Gaining weight is troublesome for most people, but especially for athletes who engage in a substantial amount of exercise to improve their sports performance or fitness enthusiasts who are using exercise as a strategy for losing weight.  You would think that exercising for 10-plus hours a week (such as in the amount of time required to train for long distance events such as half marathons or more) would induce weight loss rather than weight gain, however many athletes are surprised to experience the latter.  There are several possible explanations for this seemingly paradoxical situation.

First, athlete’s diets typically contain a high percentage of their total calories from carbohydrates.  Carbohydrates are broken down into glucose and extra glucose that is not needed immediately for fuel is stored in the muscle and liver in the form of glycogen.  Glycogen molecules hold a substantial amount of water, 1 gram of glycogen has 2.7 grams of water with it.  So, if you are consuming more carbohydrates, your body is going to contain more water. 

This additional water is not the same thing as water retention where excess water is held between cells; the water attached to a glycogen molecule is inside the cells, which makes it healthy. Nevertheless, it can increase your body weight by as much as 3 – 5 pounds.  This weight gain is only water weight, not fat weight, therefore it should not be of concern to the athlete or fitness enthusiast that experiences this type of weight gain.

In addition to the body having more water weight because of the extra consumption of carbohydrates, endurance training enhances the body’s ability to store more glycogen than it would at a normal non-trained or pre-trained state.  The average glycogen storage capabilities for muscles of non-trained individuals is about 80 – 90 mmoles/kg.  In contrast, a trained individual has muscle glycogen storage capabilities up to 135 mmoles/kg.  So, endurance athletes and fitness enthusiasts can gain water weight by consuming more carbohydrates, as well as by training their system to store more glycogen.

Getting your body to store more glycogen is a strategy for competitive endurance athletes- the more glycogen stores you have, the better your performance.  The additional benefit of the extra water is that the cells are hydrated and optimal hydration is essential for good sports performance.  So, once again, this water weight gain is a healthy weight gain.

So unless you are just blatantly overeating, the weight gain is probably just water weight.  However, there are many instances where athletes gain fat weight as well, because they are just over-eating or eating a lot of high calorie or high fat foods because they think their training will negate the extra calories and keep them from gaining weight.  The bottom line is, if you eat more calories than you burn off, you will gain weight.  A trained body can use extra calories more efficiently than an untrained body, but weight gain is still possible in highly trained athletes and fitness enthusiasts who eat excessive amounts of calories.

Finally, endurance training typically does not induce weight gain from muscle hypertrophy, however if the individual was not involved in a fitness program prior to the onset of their endurance training, he/she could experience some muscle hypertrophy from running.  In addition, already trained recreational runners, who have never trained at high intensities, could also experience some muscle weight gain provided that the muscles were worked at intensities that caused the muscles to work at near maximal force generation, such as in sprinting in interval training or running  up hills.