The human brain, despite comprising only a small fraction of total body mass, consumes approximately 20% of the body’s total energy expenditure [1]. This substantial energy demand stems primarily from billions of neurons continuously engaged in information processing, transmission, and storage [2]. Disruptions in cerebral energy metabolism are linked to various neurological disorders. In Alzheimer’s disease, reduced glucose metabolism correlates with cognitive deficits and plaque density [3,4]. In hepatic encephalopathy, both mitochondrial function and glucose utilization are impaired [5]. These metabolic alterations significantly contribute to disease progression. Other neurological conditions like amyotrophic lateral sclerosis and epilepsy also exhibit distinct patterns of metabolic dysfunction [6,7].

Under normal physiological conditions, glucose serves as the brain’s primary energy substrate, meeting most of its metabolic needs [8]. However, during glucose scarcity, the brain demonstrates remarkable metabolic flexibility. It can utilize alternative energy sources: lactate from peripheral glycolysis and ketone bodies produced in the liver, which cross the blood-brain barrier during prolonged fasting or ketogenic diets [9,10]. The proportion of energy derived from these substrates varies with context. Under typical conditions, glucose oxidation predominates, whereas during extended fasting, ketone bodies can provide up to two-thirds of the brain’s energy, with glucose supplying the rest [11]. This adaptability enables the brain to maintain energy homeostasis across different metabolic states.

Brain energy metabolism, however, extends beyond basic nutrient utilization, involving complex cellular interactions and molecular pathways. A widely recognized framework for understanding this process is the astrocyte-neuron lactate shuttle (ANLS) hypothesis, which underscores the metabolic cooperation between astrocytes and neurons as a cornerstone of brain energy metabolism [9,12,13]. According to this hypothesis, glucose is primarily taken up by astrocytes, metabolized into lactate, and subsequently shuttled to neurons. Neurons convert this lactate into pyruvate, which enters the tricarboxylic acid (TCA) cycle to produce ATP, a process suggested to be more efficient than direct glucose utilization (Fig. 1A). In addition to its role as an energy substrate, lactate exerts critical signaling functions in the brain. Recent studies have revealed its multifaceted functions in brain physiology, including the regulation of synaptic plasticity, memory formation, and neuroprotection [14]. The critical importance of lactate homeostasis is further emphasized by evidence linking perturbations in lactate metabolism to various neurological disorders, including Alzheimer’s disease [15], traumatic brain injury [16], and epilepsy [17].

Figure 1. Evolution of astrocyte-neuron lactate shuttle (ANLS) hypothesis. (A) Traditional model: The classic ANLS hypothesis proposes that neurons depend primarily on lactate produced by astrocytes for energy metabolism. This model assumes strict metabolic compartmentalization, with neurons exclusively expressing the LDH1 (LDHB) isozyme. (B) Updated model: The revised ANLS hypothesis demonstrates that neurons possess greater metabolic flexibility through the expression of both LDHA and LDHB. While neurons maintain their ability to utilize lactate, they do so less efficiently through LDHA compared to LDHB. LDH, lactate dehydrogenase; MCT, monocarboxylate transporter; GLUT, glucose transporter.

This review provides a comprehensive examination of the ANLS hypothesis, from its classical perspectives on metabolic compartmentalization to recent advances, offering an updated framework for understanding brain lactate metabolism and its therapeutic implications in neurological disorders. We particularly focus on the complexities and unresolved aspects of lactate metabolism, including the compensatory roles of LDHA and LDHB, to illuminate the current state of research in this field.

Brain energy metabolism operates primarily through aerobic processes, with mitochondrial oxidative phosphorylation serving as the principal mechanism for ATP production from glucose and other substrates [18-20]. These sophisticated organelles orchestrate numerous crucial cellular processes beyond ATP generation, playing vital roles in neuronal survival and function [21,22].

