https://orcid.org/0000-0001-5627-7322
Abstract
Ketone bodies – 3-hydroxybutyrate (3-OHB), acetoacetate, and acetone – are ancient, evolutionarily preserved, small fuel substrates, which uniquely can substitute and alternate with glucose under conditions of fuel and food deficiency. Once canonized as a noxious, toxic pathogen leading to ketoacidosis in patients with diabetes, it is now becoming increasingly clear that 3-OHB possesses a large number of beneficial, life-preserving effects in the fields of clinical science and medicine. 3-OHB, the most prominent ketone body, binds to specific hydroxyl-carboxylic acid receptors and inhibits histone deacetylase enzymes, free fatty acid receptors, and the NOD-like receptor protein 3 inflammasome, tentatively inhibiting lipolysis, inflammation, oxidative stress, cancer growth, angiogenesis, and atherosclerosis, and perhaps contributing to the increased longevity associated with exercise and caloric restriction. Clinically ketone bodies/ketogenic diets have for a long time been used to reduce the incidence of seizures in epilepsy and may have a role in the treatment of other neurological diseases such as dementia. 3-OHB also acts to preserve muscle protein during systemic inflammation and is an important component of the metabolic defense against insulin-induced hypoglycemia. Most recently, a number of studies have reported that 3-OHB dramatically increases myocardial blood flow and cardiac output in control subjects and patients with heart failure. At the moment, scientific interest in ketone bodies, in particular 3-OHB, is in a hectic transit and, hopefully, future, much needed, controlled clinical studies will reveal and determine to which extent the diverse biological manifestations of 3-OHB should be introduced medically.
3-hydroxybutyrate (3-OHB)/β-hydroxybutyric acid is a 4-carbon, 104.1-Dalton molecular weight ancestral keto-acid metabolite, which has been conserved throughout evolution as a life-preserving metabolic fuel substrate for all domains of life, delicately wavering and alternating with glucose as easily combustible energy sources during the feast-and-famine oscillations of biological life (1–5). Notably 3-OHB can replace glucose as the major central nervous system (CNS) fuel for the brain in man, thereby sparing vital carbohydrate and protein stores (Fig 1). 3-OHB has attracted the majority of scientific interest, because it is quantitatively dominating and because it, unlike acetoacetate and acetone, activates certain specific cellular signals (4, 5) (Fig 2).
Brain utilization of ketone bodies and interconversion between 3-OHB and other fuels metabolites. Brain substrate utilization in 3 obese volunteers after several weeks of starvation. Ketone bodies supply close to 60% of the fuels used and thereby spare glucose and indirectly protein. Data from GF Cahill (1). In the cell, there is a vivid exchange between 3-OHB, acetoacetate, and acetyl-CoA, as depicted in the right panel. 3-OHB, 3-hydroxybutyrate.
Intracellular signaling mediated by 3-hydroxybutyrate (3-OHB). 3-OHB binds to and activates specific hydroxyl-carboxylic acid receptors (HCAR) and inhibits histone deacetylase (HDAC) enzymes, free fatty acid receptors (FFAR), and the NLRP3 inflammasome. See main text for further details.
Among clinicians, 3-OHB has often been recognized as being a noxious, toxic, spillover agent causing diabetic ketoacidosis, but recent years have witnessed a steadily growing body of evidence that 3-OHB possesses potent, distinct, beneficial effects clinically at target organs and at the whole-body level.
The scope of this review is to capture and highlight these advances in particular as regards proven or potential clinical use and implications, based on a narrative review. A survey of the medical manifestations of 3-OHB is provided in Table 1.
