I don't know why people fixate on food so much when it comes to dieting. They seem to forget the role of metabolism and exercise. There's a reason you could eat as much shite you liked as a teenager but still be in good shape (well, within reason). If you have a high metabolism and/or exercise a lot you can eat what you want without obsessing about calories.
It's also not so much about fat/sugar but rather TYPES of fat/sugar. I eat more fat than my obese cousin yet I'm lean because the fat is in avocados and fish. The sugar you get in fruit is better than refined kinds dumped into confectionary. As a rule of thumb, ask how many machines a food has been through since it's original source. The more processing, the worse it is. And the link to cancerous free radicals is also noteworthy.
The variability in appetite is linked to exercise levels, what you ate previously, and natural rhythms. In rare cases it is due to malignant health disorders, however.