In theory, to gain one pound of body weight per week, you d need to consume an additional:
a. ten grams of fat per meal.
b. six servings of protein per week.
c. 500 calories per day.
d. 1000 calories per day.

In theory, to gain a pound of body weight per week, one must consume an additional 1,000 calories per day. An approximate 3,500 calories is equivalent to a pound of body weight. If a person continuously consumes more than 3,500 calories over a period of time it is expected that this person will gain 1 pound of weight. 

Nutritionists usually suggest that an underweight person should add between 500-1,000 calories per day to encourage a 1-2 pound increase in body weight. Because being underweight sacrifices one's immunity and overall health.