Medical Manifestations of 3-OHB/Ketone Bodies
| Fasting, Exercise, and Longevity: Animal studies show that 3-OHB prolongs life and participate in the health promoting effects of fasting and exercise Human studies show that 3-OHB is at least equal to glucose as oxidative fuel Inflammation, Shock, and Cancer: Human studies show that 3-OHB acutely preserves body protein Animal studies suggest that 3-OHB counteracts hemorrhagic shock and cancer Central Nervous System Human studies show that a ketogenic diet effectively reduces seizures in epilepsy and that 3-OHB acutely increases cerebral blood flow and decreases glucose uptake and suggest together with animal studies that ketone bodies may be useful in many neurological disorders, including dementia, stroke, Parkinson disease, and multiple sclerosis Cardiovascular System Human studies show that 3-OHB acutely and profoundly increases cardiac blood flow and output in healthy and in heart failure subjects It is uncertain whether 3-OHB has clinical effects in acute heart failure or in the management of chronic heart failure Diabetes Mellitus and the Metabolic Syndrome Human studies show that increased ketogenesis plays important roles in the pathogenesis of diabetic ketoacidosis and in the defense against hypoglycemia Human studies suggest that ketone bodies may be beneficial in the management of type 2 diabetes and the metabolic syndrome in terms of cardiovascular risk, weight loss and preservation of lean body mass |
| Fasting, Exercise, and Longevity: Animal studies show that 3-OHB prolongs life and participate in the health promoting effects of fasting and exercise Human studies show that 3-OHB is at least equal to glucose as oxidative fuel Inflammation, Shock, and Cancer: Human studies show that 3-OHB acutely preserves body protein Animal studies suggest that 3-OHB counteracts hemorrhagic shock and cancer Central Nervous System Human studies show that a ketogenic diet effectively reduces seizures in epilepsy and that 3-OHB acutely increases cerebral blood flow and decreases glucose uptake and suggest together with animal studies that ketone bodies may be useful in many neurological disorders, including dementia, stroke, Parkinson disease, and multiple sclerosis Cardiovascular System Human studies show that 3-OHB acutely and profoundly increases cardiac blood flow and output in healthy and in heart failure subjects It is uncertain whether 3-OHB has clinical effects in acute heart failure or in the management of chronic heart failure Diabetes Mellitus and the Metabolic Syndrome Human studies show that increased ketogenesis plays important roles in the pathogenesis of diabetic ketoacidosis and in the defense against hypoglycemia Human studies suggest that ketone bodies may be beneficial in the management of type 2 diabetes and the metabolic syndrome in terms of cardiovascular risk, weight loss and preservation of lean body mass |
3-OHB, 3-hydroxybutyrate.
Medical Manifestations of 3-OHB/Ketone Bodies
| Fasting, Exercise, and Longevity: Animal studies show that 3-OHB prolongs life and participate in the health promoting effects of fasting and exercise Human studies show that 3-OHB is at least equal to glucose as oxidative fuel Inflammation, Shock, and Cancer: Human studies show that 3-OHB acutely preserves body protein Animal studies suggest that 3-OHB counteracts hemorrhagic shock and cancer Central Nervous System Human studies show that a ketogenic diet effectively reduces seizures in epilepsy and that 3-OHB acutely increases cerebral blood flow and decreases glucose uptake and suggest together with animal studies that ketone bodies may be useful in many neurological disorders, including dementia, stroke, Parkinson disease, and multiple sclerosis Cardiovascular System Human studies show that 3-OHB acutely and profoundly increases cardiac blood flow and output in healthy and in heart failure subjects It is uncertain whether 3-OHB has clinical effects in acute heart failure or in the management of chronic heart failure Diabetes Mellitus and the Metabolic Syndrome Human studies show that increased ketogenesis plays important roles in the pathogenesis of diabetic ketoacidosis and in the defense against hypoglycemia Human studies suggest that ketone bodies may be beneficial in the management of type 2 diabetes and the metabolic syndrome in terms of cardiovascular risk, weight loss and preservation of lean body mass |
| Fasting, Exercise, and Longevity: Animal studies show that 3-OHB prolongs life and participate in the health promoting effects of fasting and exercise Human studies show that 3-OHB is at least equal to glucose as oxidative fuel Inflammation, Shock, and Cancer: Human studies show that 3-OHB acutely preserves body protein Animal studies suggest that 3-OHB counteracts hemorrhagic shock and cancer Central Nervous System Human studies show that a ketogenic diet effectively reduces seizures in epilepsy and that 3-OHB acutely increases cerebral blood flow and decreases glucose uptake and suggest together with animal studies that ketone bodies may be useful in many neurological disorders, including dementia, stroke, Parkinson disease, and multiple sclerosis Cardiovascular System Human studies show that 3-OHB acutely and profoundly increases cardiac blood flow and output in healthy and in heart failure subjects It is uncertain whether 3-OHB has clinical effects in acute heart failure or in the management of chronic heart failure Diabetes Mellitus and the Metabolic Syndrome Human studies show that increased ketogenesis plays important roles in the pathogenesis of diabetic ketoacidosis and in the defense against hypoglycemia Human studies suggest that ketone bodies may be beneficial in the management of type 2 diabetes and the metabolic syndrome in terms of cardiovascular risk, weight loss and preservation of lean body mass |
3-OHB, 3-hydroxybutyrate.
