In this study, we demonstrate that chronic exposure to caffeine during adulthood extends life span and healthspan of C. elegans in a temperature-dependent manner. Life span extension from caffeine is non-additive with life span extension by bacterial deprivation and independent of the hypoxia inducible factor, HIF-1, the C. elegans p53 ortholog, CEP-1, and the C. elegans Sir2 ortholog, SIR-2.1. Caffeine appears to act, at least in part, by activating the FOXO transcription factor DAF-16 in a manner similar to reduced IIS. Chronic caffeine exposure also delays paralysis in a C. elegans model of polyglutamine disease, though this effect appears to be independent of DAF-16.
Perhaps the most intriguing therapeutic value for caffeine is its potential to reduce the risk and delay the onset of age-associated neurodegenerative disease. As described in the introduction, studies in humans and rodents indicate that caffeine treatment reduces the risk of disease onset and improves cognitive decline in models of Alzheimer’s and Parkinson’s disease. Previous work has also shown that caffeine is capable of delaying pathology in worm models of Alzheimer’s disease [36, 37]. In this study, we expand upon these findings to show that caffeine is capable of delaying pathology in a worm model of polyglutamine disease. Although mammalian studies investigating the use of caffeine in neurodegenerative disorders have focused primarily on Alzheimer’s and Parkinson’s disease, research that examines the effects of caffeine consumption on Huntington’s disease in human populations is currently underway. Our finding suggests that a more detailed examination of the influence of caffeine on the progression of a broader spectrum of diseases associated with proteotoxic stress in mammalian models may be warranted.
This study and that by Lublin et al.  both identify an epistatic interaction between caffeine and IIS. A related link has been made in mammals. While acute treatment with caffeine has been shown to increase blood pressure  and reduce insulin sensitivity [53, 54], long-term coffee consumption shows a strong correlation with reduced risk of type 2 diabetes in humans , and chronic caffeine exposure prevents diet-induced insulin resistance and hypertension in rats . Chronic caffeine consumption may prove useful in mimicking reduced IIS and improving diet-induced insulin resistance.
The interaction observed between bacterial deprivation and caffeine suggests that caffeine may also emulate aspects of dietary restriction, which is particularly interesting given that caffeine is already in common use in human society. The observation that caffeine interacts with both bacterial deprivation and IIS is complicated by the fact that bacterial deprivation extends life span independently of both DAF-2 and DAF-16 [46, 48], suggesting that caffeine may activate overlapping downstream targets in both pathways. The additional observation that life span extension from caffeine is reduced in three other strain backgrounds with mutations in genes linked to aging suggests that caffeine may be activating a common set of cellular processes important for increased longevity via a range of interventions. The idea that caffeine may interact with multiple pathways involved in aging is further supported by the observation that the influences of caffeine on life span and polyglutamine toxicity are separable with respect to their dependence on both temperature and DAF-16. Alternatively, it is possible that caffeine alters the molecular or physiological state of the organisms in such a way that renders the organism unable to respond normally to signals that result in increased life span in untreated animals. Epistatic interactions provide limited information regarding the potential mechanism of life span extension , and further study will be required to unravel the complexities of caffeine’s impact on the aging process.
This study identifies genetic interaction between caffeine and both the IIS and the dietary restriction pathways. The direct molecular targets and downstream mechanisms by which caffeine influences longevity remain to be investigated. Mammalian research suggests that caffeine primarily impacts cognitive phenotypes by antagonizing adenosine receptors A1 and A2A. At sub-toxic levels of caffeine, adenosine receptors are the only clear molecular targets and many of the beneficial effects of caffeine are mimicked by specific agonists of adenosine receptor A2A. Mild inhibition of phosphodiesterase activity has only been observed at higher concentrations of caffeine . Functionally, caffeine prevents memory impairment induced by heavy alcohol consumption in rats, an effect that can be mimicked by simultaneous treatment with inhibitors of phosphodiesterase 5 and adenosine receptor A2A (but not by either inhibitor alone) . This suggests that specific subclasses of phosphodiesterases may be important for the action of sub-toxic doses of caffeine in some circumstances.
Clear orthologs of mammalian adenosine receptors have not yet been identified in C. elegans, though there are several candidate genes based on sequence homology. In contrast, the C. elegans genome contains six phosphodiesterases that fall into two functional classes: one class that specifically targets cAMP (PDE-4,6) and another that is thought to target cGMP (PDE-1,2,3,5) [59, 60]. High doses of caffeine have been shown to inhibit mammalian cAMP phosphodiesterases [61, 62], indicating that the former class is of greater interest. Further investigation will determine whether adenosine receptors, phosphodiesterases, or other targets are involved in caffeine’s effect on worm life span.
Two factors have complicated mammalian research with caffeine. In human populations, most studies determine caffeine intake by consumption of caffeine-containing foods and beverages, such as coffee, tea, soft drinks, and chocolate, all of which contain other compounds that have the potential to affect the diseases under investigation. For example, one study found that long-term coffee consumption shows a strong correlation with reduced risk of type 2 diabetes , while a later study found a similar correlation for both caffeinated and decaffeinated coffee consumption . A recently published study identified a correlation between consumption of either caffeinated or decaffeinated coffee and reduced mortality risk for a range of age-associated diseases, including diabetes . The collective findings from these studies suggest that some of the insulin-related benefit from coffee consumption may result from compounds in coffee other than caffeine. Interpretations are further complicated by seemingly contradictory effects resulting from acute and chronic caffeine treatment in some circumstances, as discussed previously with respect to insulin sensitivity and hypertension. C. elegans may be a useful model for decoupling these types of complications. For example, worm studies completed to date have already begun to separate influences from caffeine and non-caffeine sources with respect to coffee. Dostal et al.  identified SKN-1 as a primary downstream factor in the caffeine-independent delay in amyloid beta toxicity using coffee extract, while this study and that by Lublin et al.  identify IIS as an important player in life span extension by caffeine.
A growing accumulation of evidence in humans, rodents, and nematodes suggests that chronic caffeine exposure may yield significant health benefits by delaying aging and preventing specific age-associated pathologies. Recent studies indicate that C. elegans will be a useful system for dismantling the molecular events that underlie the beneficial effects of caffeine. C. elegans research may also help unravel the complications associated with acute versus chronic caffeine treatment and identify other compounds in coffee and tea with the potential to promote longer life span. Overall, based on the observations that caffeine is capable of increasing life span in worms and has been correlated with decreased mortality in humans, we anticipate the expansion of studies examining the influence of caffeine on longevity into mammalian systems.