As discussed in the previous section, the pathophysiology resulting from a sports-related concussion is complex and multifaceted as evidenced from animal models of mTBI and TBI. While pharmaceutical therapies may target only one mechanism of injury and have not shown great promise, nutritional supplementation has emerged as a potential neuroprotective agent that targets multiple mechanisms within the complex secondary sequelae. By targeting multiple mechanisms within the injury cascade, nutritional supplementation may be used prior to injury to allay damage that occurs subsequent to both sub-concussive and concussive incidents. In the following sections, nutritional supplementation as a neuroprotective agent, provided prior to injury, will be examined.

Creatine

In the days following sports-related concussion, proton magnetic resonance spectroscopy has detected reduced brain levels of creatine (Cr) [N-aminoiminomethyl-N-methylglycine] coincident with decreases in N-acetylaspartate (NAA) [57]. Changes in NAA have been shown to correlate with those of adenine nucleotides, validating the use of NAA as a surrogate marker of cerebral energy metabolism during the vulnerable period following mTBI [44, 58, 59]. In the brain and other tissues with high fluctuating energy demands, Cr serves to maintain temporal and spatial energy homeostasis in conjunction with the creatine kinase/phosphocreatine (CK/PCr) system [60,61,62,63,64]. The tight coupling of Cr to the CK/PCr system and the dysregulation of energy metabolism that follows injury explains why reduced brain levels of Cr are observed in the period following an mTBI [57, 65, 66]. However, several studies have reported no changes in Cr in the presence of decreased NAA [67, 68]. A closer examination of that study in which brain levels of Cr were reduced suggests that the reduction in Cr resulted from a more severe injury, as athletes in that study experienced a longer duration of clinical symptoms post-injury, as well as a longer period of time for the normalization for NAA [57]. Therefore, if brain Cr levels were to be made available by increasing the pool of brain Cr prior to mTBI, theoretically, the increased availability may provide an additional energy source allowing a maintenance of energy homeostasis during periods of energy fluctuations. Indeed, two reports in rodent models of TBI (controlled cortical impact) have concluded that Cr supplementation prior to insult afforded those supplemented animals neuroprotection through direct action on mitochondrial energy homeostasis [69, 70]. Cr-supplemented animals exhibited reduced free fatty acid and lactate accumulation as well as a reduction in reactive oxygen intermediate production. The apparent maintenance of energy homeostasis and resultant decrease in oxidative stress likely contributed to the cortical tissue sparing that was also observed in those animal supplemented with Cr. A reduction in oxidative stress reduces subsequent lipid peroxidation which can further damage neuronal proteins [71,72,73]. The potential neuroprotective effect of Cr may not be limited to energy homeostasis as recent reports suggested additional roles for Cr within the brain including, but not limited to, action on NMDA [74] and gamma-aminobutyric acid (GABA) receptors [75]. Further study is warranted in models of mTBI and TBI.

Cr is endogenously synthesized from glycine, arginine, and S-adenosyl-l-methionine in the kidneys, liver, pancreas [63], and to a lesser extent, the brain [76]. Cr synthesized in the brain [76] does not account for the total pool of brain Cr as Cr is also shuttled across membranes via a creatine transporter protein (CrT) [55, 77, 78]. Since the CrT is not ubiquitously expressed in the brain, Cr uptake and saturation of the endogenous pool takes longer than in other tissues, specifically muscle [55, 79], the primary site of Cr storage accounting for up to 90% of the total body Cr pool [63, 80]. This likely explains why animals supplemented for a longer period and at higher doses were afforded greater protection [69, 70] and highlights the importance of timing and dosage as factors contributing to neuroprotection (Table 1). The total body Cr pool can be increased by ingestion of foods high in Cr (i.e., meat or fish) and/or nutritional supplementation. Ingestion of Cr in solution may increase whole-body Cr to a greater degree than meat [81], which is affected by cooking processes [82]. One consideration for athletes seeking to increase Cr levels is the additional calories associated with increased consumption of meat and/or fish. The most common form of Cr found in dietary supplements, food products, and referred to in the scientific literature is creatine monohydrate (CrM) [83]. Though the authors are unaware of any study examining the efficacy of other forms of Cr on the pool of Cr in the brain, a number of studies have been carried out on the effects of various formulations on muscle Cr. Despite consistent findings demonstrating the efficacy of CrM for increases in muscle Cr [83,84,85], manufacturers continue to develop alternative forms in an effort to increase market share.

