To gain weight, your body has to be in a calorie surplus. To do that, you have to consume 300-500 calories over your maintenance calorie level. 300 is generally the accepted amount to properly gain .5lb/week, and 500 for 1lb/week.
How that works is, eating 500 calories over maintenance every day, creates a surplus of 3,500 calories at the end of the week. Scientifically, one pound of fat is 3,500 calories, and a pound of muscle is 2,500 calories. Combined with exercise, you can still gain muscle, but be aware that the weight you gain will be a combination of muscle, fat, and water weight.
Easy way of gaining weight:
*1-1.5g of protein/lb. - Protein provides nutrients for the body to maintain muscle and also to grow it.
*1-3g of carbs/lb. - Provides really efficient and easily digested energy.
*0.3-1g of fat/lb. - Sustains proper insulin levels, hormone function, and sex drive.
*300-500+ calories over your maintenance.
You can eat pizzas, ice cream, sodas and whatnot, if they somehow fit into your macronutrient (protein/fat/carb) and calorie goal. Also, the body doesn't care if you ate 2 meals or 20 meals, as long as you reach your calorie and nutrient goal at the end of the day, you will gain weight. Generally, I'd suggest 3-8 meals a day because smaller meals feel more satiating than larger ones, but if you can only eat 3-4 meals in your schedule, then build your diet based around eating 3-4 meals. Meal frequency only makes a small difference, and that difference is only aimed towards elite athletes who have trained for more than 10 years.
You should eat a balance of proteins, carbohydrates, and the right kinds of fat (unsaturated and monounsaturated fats, olive oil, canola oil, pistachios, almonds, walnuts). Drink mixed juices (apple/berry, peach/orange/banana) for a higher calorie intake. Also, make sure to snack in between meals. Nuts, dried fruits, and yogurt are good options, but there are many other nutritious foods out there haha. And then just exercise. Resistance exercises like push-ups, pull-ups and weight lifting will increase your body mass. Most of this information is from here: http://funadvice.com/r/15dpjufo5o7 Goodluck:)gottta gain weight!!!
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