Expert Discussions

A public workshop was held June 23–24, 2010, in Washington, DC, where invited civilian and military subject-matter experts (e.g., medical experts, nutritionists, and neu roscientists) presented their findings and experiences on various TBI-related topics, with a focus on potential nutrient interactions with the pathogenic sequelae of TBI. The committee also reviewed the existing literature examining the pathology and metabolic alterations of the insult with the goal of identifying nutrients that had the potential to favorably alter metabolism, oxidative stress, and tissue structure and repair, following and sometimes in advance of TBI.

Review of the Literature on Pathophysiology and Identification of Targets for Nutritional Interventions

As a background to elucidate the committee’s rationale, a summary of current knowledge about the pathophysiology of TBI was presented in Chapter 3. Briefly, brain injuries differ dramatically from patient to patient depending upon the location, type, intensity, and duration of the initiating concussion force. One common feature of severe brain injury is reduced blood flow and oxygen deficiency. Neuronal membranes become damaged and ion channels malfunction, leaking proteins and neurotransmitters. Apoptotic genes are activated, brain cells begin to swell, and mitochondrial function diminishes as the brain cells enter a “death spiral” (Scudellari, 2010). Production of free radicals increases and calcium ions are released, resulting in tissue damage and cell death (Mattson and Chan, 2003). For the purposes of this report, TBI pathology has been divided into primary, secondary, and long-term injury mechanisms to permit more specific focus on the timing and locus of the nutrient intervention. Box 4-1 shows a summary listing of adverse physiological responses as well as select potential neuroprotective targets (e.g., control of inflammation, angiogenic and neurogenic repair and recovery) discussed during the June public workshop and committee meetings that might respond to nutritional interventions. Box entries with an asterisk indicate especially promising potential targets for nutritional intervention. These potential nutritional targets and the mechanism or metabolic bridge linking them to TBI treatment or prevention have been grouped into three topic areas and will be discussed briefly in this chapter. Table 4-1 matches these potential nutritional targets with nutrients having relevant mechanisms of action; however, the reader should make no conclusions about the clinical efficacy of any of these nutrients based on this table. The information in this table is not meant to imply that there is any evidence from animal models or human trials showing benefits in ameliorating TBI. The table was constructed to illustrate that those nutrients and food components act on multiple and diverse processes that are common to brain injury, and therefore their effects (beneficial and adverse) should be reviewed.

BOX 4-1

Physiologic Responses to TBI and Protective/Reparative Targets. Physiologic responses Intracranial hemorrhage

TABLE 4-1

Plausible Mechanisms of Action by Selected Nutrients and Food Components That Might Affect TBI Outcomes.

For most nutrients, more than one mechanism is proposed. The scientific community believes that, based on the variations in injuries, responses, and outcomes after TBI, no single nutritional mechanism will serve as the magic bullet for TBI. The evidence on the ability of these putative nutrients to improve resilience or ameliorate the effects of TBI will be discussed in detail in subsequent chapters (Chapters 6–16).

Review of the Literature on Effectiveness of Nutrients

From its discussions, the committee identified an initial list of search criteria and promising pre- and postinjury nutritional interventions that might be useful adjuncts in the overall medical treatment and prevention of TBI. The committee decided on the search criteria and vocabulary included in Box 4-2.

BOX 4-2

Literature Search Criteria and Vocabulary. Inclusion Criteria Health condition: patients or animal models of TBI (including in the developing brain), subarachnoid hemorrhage, intracranial aneurysm, stroke, anoxic or hypoxic ischemia, epilepsy