Table 1 Outline of studies in which creatine or curcumin was provided prior to injury, mild traumatic brain injury, or traumatic brain injury Full size table

CrM supplementation is an effective strategy for increasing levels of brain Cr in humans. A single bolus dose of CrM (20 g) has been reported to increase brain levels of Cr which are further enhanced when supplementation is extended [86]. However, a recent study found no difference in brain Cr content following a 7-day period in young (10–12 years), healthy participants [87]. In tissues with a high pre-supplementation pool of Cr, such as muscle and the brain, a period of 2–4 weeks is recommended [79]. A high pre-supplementation pool may limit any further increases. Further, in muscle, one of the most studied tissues related to Cr uptake, it is well known that some individuals respond to Cr supplementation (responders), while others do not (non-responders). This has been suggested to be due to the physiological profile of the individual (i.e., those with high initial levels may not respond) [84, 88].

A known and valid side effect of creatine supplementation is weight gain [89] and athletes who use creatine should be cautious, particularly if competing in a sport that is weight restricted or classified. Several case reports and an array of anecdotal evidence have misleadingly purported that oral creatine supplementation increases the risk of musculoskeletal cramping and causes dehydration and renal injury. However, there is no persuasive evidence suggesting that oral creatine supplementation causes musculoskeletal cramping or adversely affects renal function in healthy or clinical populations [89]. In fact, a 2003 study found that the incidence of muscle cramping, dehydration, and total injuries over the course of a collegiate American Football season was less in those athletes who supplemented with creatine when compared with those athletes not supplementing with creatine [90]. Furthermore, administration of creatine doses of up to 0.8 g/kg/day for up to 5 years have demonstrated no adverse health risks [89].

Curcumin

In the presence of ROS, the high content of polyunsaturated fatty acids (PUFAs) make the brain particularly susceptible to lipid peroxidation [71], which can further damage neuronal proteins [72, 73, 91]. Curcumin, the bioactive component of the spice herb turmeric (Curcuma longa), has a long history of medicinal use due to its antioxidant and anti-inflammatory properties [92]. Several studies have demonstrated the antioxidant properties of curcumin in rodent models of mTBI [72, 73, 93]. Perhaps the most convincing argument for the antioxidative effects of curcumin following mTBI were demonstrated in the presence of a diet high in saturated fat [73]. Diets high in saturated fat increase free radical formation and exacerbate the deleterious effect of TBI on cognition and neuroplasticity [94]. Curcumin attenuated the increase observed in oxidized proteins and normalized levels of brain-derived neurotrophic factor (BDNF), synapsin I, and cyclic adenosine monophosphate response element-binding protein (CREB) similarly in those fed normal and high-fat diets [73]. BDNF and its downstream effectors synapsin I and CREB are pivotal for facilitation of synaptic transmission and modulation of transcription factors associated with cognitive processes [94,95,96]. The lack of cognitive deficits observed in those supplemented with curcumin on either diet in the presence of normal BDNF supports previous findings [94, 96].

Curcumin is also known for its anti-inflammatory properties. In that regard, curcumin interacts with multiple inflammatory pathways [97, 98], but it primarily suppresses inflammation by inhibiting IκB kinase (IKK) signaling complex, thereby preventing the activation of nuclear factor-kappa B (NF-κB) [97, 99, 100], which regulates the release of many pro-inflammatory cytokines, including IL-1β. IL-1β has been suggested to play a role in cerebral edema following TBI by way of aquaporin 4 (AQP4) regulation [101]. Pre-treatment with curcumin prior to injury attenuated cerebral edema concomitant with reduced NF-κB activation, IL-1β, and AQP4 expression (Table 1). However, recent findings point to the potential of curcumin to maintain energy homeostasis [102]. Though maintenance of energy homeostasis does not necessarily mean a complete amelioration of oxidative stress and inflammatory processes implicated in the pathological secondary sequelae, it would at least attenuate those secondary injury processes. Sharma et al. [102] reported that curcumin supplementation prior to injury effectively maintained energy homeostasis post-injury as evidenced by an increase in mitochondrial proteins, including AMP-activated protein kinase (AMPK) and ubiquitous mitochondrial creatine kinase (uMtCK). AMPK is an important sensor of cellular energy homeostasis [103]. Normalization of AMPK and p-AMPK following TBI is an indication of maintenance of cellular energy homeostasis. Excessive intracellular calcium is largely responsible for mitochondrial dysfunction and an increase in oxidative stress [104]. uMtCK is an enzyme responsible for the regulation of calcium and energy homeostasis [105], and the apparent preservation of energy homeostasis as evidenced by conservation of uMtCK, AMPK, and other mitochondrial proteins may therefore be responsible for the reduction in oxidative stress observed by others [72, 73, 93]. Though the exact mechanism by which curcumin may exert effects on energy regulation is unknown, it may be through modulation of the AMPK/uncoupling protein 2 (UCP-2) pathway [106,107,108].

