Importance of Muscle in Healthy Aging and Association between Muscle NAD+ Abundance and Physical Activity in Humans
Introduction – What Scares You About Aging?
If you ask people what disease they are most afraid of when they get old, you are likely to hear things like cancer, heart attack, diabetes or Alzheimer's disease (AD). I would be surprised if one out of 100 people would say that they are afraid of sarcopenia, the age- related progressive muscle loss. However, if you ask people what their idea of happy retirement is, they will probably tell you that they want to remain active, travel and play with their grandkids. The unfortunate thing is that sarcopenia can severely impact your ability to do all of these things.
People underestimate how important it is to focus on retaining muscle as we age. According to a study published in Current Opinion in Clinical Nutrition and Metabolic Care, humans lose 3-8% of muscle mass per decade after the age of 30 and the muscle loss is significantly sped up after the age of 60. By the age of 70, we lose 1% of our muscle mass every year. Indeed, such a loss of muscle mass will severely impair your ability to do many of the things you want to do in an ‘ideal retirement scenario’.
The direct cost of sarcopenia to the US healthcare system was 18.5 billion in 2000. A significant portion of the money is spent on hospitalisation after falls. The loss of muscle makes it more difficult for old people to keep their balance, and they are much more likely to fall. This is shown in a study published in Clinical Nutrition which reported that sarcopenia increases the risk of falls in older adults by 55%.
While falls do not endanger young people significantly, osteoporosis, another age-related disease, substantially increases the risk of fractures after falls in old people. A prevalent fall- related type of fracture in the elderly is a hip bone fracture which makes you immobile. The senescence of repair cells and exhaustion of bone stem cells can make your recovery too long and result in several months of immobilisation. Immobilisation leads to further muscle loss, and it is not uncommon that older adults who suffer hip bone fractures do not get fully back on their feet. The immobilisation leads to rapid deterioration of overall health, as demonstrated in a study published in the Journal of Orthopaedic Surgery and Research, which followed 3993 elderly patients who were admitted to a hospital with a hip bone
fracture. The 12-month mortality rate was a whopping 33% - a third of the patients admitted to the hospital with hip bone fractures did not survive the following year.
The loss of muscle mass can also seriously affect your metabolism. Muscle is responsible for roughly 40% of your resting energy expenditure, and it is the most significant sink for circulating glucose and fatty acids. Losing muscle makes it more likely for glucose to stay in your circulation, which explains why a study published in Diabetes, Metabolic Syndrome and Obesity: Targets and Therapy reported that sarcopenia significantly increases your risk of type 2 diabetes (T2D). Individuals in the lowest quartile of lean mass relative to body weight had a 2-fold greater risk of developing T2D. Indeed, T2D is one of the greatest risk factors for all the major diseases people usually fear, including heart disease, cancer and AD.
The progressive loss of muscle mass associated with ageing might not sound like a serious killer comparable to cancer or heart attack. However, the above-mentioned studies hopefully showed you that sarcopenia is a killer, albeit an indirect one. Moreover, most people would tell you that the length of your lifespan matters less than the quality of life you have as an older adult. This perspective makes fighting sarcopenia even more crucial because most of the goals people have for the 'ideal retirement scenario' depend on independent movement. Indeed, no other tissue is as important as muscle regarding these goals.
We have mentioned the percentages of muscle loss that occurs over the lifespan. Unfortunately, these numbers do not paint the whole picture. Loss of 1% muscle mass every year past 70 might not sound too scary. However, it is not the loss of muscle mass that matters for the functional impairment. The function declines because of the loss of strength. A study published in Sports Health reported that the loss of muscle strength is faster than the loss of muscle mass. We lose about 10-15% of strength per decade up to the age of 70 years and about 25-40% of the strength after the age of 70 years.
These numbers sound pretty bad, right? They suggest that the muscle mass we retain with ageing loses its function. This is not very surprising. Muscle proteins get damaged constantly, and the turnover of these proteins depends on autophagy, whose rate decreases with ageing. Even more importantly, the effectiveness of oxidative phosphorylation, which produces energy in the form of ATP, decreases as we age. As ATP is required for every muscle contraction, it makes sense that the strength declines.
