The effects of the ECA stack in weight loss are primarily due to the ephedrine component. Ephedrine acts both as a beta agonist and stimulates the release of norepinephrine. Increased circulating norepinephrine in the body then acts on white adipose tissue by increasing cAMP levels. This causes a thermogenic effect, raising body temperature and increasing the user's metabolism in conjunction with the rest of the stack.[citation needed]
However, the body's negative feedback system then activates to normalize the metabolism. This is done via the production of phosphodiesterase inside the cells, and prostaglandins outside the cell, which both lower cAMP levels within the cell.
Caffeine inhibits the production of phosphodiesterase inside the cell and therefore slows cAMP breakdown. It also binds with and competitively inhibits adenosine receptors in the brain, triggering the release of epinephrine and increasing cAMP levels further.
Aspirin inhibits prostaglandin production outside of the cells, which, in conjunction with caffeine, greatly prolongs the thermogenic effects and increased metabolism by sustaining elevated cAMP levels.
Ephedrine also has an anorectic, or appetite-suppressing, effect. However, these effects only last for about two weeks if the stack is not cycled, as the body becomes tolerant to ephedrine to some degree.[citation needed] It is estimated that 60%–75% of the weight loss from using the ECA stack comes from the anorectic effect, and the remainder from thermogenesis.[citation needed]
The final component in weight loss of the ECA stack is that of a simple stimulant; the higher epinephrine and norepinephrine levels result in increased aerobic exercise performance and less fatigue