And how are mitochondria immortal?
Summary
Existing theories of aging search for markers that are better predictors of life expectancy than chronological age.
Biological age is an increasingly popular measure of life expectancy, although existing chemical biomarkers of biological age are unreliable.
Because mitochondria have their own DNA that reproduce asexually, their immortality may be critical to human longevity.
Cold therapy both supports mitochondrial health and stimulates production of cold shock proteins that extend lifespan in animal models.
Longevity
A few months ago I travelled to Optispan in Seattle WA to undergo a comprehensive health assessment. They drew my blood to get a fasted lipid panel. They used ultrasound to image my blood vessels and internal organs. They measured my body composition with a DEXA scan. And they put me through a battery of tests to measure my overall physical fitness, including grip strength, standing broad jump, pushups to failure, and VO2max.
VO2max is the most grueling.
My coach Nick fitted me with a mask that covered my mouth and nostrils to monitor my breathing while I walking on an inclined treadmill. Nick gradually increased the grade of the treadmill so that I would be working harder and harder, while a machine measured the rate at which my body was consuming oxygen. The higher my oxygen consumption, the greater my cardiovascular fitness.
I walked that treadmill until I was almost certain I was going to puke into Nick's mask, at which point I gave up.
Nick said my V02max was 28 ml/kg/min.
After each of my strength tests (e.g., I did 33 pushups and a 6'2" standing broad jump) Nick praised my effort and encouraged me on to the next exercise, but after my VO2max Nick seemed... disappointed.
Laying flat on my back, panting, I texted my results to AJ Kay. She wrote me back almost right away:
For a 58-year old man, a VO2max = 28 ml/kg/min is "poor" and somewhere online there's probably a website run by Peter Attia MD that says I'll likely be dead in seven years.
That got me thinking about aging and longevity.
What is aging?
An online actuarial table says that the average 58-yr-old American male can expect to live another 21.87 years. In other words, based on my birthdate alone, the Social Security Administration (SSA) expects me to be dead at the age of 80. However, knowing my general fitness level and the age to which my parents and close relatives lived can improve upon that estimate of my life expectancy.
For example, when other factors, such as muscle mass and body composition, insulin sensitivity, bone density, cognitive capacity, and family history are taken in to account, estimates of my life expectancy might be modified up or down.
(BTW, in my case all of these markers except VO2max are consistent with a very long, healthy lifespan. For example, my Nana, who was infamous within our family for refusing to eat vegetables, lived to be 99 -- so who's to say?)
Recently, the term biological age has come into fashion. It does the same thing as estimate life expectancy, but in a more complicated way that is better for either scaring me into making lifestyle changes or appealing to my vanity, depending on the outcome. Biological age, as distinct from chronological, is a way of matching my estimated life expectancy with a population that is either older or younger than me. For example, suppose the Optispan assessment estimates that my life expectancy really is only seven years. Because that's the same average life expectancy of an 82-yr-old man, the actuarial tables might say that although my chronological age is 58, my biological age would be 82.
That's a frightening image that might get me into a spin class (or something).
Conversely, if my estimated life expectancy as a result of my assessment was really 40 years, then the actuarial tables would match me with the average 36-year-old and you could say my biological age was really 36 -- about which I'd feel great.
The emotional impact of biological age has helped support a veritable scientific cottage industry selling quick tests to estimate life expectancy expressed in terms of biological age. For example, I recently did a DNA methylation test offered by Elysium Health that told me my biological age was 51 years old -- which is to say that epigenetic markers on my DNA exhibit patterns that are consistent with a life expectancy of 27 more years, rather than the 22 suggested by my birthday.
The disadvantage of physiological measures such as grip strength and VO2max is that they're downstream of the mechanisms that control aging. For example, no one really believes that poor hand grip strength causes mortality. Rather, we all know that hand grip strength is an indicator of overall fitness level, and that it is fitness that modifies life expectancy.
But how?
Theories of aging
The purpose of a scientific theory is to generate falsifiable hypotheses that improve our understanding of how the world really works. Good theories generate lots of informative hypotheses and if we had good theories of aging, perhaps we could use the resulting knowledge to slow our own biological clocks and live longer, healthier, more satisfying lives.
The most popular theories of ageing can generally be classified into three broad categories:
somatic theories that measure declining physiological parameters, like hand grip strength, walking speed, time balancing on one leg, and VO2max.
genetic theories that measure biomarkers of accumulated damage to DNA, RNA, and/or the proteins they synthesize or the telomeres that protect them.
energetic or metabolic theories, including mitochondrial theories, that describe progressive declines in metabolic function.
