Oxygen, the very thing that gives us life, can also be life-threatening. The air we breathe is 21% oxygen at a pressure of 1 atmosphere absolute (ATA) — i.e., sea level. However, under certain circumstances, we can be exposed to up to 100% oxygen at pressures at or above 2.4 ATA, increasing the risk for central nervous system oxygen toxicity (CNS-OT) seizure. CNS-OT seizures are a limitation of both hyperbaric oxygen therapy (HBOT) and scuba diving, particularly among divers using closed-circuit rebreathers, such as Navy SEAL divers.
Limitation of HBOT
HBOT involves breathing 100% oxygen at increased pressures, up to 3 ATA. HBOT essentially saturates the blood with dissolved oxygen and is used therapeutically to deliver elevated levels of oxygen to various tissues. HBOT is an FDA-approved therapy for approximately 15 medical applications, including wound healing, decompression sickness, carbon monoxide poisoning, and more. In addition, anecdotal evidence supports a growing list of off-label applications for HBOT.
Limitation of Diving with Closed-Circuit Rebreathers
Closed-circuit rebreathers used for undersea diving recycle and filter exhaled gas, delivering nearly pure oxygen with safe levels of carbon dioxide. There are significant advantages to using this equipment. It prevents the toxic buildup of nitrogen and carbon dioxide in the blood, which can lead to decompression sickness and narcosis. Additionally, for military purposes, released gases do not form bubbles on the water’s surface, thus protecting the diver from detection.
There is also a disadvantage. Pressure, and so the risk of seizure, increases with depth. Navy SEAL divers are often required to swim for hours to cross bodies of water and are sometimes required to dive to beyond safe levels. Diving to just 50 feet of seawater for 10 minutes puts the diver at risk of a CNS-OT seizure, and these seizures come with no warning. Diving to 130 feet of seawater, which would equate to 5 ATA, will almost certainly produce a seizure within five to 10 minutes. No one is resistant to CNS-OT.
What causes CNS-OT seizures?
The causes of CNS-OT seizures remain largely unknown. It is known that high levels of oxygen at increased pressures triggers the production of oxygen free radicals and oxidative stress in the mitochondria, which ultimately impedes various metabolic processes. This metabolic crisis increases brain hyperexcitability and provokes seizure.
The military prevents CNS-OT with anti-epileptic drugs, but in order to be effective, they are required in doses that can have sedative effects and impair physical and cognitive performance.
Therapeutic Ketosis: An Armor Against CNS-OT Seizures?
Three critical observations led to the investigation of exogenous ketones:
- In very early studies of prolonged starvation, it was demonstrated that ketones could supply up to two-thirds of total brain energy requirements.
- Fasting (24 to 36 hours) was shown to delay the onset of CNS-OT seizures with results roughly comparable to high-dose anti-epileptic drugs.
- The ketogenic diet was a well-documented therapy for drug-resistant seizure disorders (e.g., epilepsy).
What is the overlap between prolonged starvation, fasting, and the ketogenic diet? Ketones!
Both fasting and the ketogenic diet work by shifting the body’s fuel source away from glucose. The body enters a state in which it becomes heavily reliant on fatty acids and ketones, otherwise known as ketosis. Ketosis can significantly alter brain energy metabolism and neurotransmitter systems.
Ketosis is achieved when glucose availability is severely limited, suppressing insulin and allowing for the mobilization of free fatty acids from stored body fat or dietary fat. Free fatty acids cannot cross the blood brain barrier, but some are converted into ketone bodies in the liver, which are then readily accessible and may be utilized by the brain.
Three primary ketone bodies are produced during ketosis:
- Beta-hydroxybutyrate (BHB) – the most stable ketone body; found in circulation in the greatest abundance due to its stability.
- Acetoacetate (AcAc) – the ketone body that enters cellular energy metabolism; BHB must be converted to AcAc in order to be used as fuel.
- Acetone – produced in small amounts through the spontaneous decarboxylation of AcAc. This ketone body may have neuroprotective effects at low levels and makes the breath smell “fruity” in a high state of ketosis.
Exploring the Effects of Exogenous Ketone Supplements on CNS-OT
Research at USF Hyperbaric Biomedical Research Laboratory (USF HBRL) uses advanced technologies to study the effects of high-oxygen environments at the cellular and whole-animal levels. We can manipulate this hyperbaric environment to simulate Navy SEAL dives, which then allows us to understand the biochemical and physiological effects of CNS-OT seizures, such as mitochondrial free radical production, EEG brain activity, respiration, body temperature, and various other parameters. In addition, we have been assessing the effects of various ketone formulations as a potential mitigation strategy for CNS-OT and other diving-related issues, such as nitrogen narcosis (Ari et al., 2018).
In our first study, we used a specific ketone ester (R,S-1,3-butanediol acetoacetate diester (BD-AcAc2)) that can significantly elevate blood levels of both BHB and AcAc. Previous research indicates AcAc and acetone display the greatest neuroprotective anti-seizure potential (D’Agostino et al., 2013).
A single dose of this ketone ester (BD-AcAc2) rapidly induces deep ketosis, similar to prolonged fasting or a strict ketogenic diet, elevating both AcAc and BHB above 3.0 mmol/L. Remarkably, this level of therapeutic ketosis in rodent model studies was achieved when feeding a standard high-carbohydrate rodent chow.
