You are gaining weight on holiday mainly due to water retention - not body fat increases. Your diet is probably containing a few extra Carbs in the form of Ice Cream, Chips, and Sandwiches when you are out and about… Carbohydrates are like a sponge.

Gaining weight on vacation is a common phenomenon often referred to as the "Travel Weight Paradox," and it is frequently caused by physiological shifts rather than just excessive calorie intake. Most immediate post-holiday weight gain is actually water retention triggered by high-sodium restaurant meals and long periods of sitting during travel, which causes fluid pooling and bloating. Furthermore, travel significantly disrupts your circadian rhythm and sleep cycles; this disruption can slow your metabolism and cause a spike in cortisol, a stress hormone that encourages the body to store fat, particularly around the abdomen. Your digestive system also relies on routine; irregular meal timings and dehydration can slow gut motility, leading to temporary weight increase and a "heavy" feeling. Additionally, even if you are walking more than usual, "incidental" calories from alcohol, sugary drinks, and unfamiliar ingredients can add up subtly. Usually, once you return to your normal routine, sleep patterns, and hydration levels, most of this temporary weight—which is often just water and glycogen—will naturally dissipate within 3 to 7 days.