The intensive metabolic demands of neural function stem from the constant need to maintain ionic gradients across neuronal membranes, support neurotransmitter recycling, and facilitate synaptic transmission. This high energy demand is met almost exclusively through oxidative phosphorylation in mitochondria, making neurons particularly dependent on mitochondrial function. Mitochondria generate ATP through oxidative phosphorylation, a process that involves the electron transport chain and ATP synthase. This method is highly efficient, producing significantly more ATP per glucose molecule compared to glycolysis. It is estimated that approximately 90% of the ATP utilized by the brain is produced by mitochondrial oxidative phosphorylation [2,23]. Unlike other tissues that can temporarily rely on glycolysis during periods of metabolic stress, neurons require continuous and efficient ATP production through mitochondrial respiration to sustain their complex functions. Recent advances in neuroenergetics have revealed that mitochondrial dysfunction lies at the heart of numerous neurological disorders, ranging from neurodegenerative diseases such as Alzheimer’s and Parkinson’s to acute conditions like stroke and traumatic brain injury [24-26]. The vulnerability of neurons to mitochondrial impairment is particularly evident in the aging brain, where declining mitochondrial function strongly correlates with cognitive decline and an increased susceptibility to neurological diseases [27,28]. Furthermore, mitochondria in the brain serve functions beyond energy production, including calcium homeostasis, redox signaling, and regulation of apoptotic pathways [29-31]. These organelles form dynamic networks that respond to cellular demands by undergoing fusion and fission, processes crucial for maintaining mitochondrial DNA integrity and adapting to metabolic challenges [32,33]. These dynamic processes govern mitochondrial distribution along neuronal axons. This organization ensures local ATP production at synapses, where energy demands reach their peak [34,35].

The ANLS hypothesis, first formalized by Pellerin and Magistretti in 1994 [36], explains how neurons meet their considerable energetic needs through metabolic cooperation with astrocytes. This model describes a sequential process. First, neuronal activity triggers glutamate release into the synaptic cleft. Then, astrocytes take up this glutamate along with sodium ions. Finally, this uptake activates Na+/K+-ATPase pumps in astrocytes to restore ionic balance. The increased activity of these pumps elevates glycolytic activity in astrocytes, resulting in enhanced lactate production. The lactate is then transported to neurons, where it undergoes oxidation to fulfill their energy requirements. It is rapidly converted into pyruvate, which is then metabolized in the mitochondria, providing a fast and efficient energy source. Unlike glucose, which requires 10 enzymatic steps to produce pyruvate, lactate requires only a single step, making the process highly efficient. This streamlined mechanism is especially vital during periods of heightened neuronal activity, allowing neurons to promptly meet their increased energy demands [37].

Astrocytes’ unique anatomical features make them ideally suited for this metabolic coupling. Their extensive foot processes serve dual roles: supporting the blood-brain barrier by ensheathing endothelial cells, thereby facilitating selective glucose uptake, and connecting to synaptic areas to ensure efficient lactate delivery to neurons [38]. This structural organization, combined with their significant glycolytic capacity even under aerobic conditions, enables astrocytes to function as active regulators of neuronal energy supply rather than mere support cells. Neurons preferentially utilize lactate as an energy substrate, particularly during periods of increased activity or glucose scarcity, establishing a symbiotic metabolic relationship with astrocytes. Over the past three decades, substantial evidence has validated the ANLS hypothesis. Two-photon microscopy using genetically encoded nanosensors has visualized lactate gradients, confirming directional flow from astrocytes to neurons via monocarboxylate transporters (MCTs) [39]. Studies on glucose metabolism reveal preferential astrocytic glucose uptake during neuronal activation, indicated by elevated astrocytic NADH:NAD+ ratios [40]. The coupling of glutamate signaling with glucose metabolism provides further evidence for the ANLS hypothesis. When astrocyte-specific glutamate transporters are downregulated, glucose utilization decreases. Conversely, glutamate uptake both stimulates astrocytic glucose consumption and inhibits neuronal glucose uptake [41,42]. Electrophysiological studies demonstrate that astrocyte-derived lactate is essential for sustaining neuronal activity [43,44], and recent investigations have linked this lactate transport to cognitive processes such as memory formation [9].

