Fasting: molecular mechanisms and clinical applications

Abstract

Fasting has been practiced for millennia, but, only recently, studies have shed light on its role in adaptive cellular responses that reduce oxidative damage and inflammation, optimize energy metabolism, and bolster cellular protection. In lower eukaryotes, chronic fasting extends longevity, in part, by reprogramming metabolic and stress resistance pathways. In rodents intermittent or periodic fasting protects against diabetes, cancers, heart disease, and neurodegeneration, while in humans it helps reduce obesity, hypertension, asthma, and rheumatoid arthritis. Thus, fasting has the potential to delay aging and help prevent and treat diseases while minimizing the side effects caused by chronic dietary interventions.

Figure 1. Fasting extends lifespans of yeast, worms and mice

A) lifespan of E. coli incubated in either LB medium or nutrient-free buffer (Gonidakis et al., 2010); B) lifespan of S. cerevisiae incubated in either medium or water (Wei et al., 2008); C) Lifespan of C. elegans in standard medium or in medium with a 90% reduction or complete removal of bacterial food (Kaeberlein et al., 2006); D) Lifespan of mal C57BL/6J mice on alternating day fasting initiated at 1–2 month of age (Goodrick et al., 1990).

Figure 2. Pivotal roles of the nervous and endocrine systems as mediators of adaptive responses of major organ systems to intermittent fasting

IF modifies brain neurochemistry and neuronal network activity in ways that optimize brain function and peripheral energy metabolism. Four brain regions that are particularly important in adaptive responses to IF include the hippocampus (cognitive processing), striatum (control of body movements), hypothalamus (Hyp, control of food intake and body temperature) and brainstem (control of cardiovascular and digestive systems). The brain communicates with all of the peripheral organs involved in energy metabolism. IF enhances parasympathetic activity (mediated by the neurotransmitter acetylcholine) in the autonomic neurons that innervate the gut, heart and arteries, resulting in improved gut motility and reduced heart rate and blood pressure. By depleting glycogen from liver cells, fasting results in lipolysis and the generation of ketone bodies resulting in a reduction in body fat. IF enhances insulin sensitivity of muscle and liver cells, and reduces IGF-1 production. Levels of oxidative stress and inflammation are reduced throughout the body and brain in response to IF.

Figure 3. Fasting in mammals

A) Concentrations of ketone bodies (acetone, β-hydroxybutyric acid, acetoacetic acid) and plasma free fatty acids (FFA) during 40 days of fasting in humans. Note the more than three orders of magnitude change in β-hydroxybutyrate and the doubling of FFA; B) Brain substrate utilization in three fasting obese volunteers after several weeks of food deprivation. Many studies suggest that human brain cells can survive with little to no glucose, but this has not been clearly demonstrated (Redrawn from: (Cahill, 2006)). C) Emperor penguins can fast for periods lasting for over 5 months. The picture shows Emperor penguins and their chicks a few weeks before fledging (courtesy of Yvone le Maho). The parents go back and forth between the open sea and their colony on sea ice, next to a glacier, which offers protection against wind, to regurgitate food conserved in their stomach to feed their chicks while they are themselves fasting. Fasting penguins undergo 3 phases (Le Maho et al., 1976; Le Maho et al., 1981; Robin et al., 1987). The first phase (phase I) represents a transition between the fed state and starvation, during which the penguin stops utilizing diet-derived energy. This phase, which lasts between several hours and several days, is characterized by a rapid decrease in protein loss. The following phase (phase II), is a ketotic phase associated with protein sparing which can last for several days in rats to several months in obese geese, king penguin chicks, bears, and seals (Adams and Costa, 1993; Atkinson and Ramsay, 1995; Castellini and Rea, 1992; Cherel et al., 1991; Cherel and Groscolas, 1999; Cherel and Le Maho, 1985; Cherel et al., 1988a; Cherel et al., 1988b; Fond et al., 2013; Reilly, 1991; Robin et al., 1987; Robin et al., 1988). Phase III is brief, since the high protein loss leads to death. During phase III glucose and total plasma protein levels are reduced, and uric acid increases while ketone bodies values remain low. Wild animals that fast for long periods are efficient at sparing proteins during long periods of fasting, with only 2–10% of total energy coming from proteins versus the 20–40% in species less adapted to fasting.

