Do "fat-burning supplements" work?
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 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 also suppress appetite and gastric function, so we're not hungry.
You'd think that it would be good to have these hormones high if you wanted to lose body fat. This is the basis for almost all "fat-burning" supplements. They contain stimulant such as caffeine, that amp up our epinephrine and/or norepinephrine. They also lower our appetite temporarily.
However, without exercise, releasing our "fight or 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're not exercising, they don't do anything. Eventually, they shrug and go back home again.
Stimulants don't give you energy (because energy can't be created or destroyed). Instead, stimulants borrow energy. And if we don't use energy, it gets put back, often leaving us feeling less energetic than before. (or worse, 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 well-known ergogenic aid (a substance that can boost performance).
Be aware that most stimulants are banned is athletic competition.