Dysfunction in the ANLS has been increasingly implicated in various human neurological and psychiatric disorders, highlighting its fundamental role in brain function. In Alzheimer’s disease, impaired astrocytic glucose metabolism and reduced expression of glucose transporters coincide with decreased lactate availability for neurons, contributing to cognitive decline [45]. Similar metabolic deficits have been observed in Huntington’s disease, where mutant huntingtin protein disrupts astrocytic glycolysis and lactate production [46]. In amyotrophic lateral sclerosis, dysfunction of astrocytic lactate transporters (MCTs) precedes motor neuron degeneration, suggesting a causative role in disease progression [47]. Epilepsy has also been linked to ANLS dysfunction, with altered astrocytic glucose metabolism and lactate transport contributing to seizure susceptibility [48]. Beyond neurological disorders, disrupted lactate shuttling plays a role in psychiatric conditions; reduced astrocytic lactate production has been observed in major depressive disorder [49], while schizophrenia shows alterations in astrocytic glucose metabolism and lactate transport [49,50]. Understanding these pathological alterations in ANLS function not only provides insights into disease mechanisms but also suggests potential therapeutic strategies targeting astrocyte-neuron metabolic coupling.

Lactate dehydrogenase (LDH) is a unique enzyme mediating the interconversion of pyruvate and lactate in mammals. LDH catalyzes both the reduction of pyruvate to lactate (coupled with NADH oxidation to NAD+) and the oxidation of lactate to pyruvate (coupled with NAD+ reduction to NADH). The ANLS hypothesis depends on this enzymatic activity and effective metabolic compartmentalization, requiring astrocytes to reduce pyruvate to lactate and neurons to oxidize lactate to pyruvate. This directional flux is achieved through the differential expression of LDH isozymes (LDH1–5), which are tetrameric combinations of LDHA (A) and LDHB (B) subunits, each with unique tissue distributions and kinetic properties [51,52]. LDH1 is composed entirely of LDHB subunits (B4) and is predominantly found in tissues that favor oxidative metabolism, such as the heart and brain [53]. This isozyme has a low Michaelis constant (Km) for pyruvate and lactate. This characteristic makes it highly efficient at catalyzing lactate oxidation [54]. Furthermore, LDH1 is inhibited by physiological concentrations of pyruvate, reinforcing its role in lactate utilization [55]. In contrast, LDH5 (A4), composed solely of LDHA subunits and found predominantly in glycolytic tissues like liver and skeletal muscle [56], is optimized for reducing pyruvate to lactate. LDHA-dominated isozymes exhibit a higher Km for pyruvate and lactate and are less inhibited by pyruvate, making them ideal for lactate production under aerobic conditions, akin to the Warburg effect [57,58].

In the brain, these isozyme-specific activities align with the metabolic roles of astrocytes and neurons. Immunohistochemical and biochemical studies reveal that astrocytes primarily express LDH5 (A4) and LDH4 (A3B1), reflecting their glycolytic phenotype and their role as lactate producers [59,60]. This high glycolytic activity supports the export of lactate to neurons. Conversely, neurons predominantly express LDH1 (B4) and LDH2 (B3A1), which are well-suited for oxidizing lactate to pyruvate due to their low Km for lactate and inhibition by pyruvate [54,61]. Single-cell RNA sequencing studies further confirm this compartmentalized expression, showing elevated LDHA expression in astrocytes and predominant LDHB expression in neurons [62,63]. This differential distribution of LDH isozymes provides a molecular foundation for the lactate transfer between astrocytes and neurons, as proposed in the ANLS hypothesis, ensuring efficient metabolic support for neuronal activity. The critical importance of this distribution is highlighted by studies of LDHB deficiency, which leads to mitochondrial dysfunction, increased oxidative stress, and neurodegeneration. LDHB-deficient mice exhibit increased neuroinflammation and demonstrate cognitive deficits, particularly in long-term memory retention [64-66].

The directional flow of lactate in the ANLS hypothesis is facilitated by the specific cellular distribution of MCTs. MCT1, predominantly expressed in astrocytes and endothelial cells, mediates lactate efflux from astrocytes and its transport across the blood-brain barrier. MCT4, also expressed in astrocytes, functions primarily as a lactate exporter due to its high Km value, making it particularly suited for lactate efflux during periods of high glycolytic activity. In contrast, MCT2, which exhibits a lower Km for lactate and is exclusively expressed in neurons, facilitates efficient lactate uptake even at low extracellular concentrations (Fig. 1A) [67]. Astrocytes express lactate exporters (MCT1/4), while neurons express the high-affinity importer (MCT2). This complementary distribution pattern creates a metabolic gradient. The gradient supports directional lactate flow from astrocytes to neurons [68]. The distinct kinetic properties of these transporters, combined with their cell-specific expression patterns, establish an efficient system for lactate delivery that matches neuronal energy demands [39].