Figure 4. Neural circuits and cellular signaling pathways the mediate adaptive responses of the brain to fasting

A) Neurons in the hippocampus play critical roles in learning and memory, and are vulnerable to dysfunction and degeneration in Alzheimer’s disease, stroke, traumatic brain injury and epilepsy. The dentate gyrus (yellow) contains neurons that receive inputs from neurons in the entorhinal cortex (EC), with the latter brain region serving as a conduit for sensory information from higher cerebral cortical regions involved in responding to sensory inputs and internally-generated cognitive processes. Increased activity in these neurons occurs in response to fasting resulting in the production of brain-derived neurotrophic factor (BDNF). BDNF promotes the growth and maintenance of dendrites and synapses, and also enhances the production and survival of new neurons from neural stem cells; the newly-generated neurons then integrate into the existing neural circuits; B) Signaling pathways by which glutamate, BDNF, insulin and glucagon-like peptide 1 (GLP-1) improve neuronal bioenergetics and protect the neurons against neurodegenerative disease and traumatic injury. Glutamate activates AMPA and N-methyl-D-aspartate (NMDA) receptors resulting in Ca²⁺ influx and the activation of Ca²⁺/calmodulin-sensitive (CaM) kinases which, in turn, activate the transcription factors cyclic AMP response element-binding protein (CREB) and nuclear factor κB (NF-κB). Genes induced by the latter transcription factor include those encoding BDNF, the DNA repair enzyme APE1, the master regulator of mitochondrial biogenesis PGC-1α, and the antioxidant enzyme manganese superoxide dismutase (MnSOD). BDNF and insulin bind their respective receptor tyrosine kinases (trkB and the insulin receptor) resulting in the activation of the PI3 kinase and Akt kinase. BDNF also stimulates mitogen-activated protein kinases (MAPK). Some of the gene targets of BDNF include PGC-1α, APE1, and the anti-apoptotic protein Bcl-2. Insulin activates the mammalian target of rapamycin (mTOR) pathway to promote protein synthesis and cell growth. Finally, GLP-1 activates receptors (GLP-1R) coupled to cyclic AMP production, CREB activation and BDNF production.

Figure 5. Differential stress resistance and sensitization in aging, disease prevention and cancer treatment

A) In both mice and humans, fasting for 2 or 5 days, respectively causes an over 60% decrease in IGF-I, a 30% or more decrease in glucose and a 5–10 fold increase in the IGF-1binding protein and inhibitor IGFBP1 (Cahill, 2006; Lee et al., 2012; Raffaghello et al., 2008; Thissen et al., 1994a; Thissen et al., 1994b). These and other endocrinological alterations affect the expression of hundreds of genes in many cell types and the consequent reduction or halting of growth and elevation in stress resistance, which may be dependent in part on FOXO and other stress resistance transcription factors. These periodically extreme conditions can promote changes, which are long-lasting and delay aging and disease independently of calorie restriction, although the cellular mechanisms responsible for these effects remain poorly understood. In the presence of chemotherapy drugs, fasting can promote the protection of normal but not cancer cells (differential stress resistance, DSR) since oncogenic pathways play central roles in inhibiting stress resistance and therefore cancer cells are unable to switch to the stress response mode; B) The extreme changes caused by fasting, and particularly the very low IGF-1and glucose levels and high IGFBP1 also generate a tumor prevention environment which promotes cancer cell death since transformed cells have acquired a number of mutations which progressively decrease their ability to adapt to extreme environments (differential stress sensitization, DSS) (Guevara-Aguirre et al., 2011; Lee et al., 2012; Lee et al., 2010).