I believe it's already well know that in order to experience mass and strenght build up you need to eat properly. This means you have to eat ay least 5 times a day (preferably 6-7) and be aware so that your meals contain the right amount of calories (which you need for bulking!) , protein and minerals as well as fat, needed for your muscles to start building.
A male adult person can consume about 38 (or more, depends the subject) grams of protein on each meal. Fat is also needed in order to make muscle fiber <>. When people are on strict diet they might notice that their skin (mostly on the belly) seems to make rolls...this is due to the fact that the don't receive as much fat as their muscles need to fill that vacuum between muscle and skin. Therefore consuming foods rich in fat is acceptable once or twice a week even to those who are on a cutting diet.
A good protein and carbon source is rice with wich it's good to combine your meat (which must be rich in protein). Chicken, Beef, Egg, Tuna, Milk (low fat 1 1/2), Yougart are some food very rich in protein.
On the other hand black bread , rice, pasta, and also some fruits are rich in carbon which are mostly needed in order to receive the right ingridients which will help your muscle rest and grow in compination with protein.
A person weighting 75 killos needs about 1700-2000 calories in order to keep his weight stable. Therefore in order to grow will need about 3000 + calories.
Supplements make a good job helping you while dieting cause not everyone has time to prepare so many meals!
There are many Protein supplements which work while dieting in order to gain mass and strength. Usually supplements with 600 calories and more are best.
Also creatine makes a difference adding some power and mass. But when on a gain diet consider buying a creatine with carbon which provides extra calories - energy.
Finally it's easy to maintain or increase your weight if you play your cards right.
Pay attention on small details you get every time about dieting and try to make them work for you.
For example it is essential that you eat a banana or a protein chocolate bar after work out cause your body needs that nutrients in order to retain lost calories and ingridients during work out. Also a good product for this is liquid rich in carbon.
