Weight loss experts used to warn us that on average, Americans gain seven pounds over the holiday season from Thanksgiving to New Year. Fortunately, more recent studies have dialed that number back to what looks like a more manageable pound or so. For example, one 2016 study published in the New England Journal of Medicine found that in the U.S., we put on an average of 1.3 pounds after the holidays. (For those who are already overweight, other studies have found it can be as much as five pounds or more. We are not alone. The researchers, from Tampere University of Technology in Finland, found that packing on pounds over the holidays isn’t a uniquely American problem. Germans gained 1.8 pounds and the Japanese put on an extra 1.1 pounds over Christmas. Everyone also overdid it on national holidays, such as Thanksgiving, Easter and Golden Week, a Japanese holiday that occurs in May. But it’s actually worse than it sounds. A full half of the pounds that Americans and their friends p...