by Siim Land
Exercise has anti-inflammatory effects and it increases SIRT1[53]. The long-term benefits of exercise are even thought to be regulated by SIRT1.
Cyclic-AMP (cAMP) pathway activates SIRT1 very rapidly to promote fatty acid oxidation independent of NAD+[54]. cAMP is linked with AMPK which gets activated under high energy demands while being energy deprived.
Heat exposure and saunas increase NAD+ levels which promote SIRT1 as well[55]. Sweating, cardio, yoga, or infrared saunas will probably have a similar effect on activating heat shock proteins.
Chronic oxidative stress and DNA damage deplete NAD+ levels and decrease sirtuin activity. This will then disrupt DNA repair and impair mitochondrial functioning. That’s why you want to keep stressors acute and followed by recovery.
Melatonin can activate sirtuins and has anti-aging effects[56]. It’s also the main sleep hormone and a powerful antioxidant that helps the brain get more recovery from deeper stages of sleep. Most of the repair processes and growth happen when you’re sleeping and melatonin plays an important role.
Sirtuins also affect the circadian clocks so keeping a consistent circadian rhythm is incredibly important for longevity. NAD+ is under circadian control and when you’re misaligned you’ll have less energy and lower SIRT1 and SIRT3 activity[57]. Circadian rhythm mismatches are linked to many metabolic disorders, glucose intolerance, and brain degeneration. That’s the opposite of what you want for longevity.
Figure 8 How SIRT6 Affects Longevity
These are the main ways of increasing sirtuin activity but they’re not exclusive to the SIRT family. In fact, most of them also interact with the other longevity-promoting pathways we mentioned beforehand. So, let’s carry on.
Caloric Restriction and Longevity
One of the few known ways of actually extending lifespan in most species is caloric restriction.
Caloric restriction and energy deprivation lower mTOR signaling, which in turn upregulate other pathways of energy homeostasis, such as AMPK and autophagy.
Autophagy and cellular turnover are essential for the life extension effects in DAF-2 mutants[58]. Suppressing autophagy in other species has been found to negate the longevity benefits of caloric restriction as well. For instance, if you block genetically modified mices’ autophagy genes, then they won’t live longer even under caloric restriction, whereas normal mice who have autophagy activated do[59].
Therefore, the life-extension benefits of caloric restriction and fasting are mostly induced by autophagy and increased sirtuin activity that promote cellular turnover and recycling of old cells. That’s an important point because it means you can side-step some of the negative side-effects of prolonged caloric restriction by knowing what you’re doing and elevating autophagy with other means. In Chapter IV, we’ll look at this in more detail.
Nematode Worms and Anti-Aging
In 2017, a study published in the journal Cell Metabolism showed that aging and age-related diseases are associated with a decrease in the cells’ ability to process energy efficiently[60]. Scientists used nematode worms who live for only two weeks to carry out an experiment on their mitochondria.
They found that restricting the worms’ calories and manipulating AMPK promoted longevity by maintaining mitochondrial networks and increasing fatty acid oxidation. This happened in communication with other organelles called peroxisomes that regulate fat metabolism (See Figure 9). Essentially, more fatty acid oxidation from their own energy stores led up to living longer because they were put under caloric restriction.
Figure 9 Dietary Restriction Increases Longevity Through Mitochondrial Network Homeostasis
The researchers proposed that fission and fusion amongst the mitochondria’s network and fatty acid oxidation are required for the longevity benefits. For intermittent fasting-mediated lifespan increase, you need the dynamic remodeling of mitochondrial networks, which happen in response to various physiological and pathological stimuli.
The life cycles of mitochondria are characterized by fission and fusion events (See Figure 10).
Fusion states happen when several mitochondria mix and organize themselves into a network. They basically merge together into a single much larger mitochondrion.
Fission states happen when the fused mitochondria get split into 2 out of which the one with a higher membrane potential will return to the fission-fusion-cycle and the one with a more depolarized membrane will stay solitary until its membrane potential recovers. If its membrane potential remains depolarized it’ll lose its ability to fuse and eventually will be eliminated by mitophagy.
Figure 10 Mitochondrial Fission-Fusion Cycles and Longevity
Changes in nutrient and energy availability can make the mitochondria stay in either one of these states for longer.
Post-Fusion State is called Elongation, which is characteristic to states of energy efficiency, such as starvation, acute stress, caloric restriction, and biological aging (senescence).
Post-Fission State is called Fragmentation, which shortens the mitochondria and keeps them separate. This is typical to bioenergetic inefficiency that’s caused by high energy supply and extended exposure to excess nutrients.
Basically, caloric restriction promotes energy efficiency because the organism is required to sustain itself with fewer calories. Having access to an abundance of energy, however, leads to inefficient mitochondrial function because every single mitochondrion must expend less effort to carry its weight so to say. That can lead to the accumulation of dysfunctional components.
