I just read this on another website, as I am also weigh myself daily! Thought it might help explain things a bit better.
Can I Gain Weight Overnight?
Dear Dr. Katz:
I weigh myself every morning at the exact same time, before I have consumed anything. Some days I'm two to three pounds heavier than the previous day. Is it possible that I can gain this much weight in one day?
Response:
The answer depends entirely on what you mean by weight. Sure, you can fluctuate by two or three pounds from day to day, but not because you've gained or lost that much body fat.
To gain a pound of fat requires taking in 3,500 to 4,000 calories (and to lose a pound requires a deficit of the same magnitude). The only way you could put on three pounds of fat in one day would be to eat roughly 12,000 more calories than you need, which is usually enough food for an entire week. The pounds that appear and disappear so quickly on the scale are due to factors like water weight, which goes up and down constantly depending on how hydrated you are, and the amount of food in your GI tract at any one time.
So don't rely on a daily weighing. Focus instead on healthful living, and look to the scale only once a month (once a week if you're trying to lose weight) for the occasional reality check.
Dr. David L. Katz, M.D.
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FAQs: Healthy HabitsQ: I have been stuck at the same exact weight for a week now (I go up a pound one day and back down the next day) and I'm not sure why. I'm really frustrated, because I started lifting weights on Monday; I've been walking on all the days and I've really been trying to watch what and how much I'm eating. Do you have any thoughts or suggestions?
A: Do not worry about daily weight when you first start muscle building. You will accumulate water in these newly worked muscles—it takes time to get rid of this new water. So weight gain (or lack of loss) is water that will go away with time. It is the opposite of weight loss when you go on a low-carb diet where you'll get rid of water in your muscles. Water loss in muscles will come back like an unwanted boyfriend when you eat carbs again. So your steady weight now is actually progress.