ER I meant if you are still growing I think 20BMI is ideal. Sorry, bit tired.
Er yes, I should have mention that the BMI calculator put a "healthy" BMI at about 109-140 lbs. So you don't need too much more to be at a healthier weight. Personally I think 20 BMI is ideal (especially if you're not still growing or thinking about having babies soon) which is calculated at about 117lbs for you- but this is my own opinion.
And yeah, anything in a box, wrapper, or Styrofoam container is almost always unhealthy. One thing to give yourself healthy fats -lean meats, diary, nuts, lentils, coconut, avocado, etc. Another thing entirely to suggest you gorge on ice cream and McD's; I'm definitely not suggesting that!
I'm 5'2" and weigh 113 so really you dont need to gain much just eat some fatty foods but DONT EAT FAST FOOD ever heard of pink slime?
First- has dr's ruled out anything that could be preventing weight gain? These can be things such as malabsorption, parasites, overactive thyroid, etc? The BMI calculator I use says you're just under 18 BMI which is underweight, so definitely you need to gain a little. First thing I suggest is ask your dr to test for thyroid problems, ask specifically for Free t3 and t4 because they're more reliable at showing what the thyroid is actually doing than the standard TSH test. Second if you typically have loose stools or other odd bowel issues you need to talk to your dr about that. Loose stools can be symptomatic of more serious medical conditions-especially when it prevents weight gain.
Assuming everything checks out the other thing to consider is what are you eating? I suggest you write down a couple days of what your food is, perhaps add it to the food tracker on here, and see where you are calorie and fat wise. This might help you figure out where you need to go from there.
To help gain healthy weight make perhaps start off your day with a good granola/museli, specifically one with nuts. For snacks, maybe go for a boiled egg. If you're a dairy drinker, don't drink skim go for 2 % or full. Another thing you can do is take chilled coconut milk, add a banana or other fruit to it and make yourself a smoothie. I found that this is a delicious and easy way to boost my fat intake up to recommended levels. Other things you can do- almond and nut meals, these don't have much substance but can easily be added to yogurts, liquids, and baking recipes to boost up healthy fats. Avocados are good too- maybe add a tbsp or so to sandwiches or salads. It also makes an awesome creamy sauce for pasta (Mash and mix the desired amount in with the meat or sauce before adding it to the pasta). Can't think of anything else at the moment though.. But because you're underweight you can always ask your dr for a recommendation to a dietician.