Summary: Amino acids are potent modulators of protein turnover and skeletal muscle cells are highly sensitive to changes in amino acid availability. During amino acid abundance increased activity of mTORC1 drives protein synthesis and growth. In skeletal muscle, it has been clearly demonstrated that of all the amino acids, leucine is the most potent stimulator of mTORC1 and protein synthesis in vitro and in vivo. As such, leucine has received considerable attention as a potential pharmaconutrient for the treatment of numerous muscle wasting conditions. However, despite a multitude of studies showing enhanced acute protein synthesis with leucine or leucine-rich supplements in healthy individuals, additional leucine intake does not necessarily enhance protein synthesis during muscle wasting conditions. In addition, long-term, placebo controlled, iso-caloric studies in humans consistently show no beneficial effect of leucine supplementation on skeletal muscle mass or function. This review, critically evaluates the therapeutic potential of leucine to attenuate the skeletal muscle wasting associated with ageing, cancer and immobilization/bed rest. It also highlights the impact of inflammation on amino acid sensing, mTORC1 activation and stimulation of protein synthesis and challenges the underlying hypothesis that the acute activation of mTORC1 and stimulation of protein synthesis by leucine increases in muscle mass over time. We conclude that leucine, as a standalone nutritional intervention, is not effective in the prevention of muscle wasting. Future work should focus on identifying and utilizing other nutrients or treatments that sensitize skeletal muscle to leucine, thereby enhancing its therapeutic potential for muscle wasting conditions.
Alex’s Notes: Skeletal muscle mass is the result of protein turnover – the balance between synthesis and degradation. There are many key regulatory factors, of which the mechanistic target of rapamycin (mTOR) plays a critical role. Activated by amino acids, growth factors, and energy status, mTOR is seen as a master regulator of protein synthesis and subsequent hypertrophy. Yet, growth factors are not very efficient when amino acids are not present, and amino acids may actually stimulate mTOR through growth factor independent mechanisms. As such, amino acid supplements have received a considerable amount of attention, and Daniel Ham of the University of Melbourne, Australia sought to determine if this attention is well placed.
The specific amino acid in question is leucine. Changes in muscle protein synthesis are likely the result of changes in extracellular, not intramuscular, amino acid concentrations. This means that it is the amino acid spikes to the bloodstream that stimulate the protein synthetic pathways. With regard to leucine, Ham suggests that there are four regulatory effects that it has on protein metabolism:
- Increasing substrate availability
- Increasing the secretion of anabolic hormones such as insulin
- Directly modulating anabolic signaling pathways in skeletal muscle
- Potential secondary effects of metabolites such as β-hydroxy-β-methylbutyrate (HMB)
Support for the above comes from several studies, the strongest of which is one performed in healthy young men where a 110% increase in skeletal muscle protein synthesis was observed after a small (3.42g) oral dose of free leucine was consumed. Point #4 came into play when these authors observed the same protein synthetic response with a smaller 2.42g dose of HMB, and point #2 stemmed from the small increase in insulin levels following the leucine consumption. However, seeing how essential amino acids (EAA) stimulate protein synthesis independent of insulin levels, and carbohydrates do not significantly increase muscle protein synthesis, insulin is likely more permissive rather than modulatory.
More than likely, insulin is involved in preventing catabolism rather than stimulating anabolism. Sure enough, increasing insulin from a baseline value of 5 μU/mL to 30 μU/mL and above does not further stimulate protein synthesis when combined with an infusion of EAAs in healthy young persons. Moreover, insulin levels of just 15 μU/mL can almost maximally reduce muscle protein breakdown with no further inhibition at 30 μU/mL.
So where does leucine come into play? In persons with insulin resistance it has been suggested that leucine can improve anabolic signaling by increasing circulating insulin levels. Overall, the required leucine dose to have maximal effects will differ depending on the physiological state of the person consuming it. For instance, older individuals suffer “anabolic resistance” and require larger boluses to achieve effects similar to smaller doses in healthy young persons. This range is about 3-5 grams on leucine, be it alone or as part of a more complete protein or meal.
Leucine & Muscle Wasting
So can this single amino acid be a lifesaver during times of muscle wasting? Chances are, probably not. As Ham summarizes,
“The importance of adequate leucine intake should not be underestimated and is considered a basic requirement of life. However, taken together, these data clearly demonstrate that supplemental leucine, beyond this basic requirement for life, acutely modulates protein metabolism by increasing substrate availability, increasing the secretion of anabolic hormones such as insulin, and directly modulating anabolic signaling pathways in skeletal muscles of healthy humans. This well demonstrated capacity of leucine to directly activate mTORC1, stimulate protein synthesis, reduce protein breakdown and thus acutely improve protein balance, has led many to the logical conclusion that over time, the summation of these acute increases in protein balance would lead to muscle mass accretion, augment training adaptations and counteract skeletal muscle wasting. However, despite the strong data to support an acute modulation of protein metabolism by leucine, there is little evidence to support a beneficial effect of leucine as a treatment for muscle wasting conditions.”
Let’s begin with disuse. We have all heard the saying, “use it or lose it,” and when it comes to immobilization of skeletal muscle, it could not be truer. Protein breakdown is upregulated within 24 hours after immobilization begins, and after the first ten days nearly all muscle catabolism is thought to occur from a blunted anabolic response to nutrients. Despite leucine’s undisputed ability to stimulate protein synthesis and prevent catabolism, the effects of bed rest on muscle wasting are more dependent on energy availability and exercise. One study put leucine and exercise head to head, with females undergoing 60 days of bed rest consuming 1g/kg/day of protein in combination with resistance and aerobic training, or consuming 1.6g/kg/day of protein including 7.2g of BCAAs (3.6g leucine) consumed three times daily with each meal but not performing exercise. The exercise group effectively prevented the reduction in muscle fiber size and power, while the high protein group did not. Given the synergistic effects of exercise and protein, it is sad that an exercise + leucine group was not included in this study. Regarding energy requirements, healthy young volunteers consuming 80% of their total energy requirements during 14 days of bed rest lost significantly more lean mass than those consuming 100% of their energy requirements.
