If the NHE diet is followed accurately, one would expect to be leaner and in improved condition. What if your underweight and want to gain some fat - do you 

  A) increase carb limit perday for non bodybuilders > 30-50g for example 80-100g

  B) increase carb load to 3 or 4 servings in one evening

  C) carb load more frequently

  D) or just follow a standard western diet and get fat like everybody not following NHE

I do not accept the premise that a person who has ready access to unlimited food needs to enlarge bodyfat. Lean is the natural state. Excess bodyfat is associated with hormonal disorders and several degenerative diseases. Therefore, it would be unethical to advise you on how to become fatter.

I believe what you are actually asking is how to “bulk-up” or “add mass” in the sense of adding muscle, which often comes along with some fat accretion. With this in mind, the answer to your question is “e” none of the above - the best way is with healthy fats. Flaxseed oil or Udo’s Choice, added to food, protein shakes, and/or supplemented directly, contains a higher concentration of calories than does carbohydrate, helps optimize omega 3/omega 6 ratio (a fault line in the average American’s diet), and furnishes essential fatty acids that millions of health-knowledgeable people pay for in pill form, i.e., fish oil. While there are specific benefits to uncontaminated fish oil for which vegetable oils cannot substitute, the oils mentioned above provide, in my estimation, approximately 70% of the heart benefits of fish oil - with better quality control, plant-specific health benefits of their own, and superior cost effectiveness.