Obesity doctor shares the simple reason you gain weight over Christmas – and it’s NOT because of overeating

You can’t blame festive weight gain on eating scores of mince pies, fistfuls of Celebrations and double helpings of roast potatoes, a leading doctor has claimed. 

Previous studies have estimated that Britons devour as many as 6,000 calories on Christmas Day alone, three times the NHS’ recommended allowance of 2,000 for men, and four times the 1,500 recommendation for women.

But Professor Franklin Joseph, Consultant Physician in Endocrinology, Diabetes, & Obesity and founder of Dr Frank’s Weight Loss Clinic in London, says festive food alone rarely leads to fat gain—but sitting around doing nothing does.

He said: ‘People massively overestimate how much damage a few days of richer food can do. 

‘Fat gain happens slowly. You don’t gain pounds of fat from a handful of mince pies.’

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.

‘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. 

‘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.’