‘Whey or Casein protein’ shakes should be consumed for their convenient and easily ingested, readily absorbable, high biological value protein. They are generally relatively low in calories (approx 100kcal made up with water) and are very useful in developing muscle, lean fat free mass, tone and definition.
‘Weight gainers’ can provide you with (on average) around 1000kcal per serving! So in consuming as little as approximately 300-400mls a day, you can consume the same number of calories obtained from 2 average sized meals! Combine this with 4-6 small meals, and depending on your activity levels...weight gain should soon follow.
An approximate way of calculating your calorie requirements is this:
25-35kcal x ‘your weight’ e.g. 80kg male = approx 2000- 2800kcal
(NOTE: This is an approximation)
You can + or - 500kcal depending on your activity levels, or whether you wish to gain or lose weight.