Restoration of Cellular Energetics

Metabolic characteristics of TBI include cerebral acidosis, hypoxia, decreased adenosine triphosphate (ATP) production, and resultant mitochondrial malfunction followed by apoptotic cell death. Apoptosis can be triggered by many different stimuli such as certain cytokines, toxins, oxidative stress, and calcium influx through damaged plasma membrane channels or release from the endoplasmic reticulum. Mitochondrial calcium dynamics are involved in the regulation of cellular energy metabolism and neurotransmitter release (Mattson and Chan, 2003). Although it is a nutrient, calcium acts in this case as a messenger to coordinate cellular reactions that drive apoptosis. Calcium release is normally under tight physiological control, but this normally close regulation is altered by the events following a traumatic brain injury. Calcium released from the endoplasmic reticulum triggers a mass exodus of cytochrome C from mitochondria in the cell, activating caspases and other nuclease enzymes that finalize cell death (Mattson and Chan, 2003). Many of the hypothetical mechanisms of nutrients included in this report may have the effect of preventing apoptosis. Neuronal apoptosis can be induced by activation of glutamate receptors confined to the postsynaptic membrane of a dendrite, permitting a local influx of calcium that triggers cytochrome C release from mitochondria throughout the dendrites and cell body (Boehning et al., 2003). The series of events that triggers the malfunctioning of mitochondria severely disrupts the energetics of the cell. Thus, restoration of mitochondrial function or providing sources of energy other than glucose is a key objective after a TBI event. In addition, nutrients or food components that counteract or prevent ionic disturbances and excitotoxicity have the potential to improve outcomes of TBI. One such nutrient is magnesium, which plays a role in the pathobiology leading to TBI (Sen and Gulati, 2010). The level of magnesium in the brain has been found to affect the influx of calcium and glutamate to the postsynaptic neuron, one potential mechanism by which magnesium exerts its effects.

Several other food components could potentially ameliorate the described effects on mitochondrial malfunction by augmenting energy metabolism. Creatine, a nitrogenous guanidine compound found naturally in vertebrates, accesses the brain via a creatine transporter. Creatine and the creatine kinase/phosphocreatine system serve to buffer energy in tissues with high and fluctuating energy requirements (e.g., muscle and brain). Since the brain requires significant ATP turnover to maintain membrane potentials and its signaling capacity, creatine metabolism and the creatine kinase/phosphocreatine system are important for normal brain functioning; these may be compromised in diseases of the central nervous system. Studies also suggest that creatine’s neuroprotective effects are at least partly due to increased cerebral blood flow and reduction in the processes associated with secondary brain damage, such as an increase in free fatty acid and lactate (Sullivan et al., 2000).

Acetyl-L-carnitine (ALC), another food component, has been shown to confer neuro-protection in models of both global and focal ischemia (see review, Zanelli et al., 2005). It is postulated that ALC serves as an alternative source of acetyl coA, and as such promotes aerobic energy metabolism and reduces tissue acidosis. With reduced tissue acidosis, oxidative stress may likewise be ameliorated. Data further suggest that ALC modulates excitotoxicity, apoptosis, and inflammation. The beneficial effects of ALC may reflect the targeting of multiple pathogenic pathways. However, a review of the literature did not deliver enough evidence for the committee to write a chapter and make recommendations. Only a few studies have been done in animal models of ischemia. In addition, improvements in neurological outcomes were only seen when ALC was administered within 30 minutes after the injury, which might not be feasible in the field. The acetate precursor glyceryl triacetate, a food additive and dietary supplement, has also been found to effectively supply acetyl coA as an alternative energy source in an animal model of TBI (Arun et al., 2010), which can contribute to more immediate energy production for mitochondria than glucose.

Essential nutrients might also be promising in the treatment of TBI. Dietary branched-chain amino acids (BCCAs) are important for de novo synthesis of glutamate, which plays an important role in maintaining synaptic neurotransmitter pools. In an experimental model of TBI, BCAAs (i.e., valine, isoleucine, leucine) were found to be reduced (Cole et al., 2010), and dietary delivery resulted in improved cognitive outcomes that correlated with restored net synaptic efficacy. Although these findings are quite encouraging, further studies, as recommended in the next chapter, are needed to determine if this benefit is sustained over time. It is postulated that BCAAs are beneficial as precursors of neurotransmitters as well as providing an alternative energy source. It is unclear to what extent the synthesis neurotransmitters might contribute to the potential effects of BCAAs in TBI, because BCAAs influence the synthesis of both excitatory and inhibitory neurotransmitters.

Other promising interventions include specific diets rather than a single food component or nutrient. The ketogenic diet is a high-fat, low-carbohydrate diet used as a therapy for epilepsy, especially in children (Beniczky et al., 2010). This diet provides an alternative source of fuel to the brain and enhances energy production in the brain when glucose utilization is impaired. Although the mechanisms of protection in epilepsy are not completely understood, it appears that following a ketogenic diet provides resistance to metabolic stress and may thereby provide protection to the injured brain. Animal studies have shown that ketogenic diets improve mitochondrial antioxidant status and protect the DNA (deoxyribonucleic acid) from oxidative damage by increasing glutathione, a major mitochondrial antioxidant, and by reducing both acetyl coA, an indicator of mitochondrial redox status, and lipoic acid, a thiol antioxidant. One drawback of ketogenic diets is that patient compliance is difficult to maintain. Alternative diets could be developed that are more organoleptically acceptable and have similar effects in the brain.

