A healthy and normal body having adequate metabolism should gain weight on consumption of high fat, high carbohydrate, high calorie diet as the same is ideal for synthesising fat in the body leading to weight gain. However it should also be remembered that in such a regime a person must undertake some exercises also to contain that weight in form of body muscles and strength otherwise it may lead to a plump physique.
There are number of supplements available in the market for quick weight gain. People who have weak digestion or have digestion related problems can go for these supplements as they are more easily absorbable in the body then the normal eatable foods. The supplements available in the market are based on the combination of carbohydrate, protein, vitamins and minerals. To substitute a meal by these supplements one has to take a quantity of these supplements corresponding to at least 50 gm of protein and 250 gm of carbohydrates. This can be found from the printed directions on the supplement container.
Those who can digest the foods in normal way can go for the natural weight gainers like rice, banana, oily food, potato, cream milk, eggs (specially yellow part), cheese, sugar, sweet potato etc. Those who can take non-veg foods, for them meat, chicken, eggs, crabs etc will suffice for the purpose.
Those who are on a quick weight gain mission should take care of their digestive system and if required should consult a doctor for any supplement to be taken in form of enzymes and digestive juices.
In general, barring a few exceptions, gaining weight is always easier than losing weight.
Knowledge is power.