According to nutritionist Vicky Warr, the more fat your body has, the slower your resting metabolism becomes, making it even easier to put on weight.
Ms Warr said that the average Briton consumes 7,000 calories on a typical Christmas Day - far more than the recommended intake.
"A normal daily intake should be about 1,800 for women and 2,200 for men," the expert revealed.
"But Christmas pudding and brandy butter alone can add up to 1,200 calories," she continued, noting that a person would have to run for two hours or complete a half marathon to burn off the energy in the pudding alone.
"It's no wonder there are so many detox diets and people rushing to join a gym in January," she remarked.