Curcumin is the most abundant phenolic compounds in turmeric, a spice commonly found in curry powders and sauces with a long history of use in cooking and in traditional Indian and Chinese systems of natural medicine.¹ More recently, curcumin has been investigated for medicinal properties, health, and possible sport recovery and performance benefits. Turmeric is “generally recognised as safe” by the US FDA, indicating that it can be a safe food additive. However, curcumin in an unstable compound and its bioavailability is very poor. As such, recent efforts have focused on developing curcumin formulations with greater bioavailability and systemic tissue distribution. Broadly, these formulations include lipid additions (turmeric oil and piperine), adsorption on matrices (lecithin), and curcumin size reduction to nano particles.²

Curcumin has a number of potential health/ performance benefits

• In health, curcumin has been shown to aid in the management of oxidative stress and inflammatory conditions such as metabolic syndrome and arthritis.³
• Curcumin has been found to have non-steroidal anti-inflammatory drug-like actions as evidenced by a largely dampened activity of markers of oxidant and inflammatory pathways (e.g. COX-2 and NF-kB signalling) in disease^{4, 5}, although clear associations between pathway markers and muscle performance in athletes remain unclear.
• Curcumin has been associated with reduced exercise-induced muscle damage (EIMD) and exercise-induced muscle soreness (EIMS), plus lowered delayed-onset muscle soreness (DOMS). As such, curcumin may be used in the treatment of pain and inflammation following exercise, reduction of oxidative stress following exercise, plus alleviation of muscle damage, improving muscle recovery, EIMS and DOMS.
• Curcumin supplementation usually has no noticeable side effects. However, there are safety concerns when consumed in doses > 2g/day.
• Effects of long-term supplementation on the magnitude-efficacy of the training response and adaptability are at the time of writing unknown, but warrant investigation.

Mechanistic evidence and nature of benefits for athletes.

• Reduced pain following heavy muscular exercise loading is reported by way of lower DOMS or other muscle soreness scores in most^6-10 but not all studies.¹¹
• The reduction in DOMS is also associated with improved joint range of motion, suggesting reduced musculotendinous stiffness⁶. Curcumin also lowers the marker of muscle damage - creatine kinase (CK) - concentration within the blood, suggesting the reduced muscle soreness findings may be associated with attenuation of muscle cell damage. More conclusively, a recent meta-analysis found a significant effect of curcumin supplementation on reducing CK (weighted mean difference [WMD] = -48.5 IU.L-1 ; 95% CI: -80.7, -16.4;p=0.003) and muscle soreness index (WMD = -0.5; 95% CI: -0.8, -0.2; p=0.001), leading to the author conclusion that “curcumin may be known as a priority EIMD recovery agent in interventions”.¹⁰
• Reduced inflammation is inferred from a reduction in markers of systemic inflammatory modulatory pathways, models which were developed largely from in vitro research models within the (health) therapeutic use setting. Additionally, advanced glycation end-products (AGEs) produced from muscle-damaging eccentric exercise (e.g. jumping, resistance training, and hard running), are associated with reduced subsequent performance, although may not be mechanistically determinant but a side effect. Nevertheless, 3 months of curcumin plus Boswellia serrata (BSE) gum resin supplementation was shown to lower AGEs and lipid- peroxidation outcomes in athletes.¹² Six days of curcumin supplementation during a bout of EMID attenuated post-exercise concentration of the inflammatory cytokines IL8 and TNF-α, but not IL6 or IL10.11 Meanwhile in a similar experimental model, curcumin increased IL6 concentrations at 0-h (31 %; 90%CI ±29 %) and 48-h (32 %; ±29 %) relative to baseline, but decreased IL-6 at 24-h relative to post-exercise (-20 %; ±18 %) 7. In contrast, Tanabe et al.¹³ reported no significant changes in IL6 and TNF-α.
• Reduced oxidative stress markers post-exercise in the urine or blood support other in vitro models suggesting anti-oxidant effects. For example, curcumin can modulate the activation of T-cells, B-cells, NK cells, neutrophils, macrophages, and dendritic cells¹⁴, and at low doses, curcumin can also enhance antibody responses, suggesting it may modulate the immune system.¹⁵ However, in athletes, results are mixed and not all chronic (e.g. 28 d) supplementation studies show consistent effects on system oxidative stress or inflammatory markers.^{7, 9}

Effects on recovery and performance in athletes.

• A selection of well-designed and executed studies suggest worthwhile improvements in skeletal muscle function following a single bout of strenuous or damaging exercise; whether these largely laboratory-based surrogate muscle-function outcomes translate to real-world performance remains to be established.
  • Nicol et al.⁷ supplemented athletes with a very high dose of curcumin stated as equivalent to 0.3 g/kg/d or apparently ~1060 mg twice daily (from 2.5 g curcumin), or control, for 2.5 days before and 2.5 days after severe EMID. Associated with moderate-large standardised reductions in pain perception during single-leg squat was a small increase in single-leg jump performance (15%; 90 %CL ± 12%) with curcumin supplementation.
  • Tanabe et al.⁷ gave 14 healthy men 150 mg/d curcumin before and 12 h after maximal eccentric arm exercise. Maximal voluntary contraction torque decreased less and recovered faster (e.g. 4 days post-exercise: -31 ± 13 % vs. -15 ± 15 %) with curcumin compared with placebo.
  • Delecroix et al.¹⁶ gave 10 elite rugby players 6 g curcumin + 60 mg piperine/d for 48 h before and following EIMD. Curcumin supplementation resulted in a moderate reduction in sprint mean power output (-1.1; 90%CI -1.9 to -0.9) at 24-h post exercise, but no other outcomes were clear. The lack of a consistent performance outcome and the addition of piperine (which may enhance bioavailability of curcumin up to 2000%), needs to be considered in the context of this study.
  • Hillman et al.6 studied the effects of a 500 mg dose of curcumin supplementation twice daily for 10 days (6 days pre, day of and 3 days post exercise) on DOMS and muscle power following plyometric exercise in 22 men and women. Following 5 x 20 drop jumps on day 7, vertical jump test performance was reduced in placebo but maintained following curcumin supplementation (curcumin benefit +1.6 inches 90%CI 0.63 to 2.6).
  • Several other studies were conducted, but used measures of performance that have poor reliability. For example, Jäger et al.¹⁷ supplemented two doses for 8 weeks but report mostly inconclusive results to muscle function, probably related to the use of isokinetic peak torque as the performance measure¹⁷, which has poor reliability.¹⁸
  • Overall, there is building evidence to suggest a benefit to recovery of muscle performance in power or explosive activities from several days of curcumin supplementation at specified doses (500-2500 mg of curcumin twice daily). However, the evidence remains mixed, coupled with confounding effects from co-ingredients, and more high-quality studies in explosive, power and sprint, plus endurance models, are required before firm dose efficiacy and performance-based conclusions can be formed.