So we are all aware of the need to exercise and no doubt we have all heard exercise helps you lose weight, but how? Does it just evaporate, turn to liquid and ooze out or disappear into thin air? Well, it isn’t quite that simple, however the last statement may not be that far from the truth, to a point.
Firstly, let establish that when we are talking about losing weight, we are actually referring to losing fat right? We certainly do not want to lose muscle as this is valuable for movement, metabolic rate and a host of other daily functions such as digestion and pumping blood. So ultimately it not specifically weight loss we want rather reduction in fat.
So, when we exercise how does this assist with fat loss? Firstly fat is a very valuable source of energy. For every gram of fat we have in our body, it contains over twice as many calories as protein and carbohydrates. Therefore, in times of famine, having extra fat was a valuable asset. Today, however it is not so necessary and sits in our body causing a host of unhealthy physiological responses.
Ok, so we all are aware during exercise our body requires extra energy to perform the various activities. Our body’s first choice of energy is glucose as this is simple and easy to utilise, however our body and muscles can only store minimal amounts of glucose before these stores need replenishing. The body then has to rely on other energy forms, which can be either fat or protein. Both of these can be broken down to be used as energy. Without getting too scientific all 3 macro nutrients can be broken down to be used in metabolism in the Krebs cycle and electron transfer chain to create energy. The Krebs cycle and electron transport chain is an aerobic pathway and requires the use of oxygen to perform the specific steps required to create energy. Generally speaking only glucose and glycogen are utilised for energy during anaerobic exercises, which explains the reason we see so many cardio machines with ‘fat burning’ zones (roughly 65% Maximum Heart Rate). In relation to this, and getting off track for a minute, you may well be thinking that if you want to lose fat, it is necessary to perform lower intensity cardio within your ‘fat burning zone’, but don’t jump to that conclusion yet. Yes, during a fat burning zone workout you will use more of your fat stores as energy as the body has the time and the oxygen to go through the process of utilizing them, however, once you stop the workout your body tends to return to its resting metabolic rate rather quickly without having a huge impact on your overall metabolism. However, if you happen to engage in higher intensity exercise, such as HIIT style training, your body will utilise more glucose to perform the activities but you will create and effect called Excessive Post-exercise Oxygen Consumption (EPOC), which is basically an oxygen debt. This results in your body having elevated metabolism for a number of hours after your workout and the body has to replenish this debt and utilises many calories to bring the body back to homeostasis. Energy from fat is utilised during this process and overall you use more calories, resulting in a reduction in fat storage.
So getting back to what happens to fat. As mentioned above, fat is put through the Krebs cycle and electron transport chain, otherwise known as the oxidative phosphorylation pathway. This pathway has a number of steps that are rather complicated, but in essence the end result of this pathway is the creation of ATP which is stored energy, water and carbon dioxide. So, when we exercise fat creates energy and we breath out the by-product as CO2, water in the body is used for a number of bodily functions and also for sweating to cool our body, so in essence perhaps our fat does disappear into thin air and oozes out of our body, just in a slightly more technical way ;-).