Ketone Body Metabolism
3-OHB, acetoacetate, and acetone are commonly referred to as ketone bodies, although strictly chemically, 3-OHB and acetoacetate are hydroxyl-carboxylic or “keto-acids” (3) (Figs 1-3). Ketone bodies are predominantly produced in the liver and are used as oxidative fuels in virtually all other tissues, in particular those with a high level of activity such as the CNS, heart, and muscle (4, 5).
Regulation of lipolysis, proteolysis and ketogenesis. The processes are under hormonal control: insulin inhibits and stress hormones in general stimulate. Ketogenic amino acids provide a minor proportion (< 10%) of the ketone bodies produced.
Ketone body formation, ketogenesis, is primarily regulated by the supply of fatty acid precursors from the portal bed to the liver and by insulin and glucagon ratios in the portal bed (Fig 3); high levels of free fatty acids (FFA) and glucagon stimulate and insulin inhibits hepatic ketogenesis (4–7). In addition, ketogenic amino acids, in particular leucine, are precursors and supply a minor part (<10%) of the ketone body pool (8). In humans, ketone bodies display a characteristic circadian pattern with low concentrations under fed daytime conditions and rising levels in the early morning, probably secondary to fasting and perhaps nocturnal growth hormone surges (9, 10). Fasting and physical exercise invariably increase circulating ketone body concentrations into the millimolar range (11, 12), as does systemic inflammation (13).
The release of fatty acids from fat depots, lipolysis, determines FFA precursor supply to the liver; this process is under hormonal control; insulin inhibits and epinephrine, cortisol, and growth hormone stimulate lipase action in adipose tissue (14). Lipolysis is exquisitely sensitive to insulin (15), implying that minor surges of insulin effectively block FFA release and subsequent ketone body formation.
D- and L-3-OHB Isoforms and Metabolic Interconversions
In the body, 3-OHB exists in 2 isoforms (enantiomers), D- and L-3-OHB, and it has traditionally been assumed that the D-3-OHB isoform was the only form biologically active and present. This assumption has been questioned and a number of studies have reported that L-3-OHB is present in various amounts in various tissues and can be metabolized, probably at a slower rate, and also converted to D-3-OHB, obviously adding to the complexity of ketone-body metabolism (3, 16–18). Many analytical methods only measure D-3-OHB.
In the cell, there is a vivid exchange between 3-OHB, acetoacetate, and acetoacetyl/acetyl CoA (Fig 1), which may entangle the use of isotope dilution techniques to quantify ketone body fluxes because of unpredictable rapid dilution and “futile” loss of label (19, 20). Such events may lead to overestimation of ketone body fluxes, so called “pseudo-ketogenesis” (Fig 1).
Cellular Signalling
Although primarily a highly versatile and efficient metabolic fuel substrate, 3-OHB also acts as a specific modulator of intracellular signaling events (4, 5) (Fig 2). Deacetylation of histones in the nucleus is promoted by histone deacetylase (HDAC) enzymes and inhibition of this process may have a broad number of beneficial effects including cancer cell-cycle arrest, reduced angiogenesis, and modulation of immune responses and oxidative stress (4, 5, 21). 3-OHB specifically inhibits class 1 HDACs and globally reduces oxidative stress in mice (4, 5, 22), which may contribute to longevity, albeit any clinical implications of this are uncertain.