These mechanisms show that the mitochondria evolved to adapt to drastic changes in nutrient availability in the form of fasting and feasting. In times when food is scarce, the body goes into semi-hibernation and repair mode so it can last until the next period when there is plenty of food and it’s a better time to procreate. Fasting and caloric restriction promote mitochondrial efficiency by fusing together several mitochondria. Nutrient excess in the eating phase fragments the mitochondria and decreases their ability to produce energy. Chapter XII will also talk about nutrient/energy density but for now, let’s carry on with caloric restriction.
Caloric Restriction of Rats and Monkeys
Caloric restriction and intermittent fasting have been linked to longevity previously already. Here are some of the main findings in other species.
In 1946, a study on rats found that fasting one day out of three increased lifespans in males by 20% and in females by 15%[61]. While they didn’t experience any retardation of growth, the death of tumors increased in proportion to the amount of fasting. Other studies on rodents have noted reduced inflammation and other age-related health issues[62].
Fasting has been shown to increase the lifespan of bacteria and yeast by ten-fold[63]. Yeast have a very short lifespan of just a few days and weeks, but a 10x boost is still phenomenal.
Caloric restriction shows increased lifespan of brain neurons in both humans and monkeys[64]. Maintaining cerebral health is a critical factor for longevity because you wouldn’t be able to enjoy your life.
In 2009, a group of scientists from the University of Wisconsin reported improved biomarker and longevity benefits in rhesus monkeys who ate less[65]. However, in 2012, a follow-up study done by the National Institute of Aging noted there to be no improvements in survival, but they did find a trend toward better health. After working through the conflicting outcomes, it’s thought that the different results were caused by several things[66].
Caloric restriction is more beneficial in adults and older monkeys but not as so in younger animals. That’s because growing organisms need more nutrition for proper development. Similar effects may be true in humans as well.
How much less food was eaten also affected the differences in survival rates. A severe caloric restriction isn’t sustainable in free-living humans who have access to cheap hyper-palatable food. In humans, malnourishment and nutrient deficiencies aren’t healthy in the long term either.
The monkeys in the National Instit
ute of Aging ate naturally sourced foods whereas the ones in Wisconsin ate processed food with higher sugar content, which made them substantially fatter. That’s probably due to the increased inflammation and insulin/IGF-1 pathway stimulation.
There were also sex differences, where females seemed to have less adverse effects of obesity than males. This makes sense, as women are more prone to carrying extra fat for their offspring and thus not be that affected by it as much.
Figure 11 Graph of increased lifespan from higher caloric restriction in monkeys
What about humans? Do caloric restriction and intermittent fasting have a similar effect on longevity in humans? We do share 93% of the genes with rhesus monkeys. Other species like Chimpanzees are also practically genetically identical to us.
One human study on 3 weeks of alternate day fasting discovered an increase in SIRT1, which is associated with longevity[67]. Why this happens is still unclear, but it’s suggested that caloric restriction induces cellular respiration, which increases NAD+ and reduces NADH levels. NADH inhibits Sir2 and SIRT1.
SIRT1 has been shown to also activate PGC-1α, which triggers the growth of new mitochondria[68]. SIRT3, SIRT4, and SIRT5 improve mitochondrial function as well[69]. This may be evidence that if not increased lifespan, then caloric restriction and fasting will improve the longevity of the cells nonetheless.
When your body faces a shortage of energy whether through caloric restriction, fasting, starvation, or anything the like, then you’re going to promote the fusion of mitochondria. This lowers your energetic demands because the organelles in your cells are better connected. It’ll also make you recycle old worn out cell components and convert them back into energy through the process of autophagy. Mitophagy is a layer deeper and happens inside the mitochondrial fission-fusion cycle.
Energy restriction also upregulates the other genes that increase energy efficiency by improving insulin sensitivity and fat oxidation. Remember – the increased lifespan of the nematode worms happened because of peroxisome-mediated fat metabolism. During states of fasting or depletion of exogenous calories, your mitochondria rev up their functioning and boost endogenous energy production from internal sources.
Figure 12 Increased lifespan in many species is linked to improved mitochondrial functioning and increased sirtuin activity
Ketosis is another vital component to the survival of the mitochondria as it allows them to become more energy efficient. This shift of starting to burn ketones preserves muscle tissue, gives adequate energy to the brain, and keeps you satiated by downregulating some of the hunger signalings.
From an evolutionary perspective, it makes perfect sense, as the mechanisms of longevity came from organisms trying to survive periods of nutrient deprivation and avoid age-related damage.
The mitochondrial fission-fusion cycles are also dependent on autophagy modulating pathways such as AMPK and mTOR[70]. We’ll be getting to know these three very well in the upcoming chapters but to give you some basic overview:
mTOR or mammalian target of rapamycin is responsible for cell growth, protein synthesis, and anabolism. It will make the body build new tissue.
AMPK or AMP-activated protein kinase is a fuel sensor that is involved in balancing energy deprived states.
Autophagy is the process of self-eating and cellular turnover in which the body recycles its old worn out components back into energy.
mTOR inhibits autophagy because it makes your body grow, which requires expending energy and upregulating the metabolism, whereas AMPK supports autophagy due to the energy-deprived state.