Moving onward to the elderly, it has been clearly demonstrated that mTOR activation in response to a bolus ingestion of EAAs is blunted in older (70 years) compared to younger (28 years) individuals. Thus, it stands to reason that diets higher in protein and leucine can overcome this “anabolic resistance.” A three month long intervention in healthy elderly persons, unfortunately, did not find improvement in muscle mass or strength compared to iso-energetic controls when 2.5 g of free leucine was supplemented into breakfast, lunch, and dinner (daily total of 7.5g). This dose represented an increase from about 2.1 grams of leucine per meal to about 4.6g, which is more than enough to stimulate protein synthesis in this population. As a follow-up study, the researchers repeated the experiment in type-2 diabetics over a period of six months and again found no improvement in muscle mass or strength with leucine treatment.
Finally, cancer cachexia is a wasting of muscle and fat associated with tumor bearing that results from anorexia, muscle inflammation, and tumor-secreted factors. While there have been no long-term human interventions, studies in mice have shown promising results. Granted, in the linked study tumor volume was reduced with treatment, so the benefits on muscle mass may have been secondary to reduced burden.Interestingly, in a separate mouse study, 0.25 g/kg of HMB was 60% more effective than 1 g/kg leucine in preventing the loss of body mass over a 4 day period. However, one must also consider the effects of supplementation in the tumor(s) itself. While acute doses of leucine have shown to reduce tumor size, it is well known that mTOR is a potent regulator of tumor growth and that leucine is a potent stimulator of mTOR. Thus, monitoring is prudent with amino acid therapy, and given the potential risks of excessive mTOR stimulation, it may be wiser to focus on other nutritional treatments to reduce muscle wasting in cancer.
Why doesn’t leucine work?
While it has been demonstrated clearly in healthy humans that increasing plasma and muscle leucine concentration potently stimulates protein synthesis, this is not necessarily the case during catabolic conditions. The reduced protein synthetic response to dietary interventions is deemed “anabolic resistance” and has been demonstrated in the conditions discussed above and more. Chronic inflammation and excessive reactive oxygen species (ROS; aka free radicals) are believed to be central to anabolic resistance and mTOR inhibition. It has been shown that the excessive inflammation experienced during sepsis reduces leucine-stimulated mTOR activation by 80%.
This begs the question, why? And it appears that the mechanism involves an inhibition of amino acid signaling by mTOR. In addition to the sepsis example above, increases in the concentration of free leucine both in muscle and plasma have been reported during catabolic conditions such as fasting, uncontrolled diabetes, limb immobilization, and cancer cachexia. Moreover, 14 days of leg immobilization in healthy persons led to a 40% increase in intracellular leucine concentrations but a 27% reduction is post-absorptive muscle protein synthesis. This strongly suggests that during times of catabolism the rise in leucine concentration has a less pronounced synthetic response.
So what can we do?
When facing anabolic resistance, there are two main strategies.
- Increase the anabolic stimulus in hopes of overcoming the resistance, or
- Reduce the factors causing the resistance
By now the most sensible route should be obvious. As Ham states,
“Irrespective of the capacity of amino acids to stimulate protein synthesis during muscle wasting conditions, based on current evidence, acute stimulation of protein synthesis by leucine does not lead to meaningful changes in skeletal muscle mass over time.”
I would further this argument and ask if chronic activation of mTOR itself would increase muscle mass. When it comes to answering this question, the evidence is slim. Genetic overexpression of upstream mTOR activators such as Akt has shown to stimulate muscle hypertrophy, and this has been shown to be dependent on activation of mTOR. Yet, in branched chain amino acid transferase (BCATm) knockout mice, plasma BCAA concentrations are chronically elevated five- to ten-fold and this is associated with increased expression of mTOR. Yet, lean body mass is completely unaffected. As a final nail in the coffin, a very recent study in healthy young men found that there was no correlation between the acute muscle protein synthetic response to a bout of resistance training and the magnitude of muscle hypertrophy following the 16 week resistance training program. They also observed no correlation of hypertrophy with activation of Akt or mTOR. Let’s also not forget that mTOR stimulation interferes with other essential cellular processes such as autophagy, but that is another discussion entirely.
The regulation of muscle mass is undeniably dependent on nutrient supply. Nonetheless, the provision of nutrients – including leucine – above what is needed by the body does not appear to further enhance muscle mass. I couldn’t say it any better than Ham does,
“The evidence supporting a central role for leucine in the acute stimulation of protein synthesis in rodent and human studies is overwhelming. Likewise the importance of maintaining a sufficient intake of leucine is indisputable. However, there are currently no well-designed, long-term, iso-caloric experiments in humans consuming sufficient dietary protein that show a beneficial effect of leucine supplementation on muscle mass during inactivity, cancer cachexia or sarcopenia. The lack of translation of the robust acute muscle protein synthetic response to leucine into meaningful changes in skeletal muscle mass in long-term animal and human studies leads us to conclude that acute stimulation of mTORC1 and muscle protein synthesis by leucine does not lead to changes in muscle mass.”