By Austin Baraki
In our first article we defined sarcopenia as a loss of skeletal muscle mass and strength. We discussed how disuse, poor nutrition, neuromuscular changes, hormonal status, and chronic inflammation contribute to loss of skeletal muscle protein. In today’s article we’ll examine the other side of the equation that affect skeletal muscle protein synthesis (a.k.a. gainzZz™), and how this manifests as hypertrophy. This article is probably the longest and most science-heavy of the series, but bear with me – it does build towards a practical conclusion!
[Before we begin, I’d like to make a clarification and correction from the prior article regarding muscle atrophy. Although I focused on the selective loss of type 2 muscle fibers with aging and disease-related atrophy, I failed to make clear that there are other contexts in which type 1 fibers might be selectively lost instead, including spinal cord injury and even some situations involving immobilization/disuse (1,2,3). Although fast-twitch fibers may hold more appeal in their capacity for force and power production, the importance of our slow-twitch muscle fibers should not be discounted. They are essential for our postural muscles and low-intensity day-to-day activity. One could argue that without adequately functioning slow-twitch muscle fibers, you’d have few opportunities to express any fast-twitch power at all! The point is, we want to lose as little of our muscle mass and strength as possible. Okay, now back to today’s article.]
Protein Synthesis: Recall from high-school biology that protein synthesis requires translation of a genetic transcript taken from your DNA, e.g. DNA is transcribed to RNA and RNA is translated to protein. Similarly, the mechanisms that stimulate muscle protein synthesis (MPS) involve activating transcription of myonuclear DNA, followed by translation into protein. So what kinds of things trigger this process? Lets take a look at the major mechanisms: exercise, dietary protein (esp. leucine), and hormones/cytokines.
- Exercise is the obvious place to start. We can divide this topic into “cardio” (aka endurance-type, aerobic, “long slow distance,” etc.), high-intensity intervals, and resistance exercise. We know that endurance-type exercise mainly improves energy efficiency by stimulating biochemical adaptations (e.g. via PGC-1a and AMPK) so we can run, swim, bike, etc. faster and for a longer period of time (4). Although endurance exercise can stimulate MPS, the effect is not particularly robust and does not sufficiently build the muscle strength or size that we need to prevent/reverse sarcopenia (5,6,7). In contrast, resistance exercise involves loading our musculoskeletal system with enough intensity to trigger structural adaptations; that is, it forces our body to build new tissue including bone and muscle that are very useful in preventing/reversing osteo- and sarcopenia. And interestingly, high-intensity interval training can provide some of both worlds, potently stimulating biochemical adaptations as well as a surprisingly robust MPS response (8,9,10).
So, how does exercise trigger muscle protein synthesis? It turns out to be quite complex, but we’ll begin with a simplified look at the direct mechanisms. Resistance exercise results in muscle tension and “microtrauma”, acute inflammation, local hormonal responses, metabolic stress, and cellular swelling (from metabolite accumulation and osmotic fluid shifts) (11). Each of these mechanisms can activate specific intracellular signaling pathways including the “master” mTOR pathway, MAP kinase cascade, as well as several Calcium-dependent pathways (11). The intricate details of these pathways are beyond the scope of this article series, but the bottom line is that they stimulate transcription of myonuclear DNA for new protein synthesis, and inhibit protein breakdown and cell autophagy (“recycling” processes normally stimulated by the counterregulatory AMPK pathway). The exercise-induced MPS response can last up to 4 hours if fasted and according to Phillips et al., MPS rates have been shown to be elevated above baseline by 34% at even 48hrs post training (4, 50)!
Before moving on let’s put this discussion in the context of sarcopenia, typically in aging/older individuals. We know that aging muscle demonstrates a relatively “blunted” response to anabolic stimuli (exercise, protein, insulin, etc.), a phenomenon known as age-related “anabolic resistance” (5,12-14). This is also known to occur in young, healthy muscle tissue during prolonged immobilization, but anabolic resistance is a unique challenge in older age. Fortunately the human body can undergo the stress-recovery-adaptation cycle at any age; careful “dose titration” just becomes more important as exercise recovery capacity and sensitivity to the stress diminishes. This situation is ideal for an experienced and qualified strength coach to apply their expertise, or perhaps one of the rare physical therapists who has an intimate and practical understanding of effective strength training.
