If you don't receive enough calories from your regular diet or suffer from having a much higher than normal metabolism, then a weight gainer supplement is something which could be exactly what you need to give you gains a jump start. There is such a vast range of different weight gainers available at the moment and it's important to choose the one which is best suited to your body, training and goals. The calories per serving can vary from 350 to more than 1000!
When you start looking for a weight gainer you should be paying close attention to several things and these include the carbohydrate source, the amount of sugar and also the fat and saturated fat content. You want the main carb source to a form of complete carbohydrates and the secondary as sugar to aid with recovery, or more preciously the replenishing of glycogen stores. Complex carbohydrates should be the main source for several reasons, firstly they are slower to digest and therefore provide a more sustained release of energy, they won't spike insulin levels like sugars or fast digesting carbs will resulting in excess fat gains, you will also get more fibre from this source of carbs, which in turn should result better digestive function. The protein source can also vary between different weight gainers so it's important that you select the one best suited to your training. For example, if you plan on consuming your shake directly after training then you should look for a gainer which uses whey protein as its main protein source because of its fast absorption in to the system. However if you looking to use it as a meal replacement i.e. twice a day, between 3 regular meals, then you'll be better suited to a protein blend, which provides a mixture of slow releasing protein like casein and along with the whey protein discussed above. Also keep a close eye out for the sugar and fat contents but if you purchase a weight gainer which is high in these then you're less likely to gain quality muscle but rather unwanted excess fat.
So to sum up, those of you who seem to be either losing weight while training or simply finding it nearly impossible to increase your weight then maybe a weight gainer supplement is something which will do the trick for you!