Reduction of Oxidative Stress and Inflammation and Repair of Cellular Damage

Oxidative stress and chronic inflammation are among the root causes of many diseases, such as cardiovascular disease, insulin resistance, autoimmune diseases like rheumatoid arthritis, and Alzheimer’s disease. Brain injuries such as ischemic brain injury and TBI have also implicated oxidative stress as a key pathological factor in secondary injury progression. Mitochondria, the main source of cellular ATP production under normal aerobic conditions, are susceptible to reactive oxygen species (ROS), which generate localized oxidative stress. Injury to the brain also promotes inflammation, which can eventually lead to further tissue damage if not well regulated and resolved promptly. These observations suggest that dietary antioxidant and anti-inflammatory therapy may be useful approaches to promote protective mechanisms in the injured brain (Wu et al., 2006).

One class of nutrients that could be important to improve TBI outcomes is niacin (i.e., vitamin B3, nicotinamide adenine dinucleotide [NAD⁺]) and its derivatives, since it plays an important role in maintaining and regulating cellular redox states, calcium ion stores, DNA damage and repair, response to cellular stress, and energy metabolism (Xu and Sauve, 2010). Enhanced oxidative stress can induce DNA strand breaks, which in turn are repaired by activation of poly(ADP-ribose) polymerase (PARP). Under certain pathological conditions such as trauma and resulting sepsis, this process of DNA damage and repair can consume large amounts of NAD⁺, leading to its cellular depletion, which in turn promotes inflammation (Wendel and Heller, 2010). The nuclear protein poly(ADP-ribose) polymerase-1 (PARP-1) functions as a molecular stress sensor and DNA repair mechanism. Overactivation of PARP-1 followed by NAD⁺ depletion is a major contributor to ischemic brain damage (Love, 1999). PARP-1 is believed to be a key mediator of cell death in excitotoxicity, ischemia, and oxidative stress (Alano et al., 2010). Reactive oxygen species strongly activate the enzymatic activity of PARP-1 (Altmeyer and Hottiger, 2009). Excessive activation of PARP-1 is believed to be a key link between oxidative stress, glutamate excitotoxicity, and cell death (Ying and Xiong, 2010; Yu et al., 2009). NAD⁺ links PARP-1 to energy metabolism (Altmeyer and Hottiger, 2009), suggesting that NAD⁺ depletion, such as that observed in TBI, is a causal event in PARP-1 mediated cell death. Preventing the depletion of cytosolic NAD⁺ that characterizes the massive disruption of energy metabolism following TBI could therefore ameliorate the chain of events that lead to cell death. Despite this role in regulating redox states, a literature search on niacin did not reveal any study to explore its effects in outcomes of TBI. For this reason, niacin was excluded from the list of promising nutritional approaches.

There are numerous other nutrients and food components that have either antioxidant or anti-inflammatory effects and that could potentially be beneficial in TBI pathology. The committee initially selected some because they have been studied extensively in connection with cardiovascular disease and other chronic diseases having inflammation as one of their etiologies. Vitamin E (alpha-tocopherol) is a potent reactive species scavenger that acts as an inhibitor of lipid peroxidation (Marklund et al., 2006). In a 2010 experimental model of TBI, dietary supplementation with alpha-tocopherol has been shown to reduce protein oxidation, increase levels of the antioxidant superoxide dismutase, and improve cognitive outcome (Wu et al., 2010). Identifying compounds such as alpha-tocopherol or alpha-eleostearic acid that mitigate oxidative stress or regulate the release of apoptosis-releasing factors from the mitochondria may yield important therapeutic approaches for the nutritional treatment of neuronal injury (Kondo et al., 2010).

Brain development and function are reliant upon n-3 fatty acids (Bazan, 2007). This group of fatty acids is of particular interest in the setting of TBI because of its potential role as a modulator of inflammation. Two fatty acids, the n-3s docosahexaenoic acid (DHA— C22:6) and eicosapentaenoic acid (EPA—C20:5), support resolution of inflammation (see review, Kidd, 2007). While their mechanisms of action are currently being elucidated, Serhan and colleagues in Appendix J describe a number of studies that led to this conclusion. A hallmark finding of this research is that DHA is metabolized to resolvins and protectins, which are hypothesized to resolve inflammation and protect the brain from further damage. Endogenous neuroprotectin D1, while elevated in a model of brain ischemia, may not be sufficient to thwart the pro-inflammatory response; exogenous administration (intraventricular infusion), however, may reduce inflammation and result in neuroprotection (Bazan, 2007). Taken together, it appears that the mechanistic effects of diets high in n-3 fatty acids, such as DHA or its analogues, warrant further investigation for ameliorating the adverse effects of TBI.

