I wrote this whole thing out for the old thread and then someone posted 😂This was for @StrikeForever
OK I will explain my imperfect understanding. Let's first briefly lay out how we accumulate fat. We digest food into carbs (glucose), fat (glycerol and fatty acids), protein (amino acids) and fibre. The glucose causes our blood sugar to rise and our body releases insulin. The insulin moves the sugar into our cells, where through processes like glycolysis, it's converted to ATP (what powers our cells). Anything we don't use right away is converted to glycogen and that is stored in our muscles and liver. When those stores are full, the liver converts the remaining glucose, along with available fatty acids (from your dinner or already in circulation), into triglycerides and stuff those into adipocytes (fat cells).
All these caches are accessed in a specific order to release energy when we need it: first, the glucose in your blood, then the glycogen stored in your liver (which shares glucose with the body) and muscles (which use it locally), and finally, the fat stored in your adipocytes (my enormous bottom). This is a bit complicated but basically a main driver of releasing stored energy when blood sugar is low is the peptide glucagon. Glucagon triggers glycogenolysis, the release of glucose from liver stores for energy, and then lipolysis, the breakdown of fat for energy using beta-oxidation. The waste products of this process are carbon dioxide and water, which you breathe out.
Glucagon drives this energy utilisation when blood sugar is low. GLP means glucagon like peptide. But interestingly, GLP-1 actually suppresses the release of glucagon by the pancreas and increases insulin release, specifically iwhen you eat carbs/sugar. This helps you use that energy from food right away really efficiently. Then, between meals or when you're sleeping, and your blood sugar naturally lowers, your body accesses your stores of fat for its energy – because lower insulin levels allow it – instead of making you feel driven to eat more energy. Because GLP-1 helps you eat less and less often by managing appetite, you get through your glycogen stores more readily and actually start burning the fat.
So why do we feel hungry when we are fat? It's basically for the same reason as T2 diabetics need insulin. When you've chronically elevated insulin in your system for a long time with high sugar, your body gets insulin resistant. Adipocytes release this signalling hormone called leptin that says "we've got enough energy stored" and stops you wanting to eat. But the more fat cells you have, the more leptin you are producing, until your brain becomes leptin resistant, or 'fat deaf'. Without the brain properly hearing leptin, the satiety hormone, you feel hungry even though you have the calories to hand, and your body drives you to eat more and more. It thinks it is starving!
GLP opposes the hunger hormone (ghrelin) in your brain and makes you more sensitive to leptin. With GLP-1, you feel less hungry, broadly, because you are able to access your stored calories, and, as your fat cells reduce and the level of circulating leptin in your system decreases, your body is able to begin recognising satiety again.
That's my basic laywoman's understanding of the process.