I think when someone simplified it for me, then it made sense. If your machine (body) burns more efficiently (and add a higher rate) than most, well, you have to feed it enough excess fuel to induce weight gain.

Say for example, the recommended intake of carbohydrates for an AVERAGE healthy female is 2200caL/day (I’m just giving an example), well unfortunately that means for you, and people like myself, consuming more than 2200caL/day. Your body will easily use up the 2200 and have something left to store. They key is using healthy options to ramp up your caloric intake instead of the quick fix (unhealthy) options like snack food, cakes, etc.
TUNA, CHICKEN, TOFU, PEANUT BUTTER, ALMOND BUTTER, YAMS, etc

The other option is to add in weight lifting, even in a minor way. Muscle weighs more, keeps your body tone while your gaining the fat weight. Your not going for Hercules, more tone and moderate muscle gain.

Boost, Ensure are things that you can add to smoothies to help the process. Banana, so protein, two to four big heaps of Greek yogurt, honey and your fruit of choice…BUZZ, and great way to start your morning BEFORE having your breakfast an hour or so later. I would often have a chicken sandwich or omelette wrap. Even a few slices of toast with peanut butter.

Eat six times a day…healthy food options. You will actually start to notice that after a few weeks, eating six times a day actually stimulates your appetite, and you will feel hungry before each anticipated meal. Good sign that your body is processing well. remember HEALTHY HEALTHY options. Eat clean, you feel great, and look FABULOUS!

...lack of appetite has been something I have struggled with all my life. Even as I child, my mom would have to convince me to eat something.

That’s why I mention the small but frequent meals. It actually helps with the appetite thing…at least I found that it did for me. Plus, lifting weights 2-3 times a week helped in this regard as well.

If you still have no appetite after a few weeks of eating at this volume, you could talk to your doctor about some antihistamines that have been used as a stimulant.