'I'm a doctor... this is how to prevent Christmas weight gain and still indulge'

The real issue, according to Prof Joseph, is the hours we spend sitting around watching television and travelling to visit relatives.

When you're inactive for extended periods, your body struggles to handle blood sugar and fat properly.

"When you sit for long stretches, your body becomes less efficient at handling blood sugar and fat," he said. "That means more of what you eat is stored rather than burned."

Staying still for ages also poses a problem for your appetite signals, which means you end up reaching for snacks without really needing them.

"You're not eating because you're hungry - you're eating because you're bored," he said.

This becomes particularly tricky over Christmas when big meals keep coming while activity levels plummet.

But it's important to understand why the number on the bathroom scales increases, something Prof Joseph says most people get completely wrong.

"Most of what people see after Christmas is water weight and inflammation, not fat," he said. "Saltier foods, alcohol and less movement cause the body to hold onto fluid."

There is a warning, though. If you stay glued to the sofa for days and carry that habit into the new year, you could start seeing genuine fat gain.

The good news is you don't need to starve yourself to dodge festive weight gain; the trick is simply keeping active with easy habits like brief strolls after eating, getting up regularly, lending a hand in the kitchen, or doing some light stretching.

"Even light movement improves how your body processes food," Prof Joseph said. "It's far more powerful than skipping dessert."

One thing you absolutely shouldn't do? Punish yourself with extreme calorie restriction on Boxing Day or New Year's Day.

"Crash dieting on Boxing Day or January 1st often backfires," he said. "It increases stress hormones, drives cravings and leads to another binge."