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Gaining a few extra pounds over the festive period may feel like part of the tradition.
But research suggests that even short bursts of overindulgence can trigger measurable changes inside the body.
Scientists who have examined the effects of modest weight gain caused by richer diets over several weeks have found it can affect the heart, disrupt the immune system and alter the balance of bacteria in the gut.
The average Briton is predicted to consume around 6,000 calories on Christmas Day alone – more than twice the recommended daily intake for men and around three times that advised for women.
Despite the annual anxiety about festive excess, however, surprisingly few high–quality studies have examined how much weight people actually gain over Christmas – and those that have report mixed results.
Experts say indulging for a handful of days may lead to temporary weight gain, but is unlikely to cause dramatic or lasting changes once normal routines resume.
Much of the initial increase on the scales is down to water retention and the body storing extra carbohydrates, as well as simply having more food sitting in the digestive system – rather than a sudden build–up of body fat.
As eating patterns and activity levels return to normal, this weight typically drops away on its own.
Dr Duane Mellor, a registered dietitian based in Leicestershire, said long–standing assumptions about calorie intake and weight gain are overly simplistic.
‘For many years it was believed that consuming an extra 3,500 calories would automatically lead to a gain of around one pound,’ he said.
‘Research now suggests this isn’t necessarily true for everyone.’
Dr Mellor explained that how easily someone gains weight depends on a range of factors – including sex, body size, muscle mass, age and physical activity levels.
‘In general, men tend to gain weight less easily than women, partly due to differences in body composition and how fat is stored,’ he said.
‘Genes and certain health conditions, such as an underactive thyroid, can also play a role.’
As a result, two people eating the same number of extra calories over Christmas may see very different changes on the scales.
One 2009 American study tracking 195 adults over the six –to eight–week winter holiday period found an average weight gain of just 0.8lb (0.37kg).
A similar study in Sweden reported an increase of around 0.9lb (0.4kg) in non–obese adults over a two– to three–week Christmas break.
Among obese participants, however, changes were far more variable – ranging from a gain of more than 13lb (6.1kg) to significant weight loss over the same period.
More recent British research, published in PLOS One in 2020, followed more than 3,500 people from late November to the end of January and found an average weight increase of 1.35 per cent – equivalent to around 1.2kg (roughly 3lb).
Crucially, the researchers also found that weight began to fall again between January and March, suggesting much of the festive gain was temporary.
Other studies have found little or no change in overall body weight, but small increases in body fat – while some have reported no measurable change in either weight or fat, despite shifts in diet and physical activity.
Dr Mellor said people who feel they have overindulged should resist the urge to rush into extreme New Year’s resolutions.
‘I would encourage small, realistic changes to diet and physical activity that are easy to stick to,’ he said.
Evidence suggests even simple interventions can help prevent festive weight gain from becoming permanent.
In a study published in the British Medical Journal, researchers from the universities of Birmingham and Loughborough tracked 272 volunteers over two Christmas periods.
Half were given basic advice, encouraged to weigh themselves twice a week and shown how much exercise it would take to burn off popular festive treats. The rest were asked to carry on as normal.
Those given no guidance gained an average of 370g – just under 1lb – between November and January.
By contrast, those who received advice actually lost around 130g, or about a quarter of a pound.
Professor Amanda Daley, of the School of Sport, Exercise and Health Sciences at Loughborough University, said: ‘Our research shows that a brief intervention over the Christmas period can help prevent the small weight gains that accumulate over time and drive the obesity epidemic.’
In other words, a few indulgent days won’t undo a year of healthy habits – but what happens after Christmas matters far more than what’s eaten on the day itself.
