Question

how much protein need to be consumed daily by a 20yr man

Answer 1

aloha user!!

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for not increasing muscle mass:

=> the body weight gives some idea about how much protein needs to be consumed.
=> Also if u do extra physical work you have got to add protein for that.
=> consume 0.8 g of protein per kg of your body weight.
=> for example if your weight is 45. you are going to multiply it by 0.8 that would give 36g of protein requirement. 

for increasing muscle mass:

=>  you are going to take 2.2 grams / kg of your weight.
=> for example a person of body weight 45 kg wishes some muscle gain. he/she is going to multiply 2.2 grams by 45 to get protein requirement of  around 100 grams. ( approx )
=> take enough fats.
=> workouts + intake of carbohydrates along with creatine will ensure stronger muscles.
=> if you want good muscle gain choose effective exercises.
=> doing certain exercises can burn away your muscle gains so be careful.

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hope it helps :^)