In adipocytes, cells of the immune system and some epithelial cells 3-OHB binds to hydroxyl-carboxylic acid receptor 2 (HCAR2), thereby effectively imposing a negative feedback mechanism inhibiting lipolysis and possibly also exerting antiatherogenic and anti-inflammatory effects broadly (23–26). Nicotinic acid and nicotinic acid derivatives such as Acipimox bind to the same receptor.
In addition, 3-OHB binds to and blocks the G-protein-coupled FFA receptor 3, thereby reducing lipolysis (6) and antagonizes the NOD-like receptor protein 3 (NLRP3) inflammasome leading to inhibition of inflammation (5). Collectively, these signaling events imply that 3-OHB has a remarkably widespread potential to retain and confine inflammation, lipolysis, atherosclerosis, angiogenesis, oxidative stress, and carcinogenesis.
Systemic Manifestations: Fasting, Exercise, and Longevity
Fasting and exercise are fundamental natural occurrences in human biological life, are both characterized distinctly by increased concentrations and utilization of ketone bodies and by striking similar capacities to increase longevity across a wide range of species, and may both be seen as natural evolutionary reservoirs for ketone body action (4, 5, 27). Prolonged aerobic exercise resembles a state of compressed fasting metabolically in terms of increased release of stress hormones and increased reliance on fat and ketone body fuels and evoke many similar effects, unless overshadowed by excessive compensatory calorie and carbohydrate intake. The concept that, ketone bodies, in particular 3-OHB, may be crucial for the life extending effects of caloric restriction and exercise, is supported by recent findings of life span extending effects of 3-OHB in Caenorhabditis elegans worms and of ketogenic diets in mice (28–30), although any underlying mechanisms remain elusive.
Although there is evidence that nutritional support with ketone bodies may conserve or modestly improve exercise performance (31), many studies have failed to show such effects, and the field remains controversial (32). Adding to the complexity a fresh report found a 15% increase in power output in the final 30 minutes of a 2-hour standardized endurance session after 3 weeks of 3-OHB supplementation (2 × 25 g/d) in fit male subjects (33). Overall, the available literature shows that 3-OHB performs at least as effectively as glucose, as regards the capacity for oxidative phosphorylation during exercise, which is important and reassuring when considering clinical investigations of acute illnesses with high-energy expenditures and oxygen demands.
Interestingly, it has recently also been reported that 3-OHB infusion to humans acutely increases circulating concentrations of erythropoietin (34), which is in line with observations that treatment with sodium-glucose transporter 2 inhibitors increases both ketone body and hematocrit levels (35, 36), suggesting that 3-OHB may improve aerobic performance by stimulating red blood cell formation.
Systemic Manifestations: Inflammation, Shock, and Cancer
Systemic inflammatory disease is characterized by a succinct metabolic response with release of all major stress hormones and punctuated surges of ketone bodies and lactate (13). These phenomena are catabolic and lead to increased loss of lean body mass and muscle protein and fat mass (37) associated with increased morbidity and mortality (38, 39). There is some evidence that 3-OHB may act to preserve protein (40), but other studies have failed to detect any effects on protein metabolism, plausibly partly because of 3-OHB-induced inhibition of lipolysis and loss of any protein conserving effects of high FFA levels. One recent clinical study reported anticatabolic actions of 3-OHB in muscle and at the whole-body level during systemic experimental endotoxin inflammation in humans (41).
In addition, data show that 3-OHB may interfere with the inflammatory process per se, tentatively by HCA receptor activation (23, 24) and by blocking of the NLRP3 inflammasome (42, 43). It is thus likely that 3-OHB contributes to the blunted inflammatory responses observed after fasting and caloric restriction (44, 45).
Studies in a variety of animal models have reported that 3-OHB improved survival after hemorrhagic shock (46, 47); it remains uncertain whether these findings can be extrapolated to humans and which mechanisms are activated.
