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In respect to this, can we gain 1kg in a day?
It's literally impossible (well, almost impossible) to gain over a kilogram of fat overnight. You would have to eat 10,500 extra calories in one day — that's almost 20 Big Macs!
Besides, how many extra calories do I need to gain weight a week?
An increase of at least 500 to 1000 calories each day will promote a one to two pound weight gain per week. It takes an extra 3500 calories to gain one pound of body weight.
Generally, overnight weight gain is usually caused from fluid retention. Consuming foods that are high in sodium (such as salt) can cause the body to hold on to water. It's not possible to gain a lot of weight over the course of one night.