NAD+ Declines As We Age
One of the most studied molecules which stand behind the decrease in ATP generation is NAD+. The whole process of ATP generation is dependent on the electron transport chain, where electron travels across the mitochondrial membrane from a high-energy state to a low-energy state and the energy released in the process is utilised to produce ATP (Electron transport chain is described in detail in the topic Oxidative Phosphorylation, Cellular Respiration - NAD+, NADH).
NAD+ is the dominant electron carrier which positions it at the very core of cellular energy metabolism. It is well-documented that NAD+ levels decrease with ageing and animal models showed that NAD+ decline correlates with functional decline.
According to the review published in Nature Reviews Molecular Cell Biology, NAD+ levels play a role in age-related processes like inflammaging, neurodegeneration, autophagy and mitophagy decline, maintenance of circadian rhythm and DNA repair. The wide variety of systems affected by NAD+ levels indicates that if our mission is to age successfully, high levels of NAD+ are very important.
The Abundance of NAD+ in Human Muscle During Ageing
As the evidence from animals looks promising, the research now started to focus on the role of NAD+ in human ageing. One of the most interesting studies in this field was published this year in Nature Aging, which will go through in detail in this topic. It can tell us quite a lot about how to prevent the functional decline of muscle in advanced age.
The goal of this study was to investigate levels of NAD+ in the muscle of young (20-30 years) and old people (65-80 years old). The elderly were sorted into three categories – trained (completing at least 3 exercise sessions a week), normal (completing one or none exercise sessions a week) and physically impaired (those who performed poorly in short physical performance tests). The activity of elderly participants was also assessed after inclusion via accelerometers. The trained group averaged about 13000 steps a day, the normal group took on average 10000 steps, and those in the impaired group averaged 6000 steps a day. Those in the young group also took an average 10000 steps a day.
It is important to note that the study recruited an abnormally active cohort of the elderly. 10000 steps a day is not a normal level of physical activity in the elderly. A study published in BMC Public Health reported that the average step count of the Japanese elderly is about 6000 steps. Taking 10000 steps a day would land an elderly person in the most active
quartile. Therefore, the trained group could be labelled as extremely active, the normal group as very active and the physically impaired group is actually the normal (average) group.
The major finding of the study is demonstrated in the picture below. There was no statistically significant difference between muscle NAD+ levels of young adults and trained (extremely active) elderly. The encouraging news is that NAD+ levels of the normally active (very active) elderly did not lag far behind. Nevertheless, NAD+ levels of physically impaired (normally active) elderly were significantly lower when compared to more active groups. Overall, these results show us that molecular changes associated with ageing are highly modifiable. We can do something about ageing. These results are in the general agreement that exercise is one of the most potent anti-ageing interventions out there, which can actually positively affect tens of age-related processes.
The advantage of this study was the extensive dataset describing both the molecular and functional status of the muscle. The available molecular parameters included mitochondrial abundance and rate of mitochondrial respiration. The functional parameters included muscle volume (determined by MRI) and muscle strength. As mentioned, researchers also knew the habitual physical activity measured by accelerometers. Remarkably, the study found a correlation between almost all of these parameters and NAD+ abundance. To understand how strong these correlations were we need to have a quick look at some core principles of statistics.
When we assess associations between values, we try to establish which variable is the response variable and which is the explanatory variable. A simple example would be a relationship between outside temperature (explanatory variable) and the rate of flower growth (response variable). In this case, it is clear which variable is the explanatory one and which is the response one (we are pretty sure that it does not get warmer because flowers start to grow quicker). However, as we will soon show, we are often not sure what is the direction between variables when it comes to complex biological relationships.
Statistical tools can tell us how much of the variance of the response variable is explained by the explanatory variable. In other words, how strongly related these two values are. This value is termed 'R2'. When R2 equals 1, the explanatory variable explains all the variation in the response variable; when R2 equals 0.5, it explains 50% of the variation. Most biological systems are complex (especially ageing) - when you get R2 over 0.1, you found a good strong association.