Finally, there is a substantial ongoing interest for the use of ketone bodies within oncology, and preclinical data have provided evidence for the existence of synergy between ketone bodies/3-OHB and conventional cancer therapies and possibly also a preventive role as regards cancer occurrence (48). Such effects have been proposed to be mediated by HDAC inhibition and HCAR activation, having antineoplastic effects, and possibly also by redirection of intracellular fuel metabolism away from glucose, which is the preferred oxidative substrate for most cancer cells. Nonetheless, a recent systematic review concluded that there is a clear need for adequately powered controlled clinical trials in the field (49).
Central Nervous System
Ketone bodies, in particular 3-OHB, may replace glucose in the brain and constitute > 60% of the energy demand during prolonged fasting (1) (Fig 1). CNS use of ketone bodies is linearly dependent on availability and, vice versa, glucose uptake is decreased in proportion to circulating ketone body concentrations (50–52). 3-OHB binds to specific receptors and lowers reactive oxygen species-dependent stress in the CNS (53) and HDAC-induced oxidative stress (22), promotes the expression of brain-derived neurotrophic factor (54), and increases cerebral blood flow by up to 30% (52).
Ketone bodies are by now an established therapeutic agent in the treatment of refractory epilepsy, in particular in the younger (55–57), and it is tantalizing that both ancient Greek Hippocratic collections and the Bible allude to fasting as being effective against seizure disorders (56). More recently, and, assumably, also more convincingly, a large, controlled clinical trial clearly showed that a ketogenic diet reduced baseline seizures in 75% of the 54 children treated (58).
Lately, there has been an immense interest in the neurological potential of ketone bodies and a cascade of studies have reported benefit in amyotrophic lateral sclerosis, Huntington disease, Parkinson disease, and multiple sclerosis, albeit mainly at the preclinical level (56).
Interestingly, high levels of 3-OHB have been shown to improve cognitive function in elderly and memory-impaired subjects (59, 60), and epidemiological studies have reported that a “keto-genic” lifestyle protects against Alzheimer’s disease (56). A new study using intravenous administration of 3-OHB reported acutely improved working memory performance in patients with type 2 diabetes and increased risk of cognitive impairment (61). It may be added that animal studies in ischemic stroke models show that ketone bodies reduce infarct volume (by up to 40%) and oxidative stress and improve mitochondrial and neurological function (62, 63). Finally, a 2020 article reported that dietarily induced hyperketonemia improved magnetic resonance imaging-determined brain network stability, which correlated with brain activity and cognitive acuity, thus suggesting potential CNS protection against dementia and aging (64). Clearly, all these exciting experimental suggestions await the outcome of controlled clinical trials.
Cardiovascular System
The myocardium displays the highest ketone body consumption per mass unit and oxidize ketone bodies in proportion to prevailing concentrations at the cost of glucose and fatty acids (65–67). There is by now ample evidence that ketone bodies, in particular 3-OHB, profoundly affect cardiovascular function. Much of the interest in ketone bodies and the heart has been sparked by the recent observation of close to 40% reduction in cardiovascular mortality coinciding with elevated levels of circulating ketone bodies after sodium-glucose transporter 2 inhibition in type 2 diabetes patients (35, 68, 69). Previously, animal studies have reported that 3-OHB prevented myocardial damage after experimental occlusion (70). A more recent report in healthy subjects employing positron emission tomography/computed tomography reported a 50% reduction of myocardial glucose uptake and an impressive 75% increase in myocardial blood flow after intravenous 3-OHB administration (71). In addition, studies in patients with chronic heart failure have established an equally impressive 40% increase in cardiac output (Fig 4) together with substantial increases in stroke volume and ejection fractions in presence of a clear dose-response relation acutely after intravenous 3-OHB (72). By implication, these results also suggest induction of vasodilation. Though these findings are striking in terms of mere magnitude and effect size, it is not known whether 3-OHB has any therapeutic role in acute heart failure or a more preventive one in chronic heart failure and subjects with predisposition.
Cardiac output in patients with heart failure. Cardiac output ± SD in 16 patients with heart failure during exposure to 3-OHB (3.3 mmol/L) or placebo. Cardiac stroke volumes and ejection fractions also increased. Data from R Nielsen et al (72). 3-OHB, 3-hydroxybutyrate.