Many phytochemicals, such as flavonoids, may also exert benefits because of their antioxidant or anti-inflammatory actions. Flavonoids consist of two aromatic rings bound together by three carbons that form an oxygenated heterocycle. Flavonoids can be classified into seven subclasses: flavonols (e.g., quercetin), flavones (e.g., apigenin), flavanones (e.g., hesperetin), flavan-3-ols (e.g., epicatechin), anthocyanins (e.g., cyanidin), polymers (e.g., proanthocyanidins), and isoflavones (e.g., genistein). The flavanol epicatechin was shown to be neuroprotective in a stroke model of brain injury (Shah et al., 2010). It is thought that this flavanol supports antioxidant defense mechanisms in the brain by activating the transcription factor Nrf2. This results in an increase in the enzyme heme oxygenase-1, which confers neuroprotection in part by degrading the prooxidant heme to the antioxidants biliverdin and bilirubin (Chang et al., 2005). In the setting of TBI, where heme arises from the breakdown of hemoglobin as well as the release of heme from dying cells, epicatechin may prove to be beneficial (Doré, 2010).

One food component with anti-inflammatory properties is resveratrol (3,4′,5-trihydroxystilbene). Resveratrol belongs to a class of polyphenolic compounds called stilbenes. Some types of plants produce resveratrol and other stilbenes in response to stress and injury, presumably to aid in their recovery process. Resveratrol has been shown to be neuroprotective in animal models of cerebral ischemia. Its exact mechanism of action is not known; there are several hypotheses, one of which is that it acts indirectly by increasing sirtuin-1 (SIRT1) activity during recovery or by sparing cells via 5′ adenosine monophosphate-activated protein kinase’s (AMPK’s) elevation of NAD⁺ levels. The NAD-dependent deacetylase, SIRT1, is an enzyme that deacetylates proteins involved in cellular regulation, including reaction to stressors. Recent evidence suggests that SIRT1 inhibitors may be neuroprotective. Based on in vitro studies, resveratrol is also hypothesized to inhibit the activity of several inflammatory enzymes, including cyclooxygenase and lipoxygenase, resulting in a suppressive effect on inflammatory and oxidative stress (Ghanim et al., 2010). Yet another hypothesis states that resveratrol may also inhibit proinflammatory transcription factors, such as NFκB or AP-1. This is supported by a human randomized controlled trial in which resveratrol was shown to exert anti-inflammatory and antioxidative stress effects in normal humans when measured by blood biomarkers (Ghanim et al., 2010). The suppressive effects of this extract on ROS generation and a range of inflammatory mediators suggest that resveratrol may induce its effects through increased expression of anti-inflammatory cytokines and a reduction in proinflammatory molecules, all potentially beneficial implications for TBI.

Curcumin, a phenolic yellow pigment from the rhizome Curcuma longa used as a flavoring and coloring agent, especially in curries, is an antioxidant and an anti-inflammatory agent. Curcumin is a potent inhibitor of the proinflammatory cytokine IL-1β and the transcription factor NFκB that are involved in cellular responses to stimuli such as stress, cytokines, and free radicals (Laird et al., 2010).

One compound that deserves attention because of its potential synergism with progesterone therapy is vitamin D. The hormone progesterone and its metabolites are considered pleiotropic drugs that confer neuroprotection by preventing the expression of inflammatory cytokines in the brain while reducing apoptosis, lipid peroxidation, and cerebral edema (Beauchamp et al., 2008) and stimulating growth-promoting factors that may play a significant role in remyelination of damaged neurons. The dosage of progesterone effective for treatment is too high to be feasibly achieved with current commercial dietary supplements. The committee therefore did not consider it as a potential stand-alone nutritional approach to the treatment of TBI; rather, it considered vitamin D₃ as an example of a nutrient that may act synergistically with a pharmaceutical product, progesterone. Cekic and Stein (2010) suggest that vitamin D, a potent modulator of cell cycle, immune function, and calcium homeostasis, can work synergistically with progesterone therapy to reduce post-TBI inflammation.

