A bite of a mince pie here, an extra helping of pudding there... It all seems so innocent at the time.   But as the calorific Christmas extravaganza continues apace, it might be worth bearing in mind that any extra helpings could still be clinging to your thighs and tummy in March.   A study has found that extra pounds we pile on over the festive period will take a depressing three months to shift.   Excessive: The average Briton consumes 2,300 calories in their Christmas lunch alone - that's 115% of a woman's RDA and 92% for a man.   According to research by MSN, for Christmas lunch alone the average Brit feasted on two to three servings, consuming 2,300 calories across one meal. That accounts for 115 per cent of the recommended daily intake for women, and 92 per cent for men - and that's before taking into account the nuts, chocolates and alcohol we consume.   According to the MSN survey, three quarters of those polled admit they over-indulge with abandon at Christmas, knowing they will suffer the consequences later as their waistbands begin to bulge.   Nutritionist Dora Walsh said the key to not putting on weight over Christmas might be to eat slowly. Keep moving over the festive season to prevent too much weight gain. She said: 'You can still enjoy your food at Christmas without going overboard and gaining weight.   "They key is to eat slowly and enjoy what you are eating without over filling, and also stay active over the Christmas period to help burn off any excess calories.   'It's about moderation, which will help avoid the extremes of gorging at Christmas and starving after.   'Any post-Christmas diet plan needs to be sensible and balanced because crash diets don't work and quick fixes won't last.'   Office worker Rachel Taylor told how she knows she will pile on the pounds thanks to festive excess - and that it will be a while before she manages to slim down again.   The 27-year-old, of Southampton, said: 'I like to keep trim but I'm not obsessive and I do make an exception for Christmas.   'Overindulging usually means I put on a few pounds and, because I don't believe in crash diets, it usually takes a month or two for me to lose it all.' in this book you will discover how the holiday weight gain and the sugar cravings.