Blaming holiday weight gain solely on indulging in festive treats like mince pies, chocolates, and extra servings of roast potatoes might not be entirely accurate, according to a top medical expert.

While previous research suggests that on Christmas Day alone, people in the UK can consume up to 6,000 calories—significantly surpassing the NHS recommendations of 2,000 calories for men and 1,500 for women—the true culprit of those extra pounds may not be the food itself.

Professor Franklin Joseph, an expert in endocrinology, diabetes, and obesity, and the founder of Dr. Frank’s Weight Loss Clinic in London, argues that it’s not the festive fare that leads to weight gain, but rather the lack of physical activity during the holiday season.

“People often overestimate the impact of a few days of eating richer foods,” Professor Joseph explains. “Weight gain is a gradual process. A few mince pies aren’t going to result in significant fat accumulation.”

In truth, the primary factor contributing to the holiday weight increase is the tendency to spend extended periods in a sedentary state—relaxing for hours, or even days, without much physical activity. This lifestyle choice, rather than the seasonal delicacies, is likely responsible for the holiday bulge.

In reality, the biggest cause of packing on the Christmas podge comes from a habit that millions of people don’t think twice about—sitting around idly for hours, if not days, on end. 

Inactivity has a huge detrimental effect on our metabolism, increases fat storage and drives overeating, all of which cause weight gain, even if what we eat over Christmas is kept in check.

Professor Joseph explained: ‘Hours spent sitting—watching TV, socialising, travelling or lying on the sofa—have a far bigger impact on weight than the food itself.

Come December, it’s hard to avoid platters of fatty, calorific foods

‘When you sit for long stretches, your body becomes less efficient at handling blood sugar and fat. 

‘That means more of what you eat is stored rather than burned.’

During the Christmas break, flopping down on the sofa to watch a box set or movie is the norm, as is tucking into an accompanying snack platter.

Inactivity can also affect the body’s natural appetite signals, making people more likely to eat mindlessly, making the last few weeks of December a perfect storm. 

‘You’re not eating because you’re hungry, you’re eating because you’re bored,’ he warned. 

So what is the solution to not ending 2025 with a few more belt notches (in the wrong direction) and a silhouette that makes Santa Claus look like Kate Moss? 

Professor Joseph says it’s not cutting calories or avoiding treats, it’s something far more simple. 

‘You don’t need to diet over Christmas,’ he said. 

Staying inactive for days on end can tip the balance toward real fat gain

‘You just need to keep your body moving.’

And before you start digging out your old Davina McCall fitness DVDs, you don’t even really need to break into a sweat to reap the benefits.

Professor Joseph says that taking a short walk after meals, standing up regularly, helping with cooking or clearing up and making time for regular, gentle stretching can help keep unwanted pounds at bay. 

He added: ‘Even light movement improves how your body processes food. 

‘It’s far more powerful than skipping dessert.’ 

It can be hard not to panic when you weigh yourself for the first time after Christmas, but Professor Joseph says that most of the time the body is swollen with fluids, not globules of yellow fat. 

However, he is quick to remind people that staying inactive for days on end can tip the balance toward real fat gain if the sedentary vibes continues into January. 

He said: ‘Most of what people see after Christmas is water weight and inflammation, not fat.

‘Saltier foods, alcohol and less movement cause the body to hold onto fluid.’

He also urges people to think twice about earmarking Boxing Day or New Year#s Day as the first day of a new, strict, healthy eating plan.  

‘Crash dieting on Boxing Day or January 1st often backfires,’ he said. 

‘It increases stress hormones, drives cravings and leads to another binge.’

‘Mince pies don’t cause Christmas weight gain – it’s caused by stopping all movement and then punishing yourself afterwards.

‘Enjoy the food, stay lightly active and return to normal habits in January. That’s how weight stays stable long-term.’