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Eat high calorie, high protein diet. Do not increase the intake of junk foods and greasy foods, this will contain unhealthy fats.
Eat healthy fats like omega-3 essential fatty acids sources like salmon, walnuts.
Eat good protein rich foods like lean meats, fish, poultry, egg white nuts, seeds, pulses and legumes.
Eat healthy carbohydrates like fruits, vegetables and whole grains.
Eat five to six small balanced meals per day rather than eating three large meals.
Protein powder and nutritional supplement drinks can be added

Eat at least these groups of food items.

Milk group: Milk, cheese, yoghurt, cottage cheese, ice cream.
Meat group: Meat, fish, poultry, eggs.
Vegetable group: Dried beans, peas, nuts, potatoes.
Fruit group: Fruit juices. Fruits like bananas, mango, chickoo.
Grain group: Cereals, breads, rolls, pasta, pancakes, rice, muffins