What I've discovered is that the first two of these these theories don't work for discovering the mechanisms of aging, and the third is difficult to measure.
The problem with somatic theories is that they confuse causation and correlation. For example, we do not age because we lose hand grip strength. We lose hand grip strength because we age. While it might be a fine thing to improve my grip strength, it won’t lead me to the proverbial Fountain of Youth. The fact is that weight training both leads to stronger grip strength and better overall health. So it’s not that grip strength slows ageing. It’s that both grip strength and health improve with weight-bearing exercise.
Biochemical makers of accumulated DNA damage don’t work any better. For example, telomeres are protective nucleotides found at the end of chromosomes. As our cells divide and replicate, they gradually lose protective telomeres, so it stands to reason that longer telomeres will be found in younger people, and that the rate of telomere shortening would be related to ageing.
Except it’s not.
There are two criteria that any effective measure of biological age must demonstrate:
For large statistical ensembles, it must correlate with chronological age.
For individuals, the departures from chronological age correlation must be predictive of expected lifespan. Put another way, wherever biological age departs from chronological age, the biological must be a better predictor of mortality than chronological.
The problem with epigenetics is that it fails both of these tests. For example, the correlation between telomere length and chronological age is poor, as is the correlation between biochemical markers of RNA function (transcriptomics), protein synthesis (proteomics), and metabolomics. The only one of these five that might pass the first test by demonstrating a promising correlation to chronological age is DNA methylation (epigenetics). Moreover, a recent study of 3000 subjects that compared all five markers found they had no correlation to one another and that none of them improve on chronological age as a predictor of expected lifespan, which defeats the whole purpose of estimating biological age in the first place (Jansen et al. 2021).
Mitochondrial theories of aging
Having ruled out the somatic and genetic theories of ageing, we are free to focus our attention on the energetic theories, which in my opinion are much more promising.
Mitochondrial DNA
Any energetic theory of the body must have its origins in mitochondria. Keep in mind that mitochondria have their own DNA, independent from the DNA in the nucleus. All cells, with few exceptions (e.g., red blood cells) contain mitochondria to convert food energy like glucose and fats into electrical energy to power the body. For example, muscles cells have hundreds of mitochondria, because they demand so much energy. However, brown fat cells are packed with even more. Brown fat cells contain thousands of mitochondria to fuel cold thermogenesis — i.e., heat production to maintain body temperature during cold exposure.
Because mitochondria are the site of energy conversion in the body, speed is essential when the body has to react fast. Keeping genetic information stored inside their own organelle walls allows the mitochondria to synthesize enzymes or other proteins right at the site where they are needed, rather than wait to signal the nucleus.
Mitochondria are immortal
According to evolutionary theory, mitochondria were once independent prokaryotic organisms. Prokaryotes don’t have a nucleus inside their cells to protect their DNA (like eukaryotes have). At some point, some poor mitochondria became subsumed by a more complex, predatory eukaryotic cell, but instead of becoming digested, the prokaryotic mitochondria was incorporated as an organelle inside the cell membrane of the eukaryote. There it stayed, in mutualistic symbiosis, producing energy in the form of ATP from the food supplied by the increasingly complex and evolving multi-cellular eukaryotic organism.
While it’s easy to see why the eukaryote would want the more efficient energy mitochondria generating power for it, you might ask, “What’s in it for the mitochondria?”
But it’s important to keep in mind that prokaryotes, without a nucleus, do not reproduce like eukaryotes do.
In a sense, prokaryotes are immortal. They reproduce asexually by replicating their own DNA and dividing into two, identical cells that subsequently operate as separate organisms. In the absence of mutation, the DNA in a prokaryote can keep replicating itself for perpetuity so what difference does it make to the prokaryote if it occupies an environment inside a eukaryotic cell, or floats freely in some primordial soup? Either way, the prokaryote can perpetuate its own genes.
By contrast, in complex, multi-cellular eukaryotes (e.g., human beings), cell division (mitosis) is a mechanism of growth, not reproduction. For example, sexual reproduction requires combining complementary strands of DNA from two parents, a Mother and a Father, to form a fertilized egg (zygote). Therefore, the offspring will always have a different genetic endowment than either parent, recognizable as unique individuals distinct from their parents.
