Non-communicable diseases (NCD) are the main causes of death in developed countries and are largely associated with aging. The worrying trend is that they becoming more common in younger age groups and the increase in life expectancy we have seen in people born in the 1920’s and 1930’s will likely be reversed in the age group born in the 60’s, 70’s & 80’s caught up in the obesity epidemic (1). Obesity is a risk factor for NCD but does not explain the cause as NCD are also the major killers of normal weight people as well. The difference between long living mammals such as naked mole rats and their shorter live cousins is thought to be due to their superior cellular protection and repair processes (2) and age related diseases in humans may be related to deficiencies in these processes too.
Constant Turnover of Protoplasm
The material makeup of living things is gradually broken down and rebuilt on a daily cycle resulting in profound changes over time during the different phases of life from growth and development to the long involutionary period of aging in adulthood (3). Cleaning up residual protein fragments generated as a byproduct of metabolism is thought to be the underlying function of sleep in virtually all animals (4) and explains the molecular basis of impairment associated with sleep deprivation. Efficient removal of protein residues is an essential life function and the regulation of these processes is inextricably tied up with nutritional status of cells (5) as is the protein synthesis side of the recycling equation (6). Autophagy or “self-eating” is the term given to cell refuse disposal and is regulated by recognition of the fasting (7) and feeding cycles, which is dependent on glucose abundance in foods. In this way glucose abundance in the diet can be linked to underlying causes of NCD (8).
Health Benefits of Calorie Restriction:
It is generally agreed that over-nutrition is responsible for obesity, diabetes and many age related diseases and it is well known that calorie restriction, even intermittent fasting, benefits all of these things and it is thought that possibly the main mechanism of these benefits is due to up-regulation of autophagy (9) which recycles redundant, potentially toxic material (10). Sensing mechanisms of nutritional status trigger cell signaling that regulates basal autophagy where survival requires mobilization of body stores during fasting. Fat mobilization (lipophagy) takes place by a process akin to autophagy (11) and mobilization of glucose from glycogen is also accomplished via autophagy (12). It may be an opportunist adaption of evolution that autophagy is employed for mobilization of substrate stores during fasting as well as protein recycling and intracellular “refuse disposal” processes. Feeding inhibits and fasting stimulates autophagy through the actions of insulin and glucagon respectively and the primary determinant of insulin and glucagon secretion is blood glucose. Insulin and glucagon control autophagy via their opposing effects on mTOR (mammalian target of rapamycin) whereby mTOR activity suppresses autophagy (5). Glucagon via glucagon receptor activates adenylate cyclase which increases cAMP and activates PKA (cAMP activated protein kinase) which inhibits mTOR thereby stimulating autophagy in situations of fasting. On the other hand glucose stimulated insulin secretion suppresses autophagy via Akt (PKB) activation of mTOR. A ketogenic diet was found to inhibit the mTOR pathway via decreased Akt signaling as well as increased AMPK signaling in the liver of rats (13). Through similar pathways Insulin and glucagon also have a role in regulating mitochondrial biogenesis. Mitochondrial biogenesis is regulated by the master-controller, PGC1 nuclear receptor coactivators. Fernandez and Auwerx (14) discovered how the pancreatic hormones insulin and glucagon play an opposing role in PGC1a transcription. Insulin secretion which by activating Akt(PKB) depresses mitochondrial biogenesis by inhibiting PGC1a transcription while PGC1a transcription is increased via the glucagon receptor-PKA pathway.
NRF2: Recent research by Rochelle Buffenstein’s group into the differences between long lived species the naked mole rat with a 40 year maximum life expectancy compared with 4 years in their short lived relatives, focuses on their superior detoxification and repair abilities largely mediated by the master controller NRF2 (NFE2L2) enhancing transcription of multiple proteins involved in cell protection and detoxification as well as chaperones involved in autophagy and protein stability (2). The activity of two master controller transcription factors NRF2 and PGC1a appear to function in tandem, as they are increased by the same environmental stimuli and cell signaling pathways regulating multiple genes involved in autophagy (15) and mitochondrial regeneration (14) respectively. Both NRF2 and PGC1a are increased by a ketogenic diet (16) (17) (18).
AMPK Another sensor of cellular energy levels is AMP activated protein kinase (AMPK) functions in a different way to the glucagon receptor activated cyclic AMP activated protein kinase (PKA). AMPK responds to increased AMP/ATP levels that occur with exercise. A recent review highlights a central role for AMPK in disease resistance and longevity (19) promoting transcription of FOXO dependent proteins such as PGC1a and NRF2 while promoting autophagy by inhibiting mTOR. Of particular relevance to the mechanism of ketogenic diets is that insulin signaling powerfully suppresses AMPK activation via Akt/PKB (20) while glucagon activates AMPK by activating CaMKIV (21)
Autophagy is induced with ketogenic diet
Because a ketogenic diet profoundly suppresses insulin secretion even in the presence of adequate calorie intake (22) it follows that ketogenic diets enhance basal autophagy (13). A ketogenic diet as in fasting also requires glucose production from glycogenolysis and gluconeogenesis which is controlled be glucagon secretion which via the PKA pathway suppresses mTOR to enhance autophagy and on the other hand carbohydrate foods that increase glucose and insulin levels activate mTOR via akt/PKB pathway to suppress basal autophagy.
Widespread appreciation of the emerging importance of autophagy in life and disease is likely to focus attention on ways to optimize these processes and macronutrients and phytonutrients have a profound impact as seen from the lessons from epidemiological and basic science studies on restriction of glucose abundance through low glycemic and ketogenic diets (23).
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