2) Dietary protein plays a major role in stimulating MPS, especially the branched-chain amino acid leucine (4,5,12,13,15-20). As whole dietary proteins are broken down into constituent amino acids and absorbed into the bloodstream, these circulating amino acids are readily taken up into our cells through specific transporters. An adequate dose of leucine directly activates the mTOR pathway, initiating transcription/translation of new muscle protein AND inhibiting protein breakdown (= gainzZz). This “adequate dose” of leucine is age-dependent, due to the anabolic resistance phenomenon described above (details to come in a later article) (5). Nutrient-driven MPS typically lasts about 1.5-2 hours on its own, after which it enters a “refractory period” for several hours before it can be re-stimulated. As mentioned above, however, circulating amino acids can synergize with resistance training to produce elevations in basal MPS for over 24 hours (4). Jordan has already written an overview article on how to optimize protein intake, and we’ll discuss the science and specifics of this topic later in the series when we get to the treatment of sarcopenia.
3)Hormones & Cytokines have a major role in stimulating muscle growth, as was alluded to in part 1 of this article series. We’ll choose a few in particular to discuss here: testosterone, insulin-like growth factor 1, growth hormone, insulin, and pro-inflammatory cytokines.
4) Testosterone has a number of effects on the body by way of the androgen receptor, and even more downstream effects after conversion to dihydrotestosterone (DHT) or estrogen. Circulating testosterone binds androgen receptors and is transported into myonuclei where it interacts directly with DNA to stimulate transcription for muscle protein synthesis (11). It also inhibits protein breakdown pathways and, as discussed in our last article, is critical to support satellite cells, the self-renewing stem cells that contribute to muscle cell growth and repair (21). Besides these typical mechanisms that we keep coming across, Testosterone also does a few other interesting things. Resistance training up-regulates the expression of androgen receptors particularly in fast-twitch fibers, making them even more sensitive to androgen-stimulated MPS and growth (11)! Testosterone also promotes maintenance, growth, and repair of existing motor neurons (recall from our last article that motor neuron loss is a major mechanism of sarcopenia) (21-24). Finally and perhaps most interestingly, it appears that testosterone helps to “commit” undifferentiated mesenchymal cells to a myogenic lineage while inhibiting adipogenesis – in other words, in cells that haven’t “grown up” yet, testosterone nudges them towards developing into muscle cells, while blocking them from turning into fat cells (21,25-27)! Before moving on, let’s make one thing clear: elevations and reductions in serum testosterone levels within the physiologic range (approx. 300-1000 ng/dL) do not appear to have a significant effect on muscle mass, so we shouldn’t worry about physiologic variations in serum testosterone after exercise or any other normal activity, regardless of what your local gym-bro says (28,29). And although we could delve into a longer discussion on the effects of DHT and estrogen, I think we’ll save it for a future article. For now, understand that (perhaps contrary to popular belief) they are both essential for building muscle, strength, and preventing atrophy!
5) Insulin-like Growth Factor 1 (IGF-1) is one of our most potent anabolic hormones; it has the capacity to stimulate growth in almost every cell in the body. We typically think of systemic IGF1 coming from the liver in response to Growth hormone stimulation, but this doesn’t actually have much effect on skeletal muscle. In fact, muscle locally produces and uses even more IGF1 than the liver in response to resistance exercise (11). This local production of IGF1 is closely linked to the acute inflammation of usual exercise-induced muscle damage (“microtrauma”) (32,33). So IGF1 can act in an “autocrine” and “paracrine” manner, meaning it can have effects on the cell that produced it and its neighboring cells, in addition to affecting distant organs in an “endocrine” fashion. So how does this work? It turns out that IGF1 has multiple systemic isoforms as well as a locally-acting variant commonly known as Mechano-growth factor (MGF) (33). This version of IGF1 is produced exclusively in response to mechanical stress and microtrauma. MGF and local IGF1 modulate the acute inflammation and accelerate anabolic effects in muscle tissue for up to 72 hours, chiefly due to a few familiar mechanisms: increasing MPS (via PI3k/mTOR), suppressing protein breakdown, and stimulating satellite cell proliferation and differentiation. Seeing a pattern here? In addition, the other isoforms of IGF1 also play a role in myofiber maturation and may synergize with testosterone’s effects.