Despite extensive evidence supporting the ANLS hypothesis, significant debate persists regarding its universal applicability and mechanistic details. A primary criticism is the suggestion that neurons may not be entirely dependent on astrocyte-derived lactate for their energy needs. Multiple studies have highlighted that neurons are capable of directly utilizing glucose as an energy substrate, particularly during periods of heightened energy demand. For example, experimental data indicate that neurons possess the enzymatic machinery required for glycolysis, allowing them to metabolize glucose efficiently to generate ATP [69]. This capacity challenges the premise that neurons rely predominantly on lactate supplied by astrocytes under physiological conditions. Recent research has further called the ANLS hypothesis into question. Synaptic activation has been shown to directly promote glucose uptake in presynaptic terminals [70], and studies have demonstrated that lactate alone is insufficient to sustain the energy demands of neurons during high-frequency gamma oscillations, which are critical for cognitive processes. Instead, glucose plays an essential role in maintaining these rhythms [71,72]. Additionally, Rothman’s “Glucose Sparing by Glycogenolysis” model proposes that astrocytes actively conserve glucose for neuronal use during states of high activity [73].

Further complicating the ANLS framework, cell-type-specific analyses have detected significant LDHA expression in neurons, undermining the previously assumed strict metabolic division between astrocytes and neurons [74,75]. This enables neurons to either metabolize astrocyte-derived lactate or produce lactate glycolytically independently, depending on LDHA levels or activity. In older mice, LDHA induction caused cognitive deficits while its knockout led to cognitive improvements [76]. Furthermore, studies have demonstrated that LDHA and LDHB can compensate for each other’s absence to some extent. For instance, in the human colon adenocarcinoma cell line LS174T and the murine melanoma cell line B16-F10, only the combined genetic disruption of both LDHA and LDHB fully suppressed lactate secretion, indicating that the presence of either isoform can partially maintain lactate production [77]. These findings suggest that the metabolic compartmentalization proposed by the ANLS hypothesis may be more flexible in neurons than previously thought.

Our recent research on LDHB-deficient neurons provides further insights into neuronal metabolic flexibility. We found substantial LDHA expression in these neurons, leading to LDH5 isozyme formation, confirmed by both immunohistochemical and biochemical analyses [66]. LDHB-deficient neurons showed reduced efficiency in lactate-driven energy metabolism when cultured, evidenced by lactate accumulation in the brain. However, under physiological glucose conditions, these neurons maintained stable energy metabolism, likely due to LDHA’s ability to catalyze both the pyruvate-to-lactate conversion and, albeit less efficiently, the reverse reaction. Behaviorally, LDHB deficiency resulted in impaired long-term memory retention while preserving short-term memory, with only mild neuropathological changes. These findings suggest that while LDHA can partially compensate for LDHB deficiency in basic energy metabolism, it may not fully support the high-efficiency lactate utilization required for specific cognitive functions, particularly in stressed aged neurons. Rather than contradicting the ANLS hypothesis, these results suggest a more nuanced model where neurons possess metabolic flexibility through LDHA expression while still benefiting from efficient lactate utilization through LDHB under physiological conditions (Fig. 1B).

Research on therapeutic interventions for LDHB deficiency reveals both opportunities and challenges. While LDHA provides partial metabolic compensation, this is insufficient for high-demand cognitive functions, especially in aged neurons. Both direct lactate supplementation and glucose-based interventions show limitations: lactate supplementation is hampered by reduced utilization efficiency and potential accumulation, while glucose interventions may not address specific cognitive deficits [64].

