Our body is immensely complex and in fact a whole range of factors may influence fat storage. The bottom line is that no matter the cause of an increase or reduction in fat mass, it must be explained through its influence on glucose and/or insulin metabolism.
On our way to understand overweight we have (that is I have, but I thought you should be included) defined overweight as excess storage of energy in fat tissue and we asked the obvious; what factors control the storage of energy in fat tissue? The answer is glucose (a simple sugar that when it’s in our blood is called blood sugar) and insulin (a hormone). The reason glucose and insulin are the prime regulators of fat storage has to do with the function the fat tissue holds as a regulator of blood glucose levels and how insulin regulates energy storage.
We know that lifestyle factors affect fat storage. We get fat and thin by doing different things in life. We all know this. We get fatter during Christmas and lose weight (although usually not permanently) when desperately clinging to our new year’s resolutions. Scientists are talking about the obesity epidemic. If humans suddenly gain a lot of weight (and we are gaining weight) during a few decades, than we know for certain that it is caused by some lifestyle factor rather than by a genetic mutation or some other. So by asking the second obvious question we will actually know what causes overweight and also how to treat it or avoid it.
What lifestyle factors affect our glucose and insulin levels?
Although the answer to this is slightly more complex than the last one, it is also, from a scientific point of view, quite easy to answer. There are two main influencing factors. One is physical activity (also known as exercise) and the other is dietary carbohydrates. You might be thinking that this is some sort of Atkins tribute based on a positive personal experience with carbohydrate restriction. It is not. This is as strictly scientific as is gets, and neither Atkins nor any other commercial weight loss diet has anything to with this. Dietary carbohydrate is simply the number one lifestyle factor influencing blood sugar and consequently insulin levels thus increasing energy storage in fat tissue. In addition carbohydrate provides structural molecules for the formation of triacylglycerol.
So here it is. The answer. This is all somewhat simplified, but still as close to the truth as we can get. If you want to lose superfluous fat tissue, restriction of dietary carbohydrate and or exercise (doing both does give the best results) is the best way to do this. If you are gaining weight (as fat tissue) it means that you are taking in more carbohydrates than your body can use in your current condition. Exercise will make our body tolerate more dietary carbohydrates and trough its effect on skeletal muscles, will reduce the risk of high blood glucose levels. A fit muscle will absorb blood glucose much more efficiently than an unfit one.
Time for a small digression here. Remember that insulin is released in our body as a direct response to our blood glucose levels. Only carbohydrate (not fat, not protein) has any real influence on our blood insulin levels. Some carbohydrates increase blood sugar and insulin more than others. How large this increase is, is measured in glycemic index or glycemic load.
So, any factor that increases our blood sugar or insulin levels will increase the storage of fat and thus reduce the use of fat energy. Calorie restriction usually also causes a drop in glucose and insulin levels (largely because of lower total intake of dietary carbohydrates and a drop in the intake of the carbohydrates with high glycemic index), and will thus also often result in weight loss (although as it seems, almost never permanently). Many factors can affect our glucose and insulin metabolism. Dietary carbohydrates and exercise (most likely in that order) are the most important lifestyle factors. But other hormones such as thyroid hormones and cortisol also influence our glucose and insulin metabolism and thus our fat storage.
I’ll try to sum up again. Overweight explained through one definition and two questions goes like this:
Definition: Overweight is excess storage of energy in fat tissue.
Q: What factors influence storage of energy in fat tissue?
A: The two most important are glucose and insulin.
Q: What lifestyle factors influence our glucose and insulin levels the most?
A: Dietary carbohydrates and exercise.
If you want to gain weight by increasing the size of your fat tissue, then you should lay back and relax. Be passive and eat a lot of carbohydrates, especially those who influence blood glucose the most. If you want to reduce the size of your fat tissue, do the opposite.