First, please pat yourself on the back for asking this. I think it very much needed asking.

I'll answer the question, but before that I have some observations to offer. They may sound harsh; I would soften it if I knew how.

2 kg (4.4 lb) isn't much of a weight gain. I have my doubts that such a slight gain would be enough to make a lasting difference, and I wonder if you are setting your "ideal weight" too low for your height and age.

I'm also concerned by the fact that you seem to think that a 30-minute meal is too much eating. For many people 30 minutes means a rushed meal, without the time to properly enjoy it.

I also note from another question you've asked that you measure your weight down to a fraction of an ounce, and that you seem to think that 112 lbs is a healthy weight for a 5'7" 16-year-old female. (Again, this sounds rather low.)

So all in all, I can't help but wonder if you have an unhealthy aversion to food and/or weight gain. If you were in the U.S., I would be posting the number to our National Eating Disorders Hotline and asking you to seriously consider calling it.

As it is, I hope you'll consider the possibility that you need more help than a forum like this can give, and I hope you'll research the resources in your country. (I know the UK has such a hotline; not sure about the resources in other countries.)

Answer:

Obligatory disclaimer: I am not a doctor, and this is not medical advice!

Loss of menstruation is complicated, and scientists are still trying to understand it. They believe there's a minimum body fat percentage involved; in addition, they know that the hypothalamus and the pituitary can contribute to the problem.

As near as I can tell, it's possible that sometimes the glands shut things down when they "think" you don't have enough body fat. It's also possible that sometimes those glands get "out of whack" because of malnutrition. And sometimes it could be a combination of both, with some other complications thrown in.

You really need an experienced health care professional to help sort it all out.

Amenorrhoea can a symptom of a lot of things--weight change (loss OR gain), chronic stress (your body tells it's not a good time to have a child), thyroid problem, brain tumors, etc etc. Either way, it's your body's way of telling you that something is wrong.

There is no "body fat percentage" necessarily. They are taking amenorrhoea out of the DSM-V because it does not mean that anyone is sicker because of it after realization that some women can have their periods at BMIs of 13. Every body is different. Listen to your body. When you DO get your period back, remember that it isn't a sign that everything is all better. Continue to take care of yourself. It takes a while to heal.

Tips to gain weight:

For you, throw out the scale. Or rather, have your parents hide it (I just read that you are 16). Have a doctor or nutritionist track your weight. Please, I think that this is probably more important advice than what to eat, how to eat, etc. Focus on health, mindfulness, etc and don't dive into weight. It is not about weight, it is about taking care of YOU physically and mentally.

I'm no expert on amenorrhoea so I'll leave that matter for others to comment on.

To put weight on you're going to have to eat, it's as simple as that. As a rough guide I would be aiming for around 1000 calories per meal, lots of fat, and a decent whack of carbs and protein. Something like 50% fat, 25% carb, 25% protein.

So you don't necessarily need to eat a massive dinner, you just need to eat decent meals 3x a day.

Oh, it generally takes me around 30min to eat a decent meal - I'm not sure I see the issue with that? Just take your time, enjoy what you're eating and do a good job of chewing it properly.

Do you know how you became be to underweight? That should also be addressed, i.e. hyperthyroidism, over training, under eating etc?

Best of luck.

It helps to have different things to eat. It can be hard to force yourself to keep eating with a proper diet if your body genuinely is full, but throwing together a smoothie with a can of coconut milk gives you a good whack of calories in a more palatable form. In fact, if I was desperate (or even if not) I can down a pint of ice cream usually regardless of what else I've eaten. If it's good quality with 4 or 5 ingredients then it's one of the more benign ways to overeat in my opinion. Of course it shouldn't be too difficult to eat enough, and I wouldn't especially worry about a large meal so long as you take it easy while you're digesting it. So, er, my advice is to veg out on the coach all evening with a tub of ice cream I guess.

Liquid calories are very easy to ingest (300 calories of juice feels like nothing!) - even if you're drinking all-natural fruit juice without any sugar added, you can still get a lot of calories without that disgusting "overfull" feeling (I hate that too! Grossest thing ever!). You can rack up 400-500 calories in a Grande Starbucks drink without even thinking about it - the default for drinks is 2% milk but you can ask them to make it with whole. Go for calorie-dense foods so you don't have to eat a lot of mass. Nuts and nut butters; coconut milk; fats. You can stir a tablespoon of olive oil into any kind of savory thing for a totally unnoticeable 100 calories extra.

If you're trying to gain weight then obviously you don't want to do a lot of super intense exercise, but I find that gentle walking or slow stretching can really help me digest and not feel overfull.

Also, can you enlist your friends? They should have no problem helping you eat more food, especially if you eat lunch with them at school. Maybe if you can eat a little more at lunch you won't have to stuff yourself so much at dinner?