You can lose weight on their system, because it is simply introducing a calorie deficet. For example, from the sample kit you could have the following during a day (all calorie figures from Jenny Craig's):
- Breakfast - Cinnamon French Toast - 240 calories
- Snack - Yogurt Bar - 130 calories
- Lunch - Turkey Burger - 280 calories
- Snack - Cinnamon Twists - 120 calories
- Dinner - Macaroni & Cheese - 230 calories
- Dessert - Walnut Brownies - 180 calories
The total daily calories is 1180. That might be a little more if you have a little fruit or a side salad with one or two of the meals. It might also be less because you only receive one dessert for every three days of meals. Anyone who is accustomed to 2000 calories or more a day (average being estimated to 2600 a day in the United States (livestrong)) will definitely be running a calorie deficet, and therefore will begin to lose weight.
Last year I personally purchased two 3-day trial kits from Jenny Craig's website and proceeded to follow the diet for six days (as part of my 52 diets over the course of a year goal). In addition to eating the low calorie meals I also jogged 4 days that week.
I ended up losing 3 pounds overall, although at one point I was as much as 5 pounds down from the previous week. The diet is very calorie restrictive and I was extremely hungry the entire week. The food is tastes good, but the portions are quite small.
The diet is also quite expensive. On the normal plan I believe you pay between $80-$100 for the prepackaged meals. This is in addition to the cost of one of their programs.