To study the effects of the ketone ester on CNS-OT, rats were administered the ketone ester and then placed in the hyperbaric chamber, pressurized to 5 ATA, which is equivalent to 132 feet of sea water.
Our results showed the ketone ester BD-AcAc2 delayed time to seizure by 574%:
- Without ketone ester (control): seizure within 5-10 minutes
- With ketone ester (BD-AcAc2): seizure after 50+ minutes
Note: Results like this have only been achieved with anti-epileptic drugs at doses that would leave the individual generally incapacitated from the high level of sedation.
We tested agents that elevated BHB alone (like 1,3-butanediol) and confirmed the protection against CNS-OT seizures was due to the elevation of AcAc and acetone; the results from elevating BHB alone seemed insignificant.
More recent experiments by Dr. Csilla Ari D’Agostino et al. (2019) have expanded this study in older laboratory animals to mimic human middle age using combinations of ketone supplementation. The results with the previously used ketone ester were less dramatic (179% increase compared to 574%), indicating a possible age-dependent factor in seizure protection. In this study, medium-chain triglycerides (MCTs) combined with KE resulted in the greatest anti-seizure neuroprotection (219% increase in time to seizure), and BHB levels were found to correlate with seizure protection. Additionally, the ketogenic supplements not only increased the latency to seizure but also reduced the severity of the convulsions.
How do Ketones Protect Against CNS-OT?
Conceivably, ketones are working synergistically through many mechanisms (unlike anti-seizure drugs), providing a unique multifaceted therapy that would be difficult to replicate with any one drug.
As mentioned previously, high-oxygen environments can promote the production of excess oxygen free radicals, ultimately provoking neuronal hyperexcitability, metabolic damage, and excitotoxicity-induced seizures. The mitochondria are the source of these free radicals and are vulnerable to oxidative damage. By targeting energy metabolism with ketones, we are targeting the potential root cause (i.e., metabolic dysregulation). In addition, when neurons become hyperexcited, they fire at an increased rate and therefore have greater metabolic demands. Ketones not only serve to meet this energy demand but also to promote neuronal stability simultaneously through various other mechanisms.
Some examples of how ketones may be working to protect the brain from CNS-OT seizures:
- Preserving brain energy metabolism: The metabolic crisis and dysfunction associated with oxygen toxicity increase the metabolic demands of the brain, which can, if glucose availability is compromised in the absence of ketones, promote the onset of seizure. Ketones preserve brain metabolism in the face of oxidative stress.
- Reducing oxidative stress: Ketones have been shown to reduce the amount of reactive oxygen species (ROS) induced in high-oxygen environments, which, as mentioned, promote oxidative stress and metabolic dysfunction. In this regard, ketones may be promoting synaptic stability through antioxidant protection effects.
- Suppressing neuronal hyperexcitability: The neurotransmitter glutamate in high levels can be excitotoxic and contribute to the genesis of seizures. Ketones have been shown to reduce the levels of glutamate and promote the production of GABA, the brain’s calming neurotransmitter. Increasing this GABA-to-glutamate ratio can have a stabilizing effect on the brain. Recent evidence supports the role of ketone-induced changes in adenosine (adenosine A1 receptor) in mediating the anti-seizure effects (Kovács et al., 2017).
- Reducing inflammation: The ketone body BHB has been shown to inhibit the NLRP3 inflammasome which upon activation promotes the release of pro-inflammatory cytokines. Therefore, through BHB-mediated inhibition, it’s possible that ketones can reduce neuro-inflammation. Emerging evidence suggests neuro-inflammation could be contributing to the onset of seizures (Kovács et al., 2019).
- Hyperpolarization of cell membranes: It is suggested that in times of neuronal metabolic stress, the activation of ATP-sensitive potassium (KATP) channels can mitigate excessive neuronal firing. These channels play a role in the interface of metabolism and brain excitability and when activated can lead to membrane hyperpolarization, which has a stabilizing effect. Ketones have been shown to activate these channels and, in turn, reduce neuronal firing.
Our research suggests a clear relationship between ketone metabolism and the delay in onset of CNS-OT seizures. It also highlights a potential therapeutic application for exogenous ketones with immediate implications in HBOT and diving with closed-circuit rebreathers. Although these uses are very specialized the science and application of these ketone technologies is paving the way for more widespread use of nutritional and supplemental ketosis for other widespread neuroprotective and even performance-enhancing effects.
Dr. Dominic D’Agostino is a tenured Associate Professor in the Department of Molecular Pharmacology and Physiology at the University of South Florida Morsani College of Medicine. He is also a Research Scientist at the Institute for Human and Machine Cognition (IHMC). His laboratory develops and tests nutritional strategies and metabolic-based supplements for neurological disorders, seizures, cancer, and metabolic wellness. He was a research investigator and crew member on NASA’s Extreme Environment Mission Operation (NEEMO 22) and has a personal interest in environmental medicine and methods to enhance safety and physiological resilience in extreme environments. His research is supported by the Office of Naval Research (ONR), Department of Defense (DoD), private organizations, and foundations.
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- D’Agostino D, Pilla R, Held HE, Landon CS, Puchowicz M, Brunengraber H, Ari C, Arnold P, and Dean J. 2013. Therapeutic ketosis with ketone ester delays central nervous system oxygen toxicity seizures in rats. Am J Physiol Regul Integr Comp Physiol, 304; R829-R836.
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