Two years ago, I was 5'4" and 109 pounds. I was extremely toned and skinny; I barely had an ounce of fat on my body. This is how I did it:
1. I would try to limit myself to under 1,800 calories a day. This was a general rule, and one that I didn't follow strictly. I would simply keep a running count in my head throughout the day, and if I didn't know the exact value of something, I wouldn't fret, I'd just estimate the amount and add it on. I never actually wrote what I ate down in a food log, though. I would eat around six meal of 300 calories each every day. If, for some reason, I had a higher calorie meal, I would simply omit another meal.
2. I would eat the same thing for breakfast each day, and I would always eat breakfast. I always had a Nature Valley Crunch granola bar, the peanut butter kind. They have 180 calories each, and would fill me up until lunchtime.
3. I would never eat more than 300 calories at lunch. I would check the nutrition facts on everything that was pre-packaged and if it had more than 300 calories, I wouldn't finish it. For instance, if I got a bagel for lunch, I wouldn't get cream cheese, because that would bring the meal over 300 calories. If I had an uncrustable, which has about 340 calories if I remember right, I wouldn't eat anything else for lunch that day. Sometimes for lunch I'd get something as light as a bag of Sun Chips, about 140 calories, and a small cup of soft serve frozen yogurt, about an additional 100 calories. This would hold me over through cross country practice until dinner around 5:00.
4. I would never eat anything with Trans Fat. As a general rule, I knew that things with Trans Fat are unhealthy, so I'd just avoid them completely. Cheese Its, for instance, have half a gram per serving, so I wouldn't eat them. Hydrogenated oils are basically the same thing as Trans Fat, so I'd avoid those, too.
5. I would have at least two water bottles a day. They kept me feeling full so I wouldn't mistake my hunger for thirst. It was very important since I was running so much. I would have some water at dinner, an entire water bottle at lunch to help me feel full, and another before, during, and after cross country practice.
6. I didn't allow myself to eat anything after 8:00 at night. There wasn't really any reason for this rule, it was just good for keeping the calories down. Even if I went to bed at 1:00, I'd just make myself suffer. Typically while I was this skinny I was going to bed around 10:00, though, and getting a good 8 hours of sleep a night. I think sleeping that much probably played a big role in keeping my weight down as well.
7. I didn't allow myself to drink anything with calories in it, except for on special occasions. So, my drink options were mainly: Diet Coke and Pepsi, Water, Crystal Light, and other diet beverages. If I went out to eat on my birthday, I would allow myself to get a strawberry lemonade or a smoothie. Cutting out calorie laden beverages was probably extremely beneficial.
8. I did 100 situps each night, along with a full pilates ab routine. There were no exceptions to this rule, no matter how tired or sick I was. I did it no matter what. It only took about ten to fifteen minutes once I got the entire thing down. The pilates targeted every area of my abs, so they were extremely toned. I had a four pack, and I had no rolls, only little skin folds. I fit into size two jeans.
9. I ran 25-30 miles a week at a relatively fast pace. This was probably one of the most crucial parts of keeping my weight down. I burned a ton of calories this way. Every weekend I competed in a 5k, which I would complete around 22:00. I would run miles around 6:00. Running long, fast, and often certainly kept my weight way down. It also helped me feel good.
10. I weighed myself every single day. This was I could know where I was headed. If my weight went up, I'd pay extra attention to what I would eat the next day. If it went down, I would celebrate.
One day I had an epiphany and decided I could weigh whatever I wanted, so I broke, gradually, all of these rules. My weight slowly and surely increased from 109 to 112 to 115 to 118. It then went from 120 to 123 to 125 go 128 to 130 to 132 to 134, where it typically stands now. It took two years for my weight to increase that much. I am now a size 8.
As a warning this diet may not have been too healthy. While at this weight, I stopped getting my period for six months. I did not start getting my periods again until I hit 115 pounds. The cessation of a period due to lack of body fat is known as amenorrhea, and it's not healthy. So, this diet as a whole may be dangerous, but maybe you can take away bits and pieces of it that might be helpful for your own diet. Everyone's body is different, though, so go with what you think is healthy.
I am now 135 pounds and still 5'4". I eat whatever I want and do not exercise compulsively anymore.
These aren't the best pictures from back then to show it, but I was around 109 pounds at this point in the two photos above. That dress no longer fits me; my boobs pop out and because of the size of my hips it's too short and barely covers my butt. That's okay because I find the dress pretty hideous and trashy these days.

This is me at 135.
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