It's strange how when a theory seems plausible, numbers are easy to swallow whatever their size. Most people know they spend more time eating more food with more calories on 25 December than they do on an average day. So, whether they're told that they'll gain 1kg or told that they'll gain 4, they're generally persuaded.
But is it 1 or is it 4? The difference matters. It matters to the people selling you the Christmas food, it matters to the people selling you the New Year's diet and it normally matters to you, the person gaining the weight.
We compare the different claims here to see which seems the most reliable.
2.3kg (5lbs)
According to the British Dietetic Association, the average person will consume 6,000 kcals on Christmas day. But wait, there's more:
Over the festive period, which seems to kick off earlier and earlier every year, the average person could consume an extra 500 kcals per day, equating to a weight gain of around 5lb by the time we reach the beginning of the New Year
Those numbers seem dubious. Only 59% of the UK are Christian. Admittedly, many non-Christians celebrate Christmas (and there are Christians who don't mark Christmas with a mammoth meal) but the fact that not everyone eats the same meal on the 25th December still points to a problem: if the 'average' person consumes 6,000 kcals, then tens of thousands of people must be eating significantly more calories to counterbalance all those people eating a lot less.
In any case, is it really plausible that in the space of 24 hours people could be digesting the equivalent of 11 McDonald's Big Macs?
Almost certainly not. These numbers have been doing the rounds for years and they're almost always inflated. That's because diet companies like Forza Supplements get the chance to conduct surveys that give them exposure, newspapers like the Daily Star get to publish colourful infographics that make them look fact-focused and readers get to see pictures of a woman with her nipples peering out through a sheer dress for no apparent reason.
0.5kg (1.1lbs)
Luckily, various scientific studies (usefully summarised by a nutritional science graduate here) have sought to measure festive weight gain using more rigorous methods.
Each study reaches a different conclusion about the total amount of weight gained (you can explore them using the chart below) but every estimate is considerably lower than the numbers cited in the press each winter.
Months to lose
Unfortunately, the good news ends there. Though people consistently over-estimate the amount of weight they'll gain over Christmas ,that doesn't result in speedier or more effective action to undo the effects of over-eating.
Titled 'A Prospective Study of Holiday Weight Gain', research by scientists at the National Institute of Child Health and Human Development found that adults weighed more in February and March than they did in September and October of the previous year. More worryingly, their weight hadn't changed significantly by the following September.
They conclude that a large part of annual weight gain in the US is attributable to the festive period. That's an important finding when you consider long-term national trends in calorie consumption and weight.
Writing in the journal American Society for Nutrition, scientists showed that the average US adult weighs 8.6kg more than they did in 1970. They claim that adults would need to eat 500 fewer kcal each day to get back down to 1970s levels- although increased daily physical activity could achieve similar results.
Verdict
Your chances of gaining weight this Christmas are high. Though you won't gain anywhere near as many pounds as scaremongers would have you believe, it's still worth being cautious since evidence suggests that Christmas weight is particularly hard to lose.
