In the human adult there is as little as 200 grams (in a tiny adult) all the way up to 500+ (in an average/larger adult) of glycogen: https://bionumbers.hms.harvard.edu/bionumber.aspx?s=n&v=1&id=110214
This is readily-accessible energy in tissue. There is a longer explanation regarding the citric acid cycle and ATPs. But let’s just be conservative and say you need to deplete these 2,000+ calories of energy BEFORE your body has any good reason to burn stored fat.
Keep in mind that, meanwhile, your liver will continue shuttling sugar into your blood stream in a nearly-never-ending supply: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4050641/. In the Cahill studies, we find that starved individuals continue making positive energy adaptations while IN STARVATION for 6 weeks: https://thehealthsciencesacademy.org/wp-content/uploads/2015/07/Fuel-Metabolism-in-Starvation_ReviewArticleTIMM2008-9Lazar-1.pdf. Never forget that since the 1970s we have had a case study of a successful 382 day fast: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2495396/.
The average person who loses weight never even touches body fat. They just keep bobbling around in the same glucose/glycogen figure, burning off healthy tissue and getting weaker. Sure. They get lighter. But can they run faster, lift more, and improve capability? If not, “weight loss” was primarily NOT FAT.
When referencing these known biological figures, it’s unclear that anyone will touch a significant portion of body fat until they’ve dramatically altered metabolic signaling. Although it CAN be done while eating a decent amount of food, especially if the person is doing a lot of intense exercise (ie - some sort of sprints and/or heavy lifting), we can plainly see that moderate food reduction and moderate activity will likely only burn off muscle and bone tissue while still preserving body fat. It results in lighter but weaker/fatter outcomes.
Thus, sadly, we have to clear out the glycogen to a degree that we OBLIGATE fat loss. This isn’t a hoping. This isn’t trying. This is FORCING the outcome of fat loss. And once we have forced the body to burn stored fat, we have to be very careful about coaxing it back into sugar-reliance. While still wanting to lose fat, the amount of food obviously must stay low, and perhaps more importantly the TYPE of food must err on the side of not-rapidly-refilling glycogen stores; otherwise, we simply return to the initial state of never-burning-stored-fat. This is the classic yo-yo. In fact, before someone is down about 40lbs, I’m unconvinced they’ve burned one ounce of body fat. The glycogen’s accompanying fluid and inflammatory bloat can easily account for 15-30lbs in most Americans. Weight loss numbers in this range are generally not fat loss at all, but the up and down of partial glycogen removal and replacement.
From a theoretical stand point, the best option for people is going to be close to a total fast for 3-10 days, guaranteeing some degree of obligatory fat burn, followed immediately by strictly greens, fats, proteins, and nearly zero starch/sugar. This means that if you want to keep the fat-burning train moving, even moderate fruit intake and natural sugars like honey are counterproductive. They will thrust you right back into sugar-reliance, giving the body NO REASON to touch stored fat.
Once you’re leanER than you want to be, have at it with the fruit and natural sugars. Even learn to integrate ACTUAL moderation of treats. But if treats keep coming in every 3-10 days or more frequently, the body just goes right back to square one for most people. Glycogen is so consistently full that there exists NO REASON why the body would lose fat. It’s not genetics. It’s not heredity. It’s not a crappy metabolism. It’s that you never clear glycogen enough to convince your body it SHOULD use fat.
There is a stress management consideration, an athletic consideration, and more. But as far as flipping the fat loss switch for most people, this is a good first landscape to ponder. Clear glycogen and keep it cleared.