Many people gain weight during the cold months even if they exercise and watch what they eat. One reason may be that you burn fewer calories when you exercise in cold weather than you do when it's hot. The hotter it is, the more extra work your heart must do to prevent you from overheating. More than 70 percent of the energy produced by your muscles during exercise is lost as heat. So the harder you exercise, the hotter your muscles become. In hot weather, not only must your heart pump extra blood to bring oxygen to your muscles, it must also pump hot blood from your heated muscles to your skin where heat can be dissipated.
On the other hand, in cold weather, your heart only has to pump blood to your muscles and very little extra blood to your skin to dissipate heat. Your muscles produce so much heat during exercise that your body does not need to produce more heat to keep you warm. So your heart works harder and you burn more calories in hot weather. This information should not discourage you from exercising when its cold, because staying in shape is a year-round proposition. However, it may help to explain why so many people find the pounds creeping on in the wintertime, even when they stay active. More on weight loss
That's a good point. Another issue for me is the decreased number of daylight hours. It curtails my normal outside activities. Also, I cook more comfort foods during the winter. Especially homemade bread. Winter definitely adds to my waistline.