One of the subunits of AMPK, AAK-2, is required for DAF-2 mutations to promote longevity in C. Elegans[71]. The mechanism by which this happens is unknown but overexpression of AAK-2 increases lifespan in worms by 13%.
Nutrient starvation allows unneeded proteins to be broken down and recycled into amino acids that are essential for survival. That keeps the organism alive longer because of increased mitochondrial efficiency. Therefore, the key to longevity and increased lifespan still gets traced back to decreased energy intake and improved energy usage within the body itself.
Mice who lack the insulin receptor in adipose tissue live longer because of increased leanness[72]. mTOR interacts with the insulin pathway to regulate the lifespan and development of C. Elegans and fruit flies[73].
There’s some evidence to show how excessive mTOR and insulin signaling are related to accelerated aging and disease but it’s not that black and white. In some situations, they can actually be beneficial and even pro-longevity. The same applies to caloric restriction and intermittent fasting. One of the core principles of this book is that you want to use these anabolic hormones only at the right time under the right circumstances. That’s why it’s called Metabolic Autophagy. Let’s carry on with more of the pathways related to longevity.
Stress Adaptation and Longevity
Adaptation to harsh environmental conditions has been mentioned a few times already. Heat stress seems to have many health benefits and some of it has to do with the insulin/IGF-1 signaling.
In C. Elegans, activation of heat shock transcription factor 1 (HSF-1) is also required for the DAF-2 mutations to extend lifespan[74]. This is thought to be because HSF-1 and DAF-16 activate specific genes that turn on small heat-shock proteins and promote longevity.
Essentially, exposure to stress whether that be the cold, caloric deprivation, or the heat makes the organism live longer because of forcing hormetic adaptation. Hormesis through heat stress increases the lifespan of flies and worms[75].
Interestingly, increased insulin/IGF-1 signaling mutations prevent the localization of DAF-16 by heat shock, which raises the possibility that the increased lifespan due to stress adaptation occurs because of lower insulin[76].
Stress resistance has been found to be a major contributing factor to increased longevity in animals with mutated insulin/IGF-1 signaling. Activation of the stress-response JNK pathway in fruit flies increases their lifespan by up to 80%[77]!
The adaptation to stress and harsh conditions is mediated through certain transcript factors that regulate energy homeostasis and longevity. They’re called FOXO proteins.
Increase FOXO Factors for Longevity
’FOX’ stands for ’Forkhead box’ and it represents a class of proteins and transcript factors that have many functions in the human body.
FOXO proteins are transcript factors that regulate longevity through the insulin and insulin-like growth factor signaling[78].
Invertebrates have a single FOXO gene, whereas mammals have four: FOXO1, FOXO3, FOXO4, and FOXO6. In mammals, FOXO proteins regulate stress resistance, cellular turnover, apoptosis, glucose and lipid metabolism, and inflammation[79][80].
FOX represents the class of proteins, the letter ’O’ is the subclass, and the number represents the member of that group. There are over 100 subclasses of FOX proteins in humans, such as FOXA, FOXR, FOXE, etc. and they have many functions. FOX proteins with the class ’O’ are regulated by the insulin/Akt/mTOR signaling pathway.
Theoretically, upregulated FOXO pathway activities increase lifespan in many species because of promoting stress adaptation in harsh environments. The FOXO pathway is an evolutionarily viable mechanism for adapting to low levels of insulin and energy deprivation.
Figure 13 Blocking the Insulin/IGF-1 Receptor promotes FOXO proteins, which leads to stress resistance, tumor suppression, and longevity
Anabolic mechanisms such as insulin, mTOR, and IGF-1 tell the body to grow and replicate but this may come at the expense of longevity and accelerated aging. Which is why you’d want to know how to balance it with the catabolic processes of autophagy, AMPK, and FOXO factors.
In fruit flies, an unhealthy high sugar diet in early adulthood curtails survival later in life despite subsequent dietary improvements[81]. Meaning that having a bad diet in your earlier youth may cause irreversible damage to your metabolic health and longevity. One of the reasons for that is the suppr
ession of FOXO transcription factors and insulin resistance.
SIRT1 increases FOXO DNA binding by deacetylating FOXO in response to oxidative stress. FOXO proteins get increased in response to cellular stress and increased energy depletion.
Calorie restriction increases sirtuins as well as FOXO factors[82][83], which is something we already know.
Fasting for 48 hours elevates FOXO1,3, and 4 by 1.5 fold and refeeding drops it back to baseline[84]. FOXO1 is also critical for adapting to fasting by activating gluconeogenesis in the liver[85]. This makes the liver produce its endogenous glucose whether from amino acids or fatty acids[86].
Even just acute exercise increases FOXO1 phosphorylation, improves insulin sensitivity and promotes mitochondrial biogenesis[87]. However, chronic exercise may decrease this exercise-induced FOXO expression[88]. FOXO factors are important for regulating muscle energy homeostasis and adapting to the stimulus[89].
In response to heat stress, Drosophila dFOXO contributes to increased heat shock protein levels, which will protect DNA damage and maintains cellular resistance[90]. Taking a sauna, exercising and sweating can promote FOXO activation.