“Burning fat” is really a two-part process: releasing energy from fat stores into the blood (lipolysis) and cells taking those molecules in and utilizing them (oxidation).
This first step, lipolysis, is triggered by chemicals known as “catecholamines.” Adrenaline is a catecholamine, for example, and once these chemicals are in your blood, they attach to “receptors” (attachment sites) on fat cells, which causes them to release some of their energy stores.
These molecules so released (free fatty acids) are then used as fuel (burned or “oxidized”) by various types of cells in the body. Well-trained muscle is particularly good at oxidizing fats, by the way, which is why it’s easier to lose fat when you have a good amount of muscle.
Now, here’s where we get to the difference between belly fat cells and fat cells that are easier to lose.
Fat cells have a certain number of receptor sites for catecholamines, but they aren’t all the same. One type of receptor is known as an “alpha-2″ receptor, and another is a “beta-3″ receptor. The physiology gets pretty complicated, but here’s the bottom line: beta-3 receptors accelerate lipolysis, and alpha-2 receptors hinder it.
What this means is fat cells that have more beta-3 receptors than alpha-2s are relatively easy to mobilize, whereas fat cells that have more alpha-2 receptors than beta-3s are harder to mobilize.
This is the problem with belly fat, and all other forms of “stubborn fat“: the ratio between beta-3 and alpha-2 receptors is heavily weighted toward alpha-2 receptors (it has many more alpha-2 receptors than beta-3).
Thus, when you’re losing fat, you immediately start seeing reductions in fat masses with high amounts of beta-3 receptors, but the masses with amounts of alpha-2 receptors are slow to respond.
For most of us, this means rapid reductions in places like our arms, shoulders, chest, face, and legs, and slower reductions in our stomachs, hips, lower back, and thighs. These latter areas are always the last to really get lean, because they contain the most fat cells high in alpha-2 receptors.