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The principles are the same, funny as it sounds. And I imagine that “weight gain” isn’t so much your goal as is an increase in lean body mass. You could go from 125 lbs to 160 lbs on the Twinkie diet, but you’re going to look and feel like garbage.
Try and find your maintenance level. This is the amount of calories that you require to maintain your current weight. There are literally hundreds of calculators online, and while none of them is going to be 100% accurate, they’ll give you a good idea. TDEE fits in a bell curve, so yours is likely similar to other peoples’ with a similar weight and body fat %.
Once you have that number, then aim to eat above it. The foods you choose to eat will impact your health and your body’s ability to increase lean body mass as a response to resistance training. Like I said, you could eat 3000 kcals of twinkies, and you’ll put on weight, but probably not in the way you want to. If you choose a reasonable surplus (~300 kcals per day) coming from healthy foods - vegetables, lesser-processed meats and carbs, and healthy fats - then you’ll be providing your body sufficient nutrients to add muscle.
They say that fat loss is done 90% in the kitchen and 10% in the gym. Increasing muscle mass is nearly the opposite. Unless you’re still an adolescent (because they can spontaneously generate muscle tissue), it is necessary to do resistance training to build muscle. Lifting weights and other anaerobic exercise is how it’s done. You cannot and will not see results unless you are stressing your muscles this way. If you increase your food intake to above maintenance without exercise, your body stores the excess as body fat. If you do only cardio exercise, then you’ll continue to decrease fat, but only to the extent that your body has use of the excess energy. You won’t build any muscle. If you want to build muscle you have to lift weights, period.
I would search fro WWII items on health an nutrition. They were geared to producing strong, fit young men for service in wartime with basic nutrition rather than an emphasis on weight loss or bodybuilding for competition. The 1943 USDA Basic Seven is a good place to start. Personally, I like the Weston-Price materials (and the Nourishing Traditions cookbook) for nutrition, but to each their own. Nobody is a one size fits all.
i would absolutely look at Eric Helms’ muscle and strength pyramid - Nutrition - it shows you foundations of getting your nutrition right in line with your goals - i found a free download here
Nutrition textbooks will have a section dedicated to weight gain. Look into diets for recovering from eating disorders, or for athletes bulking. Source: degree and a half in nutrition
Edit: also look into diets for people with cystic fibrosis. its an immune disease where one of the symptoms is problems digesting fat and other nutrients, and therefore they require 3-4x the average caloric intake. Lots of good tips on how to pack in calories and increase absorption.