Festive weight gain often temporary, studies show small, reversible changes

The holiday season can produce measurable, if usually temporary, changes in body weight and composition. Scientists say even modest weight gain from richer diets over a few weeks can affect the heart, immune system and gut bacteria. 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 — underscoring how festive eating patterns can diverge from normal routines.

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 buildup of body fat. As eating patterns and activity levels return to normal, weight typically falls away on its own. Researchers say that much of the festive gain is temporary and that the post-Christmas period matters more than the day itself.

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 noted. “Research now suggests this isn't necessarily true for everyone.” He added 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.”

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As a result, two people eating the same number of extra calories over Christmas may see very different changes on the scales. A 2009 American study tracking 195 adults over the six- to eight-week winter holiday period found an average weight gain of just 0.8 lb (0.37 kg). Scientists who have studied the effects of a “small amount” of extra weight gain from feeding people a richer diet over several weeks reported effects on the heart and immune system, and a Swedish study noted about a 0.9 lb (0.4 kg) increase in non–obese adults over a two- to three-week Christmas break. Among obese participants, however, changes were far more variable — ranging from more than 13 lb (6.1 kg) gained 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 1.2 kg (roughly 3 lb). 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 advised a cautious approach after indulging. “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.” 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 370 g — just under 1 lb — between November and January. By contrast, those who received advice actually lost around 130 g, 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 short, 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.