Most of my life I was always tiny. I weighed 69 lbs until I was 17 then 100 lbs til I was 20 and maintained that weight until I had a baby.
Ten years after my daughter was born I somehow got up to 187lbs (yikes)... I didn't even weigh that much when I was 9 months pregnant.lol So I started kick boxing for a few years, when I had the time, didn't eat more than 400 calories a day but would drink alot of coffee every day at work (which is loaded with calories). There have even been days even a wk/wk & a half were I'd forgot to eat anything, just had coffee. I'd get up @ 6 a.m., go to work, have some coffee, work through my break and lunch time,more coffee, leave work sometimes after 9 p.m., before I knew it..time for bed, day in and day out...after a few days i'd realize I hadn't had anything to eat for days...lol.
Now during the week I drink alot of water & coffee, try to eat at least 1 280 - 300 calorie Smartone meal and at least 1 bag of steamed vege's (loaded with butter of course), which is another 100+ calories a day. On the weekends I splurge a little sometimes and have pizza or a sub but nothing too crazy so I can maintain my weight.
I'm not sure if this is healthy or not, but it's what has worked for me. I'm 5'5", 113 lbs now & 42 years old; I don't work out anymore, haven't since I blew out both my shoulders and herniated a couple discs kick boxing.