Energy deprivation in youth and old age.

In youth, energy deprivation primarily results from fasting. Because inconsistent nutrient availability is common for most organisms, natural selection has provided mechanisms that detect nutrient-deprived states, followed by adaptive responses that increase the likelihood of survival until nutrients are restored. Organisms respond to fasting first by oxidizing the cellular cytoplasm, then by activating redox-sensitive kinases - namely the c-Jun N-terminal kinases (henceforth collectively termed JNK) and AMP-activated protein kinase (AMPK) - and Foxo transcription factors (henceforth referred to collectively as Foxo). Together, JNK, AMPK and Foxo induce autophagy. This fasting response is beneficial because autophagy supplies substrates for metabolism that replace missing nutrients and enhances removal of damaged organelles such as mitochondria, which increases lifespan and enhances survival through the fast. Although this response is adaptive in the context of acute nutrient deprivation, it can have harmful consequences when activated chronically. Here, I propose that cells from old organisms are constitutively energy deprived because of lifetime accumulation of dysfunctional mitochondria. As a result, these cells reactivate the fasting response seen in youth. Hence, old organisms constitutively oxidize the cellular cytoplasm and activate JNK, AMPK, Foxo and, finally, autophagy. However, because energy deprivation in old age is driven by mitochondrial insufficiency rather than nutrient deprivation, this response fails to restore ATP production and becomes chronic and deleterious. I suggest that many age-related pathologies, such as oxidative stress, neurodegeneration and sarcopenia, result from aberrant activation of the fasting response.