Wow there is a lot here.
There are two metabolic pathways for producing the energy we need to survive. Ingested glucose and foods that digest into glucose, and fats if varying length carbon chains. Our bodies operate on fats, it’s the preferred energy source and the way we store energy long term. Free fatty acids produced by the liver and absorbed in our lymph lacteals are stored in adipose tissue. The hormone that causes issue with the system is insulin. Let’s imagine we are foraging, scarce food, no food storage, and we find a fruit tree full of ripe fruit. We eat as much as we can, sleep, eat some more, invite our tribe, and the fruit is spoiled or eaten quickly. Our blood sugar spikes, insulin is excreted to force stock blood glucose in the cells of the body. If blood glucose levels don’t retreat the liver starts converting blood glucose to free fatty acids that are stored as adipose tissue, because high blood glucose will poison you. No more fruit for a while, maybe no more food for a while. Insulin levels drop opening the metabolic pathways for FFA. This is key, when insulin is present you cannot metabolize fat.
Most people don’t realize that even complete protein is partially digested into glucose, and glucose always inspires insulin secretion. Our society eats all the time, therefore insulin is pretty much always present. Exercise all you want, when insulin is present adipose tissue cannot be metabolized.
Regarding fiber, feces are waste and often antagonistic to the epithelial cells in the gut. Keeping them moving makes irritation and immune response less likely preventing things like colon cancer and IBD. Also, feeding friendly bacterial colonies in the gut is key to having those cultures ie. lactobacillus outcompete destructive cultures like helicobacter pylori.
In short fasting is the healthiest and most effective way to lose adipose tissue, because it’s the way we evolved to function for survival, it also triggers autophagy at intervals over 36 hours which makes your immune system go in cellular trash patrol, metabolizing things like cancer cells for fuel. I hope this helps.