Unfortunately, we have to say from the outset that the mechanisms by which you resist weight gain are FAR less efficient than their opposites. This makes a lot of sense from an evolutionary standpoint – at no other point in history has long-term overconsumption been an issue, and therefore it could be argued that our current environment is drastically different to the one in which we evolved. Food availability, marketing, calorie density and palatability is significantly higher than ever before, and we aren’t equipped to deal with it as efficiently as we’d perhaps like. There is some amount of fightback, however.
As you eat too much, a number of different things happen:
Unfortunately for us, the above is largely mitigated by modern lifestyles. Hyper-palatable foods which cause extremely large reward responses in your brain and very calorie dense, low fibre processed foods make it easy to overcome a reduction in appetite while ‘cancelling out’ reduced absorption. Liquid calories make this a lot worse and of course office jobs, internet shopping and on demand TV make it easy to keep your overall activity nice and low.
The magnitude to which the above all affect you as an individual is very difficult to predict. That guy you know who ‘can eat anything and never gains weight’ almost always has a very wasteful metabolism which means he’ll fidget like crazy. On top of that, he will generally eat a lot for a few days but also regularly skip meals and be very inconsistent with his food intake, the result being he moves more and eats less than may be immediately obvious. In overfeeding twin studies where they take genetically identical twins and keep them in a ward with other twins to feed them up and see what happens, the difference in weight gain between the twins is tiny whereas the difference between pairs is huge – some gain a lot of weight, others lose weight, all while eating an identical calorie surplus. This shows that your genetic makeup has a huge impact on how you will respond to a given food intake, and of course beyond that your genes and your mentality will impact your response to the modern food environment.
When you think about it, a combination of the modern world and our inability to handle it makes it difficult to argue that it’s surprising we have the obesity issue that we do.