Does protein shake help in maintaining fit body without weight gain?

MostlyMarketing, not. health. Best learn how to eat foods not highly processed for profit, addiction, convenience (ie most low fat foods). Healthy human foods are high saturated fats & protein, with very low carbohydrates, esp. processed from grains, fruits, etc. Study: etc.
It's marketing. Your weight is calories-in versus calories-out. Protein shakes are pricier than lean meat and far pricier than protein supplement powders; they will protect against your becoming protein deficient, which does happen to some dieters. Make friends with folks who exercise and get into it yourself; get used to eating only to satisfy hunger and avoiding very sugary, salty, and/or greasy stuff. Cheers.

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Is Apetamin good to drink if youre trying to gain weight? Also is it good to mix with weight gain protein shake and working out 4 days a week?

Caveat emptor. Cyproheptadine in this drug can cause confusion, hallucinations, unusual thoughts or behavior; seizure (convulsions); ringing in your ears; fast or pounding heartbeats; easy bruising or bleeding; urinating less than usual or not at all; or pale or yellowed skin, dark colored urine, fever, or unusual weakness. mild drowsiness, dizziness, And much more. . .Are you willing to risk it? Read more...