For mitochondria, prokaryotic reproduction inside a eukaryotic human being presents an evolutionary conundrum. Given that both the Mother’s eggs and the Father’s sperm carry different mitochondria, it’s not obvious how the zygote should carry out mitosis with two sets of donor mitochondria.
Evolution resolves this conundrum by destroying the Father’s mitochondria before the zygote begins cell division in earnest. Thus, we inherit our mitochondrial DNA exclusively from our Mother.
That means, absent mutation, our inherited mitochondria is identical to our Mother’s, and our Grandmother’s, and our ancient Great-great-great-grandmother’s, going all the way back to Eve — because prokaryotes are immortal in this way.
In practice, mutations are happening all the time. There are even cases of paternal mitochondria getting mixed up with maternal, confusing mitichondrial lineage (Brondham et al. 2003). Most of these aberrations are fatal, or result in disorders that make it very unlikely for the resulting human being to mature to reproductive success.
But not all of them.
So mitochondrial DNA are subject to their own selection pressures and do evolve over time — just not in the same way that the eukaryotic DNA inside our nucleus does.
Mitochondrial immortality would seem to rule them out as critical to ageing and lifespan in complex, multicellular eukaryotic creatures like humans. If the problem does not reside in the mitochondria, then it must be in other aspects of the eukaryotic organism, right?
Wrong.
How mitochondria hold the key to human longevity
Calorie restriction is the only reliable strategy for extending lifespan in laboratory-controlled animal models. Nothing else works as well, although some of the most highly touted contenders (e.g., rapamycin, Blagosklony 2019) also work energetically — i.e., on the metabolism.
What’s not yet clear is whether intermittent fasting and/or carbohydrate restriction can have the same beneficial, life-extending effect as calorie restriction.
It might, because carbohydrates are more dangerous to mitochondria than fats and proteins.
When we eat too many carbohydrates, the glucose levels in our bloodstream spike up. That glucose spike causes several things to happen, including a release of insulin from the islet cells of the pancreas.
Insulin promotes the transport of glucose from the bloodstream, across the cell membrane, into the cells where it can reach the mitochondria. Except in Type 1 diabetics, who do not make insulin, a blood glucose spike will create an insulin spike to help shuttle the glucose across the cell wall to get it to the mitochondria where it can be processed for either muscle energy (exercise), cold thermogenesis (heat production), or synthesis of lipids for storage in white fat cells.
As mitochondria process excess glucose, they inevitably create electron imbalances as a consequence of the additional energy conversion load. Those imbalances produce what are called reactive oxygen species (ROS, Turrens 2003). Under ordinary circumstances, mitochondria produce melatonin to correct these electron imbalances by donating an electron to the ROS, reducing them to a more stable state.
Unfortunately for the mitochondria, when production of ROS exceeds the capacity of melatonin to donate electrons, the ROS will find their way to mitochondrial DNA and cause damage. Without a nucleic membrane for protection, mitochondrial DNA are ten times more vulnerable to damage than nucleic DNA.
However, mitochondria protect themselves against DNA damage in several ways. First, they carry multiple copies of DNA so that they can select the best for use and reproduction. Second, they have mechanisms to repair damaged DNA, and other mechanisms to destroy mitochondria that are beyond repair.
Also, the body protects mitochondria from the damaging effects of glucose overload. For example, fatigue is a way of signaling the body to stop exercising and rest the mitochondria without overheating them. And insulin resistance is a way of preventing excess glucose from reaching the mitochondria by keeping the glucose in the bloodstream until the mitochondria inside the cells are ready to process it.
Nonetheless, chronic carbohydrate overload will result in chronic mitochondrial damage. Ultimately, when the eukaryotic human being can no longer count on healthy mitochondria to provide energy conversion to support life, it dies.
All of which is to say that mitochondria may be immortal, but they are not invulnerable. They can be killed by overwork.
Life extension as death prevention?
Most of the leading causes of death in the United States result from metabolic disorders, including heart disease, most cancers, Alzheimer’s, obesity, high blood pressure (hypertension), and Type 2 diabetes.
There are several clinical names for what amounts to a problem of chronic carbohydrate overload. Sometimes it’s called metabolic syndrome, and sometimes hyperinsulinemia, or insulin resistance, or pre-diabetes.