Though the effects of curcumin appear to be dose dependent, with those fed higher doses afforded greater protection [72], curcumin is not highly bioavailable. A major limitation to the therapeutic potential of curcumin is the poor solubility, low absorption from the gut, rapid metabolism, and rapid systemic elimination [109]. Curcumin is primarily excreted through the feces, never reaching detectable levels in the circulation [110]. High doses of orally administered curcumin, upwards of 10–12 g, have been reported to result in little to no appearance in the circulation [111]. Various methods have been developed to increase the bioavailability of curcumin involving emulsions, nano-crystals, and liposomes with varying degrees of success [112]. As an example, a recent formulation of curcumin in combination with cellulosic derivatives and natural antioxidants (tocopherol and ascorbyl palmitate) was reported to result in a 46-fold increased absorption over a standardized curcumin mixture and improved absorption over other formulations (5.8- to 35-fold), and was well tolerated with no adverse effects reported [113]. Thus, improved bioavailability is possible and a number of unique formulations have already earned the distinction of being generally recognized as safe (GRAS) from the US Food and Drug Administration.

Omega (ω)-3 Fatty Acids

A unique pathological consequence of TBI is that there is a reduction in the quantity of neuronal docosahexaenoic acid (DHA) following injury [93, 114]. Further, deficiency of brain DHA content (70%), induced by dietary restriction, heightens the response to TBI (cortical controlled impact in rats) as evidenced by greater breakdown of neuronal cytoskeleton protein (alpha spectrin II), exacerbated cell death (fewer NeuN positive cells), slower recovery of motor function, more anxiety-like behaviors, and cognitive deficits [115, 116]. DHA, an ω-3 long-chain PUFA (LCPUFA), is present in a variety of tissue types, but is most highly concentrated within the mammalian central nervous system (CNS) and is over 100-fold more abundant within the mammalian CNS than eicosapentaenoic acid (EPA), another ω-3 LCPUFA [117]. As such, the study of ω-3 fatty acids (FAs), specifically DHA intake and supplementation as it pertains to neurological development, disease, and functionality, has been extensively investigated [118,119,120,121]. Despite the fact that EPA is not an integral constituent of the CNS [119, 120], many studies examining the effects of ω-3 FAs on brain function have supplemented subjects with fish oil, which comprises both EPA and DHA. For a more extensive appraisal regarding the role of ω-3 FAs and brain health, refer to the following reviews [122,123,124,125].

In animal models of mTBI and TBI, prophylactic supplementation with ω-3 FAs, specifically DHA, mitigates white matter damage, a characteristic of mTBI, as evidenced by fewer β-amyloid precursor-positive axons; enhanced preservation of myelin; and enhanced protection of neurofilament morphology [126,127,128] (Table 2). The mechanism by which a neuroprotective effect is realized is multifaceted and not completely understood. A series of mutual mechanisms by which DHA may convey neuroprotective effects are those characteristic of the complex pathological sequelae that occurs post-injury. DHA has been shown to allay glutamate cytotoxicity [129, 130], suppress mitochondrial dysfunction and the eventual development oxidative stress [131], decrease calcium influx [130], and downregulate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits [132]. Animals supplemented with ω-3 FAs consistently exhibit enhanced resilience to TBI with functional outcomes mirroring those biological indicators of injury, even following multiple mTBIs [127], similar to that which would be observed in repetitive sports-related concussive injuries over a lifetime of play. While the obvious preservation of white matter undoubtedly aids in maintaining neurocognitive function following injury [126, 127, 133], the blunting of injury-induced reductions in molecular elements important for learning, including BDNF, synapsin I, and CREB also plays a role [133].

Table 2 Outline of studies in which ω-3 fatty acids or ω-3 fatty acids plus curcumin were provided prior to injury, mild traumatic brain injury, or traumatic brain injury Full size table