Diabetes Mellitus and the Metabolic Syndrome
Diabetic ketoacidosis (DKA) remains a common complication to type I diabetes, and 1 of the most serious acute medical conditions in the field of endocrinology; DKA and diabetic coma-related events constitute the single largest proportion of the excess mortality (~ 25%) in type 1 diabetic subjects younger than age 50 (73). DKA is caused by a combination of low levels of insulin and high levels of stress hormones leading to uncontrolled rates of lipolysis and ketogenesis (74–77).
Conversely, ketone bodies also play an important role in the metabolic defense against hypoglycemia and it has become increasingly clear that the formation and liberation of free fatty acids and ketone bodies constitute distinct counterregulatory defense components, providing alternative oxidative fuels particularly for the CNS (78–80).
It is in addition possible that ketone bodies may exert beneficial effects in the treatment of type 2 diabetes and the metabolic syndrome. Clinical studies have reported that application of low-carbohydrate ketogenic diets promote weight loss, decrease insulin levels, and improve cardiovascular risk profile (81–83) and that these effects may be linked to increased satiety and preservation of lean body mass in patients with multiple sclerosis (84); not all studies in the field have been optimally controlled and there is a lack of inclusion of hard endpoints and state-of-the-art methods for assessment of, for example, insulin sensitivity. Studies in animal models suggest that ketogenesis also may prevent fatty liver injury (85) and that 3-OHB has an anti-inflammatory, hepato-protective role mediated by the HCA2 receptor (86). Clearly, large-scale, long-term clinical studies combining targeted ketogenic, caloric restriction/quasi-fasting, and exercise programs in the type 2 diabetes/metabolic syndrome/obesity fields, are of major medical interest.
Notably, ketone bodies also are an integrated part incorporated in the original Randle/glucose–fatty acid cycle hypothesis, according to which fatty acids and ketone bodies inhibit intracellular glucose metabolism and oxidation and introduce insulin resistance and prediabetic state (66, 87, 88). Subsequent studies have in many cases failed to confirm any prominent role for ketone bodies in this context, plausibly because 3-OHB inhibits lipolysis by HCA2 receptor activation, thereby removing the fatty acid contribution to insulin resistance (23, 25).
Perspectives, Challenges, and Uncertainties
As outlined previously, it is clear that 3-OHB has profound clinical effects in the fields of neurology and cardiovascular disease, often with remarkable effect sizes and that ketone bodies play a distinct role in the development of diabetic ketoacidosis and in the defense against hypoglycemia. Given this and the many intriguing and promising results of preclinical experiments there is a compelling case for lege artis controlled clinical trials of 3-OHB, perhaps as an adjuvant to conventional therapy, in a broad variety of disease states including inflammation, cancer, sarcopenia, and neurological and cardiovascular illnesses. Being a small, natural, and abundant metabolite, like glucose, 3-OHB circumnavigates the need for major toxicological safety studies, but coincidentally also inflicts a lack of any major economic incentive and enthusiasm from the pharmaceutical industry.
At present 3-OHB is available as a sodium salt ((D)- and (L)-Na-3-OHB) and as a monoester ((D)-3-hydroxybutyl (D)-3-hydroxybutyrate. Na-3-OHB has been given intravenously, the ester orally. There is an unmet need for development and manufacturing of new formulations with increased palatability and without infliction of any added sodium load for acute intravenous use. This need includes both more suitable preparations for parental and oral administration, perhaps as nutritional supplementation in dairy products.
Finally, new investigations could aim at establishing methodologies to quantify ketone body flux rates (e.g., positron emission techniques, 3-OHB dilution techniques with stable or radioactive labelling) and at differentiation between concentrations and impacts of (D)- and (L)-3-OHB enantiomers.
Abbreviations
- 3-OHB
3-hydroxybutyrate
- CNS
central nervous system
- DKA
diabetic ketoacidosis
- FFA
free fatty acid
- HCAR2
hydroxyl-carboxylic acid receptor 2
- HDAC
histone deacetylase
- NLRP3
NOD-like receptor protein 3
Additional Information
Disclosure Statement: The author has nothing to disclose.
Data Availability: Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.
References
