To build muscle mass, in principle, you need to eat more, increase the number of calories consumed by about 20%. But this really does not mean that you can eat everything.
To gain weight precisely at the expense of muscles, the diet should be emphasized on high-quality protein products : lean meat (chicken, turkey, beef, veal), fish, mushrooms, cottage cheese, soy products, legumes. Thanks to them, muscles will grow. But it is also important to get energy for the same workouts from complex carbohydrates - any cereals, pasta from t / s wheat. You can eat any vegetables, berries, fruits, but seasonal is better - they still contain more vitamins.
You need healthy fats, they can be obtained from fish oil, flaxseed oil, olive oil.
Harmful foods - fried foods, purchased sauces, semi-finished products, fast food, sweets, cookies - are best removed from the diet. Yes, this is not as critical as when losing weight, but it is better to get the calories you need from the right foods : they are well absorbed and, importantly, do not harm the digestive tract. And the rate of muscle mass gain largely depends on the health of the digestive tract.
The process of gaining muscle mass is not as easy as you think. You really need to consume enough quality fats, if you are a man, then you simply cannot do without saturated fatty acids, because it is from them that your body synthesizes testosterone, which is an anabolic hormone. After training, be sure to consume carbohydrates with an average glycemic index as well as protein foods. All the myths about carbohydrate-free diets are bullshit. If, after physical exertion, the body is not provided with the necessary amount of carbohydrates, then it will replenish glycogen reserves from amino acids obtained during the breakdown of protein, that is, the protein is not used for the construction needs of your body, but for energy. This process is called glyconeogenesis and it seems to me that it is absolutely ineffective, moreover, an extra load on the kidneys. You need to consume about 8 grams of carbohydrates per 1 kg of your body weight per day, if you have a slow metabolism, then you should avoid simple carbohydrates, foods with a high glycemic index. For successful muscle building, it is also important to observe the protein intake rate, for experienced athletes it is 2 grams, for beginners 3-4 grams per kilogram of body weight. There is a little secret here, namely, there are two intervals during the day when the body assimilates protein as efficiently as possible, this morning (immediately after waking up), since the level of anabolic hormones in the blood is at its peak and night, for the same reason ... Therefore, it is recommended to take slow protein casein at night. You see, there is a huge variety of products, and you can't just say "you can eat this, but you can't". If you want to achieve results, then you need to figure out the principles of nutrition and training yourself, now it is so easy to find information on these topics in a format convenient for you, watch video seminars, read articles, generally dig deeper, good luck.
Why exclude? Let's add and replace!)) Add long (high-quality) carbohydrates, necessarily for dinner and before training, for example buckwheat :) Add fast-digesting protein and unsaturated fats;) You need to gain weight with high quality so that the process of anabolism was just muscle and not visceral fat :)
Remove oil from the menu (do not fry on it) and generally maximize vegetable fats, all foods containing them.
Eat protein foods, but do not overdo it - excess protein badly affects the kidneys.