"You really undermine your body's immune system. You undermine your energy system," said Ellie Wilson, R.D.
Dieting can wreak havoc on your body but many people don’t care if they are not ready to bear their bodies on the beach, they may look for quick weight loss with a liquid fast.
"Fasting turns down your metabolism. This is not a good idea and it won't be protecting in the long run. It creates fatigue and dehydration," Wilson said.
Although Slim Fast products are not good alone, they're shown to an effective weight loss tool when used with food, not in place of.
"Slim Fast has good clinical data now they have changed a lot of how they create and work with meal replacement programs," said Wilson.
They come high protein, low fat, lactose and gluten free, as well as low carb.
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"They are good in that they help you manage proteins and carbohydrates. They are often pretty well balanced which is also good is many of them are including fibers, particularly soluble fibers that can help you protect heart health, helps manage hunger," said Wilson.
Food plans for weight loss are on the label.
"Look at the plans, typically they something like in a Slim Fast product they will recommend two of their meal replacement shakes that will be some where in between 180 and 220 calories per shake plus a sensible lean and green dinner, meaning lots of vegetables and low fat meats," Wilson said.
There are two protein options -- whey versus soy. Soy is a plant source, whey is a liquid component of milk. Which one is better for weight loss?
"My pick would be the whey. Whey proteins actually have some data behind them showing that they do help us reduce body fat percentage," Wilson said.
Most plans vary. Some recommend 15 to 20 grams per meal per day. Protein bars are also popular as meal replacements.
So whether or not you are taking protein powder or a protein bar, a word of caution, know how many calories are in each.
Although it's less filling than a shake, one bar can add up to a whopping 370 calories. And even if you do not need to lose weight and but still enjoy the shakes, drink up. Reports show those who are on the shakes tend to have a higher level of calcium.