6) Growth hormone (GH), despite its name and reputation, does not appear to have significant direct anabolic effects on skeletal muscle tissue. It may contribute to increased local expression of IGF1 and MGF, but this remains to be proven, along with any other clinically significant anabolic effects (11). Similar to our discussion of serum testosterone variations above, it also does not appear that physiologic variations in circulating Growth hormone (including “spikes” after resistance training or while asleep) have any significant effects on MPS or skeletal muscle hypertrophy… so don’t listen to the clueless bro working at your local GNC store (30,31). Jordan has ranted at length on this topic as well. This is not to discount Growth hormone’s multitude of functions within the body, just that the role of systemic growth hormone (and even systemic IGF1) in MPS/hypertrophy appears to be misunderstood and blown out of proportion in the fitness world.
7) Insulin is also known to have anabolic effects, mainly by regulating protein metabolism through the usual mechanisms of suppressing protein breakdown and stimulating Akt/mTOR (11). However, the research around this topic has been conflicting as to whether physiologic insulin levels (vs. supraphysiologic levels) can independently exerts these effects, or whether sufficient amino acid concentrations in the blood are necessary for this effect to occur. At this point it appears that the latter is closer to the truth; that is, insulin does not independently trigger a significant MPS response without concurrent sustained amino acid availability (34,35,36).
8) Inflammatory Cytokines (e.g. tumor necrosis factor, IL-6, and many others) were mentioned in our last article as mediators of muscle protein loss and atrophy. However, this was in the setting of chronic elevations due to disease-related systemic inflammation. In contrast, the acute elevations induced by resistance exercise microtrauma help attract inflammatory cells (macrophages, monocytes) to begin rebuilding muscle tissue together with satellite cells. As discussed above, local secretion of IGF1 accelerates anabolism and modulates the inflammatory process, bringing the acute inflammatory process to an end (which prevents chronic fibrosis of muscle tissue) (32, 37-40). Together, these processes act synergistically to support satellite cell function, muscle repair, and MPS.
We’ve now covered the major players in the balance between muscle protein synthesis and muscle protein breakdown that determines total skeletal muscle mass (sarcopenia criterion #1). These newly synthesized proteins contribute to the contractile apparatus (myosin, actin, titin) and other structural and regulatory elements (troponin, tropomyosin, etc.). They organize into sarcomeres (the “functional units” of muscle), which are added in parallel (contributing to muscle cross-sectional area) or in series (contributing to total muscle length) (11).
But what really happens during hypertrophy? Here’s where we get really sciencey, but we’re almost done so bear with me. I promise there’s a practical point to all of this at the end!
1) Expansion of the Myonuclear Domain (MND). This refers to the volume of territory within a muscle cell that is “governed” by a single nucleus (remember, skeletal muscle cells can contain multiple nuclei, in contrast to other human cells) (11,41-42a). The size of this territory is affected by the balance between the rates of protein synthesis and protein breakdown. Furthermore, the exact volume of sarcoplasmic territory per myonucleus can vary over time and also depends on fiber type, varying inversely with the parent cell’s oxidative capacity. Okay, that’s a lot of complex words – simply put:
“Slow-twitch” fibers (high oxidative capacity) = LESS sarcoplasm per myonucleus = less growth potential.
“Fast-twitch” fibers (low oxidative capacity) = MORE sarcoplasm per myonucleus = more growth potential.
Expansion of the myonuclear domain is a component of muscular hypertrophy and can occur independent of satellite cell activity (43,45). Changes in MND size correlate to changes in muscle cross-sectional area, whether during maturational growth or aging/disuse atrophy.
