Weight Gain

Whilst many people are looking for ways to lose weight, some suffer from a lack of weight. This is especially so for those playing contact sports where a little extra weight benefits the impact area.  When gaining weight, there needs to be caution that the gain the right kind of weight. Putting on fat solves the problem but it does general health and physical well-being no good. So when someone is trying to put on weight, what they really trying to do is put on muscle - noting that muscle is also kilo for kilo heavier than fat.
Luckily, muscle gain is a relatively simple equation, and is a result of three main factors; resistance training, nutrition and rest.

Resistance Training
Training with weights, especially heavy weights, causes slight damage to the muscle fibres. The body repairs this damage and compensates by making the muscle stronger, thus bigger. This is why, over time, athletes get stronger (and bigger) as the damaged muscle tissue repairs itself, it also grows stronger and bigger. 
The best exercises to support weight gain are ones that use multiple muscle groups, like the bench press, squats and deadlifts, and when you start off you should be doing only about 3-4 sets of about 8-12 repetitions per exercise.
It is advised that you get instruction on the correct form, as poor form can lead to injury.
Initially, training needs to be only 3 times a week – as your body adapts this may increase but as muscle growth happens during recovery, too much too soon can have a detrimental effect.

Nutrition
Muscles need protein. To gain weight (muscle) adding extra protein to a diet is important. Protein can come from foods such as dairy, eggs, meat, fish or beans. Also, due to training more often, there is a need for carbohydrates and healthy fats or else the body will start burning protein for energy (and you want all that protein building muscle), so foods such as nuts, seeds, vegetables, fruits and oats help maintain energy and glycogen levels.Most weight gainers like to eat consistently throughout the day (between 6-8 meals) and often they will supplement their nutrition with a protein powder (with carbs). Something else to consider is a Branch Chain Amino Acids (BCAA) supplement, BCAA are the building blocks of protein and help utilise the protein best to build muscle.

Recovery
Often overlooked is recovery. All exercise should look like this: 

Training + Recovery = Improvement.

And this can be applied to all types of training, be it weight management, endurance or weight gain. Over time, the body adapts to training loads and the length of recovery may be shortened, but for the first 4-6 weeks it is suggested that, to gain weight, you should only train with weights 3 times a week.

What is sometimes difficult to manage is if you’re playing other sport. The best example is a person playing contact sport (Rugby or Rugby League). They need bulk and power, but if they’re doing other fitness training throughout the week, often they may be losing weight rather than putting it on. If this is the case, then additional high quality nutrition needs to be explored.

So to summarise, weight gain means muscle gain, and to add muscle you require resistance training, good nutrition and adequate recovery.