Research has shown that tea is able to cause weight loss in those who regularly consume it, and some of this effect is known to be because of the caffeine it contains. It is also known that coffee is able to aid weight loss for the same reason. Ephedrine in combination with caffeine has also been shown in research to cause significant weight loss. One way caffeine is thought to aid weight loss is via a stimulatory effect on the sympathetic nervous system (SNS). Activation of the SNS causes the release of catecholamine hormones which activate β-adrenergic receptors which in turn increase the rate of lipolysis. In addition caffeine may increase cellular levels of cyclic AMP (cAMP) because it can inhibit phosphodiesterase, the enzyme responsible for converting cAMP to AMP. The increased cellular cAMP then activates inactive hormone-sensitive lipase which results in increased lipolysis.
Research1 has attempted to determine the metabolic effects of caffeine by using respiratory exchange and [1-13C]palmitate to trace free fatty acid turnover and lipid oxidation in healthy individuals. Eight young men (mean age 23.1 y) consumed either a placebo, 10 mg/kg caffeine or 10 mg/kg caffeine during a propranolol (β2-adrenergic antagonist) treatment (100 µg/kg bolus and 1 µg·kg-1·min-1 intravenous infusion). The experiment was carried out in the post absorptive state, after an overnight fast. Caffeine ingestion significantly increased lipid turnover (2-fold), thermic effect (13%), oxidative free fatty acid disposal (44%) and non-oxidative free fatty acid disposal (2.3 fold). Caffeine ingestion also increased energy expenditure 13% and caused a doubling in the turnover of lipids. Increased lipid turnover consisted of 24% that were oxidised and 76% that were recycled. The propranolol treatment reduced but did not inhibit these effects of caffeine.
This research supports previous findings where caffeine ingestion has been shown to increase energy expenditure, lipid turnover and oxidation. β-adrenergic blockade decreased but did not eliminate the metabolic activity of caffeine, indicating that some of the metabolic effects of caffeine mediated through the SNS. The authors concluded that ≥ 50 % of the energy expenditure and 60% of the lipid turnover are accounted for by the activation of the SNS. Caffeine is known to act synergistically with adrenaline, suggesting that caffeine works via a mechanism other than SNS activation. It is probably that caffeine can also stimulate lipolysis via antagonism of the anti-lipolytic adenosine receptor. The large increase in free fatty acids seen in this study was therefore likely as a result of both β-adrenergic activation and andenosine inhibition.
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