Although microbial proteases have been used as anti-inflammatory agents (e.g., in wounds and hematomas), a review of their effects using the criteria in Box 4-2 did not identify any active research with proteases and TBI or associated conditions; therefore, they were neither considered as promising interventions nor reviewed in this report.

Repair and Recovery

Studies in the late 1990s showed that the brain has the capacity to generate new cells and that these cells can, under defined conditions, become functional. These findings, together with key studies showing the plasticity of the somatosensory circuits, serve as a foundation for understanding mechanisms of adaptability after a brain injury (see review, Kidd, 2009). Repair and recovery mechanisms likely involve revascularization of damaged tissue, neurogenesis, axonal/dendritic plasticity, and changes in neuronal networks. These events may be susceptible to nutritional augmentation approaches. For example, neurotrophins regulate neuronal survival, growth, and synaptic plasticity and neurotransmission, and protect against neuronal injury. One neurotrophin that has received particular attention is brain-derived neurotrophic factor (BDNF), modulated by diet and exercise. Brain-derived neurotrophic factor has been shown to regulate survival, growth, and differentiation of neurons during development, and is associated with learning and memory skills.

The nutrient vitamin E may act to improve neuroplasticity. Utilizing a fluid percussion model in rats, Wu et al. (2010) found that rats receiving dietary supplementation with vitamin E prior to TBI suffered less loss of BDNF and showed better-preserved brain cognition and synaptic plasticity than rats receiving no vitamin E supplementation. Thus, in addition to its antioxidant activity, vitamin E may exert its action via other targets, such as those supporting neuroplasticity.

Dietary curcumin has also been investigated for its potential to counteract the outcome of TBI on synaptic plasticity and cognition, as well as for its antioxidant properties (Wu et al., 2006). Dietary supplementation reduced oxidative damage and normalized levels of BDNF and cyclic AMP-response element-binding protein. It was therefore suggested that oxidative stress may influence synaptic plasticity and cognitive recovery through its actions on BDNF.

Cytidine 5′-diphosphocholine (citicoline or CDP-choline) is an essential intermediate in the biosynthetic pathway of cell membrane phospholipids. It is naturally present in eggs and liver, and also sold as a dietary supplement. It is readily absorbed by the oral route, with no reported serious side effects, and is subsequently widely distributed throughout the body where it is incorporated into membrane and microsomal phospholipids. The two principal components of CDP-choline, cytidine and choline, can cross the blood-brain barrier where they then become incorporated into lipid membranes (Jain, 2008). CDP-choline has been shown to inhibit apoptosis associated with cerebral ischemia and edema and to potentiate neuroplasticity in experimental models. It exerts neuroprotection in hypoxic and ischemic conditions (Secades and Lorenzo, 2006) as well as in rodent models of TBI (Kokiko and Hamm, 2007; Marklund et al., 2006), and is currently in phase III clinical trials for TBI, sponsored by the National Institute of Child Health and Human Development (NICHD) (Jain, 2008; Zafonte et al., 2009).

The antioxidant effects of flavonoids, polyphenolic compounds found in fruits and vegetables as well as wine, tea, and other beverages, are well known. They also exert a multiplicity of neuroprotective actions within the brain, including a potential to protect neurons against injury induced by neurotoxins; an ability to suppress neuroinflammation; and the potential to promote memory, learning, and cognitive function (Spencer, 2009). Flavonoids are believed to interact with critical protein and lipid kinase signaling cascades in the brain that lead to an inhibition of apoptosis and to a promotion of neuronal survival and synaptic plasticity (Spencer, 2009). Based on their actions, it is possible that flavonoids also hold potential to aid in recovery from brain trauma, although this particular aspect of flavonoid therapy has not been the subject of human clinical trials. Spencer (2009) suggests that flavonoids increase peripheral and cerebral blood flow, which in turn support angiogenesis and neurogenesis. Flavonoids likely have other effects on the injured brain, targeting post-transcriptional factors and gene expression, and scavenging neurotoxic and proinflammatory species. Yao and colleagues (2010) also observed that Epimedium (a genus of more than 60 species of herbaceous flowering plants in the family Berberidaceae) flavonoids can have a direct stimulatory effect on neuronal stem cell proliferation and differentiation in a rat cell culture model. Altogether, these observations hold promise for the application of flavonoids to neuroprotection.