2) Satellite cell recruitment. Once a particular myonuclear domain reaches a critical size (depending on fiber type, as discussed above), any further expansion requires a new nucleus to govern this new territory (11,41-42a,44). Myonuclei contain the DNA blueprint for new muscle protein destined for their assigned domain. Muscle cells therefore consist of multiple myonuclear domains, each of which has its own nucleus. Rather than “dividing” like most other cells of our body, this new nucleus is “donated” by a friendly neighboring satellite cell. This idea makes sense from a “management” perspective; the larger a cell’s contents grow, the more nuclei needed to effectively manage it. Conversely, if a MND continues to expand unchecked without incorporation of new nuclei, the lone nucleus will be overwhelmed and unable to coordinate its assigned sarcoplasmic territory, resulting in a dysfunctional muscle cell (45). In fact, it appears that myostatin acts to “rein in” MND expansion for this exact reason. Experimental myostatin-deficient mice do indeed experience uncontrolled MND expansion (growth), but have a subsequent deterioration in muscle function (46)! This unfortunately means that any future pharmaceutical myostatin inhibitor may make you hyooge, but it probably won’t make you as strong as you’d expect (…perfect for bodybuilders, perhaps? 🙂 ).
“Too long; didn’t read” – summarizing this science-heavy section:
- When we stimulate MPS from the myonuclear DNA blueprint, much of the resulting protein ends up in a growing myonuclear domain assigned to that particular “parent” nucleus.
- As this domain grows beyond a critical volume, additional nuclei are acquired from neighboring satellite cells in order to maintain a relatively consistent ratio of muscle fiber size to myonuclear content. This ratio is kept in check by myostatin (among other things) to ensure organized, functional growth.
Now that we have a better understanding of the theories of skeletal muscle hypertrophy, we should consider how this relates to atrophy. It turns out that atrophy is not just the “reverse” of hypertrophy! There has been tons of research examining muscle tissue changes after immobilization, denervation, spinal cord transection, limb suspension, mechanical ventilation, and microgravity. The multitude of techniques combined with methodological variation has resulted in quite a lot of conflicting literature on exactly what cell-level changes occur during muscle atrophy. While some of the literature indicates that the primary drivers of hypertrophy and atrophy are changes in myonuclear number, other studies show MND size changes without loss of myonuclei. The most recent evidence appears to show that age-related muscle atrophy likely does not involve significant myonuclear or satellite cell loss (41-42a, 47-49). This means that myonuclei/satellite cells gained from hypertrophy are there to stay for a long time (if not for life), they probably aren’t lost during detraining, and they can therefore respond to subsequent stimulation through all the usual mechanisms – even in old age! We just have to provide an age-appropriate stimulus of protein, exercise, etc. This is a very exciting idea.
That is the practical point I sought to make after all the science talk above.
- Muscle nuclei contain the DNA blueprints for synthesis of new muscle protein.
- More contractile muscle protein = more force production = more strength.
- Although still controversial, aging/disused muscle doesn’t appear to lose many (if any) of its nuclei or satellite cells, so it still has just what it needs to get stronger – a sort of “muscle memory”…
- …As long as we provide an adequate stimulus to overcome age-related “anabolic resistance.”
- This confirms our suspicion that “It’s never too late”!
So at this point we’ve slogged through the science looking at how protein synthesis and breakdown affect total skeletal muscle mass (sarcopenia criterion #1). In our next article we’ll explore a few other determinants of muscle “quality” that can impair muscle function (sarcopenia criterion #2), and we’ll look at current methods and clinical tools that have been proposed for clinical diagnosis of sarcopenia. This will prepare us to look at how we can optimize treatment in the fourth and final article of the series.
Austin Baraki is a 25 year old student just three months away from completing his M.D. degree. He came to the sport of powerlifting after 15 years of experience competing in and coaching competitive swimming through the collegiate level. Since transitioning to powerlifting he has achieved personal best lifts of a 480 lb squat, 365 lb bench press, and has deadlifted 525×4 at 5’10”/185 lbs. He is specializing in Internal Medicine and has a passion for coaching, teaching, preventive medicine, a nice rare steak, and making gainzZz™.
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