Only minor changes are needed to make weight gain. Consider one extra soda per day (150 Calories per day) and a bit less walking around (walking a mile for an adult is about 100 Calories, those kids are smaller but probably used to walk more going from class to class at school, so let's put it at 100 Calories).
250 Calories x 30 days in a month = 7500 extra Calories a month. 3500 Calories is about a pound of fat, so that's roughly an extra 2 pounds a month, or 24 lbs per year, assuming their exercise and food consumption otherwise stays the same as it would have. Throw in another extra snack or something, they could reach an extra 36 lbs in a year.
Easy to see how it adds up. Also adds up for adults.
A large egg is 78 calories. Over the course of a year a single egg per day adds up to 8 lbls. However, many people are able to maintain a remarcably consistent weight without thinking about it.
This suggests that humans have an innate ability to regulate body weight. The obesity epidemic can't be explained by a simple calorie analysis; it needs to account for why our innate ability to regulate body weight isn't working.
At this point, I think it is pretty well established that added sugar is a significant culprit, so focusing on soda is still probably reasonable.
I agree with you entirely on sugar. My point was more about small barely noticeable changes adding up easily. In this case the kids may have eaten until they felt full as they normally did, which is how their bodies normally regulated their weight, but because they moved slightly less and may have had some extra snacks or soda that didn't really satiate them, the extra calories added up.
American adults also gained about a pound and a half per month on average as well. Maybe not noticeable after one month, but definitely after twelve. Same situation, you're used to having a certain size breakfast, lunch, dinner, snack, drink, or whatever, your body has been used to doing that for however many years, but then you move slightly less and you're slightly closer to your kitchen all the time.
Humans have an innate ability to eat until they're full. However, due to food choices, the amount of calories eaten before feeling sated can be highly variable. Modern processed food is far more calorie dense than our bodies "fullness" sensors are able to process, like a laser pointer with a cat. Some cats can't help themselves and lose their shit over them. Others can't be bothered.
250 Calories x 30 days in a month = 7500 extra Calories a month. 3500 Calories is about a pound of fat, so that's roughly an extra 2 pounds a month, or 24 lbs per year, assuming their exercise and food consumption otherwise stays the same as it would have. Throw in another extra snack or something, they could reach an extra 36 lbs in a year.
Easy to see how it adds up. Also adds up for adults.