No matter what it’s called, the underlying condition is mitochondrial distress. The consequence is decreased lifespan — in other words, reduced biological age. In most cases, the mitochondria can be rescued by some combination of carbohydrate restriction, fasting, increased exercise, and cold therapy. Each of these alters the carbohydrate intake versus consumption balance in ways that give mitochondria time to repair and restore. For example, one study of German men diagnosed with Type 2 diabetes found that just ten days of mild cold exposure was sufficient to reduce insulin resistance by more than 40% (Hanssen et al. 2015), without changes in diet or exercise.
In those ten days, the German men who restored their mitochondrial function were biologically ageing in reverse.
Measuring your mitochondria
The field of mitochondrial-omics is less well developed than genomics, proteomics, transcriptomics (RNA), or metabolomics (Picard et al. 2019). Although the human mitochondrial genome has been sequenced, the fact that mitochondria keep multiple copies of their own DNA within their organelle walls can complicate analysis. The condition in which these multiple copies do not match is called heteroplasmy and it is especially evident in cancer cells, given that “primary dysregulation of mitochondrial function via mtDNA mutation is a pervasive feature of cancer” (Gammage & Frezza 2019). Nevertheless, “experimental approaches to determining a role for mtDNA mutations in cancer-associated mitochondrial dysfunction have yet to yield conclusive data, mostly because of the genetic intractability of the mitochondrial genome and consequently limited experimental tools.”
Mitochondrial sequencing is now available from commercial labs, although I haven’t yet tried them. In general, this approach is similar to other searches for biochemical markers like DNA methylation (epigenetics) and telomere length — except that it’s looking at mitochondrial markers, instead of nucleic.
Nonetheless, my sense is that looking for material markers may be the wrong approach. Given the role of mitochondria in energy conversion, it is likely that healthy mitochondria have an energetic signature different from unhealthy. In a subsequent article, I’ll describe one estimate of my biological age based on the electrical field generated by my body that suggests my biological age is 30 years old.
It sounds preposterous to suggest I'm a 58 year-old man in a 30 year-old body, but remember that's not really what it means. Biological age is nothing more than a backwards calculation of life expectancy. For example, the average 30 year-old man can expect to live another 45 years, so to say that my biological age is 30 means that I can expect to live another 45 years, too.
Adding 45 years to my current age of 58 would mean I'd live all the way to 103 -- which is four years older than my Nana died.
Does that sound so preposterous?
Can cold therapy protect against aging?
In Ice Baths for Mitochondrial Therapy I wrote about the anti-aging benefits of a regular ice bath practice that rejuvenates mitochondria and maintains metabolic health. However, since then a new study has come out that suggests there may be a second mechanism by which cold therapy can protect against the effects of aging -- cold shock proteins.
A group of doctors at the University of Cologne in Germany investigated the effects of cooling human cells to levels slightly below the temperatures typical of deep sleep, but above hypothermia. They discovered that cooling stimulated production of the same proteins associated with life extension in C. elegans worms. They concluded "cold temperature could be a converging mechanism to prevent distinct human disorders with age" (Lee et al. 2023).
References
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Gammage PA, Frezza C. Mitochondrial DNA: the overlooked oncogenome?. BMC biology. 2019 Jul 8;17(1):53.
Hanssen MJ, Hoeks J, Brans B, Van Der Lans AA, Schaart G, Van Den Driessche JJ, Jörgensen JA, Boekschoten MV, Hesselink MK, Havekes B, Kersten S. Short-term cold acclimation improves insulin sensitivity in patients with type 2 diabetes mellitus. Nature medicine. 2015 Aug;21(8):863-5.
Jansen R, Han LK, Verhoeven JE, Aberg KA, van den Oord EC, Milaneschi Y, Penninx BW. An integrative study of five biological clocks in somatic and mental health. Elife. 2021 Feb 9;10:e59479.
Lee HJ, Alirzayeva H, Koyuncu S, Rueber A, Noormohammadi A, Vilchez D. Cold temperature extends longevity and prevents disease-related protein aggregation through PA28γ-induced proteasomes. Nature Aging. 2023 May;3(5):546-66.
Picard M, Trumpff C, Burelle Y. Mitochondrial psychobiology: foundations and applications. Current opinion in behavioral sciences. 2019 Aug 1;28:142-51.
Reiter RJ, Rosales-Corral S, Tan DX, Jou MJ, Galano A, Xu B. Melatonin as a mitochondria-targeted antioxidant: one of evolution’s best ideas. Cellular and molecular life sciences. 2017 Nov;74:3863-81.
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