing ever-eat those things in MODERATION. So if doing a fast will help you kick the habit and gradually add back HEALTHY foods in moderation when it ends I suggest you stop reading this blog and get your butt to the market (okay...maybe finish reading if you want to be nice.)
It makes obvious sense to only use homegrown and/or certified organic produce to make your juice. By using conventionally grown produce you will be getting wonderfully concentrated pesticide and herbicide residues to fuel your body. Unfortunately this is not always accessible or affordable for many people so I would advise against juicing if this is not possible. There is a much lower ingestion of these potentially harmful chemicals when we consume the whole fruit or vegetable simply because we would eat smaller portions.