Can you really gain 5 pounds in a day?

Fluid, can cause...: A rapid weight gain. In patients with a tendency to hold on to fluid (heart failure, cirrhosis, eg) who stop their meds and are not attentive to their diet a rapid weight gain can occur. Often, however, this a sign of inaccurate or imprecise scales.

You could: You could theoretically gain that much if you had staved yourself and sat in a sauna to reduce water weight and then thereafter started eating and drinking again to regain the weight you lost. You can easily put on some weight from water retention normally as well.

It is possible: It is possible in pathological condition to gain massive amount of weight . The scenario could be untreated pulmonary edema , or congestive heart failure , acute renal failure , massive transfusion with out urine output etc. In physiological ( normal condition) it is difficult if not impossible to gain a healthy 5lb per day.