Hormones like epinephrine (aka adrenaline) and norepinephrine (aka noradrenaline), part of our 'fight or flight' hormone roster, stimulate lipid mobilization.
In other words, when these hormones go up, it tells the body to release fatty acids into the bloodstream for energy.
These hormones can go up for a few reasons. Exercise and and stress are two of the most common. In both cases, the 'fight or flight' hormones release fuel for quick energy to the muscles. These hormones in turn suppress appetite and gastric function, so we're not hungry.
You would think that it would be good to have these hormones high if you wanted to lose body fat. This is the basis of almost all "fat-burning" supplements. They contain stimulants, such as caffeine, that amp up your epinephrine and/or norepinephrine. They also lower your appetite temporarily.
However, without exercise, releasing our "fight-flight" hormones isn't as effective for losing fat, since the signal is "artificial". Although the fats are available to use as an energy source, there is no increased muscle activity that needs the energy. Fats simply recycle back into fat storage.
So the fats may be released, but if we are not exercising, they don't do anything. Eventually, they shrug and go back home again.
Stimulants don't give you energy, remember, energy cannot be created or destroyed, only transferred. Instead, stimulants borrow energy. And, if we don't use energy, it gets put back, often leaving us feeling less energetic than before. (or, hungrier)
But, if we combine mild stimulants (such as a cup of coffee) with an exercise session where those fatty acids can be used, now we are in business. Caffeine is a well-known ergogenic aid (a substance that can boost performance).
So, next time you are tempted to try a fat burning supplement for quick fat loss, think again!