Below you can see the graphs and R value (R2 is simply R*R) of the association between NAD+ abundance and muscle mitochondrial respiration and steps a day. All R2 values are remarkably strong - 0.32 (0.57*0.57) in the case of the relationship between NAD+ abundance and maximal respiration. This means that NAD+ levels can explain 32% of the variance in maximal mitochondrial respiration. Such a high R2 tells us that NAD+ levels might be very important for maintaining mitochondrial function, and preventing the fall of NAD+ might prevent mitochondrial dysfunction. This is very important as mitochondrial dysfunction accompanies the majority of age-associated chronic diseases.
The R2 of 0.2 (0.45*0.45) in the case of steps per day and NAD+ suggests that activity significantly affects NAD+ levels, which confirms that activity can increase NAD+ levels as shown in the aforementioned boxplot. Indeed, the challenge in the relationship between steps a day and NAD+ is that we can't be completely sure which variable is the explanatory variable and which is the response variable, as both directions of the relationship are possible. It is more likely that the number of steps taken in a day is the explanatory variable here. However, considering the crucial role of NAD+ in energy production, it is not implausible that higher levels of NAD+ help the elderly to remain active. The most plausible hypothesis is that the relationship is bidirectional. It is also worth noting that the study also reported a strong correlation between NAD+ levels in strength which shows that NAD+ levels are related to the functional status of muscles in the elderly.
The age-related decline of NAD+ in muscle reported in this study is in agreement with other human studies, which showed that NAD+ levels decline with ageing in the brain and pelvic skin. It can be expected that future studies will confirm the age-associated decline of NAD+ in other human tissues. Such findings will increase the chances of NAD+ boosters affecting ageing in multiple physiological systems.
Indeed, revealing that the decline of NAD+ can be significantly slowed down by physical activity is an important finding. The crucial role of exercise in maintaining NAD+ levels was also shown in a study published in Physiological Reports. This study reported that 12 weeks of either aerobic or resistance training increased levels of NAMPT by ~30% in older people (over 55 years of age). NAMPT is the rate-limiting enzyme for producing NAD+ from its precursor nicotinamide mononucleotide. Increasing levels of this enzyme should significantly increase NAD+ levels.
There is one important thing to note here. People who don't have any further context might intuitively think that resistance exercise is much more beneficial than aerobic exercise for preventing sarcopenia. While resistance exercise is excellent for gaining muscle, we have mentioned that it is not only the muscle mass but also the muscle function that matters. As aerobic exercise depends on oxidative metabolism, which utilizes NAD+ for energy production, the adaptation to aerobic exercise will greatly contribute to the prevention of NAD+ decline. The previous study, which showed that both aerobic and resistance exercise increases NAMPT levels, highlights what we try to say every time – it is not important which kind of exercise you do. Do the exercise which you like better and you are more likely to stick with. Ideally, combine both resistance and aerobic exercise to get the best from both worlds - increasing muscle mass and also increasing the functional status by increased NAD+ levels.
This study is a very important contribution to longevity research. It shows that age-related molecular changes observed in mice are relevant in humans. Even more importantly, it shows that the premise of treating ageing is feasible. As described in one of the previous topics, ageing-focused medicine would start to treat people not when they develop the first age-related chronic disease but at the moment of dissociation between chronological and biological age. The measurement of biological age is likely to be based on a composite score, which will take into account tens of different biological parameters pooled together.
We currently know that diet, exercise and sleep are the most important lifestyle factors which can slow down your biological ageing, and they should be the first line of treatment. Results of the above-discussed study show that this is a correct approach because molecules
directly involved in ageing like NAD+ are very strongly correlated with physical activity status. Even without a reliable measurement of biological age, it is scientifically sound to claim that exercise slows down biological ageing. Considering all the known benefits of exercise, it might take decades to develop a pill which will be as effective as exercise in slowing down ageing.
While the concept of sarcopenia and progressive loss of muscle mass and strength during ageing is scary, and the mortality risk associated with sarcopenia-related immobilisation is even scarier, the above-described study brings good news. Ageing sucks, but our lifestyle choices significantly influence if we age successfully.