Worldwide consumption of ω-3 FAs is low [134]; hence, supplemental intake may be recommended. Dietary sources of DHA are limited, with cold-water algae being the primary producers of DHA and EPA. Fish are also rich sources of DHA and EPA due to a diet consisting of algae [117]. Dietary intake of the essential fatty acid and precursor to DHA, α-linolenic acid (ALA), is generally much higher [134]. Although ALA can be metabolically converted to DHA, the conversion rate is low [135, 136]. Further, diets higher in ALA seem to limit the conversion rate by increasing the rate of ALA oxidation [137]. A dose-dependent relationship exists whereby plasma phospholipid DHA concentrations increase up to a dosage of ~ 2 g/day after which any further increase in dose negligibly increases plasma phospholipid concentration [117]. Plasma phospholipid DHA content correlates with DHA status and has been demonstrated to be a useful biomarker of DHA status in adults [138]. The neuroprotective effects of supplemental DHA observed in rodent models of TBI demonstrate the greatest efficacy when administered at a dose corresponding to 40 mg/kg/day [126], which corresponds to a dosage of approximate 3.6 g in a 90-kg athlete. Physical activity, particularly that which is long in duration and high in intensity, is known to affect plasma phospholipid composition [139], another important consideration for athletes. To determine if athletes performing heavy physical activity require higher doses, our research group recently examined the dose–response effect of supplemental DHA in American Football athletes. The lack of an observable increase in EPA, and the fact that retroconversion of DHA to EPA is regularly observed [117, 140] in those receiving a higher dose, led us to conclude that American Football athletes may require a higher dose. However, in addition to performing heavy physical activity, American Football athletes are larger than the average population with greater increases in height, weight, and body mass index when compared with all other sports over the last several decades [141]. Thus, while the optimal dosage for athletes, particularly those of larger size (i.e., ice hockey, rugby) may be higher than that reported for the average population, athletes of smaller stature and body mass may only require 2 g/day. The side effects of ω-3 FA supplementation are largely limited to gastrointestinal symptoms and poor palatability and may include malodorous belching, nausea, diarrhea, and acid reflux [29]. While concerns of significant bleeding have been raised, there is no convincing evidence of clinically significant bleeding associated with ω-3 FA supplementation [142] and those aforementioned gastrointestinal symptoms are experienced by a small percentage of those supplementing with ω-3 FAs [29].

Combination of DHA and Curcumin

The likely differing mechanisms by which the aforementioned nutrients and nutraceuticals provide neuroprotective effects suggest that a combination may afford greater protection. Wu et al. [93] recently reported that the combination of curcumin and DHA potentiated the effect of either alone. Rats fed curcumin plus DHA reportedly had higher levels of BDNF and reduced markers of lipid peroxidation compared with control animals or those fed DHA or curcumin alone. Further, as evidenced by enhanced stability of DHA, curcumin plus DHA effectively normalized enzymes required in the metabolism of DHA, Δ6-desaturase, and 17β-hydroxysteroid dehydrogenase (HSD). Cognitive outcomes reflected those of biological markers with the combination enhancing learning ability to a greater extent [93]. Though the exact mechanism by which the combination enhanced outcomes is unknown, the normalization of enzymes associated with DHA metabolism is likely the cause. In a more recent study, researchers from that same group demonstrated that curcumin enhanced the conversion of ALA to DHA in animals fed a diet rich in ALA in combination with curcumin [143]. Similar to that observed in their previous study [93], an increase in enzymes necessary in the metabolism of DHA, Δ6-desaturase, and elongase 2, was observed. The increase in conversion resulted in a higher content of DHA in the brain [143]. Therefore, there is precedent for the potential augmentation of neuroprotection through the combination of nutrients and nutraceuticals.

Human Studies

Up until this section, studies that have been highlighted have involved the use of animal models, specifically rodent models, of mTBI and TBI. Despite evidence as early as the year 2000 suggesting a neuroprotective role of supplementation, no study to date has been conducted in humans. This is likely due to the difficulties of conducting well designed, large-scale clinical trials in an athletic population. Athletes participating in contact sports are at risk for sustaining sports-related concussion and also show evidence of neurological damage in the absence of a concussion diagnosis. Thus, contact sport athletes represent a unique population in which to examine the potential neuroprotective effects of a nutritional intervention.

To that end, our research group recently published results from a study examining the effects of DHA supplementation on a biomarker of head trauma in American Football athletes [29]. These athletes are routinely exposed to head impacts that vary in magnitude and number over the course of the season [144, 145], resulting in some level of damage as documented via advanced neuroimaging techniques [20, 23, 24, 31, 146] and blood biomarkers [28]. American Football is associated with the highest incidence of concussion [15]. Based on our data, we concluded that DHA attenuated damage as measured by serum neurofilament light (Nf-L), the most sensitive and specific marker in regard to detecting neuroaxonal injury in concussion [147], irrespective of dose [29]. However, inference from those data was limited due to a number of constraints. In an attempt to identify an optimal dosage, as highlighted in a previous section (Sect. 3.3), athletes were randomly assigned to three different treatment groups. That reduced the number of athletes in each treatment group, which was further reduced when athletes were separated by number of repetitions performed during competition. Indeed, further examination of those data suggested that the low-dose treatment group actually experienced the greatest attenuation in head trauma. There is no doubt that large-scale clinical trials are necessary to fully elucidate the potential neuroprotective effect in athletes.