Many times people get into triathlons because they are trying to lose weight. At first they do lose weight while doing some of the shorter distances especially when they go from no activity to regular exercise. After doing the shorter distance many people inevitably want to do longer stuff like the half and full ironman distances. Many times the weight loss slows or stops and the training volume has increased. Most times the appetite increases with more training and people are eating the wrong foods so they don't lose weight. When training long hours you have to fuel your body to make it through the long rides which many times are followed by a run. I find that sometimespeople who are trying to lose weight don't want to take in calories while they are training. They would rather save those calories for goodies after their training and reward themselves with something good. Some people just want to cut calories so they don't take too many calories while working out. This is a huge mistake and will likely lead to a bad workout and poor recovery. If you are training for an ironman and not losing weight you should look at the type and quantity of calories you are consuming throughout the day. You need to fuel properly during your workouts and this is NOT the time to cut calories. The bike is the place to consume your calories during a full or half ironman. Try and take in between 150-250 calories per hour depending on your weight it may be lower or higher. Part of the calories should come in liquid form and part in solid or gel. It is also important to begin your workout with a good pre-workout meal. If you are eating healthy throughout the day you will continue to lose weight and your body will become more efficient at utilizing your fat stores for energy. If you don't consume enough calories during your workout you will most likely not be able to finish your workout, have a crappy workout, or not recover well from the workout. The end goal is to finish the race and you may not finish the race if you don't fuel properly for it.
Train Hard. Have Fun. Recover Well.