Does Acute Improvement in Muscle Recovery with Curcumin Supplementation Translate to Long-Term Training?
Dietary polyphenols, such as curcumin, green tea catechins, and pomegranate extract, may have the ability to enhance the effectiveness of long-term training programs by managing training-induced inflammation. Enhanced recovery may translate to increased capacity to train and perform more effectively. The dietary polyphenol curcumin has been reported to block the action of COX-2 and NF-kB signaling and would allow for increased capacity to train leading to heightened adaptations and the potential for ergogenic outcomes. These actions are very similar to the targeted actions of non-steroidal anti-inflammatory drugs (NSAIDs), but without the side-effects of NSAIDs. This review will compare and contrast the known effects of curcumin and identify common design elements between existing studies. Through this critical review of the existing literature it is our goal to establish a set of best practices that could be applied to an athletic population that is interested in using oral curcumin supplementation as a recovery agent.
KeywordsExercise-induced muscle injury Inflammatory cytokines Inflammation Soreness Distance running Exercise training
A comprehensive training program includes a variety of exercise modes, intensities, and durations [9, 12, 25, 28, 33, 35]. While using a variety of exercise stimuli may improve training efficiency, greater stimuli should lead to greater potential for harms such as overtraining or incomplete recovery . This is not a fault of the person training as it is often not recognized that a given volume of exercise might be too much until an injury occurs. In turn, these higher levels of inflammation and soreness limit performance in subsequent training sessions or competitive events . Traditional treatments (i.e. cryotherapy, massage, stretching, etc.) lack the evidence-based support to consistently improve recovery time course [16, 31, 36]. Improved recovery time course may be defined as fewer days to recovery, reduced severity of soreness/inflammation during recovery, or a combination of the two. Some regular exercisers turn to NSAIDs to relieve soreness and inflammation associated with an excessive training session, but this approach blunts inflammation, potentially blocking the first stage of healing [1, 32, 37]. Also, despite the perceived benefits of NSAIDs, clinical research demonstrates that NSAIDs do not necessarily restore muscle performance and may hinder recovery . Given the negative effects of habitual NSAID treatment on the central nervous system, long-term use is not advised even if NSAIDs did improve recovery . As regular exercisers continue to push the boundaries of athletic performance, critical research and development is needed to identify substances, such as dietary polyphenols like curcumin, that can limit inflammation and oxidative stress without the negative consequences associated with NSAIDs. This review has been structured to provide the biological mechanism of action for oral curcumin supplementation in the context of exercise recovery. Current literature is limited with regard to the role of curcumin supplementation as a component of a comprehensive training program.
Curcumin is a yellow pigment derived from the root of the turmeric (Curcuma longa) plant. Unfortunately, when consumed orally (in humans) curcumin has low bioavailability in its naturally occurring form, thus it is ideal to use a commercially “optimized” form of curcumin [13, 27]. Naturally occurring curcumin has been administered in doses as high as 8 g/day in humans with no side effects; however, roughly 75% was excreted in feces, indicating poor gut absorption . Furthermore, once absorbed, curcumin has been known to bind to serum proteins and be metabolized by the liver into fragments of curcumin that may not retain biological actions . Research in animal and cell culture models has demonstrated that curcumin has anti-inflammatory actions via suppression of cycloxogenase-2 (COX-2) signaling and prostaglandin formation [2, 5, 7, 17, 19]. Prostaglandins are biological substances that contribute to pain and soreness after training, thus suppression of their formation could favorably impact these symptoms. The biological mechanism of action for curcumin is similar to that of NSAIDs [7, 8, 20, 21, 29], albeit less pronounced and without NSAID side-effects. The only side effects known with curcumin are associated with very high doses (>16 g/day) and include indigestion and development of kidney stones . These symptoms are mild considering that NSAID side effects include liver and central nervous system damage.