Lactate, traditionally regarded as a byproduct of glycolysis, has emerged as a key metabolic and signaling molecule in the brain, playing critical roles in neuronal plasticity, excitability, and integrated physiological responses [78-80]. The ANLS hypothesis highlights lactate’s fundamental role in energy transfer, proposing that astrocytes metabolically support neurons by converting glucose into lactate, which is then transferred to neurons for oxidative metabolism. Lactate modulates neuronal excitability through dual mechanisms: as an energy substrate and as a signaling molecule. Studies in rodent models have demonstrated lactate’s crucial role in synaptic plasticity and memory formation. Inhibition of astrocytic lactate production disrupts memory processes and long-term potentiation, effects that can be reversed by lactate supplementation [78]. At the cellular level, lactate influences synaptic transmission through its interactions with NMDA receptors and redox-sensitive mechanisms, particularly at excitatory synapses [81]. Beyond synaptic functions, lactate plays a broader role in regulating physiological processes, including glucose sensing, osmoregulation, and respiratory control, by acting on key neuronal circuits [82-84]. Recent research also highlights lactate’s therapeutic potential in neurological and psychiatric conditions. In models of ischemia and traumatic brain injury, lactate demonstrates neuroprotective effects through its impact on energy metabolism, signaling pathways, and gene expression [85]. Similarly, lactate shows promise in treating depression, although its complex interactions with the nervous system, such as triggering panic attacks in certain anxiety disorders, underscore the need for cautious therapeutic application [49,86].

Our recent research on LDHB-deficient brains provides additional insights into lactate’s neuroprotective potential in ischemic injury. Following ischemic insults, we observed significantly enhanced neuronal survival correlating with increased brain lactate levels. This neuroprotection was associated with cerebral vasodilation and elevated extracellular prostaglandin E2 (PGE2) levels, which is achieved by lactate blocking the prostaglandin transporter (PGT), thereby preventing PGE2 uptake and degradation in astrocytes (Fig. 2) [87]. Our findings suggest that elevated lactate inhibits PGE2 reabsorption, thereby promoting vasodilation and neuronal protection, highlighting promising therapeutic implications for ischemic stroke intervention.

Figure 2. Mechanism of lactate-induced cerebral vasodilation. Lactate in the extracellular space enhances vasodilation through its interaction with prostaglandin E2 (PGE2) signaling. This occurs when lactate inhibits the prostaglandin transporter (PGT), which prevents astrocytes from taking up and degrading PGE2, thereby prolonging PGE2-mediated vasodilatory effects. AA, arachidonic acid.

Brain energy metabolism exhibits remarkable complexity and adaptability beyond the classical ANLS hypothesis. While this model provided a fundamental framework for understanding brain bioenergetics, recent evidence reveals sophisticated metabolic flexibility in neurons, challenging traditional views of strict compartmentalization. Here we show that neurons maintain substantial LDHA expression and direct glucose utilization capabilities while retaining efficient lactate metabolism through LDHB. This metabolic duality, rather than undermining the ANLS hypothesis, suggests an evolved strategy ensuring robust energy supply under varying physiological demands.

Lactate emerges as more than an energy carrier, functioning as a crucial signaling molecule that modulates synaptic plasticity and provides neuroprotection. Our work with LDHB-deficient brains demonstrates that elevated lactate levels enhance neuronal survival during ischemic injury through PGE₂-mediated vasodilation, revealing a previously unrecognized mechanism of metabolic neuroprotection. These findings have immediate implications for therapeutic strategies in stroke and other neurological disorders where metabolic dysfunction plays a central role.

The dual role of lactate in energy metabolism and signaling presents promising therapeutic opportunities for various neurological and psychiatric disorders. However, critical knowledge gaps must be addressed before developing effective treatments. The precise mechanisms by which LDHA and LDHB manipulation affect different neurological conditions remain unclear, particularly their context-dependent impacts on cognitive function and neuroprotection. Key research priorities include: developing cell-type specific metabolic modulators; identifying reliable biomarkers for treatment monitoring; and understanding the interaction between lactate metabolism and other metabolic pathways. Special attention should be directed toward age-dependent changes in metabolic flexibility and the mechanisms governing glucose-lactate utilization switching. These insights will advance both our fundamental understanding of brain energy metabolism and our therapeutic capabilities in treating disorders with underlying metabolic dysfunction.

This work was partially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (RS-2024-00416536, 2019R1A2C1087623, and 2021K1A3A1A210402481).

We thank Dr. Arnaud Mourier for suggestions and comments on the manuscript.

The authors declare no conflicts of interest.