Building up the endurance needed to make it all the way to the finish line of a marathon race requires daily long distance runs that amount to several thousand calories burnt per week. For most marathoners, this demanding training usually goes on for months on end. So, it only makes sense that anyone crazy enough to take on a marathoners training will end up looking like a toothpick by the time he/she crosses the finish line of the 26.2-mile distance. However, that is not always the case. 

Cases of individuals gaining weight while training for a marathon are becoming more and more common. So, why do some people gain weight when training for a marathon? Well, as it turns out, there are actually several logical reasons as to why weight gain can occur during marathon training. Read on to get a better understanding of why that happens. 

Muscle Buildup:
Any form of intense physical activity done on a regular basis results in the growth of muscles. In the case of running, this muscle growth occurs in several areas including the stomach, thighs, butt, and lower legs. From a fitness point of view, a lean body is a great advantage as it helps improve on running performance. On the scales however, it is a totally different story. Muscle is denser than fat despite the fact that it takes up less room on the body. Therefore, if you are a runner in training who is weighing heavier on the scales despite getting trimmer, the increase in muscle mass could be to blame. 
Overeating and Over-hydrating:
Although marathon training is often rigorous, it sets you up to make poor dietary decisions that can wipe out all calories burnt during a training session. For one, long runs leave you feeling very hungry and therefore put you at a higher risk of engaging in mindless eating. The fact that some people do not eat soon enough after a run only works to make matters worse. The longer you wait to replace what was lost during training, the hungrier you will get and the more attractive sugary and fatty foods will become. So, at the end of the day, you will end up eating more than you should or overindulging in junk food. 

Cautious eaters can also fall on the wrong side of proper dieting. Carbohydrates and liquids are the most important foods for a runner. Carbs provide the much-needed fuel for intense training while liquids ensure adequate hydration, which is critical for optimal performance. In a bid to ensure proper nutrition, a long-distance runner in training can end up eating more carbs than needed or over indulging in sugary sports drinks or fruit juices. 

Another area where long distance runners can get into trouble is thinking they have earned the right to treat themselves to whatever their hearts or stomachs desire after an intense training session. Unfortunately, these desires are usually calorie loaded junk foods.
Changes in Metabolism:
Marathon training is designed to enhance endurance and as such mostly involves running regularly at a steady, moderately intense pace. The downside to this form of training is that the body is designed for efficiency and adapts quickly to changes. So, if you engage in the same routine day in day out, your metabolism will eventually adapt in such a way that fewer calories will get burnt with the same exercise output. This can easily lead to weight gain even if the body is getting leaner. 

Buildup of Glycogen Stores:
Another way in which the body adapts to a marathon-training regimen is by increasing its ability to store muscle glycogen. While glycogen stores are great from a performance perspective, they tend to add some weight to the body. This is because with every ounce of stored glycogen, the body also stores 3 ounces of water. The stored water will show up as extra weight on the scale. 
While a good number of marathoners aim at maintaining current weight or dropping a few extra pounds when they begin training, weight gain is more often than not the resultant effect. Depending on the root cause of the gain, the extra weight can be a good or bad thing. For instance, stored glycogen and increased muscle mass is a good thing while weight gain triggered by overeating is something that can and probably should be fixed. So, if you are a marathoner gaining weight from training, it is essential that you establish the cause and make any required changes to your training regimen or eating habits.