Biological Mechanism of Action for Curcumin
Previous Applications of Curcumin for Muscle Recovery
Due to curcumin’s mechanism of action, curcumin has established itself as a relevant ingredient to support sport nutrition and ergogenic outcomes [7, 8, 19, 20, 26, 28, 32, 39]. Two of these previous studies have used a mouse model system [7, 20] and as such don’t have direct application or translation to exercising humans due to species differences in bioavailability and downstream processing of oral curcumin in the liver. The mice studies are of note because other human studies have used them to justify the dosing selected in humans, which may be problematic because of the previously noted species differences in bioavailability. One design variable that can be gleaned from rodent studies is that in order for curcumin to be successful, it appears to have the best efficacy when the quantity of soreness/inflammation is moderate, but not severe. Excessive soreness/inflammation completely overwhelms the biological system, resulting in changes that are unlikely to be managed with oral curcumin supplementation or any other treatment [11, 30]. Soreness is a difficult result to quantify and accurately assess due to individual differences in perception of pain and soreness . One potential solution that may aid with removal of individual bias is to ensure that studies always include a measurable biological indicator of muscle inflammation (i.e. serum creatine kinase, myoglobin, etc.) that isn’t prone to individual perceptual differences. Unfortunately, the measurement of biological outcomes isn’t practical in field-based training studies. This is a critical gap in the scientific literature that needs to be addressed to provide better means of tracking exercise training recovery and effectiveness.
Of the published human curcumin studies, complete comparison of results is limited due to the differences in the timing of sample collection, curcumin dose, curcumin type (i.e. natural vs. optimized), and/or measurements made [8, 26, 28, 33, 34]. Drobnic et al.  reported that when consuming curcumin (5.3 mg/kg body weight/d; total dose = 16 mg/kg body weight) 48 h prior to a bout of downhill running (45 min at − 10% grade) subjects reported significantly less subjective muscle soreness and less muscle damage (as measured by MRI) compared to a placebo. They also reported a significant decrease in serum IL-8 (pro-inflammatory cytokine) at 2 h after exercise, but found no other changes in biological measures of inflammation (CRP or MCP-1). One limitation of the study by Drobnic et al. is that no measurements were made later than 24 h after exercise-induced muscle damage, so this sampling design may not allow for a complete assessment of the muscle recovery profile (which may last up to 96 h). Sciberras et al.  reported that 3 day of curcumin consumption (5.7 mg/kg body weight/d; total dose = 17 mg/kg body weight) failed to alter serum inflammatory cytokines (i.e. IL-1RA, IL-6, and IL-10) within 1 h following a 2 h bout of cycle exercise. The authors did report significant improvements in psychological mood following exercise with curcumin, but not placebo supplementation. The lack of change seen in cytokines is not surprising due to the invoked exercise stimuli (minimal eccentric contractions) in this study.
In a study from our laboratory using a leg press muscle injury model (60 repetitions of inclined leg press at 110% of the 1RM, eccentric only) , we found that oral optimized curcumin supplementation (Longvida®; 400 mg/day for 2 days prior and 4 days after) resulted in decreased serum creatine kinase (biological muscle damage indicator) and decreased inflammatory cytokines (IL-8 and TNF-α). These biological changes were observed despite numerical decreases in soreness that did not reach statistical significance. Our response are similar to Jager  who reported that a higher dose of optimized curcumin (CurcuWIN®; 1000 mg/day) resulted in maintenance of muscle strength following downhill running, while a lower dose (250 mg/day) or placebo resulted in loss of muscle strength. To test our laboratory findings in a field model, we conducted a half-marathon training study to determine if optimized curcumin (Longvida®; 500–1000 mg/day) in combination with another dietary polyphenol (pomegranate extract; Pomella®; 500–1000 mg/day) could manage training-associated changes in inflammation [12, 35]. When compared to a placebo, we found that the treatment allowed for 11% more training mileage and 20% more caloric expenditure despite a similar number of training sessions in the 30 days prior to a half marathon race. Since the later study involved combined curcumin and pomegranate, it is difficult to determine the unique contribution of each component. We also noted that the treatment reduced a variety of inflammation-associated RNA and proteins prior to and after the half marathon race, which is the most likely cause of improved training efficiency.
To summarize, oral optimized curcumin supplementation prior to a single exercise session or during prolonged training reduced biological indices of muscle injury and inflammation. Also, despite a similar volume of exercise, individuals who consumed curcumin had lower serum CK with curcumin, which is indicative of less muscle damage. Unfortunately, the reporting of reduced muscle soreness has been inconsistent, which may be partially explained by subjective individual differences in the perception of soreness . These findings collectively outline the preliminary potential for oral supplementation of optimized curcumin as one part of a comprehensive training program. Future work must continue to explore more research in this area to better understand how dosing and timing of dosing will influence outcomes related to curcumin’s potential to favorably impact exercise outcomes and adaptations.
Future Potential of Curcumin for Long-Term Training?
Some of the research from UNT cited in this article was conducted via a Research Grant Awarded to UNT from Verdure Sciences Corp (Indianapolis, IN). The research team was not directly compensated for this work and Verdure Sciences was not involved with the writing or review of this manuscript.
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