Are you a bear? It depends what you call "natural". Gaining more weight is a direct result of eating more calories than you burn. Anything you do to alter this balance will cause a change. So, In the fall when the days are shorter and your physical activities could decrease the "natural" tendency is to gain weight. To keep from gaining weight at this time of year, increase your physical exercise or cut back in your consumption of food and calorie rich drinks.

People do not hibernate, so we do not have to build up stores of fat to see us through the winter. However, it could be considered normal for people to gain weight during the winter months, how much depends on the individual and where one lives. A person's activity level is reduced in winter because of the weather conditions and the shorter days. Outdoor sports, except for activities such as skiing, skating, or snowshoeing, are non-existant. You cannot walk around your yard pushing a lawn mower every week and it is a long walk to the store in the deep cold. When the outside temperature falls to -30C or lower, you do not spend any more time outside than you have to. You have not lived until you have experienced a wind chill of -60C: bitter, harsh weather. Working in the cold can be unpleasant, since it is hard on the lungs and uncomfortable when you sweat - it also freezes on your facial hair. Sitting around a fireplace for warmth does not burn any calories, regardless of how nice it feels. However, any small weight gain you experience is reversed in the spring when you resume normal activities.

���Temperatures start to fall, the days get shorter -- and people put on weight. The reason is that when the weather gets colder, the blood vessels in the skin contract, and as the extremities cool, blood moves to the center of the body. There it stimulates internal organs like the digestive tract, and their activity increases. When the digestive tract grows more active, appetite increases, and if the appetite increases, so does weight. That change is entirely natural. It's a time-honored belief that fall is a time for horses to get fat, and for people as well. Seen from the perspective of Eastern medicine, fall is the season to store energy. It is the time of the most nutritious food, when we harvest all the grains and fruits that have stored up a year's worth of energy. In spring and summer, people are afflicted with a lack of energy thanks to the unstable balance between yin and yang energy, but fall is a time for our bodies to stabilize. As the cool wind blows, the sweat cools and the yin-yang balance in our bodies is restored, and we begin to store up fat for winter. But that also means that people who want to lose weight have to be particularly careful in the fall. That is true even if people now wear long sleeves in summer and short sleeves in winter because they spend most of their time in temperature-controlled environments that erode the distinctive nature of the seasons. Our lives may be less in thrall to seasonal changes as they once were, but some constants remain.” http://english.chosun.com/w21data/html/news/200510/200510270002.html

As Red John says, humans do not hibernate. However, horses, deer, and other animals do not hibernate either but their bodies do undergo changes in preparation for the cold weather - they put on weight and grow thicker coats. Even dogs that spend a lot of time outdoors will do this. Humans differ from other animals in many ways, but in physical adaptation to external forces like weather , we are all very similar. This is, of course, compounded by things like Thanksgiving, Christmas and the start of hockey, football, basketball season(s) - at least on television! - when we become sedentary and, on occasion, overindulge in a ritual way, gaining weight that is just harder to get rid of in the winter. I would be interested to see if any studies have been done on this subject and if any co-relation exists with the degree of coldness in particular geographical areas; does it happen less in California and Florida than in Alaska and Nunavut, for example.

Gaining weight as winter approaches and durng winter is quite normal.Because external temperatures are low during this time and our bodies need to give out more heat,the metabolism rate of our body increases.For cells to respire at a higher rate,there has to be a higher supply of food,meaning that we have to consume a lot more food than usual which therefore leads to a slight weight gain.

Although it seems counterintuitive, the only time I have succeeded in losing wright is in the winter or early spring. I always get excited thinking that by fall, I will be at my goal weight but I have always fallen "off the wagon" after hitting a summer plateau. I have wondered if there might be a correlation between a diminishing metabolic rate as summer progresses to ensure adequate fat deposits to make it through winter. It is only recently in human evolution that we have had access to abundant foods during the winter months. Calories in or out, it makes some sense that we might begin to build fat stores just as our animal cousins do.

Sorry guys I have to add to this debate! Since January last year I have drastically changed my diet to lose 2 stone of excess weight. I'm also thoroughly active now, training to run 10k events at least twice a week. Now I wouldn't say my diet is under strict calorie control but my eating pattern has stayed constant since I lost my excess weight, and my weight has been stable since then too until...the winter set in and the first chills appeared in the air. without a change to my routine I've put on a pound of weight. I attribute this to a metabolic adjustment linked to seasonal change. I believe it is true that in colder conditions your body adjusts to keep weight on – obviously this will vary from individual to individual due to genetics etc I gues I have to increase my mileage in the winter!

Humans traditionally put on weight as winter approached. They needed the fat reserves to keep warm and they needed the reserves to get through lean times. That being the case, we have to be careful not to overdo it. We do not need the extra fat (we have warm clothing) and nor are we likely to run out of food, so we have to be careful at this time of year.