Calculate proper dosage of supplements to take
Here is the list of things I would improve:


for handling the proper answer on the "Are you going to the gym today?", I would start the while loop until "Yes" or "No" is entered
put the execution logic under if __name__ == '__main__':
define a message template with appropriate placeholders - format it with "day on" or "day off" objects and the entered username
define constants in upper case as per PEP8
let the "dosage" function return the dosage object - print the message outside of the function - for the better logic separation 
add meaningful comments and docstrings


The code with all the suggested changes applied:

DAY_ON = {
    "BCAA": "10g",
    "White": "50g",
    "Glutamine": "20g",
    "Vitamins": 10
}

DAY_OFF = {
    "BCAA": "5g",
    "White": "25g",
    "Glutamine": "10g",
    "Vitamins": 5
}

MESSAGE_TEMPLATE = """
{username}, you have to take:
    BCAA in dosage of: {obj[BCAA]}
    White in dosage of: {obj[White]}
    Glutamine in dosage of: {obj[Glutamine]}
    Vitamins in number of: {obj[Vitamins]}
"""


def get_dosage(is_training_day):
    """Returns appropriate dosage depending on whether it is a training day or not"""
    return DAY_ON if is_training_day else DAY_OFF


if __name__ == '__main__':
    username = input("What is your name?\n")

    while True:
        choice = input("Are you going to the gym today?\n")

        if choice in ('Yes', 'No'):
            break
        else:
            print("You can only enter 'Yes' or 'No'.")

    dosage = get_dosage(is_training_day=choice == 'Yes')
    print(MESSAGE_TEMPLATE.format(username=username, obj=dosage))


Demo:

$ python3 test.py 
What is your name?
Chubaka
Are you going to the gym today?
What?
You can only enter 'Yes' or 'No'.
Are you going to the gym today?
No

Chubaka, you have to take:
    BCAA in dosage of: 5g
    White in dosage of: 25g
    Glutamine in dosage of: 10g
    Vitamins in number of: 5




If you are using Python3.6+, you can also use f-strings for string formatting:

dosage = get_dosage(is_training_day=choice == 'Yes')
print(f"""
{username}, you have to take:
    BCAA in dosage of: {dosage['BCAA']}
    White in dosage of: {dosage['White']}
    Glutamine in dosage of: {dosage['Glutamine']}
    Vitamins in number of: {dosage['Vitamins']}
""")