Brain Fuel Utilization in the Developing Brain
Key InsightIn humans, the brain is the single organ with the most protracted development and maturation time and the highest energetic needs. Glucose is the primary metabolic substrate used by the brain. During early brain development and maturation, however, the energetic demands exceed the availability of blood glucose. This energetic challenge is solved in part through the mobilization of ketone bodies (KBs) as fuel.
Glucose and KBs are not only the main sources of energy, but are also used for the biosynthesis of macromolecules essential for neuronal cell proliferation, synapse formation, and myelination. Thus, besides meeting energy demands, glucose
and other fuel substrates, including KBs, may play a broader role in brain development.
Current knowledgeBrain development is characterized by a period of rapid growth beginning in utero up to 3 years of age, followed by a slower pace of reorganization and development that continue up to the third decade of life. During the latter period, the growth rate slows, and the maturing brain undergoes significant reorganization, dominated by synaptic pruning and myelination. While the adult human brain requires up to 20–25% of total energy provided by basal metabolism, the energy demands of the newborn human brain are around 50% of the body’s daily energy consumption. Not surprisingly, the brain energy requirement mirrors its development and maturation, with peak energy demands reached during the first 3 postnatal years when the rates of synapse formation and myelination are at their highest.
Practical implicationsUnder normal conditions, glucose is used for generation of energy from glycolysis or oxidative phosphorylation. In the newborn, glycolysis seems to be particularly important to support the generation of macromolecules involved in brain structural changes. Moreover, ketones (i.e., β-hydroxybutyrate and acetoacetate) are derived from oxidation of body fat by the liver. In addition to serving as an extra fuel substrate, ketones may also play a role in the synthesis of macromolecules, such as cholesterol
and fatty acids (which represent around 50% of brain gray matter). Ketones can also act as signaling and epigenetic mediators regulate brain plasticity and reorganization. The nutrients required to support brain development are found in complex food matrices, such as breast milk. Understanding how these nutrients interact to affect brain metabolism is a key to defining the nutritional requirements for supporting optimal brain development.
Goyal MS, Iannotti LL, Raichle ME. Brain Nutrition: A Life Span Approach. Annu Rev Nutr. 2018 Aug;38(1):381–99.
• The brain consumes up to 60% of the total energy available to
the body during development.
• While glucose is the main source of energy for the brain in
adults, ketone bodies are essential to complement glucose to fulfil the metabolic and energy needs of the brain during its development.
• During brain development, glucose and ketone bodies are not
only the main sources of energy but are also utilized for the biosynthesis of macro-molecules indispensable for neuronal cell proliferation, synapse formation, and myelination.
Brain development · Brain metabolism · Neurogenesis · Synapse · Myelin · Glucose · Ketone bodies · Aerobic glycolysis · Oxidative phosphorylation · Metabolism · Energetic and anabolic needs
AbstractDuring pregnancy and infancy, the hum an brain is growing extremely fast; the brain volume increases significantly, reaching 36, 72, and 83% of the volume of adults at 2– 4 weeks, 1 year, and 2 years of age, respectively, which is essential to establish the neuronal networks and capacity for the development of cognitive, motor, social, and emotional skills that will be continually refined throughout childhood and adulthood. Such dramatic changes in brain structure and function are associated with very large energetic demands exceeding by far those of other organs of the body. It has
been estimated that during childhood the brain may account for up to 60% of the body basal energetic requirements. While the main source of energy for the adult brain is glucose, it appears that it is not sufficient to sustain the dramatic metabolic demands of the brain during its development. Recently, it has been proposed that this energetic challenge is solved by the ability of the brain to use ketone bodies (KBs), produced from fatty acid oxidation, as a complement source of energy. Here, we first describe the main cellular and physiological processes that drive brain development along time and how different brain metabolic pathways are engaged to support them . It has been assumed that the majority of energetic substrates are used to support neuronal activity and signal transmission. We discuss how glucose and KBs are metabolized to provide the carbon backbones used to synthesize lipids, nucleic acid, and cholesterol, which are indispensable building blocks of neuronal cell proliferation and are also used to establish and refine brain connectivity through synapse formation/elimination and myelination. We conclude that glucose and KBs are not only important to support the energy needs of the brain under development, but they are also essential substrates for the biosynthesis of macro-molecules underlying structural brain growth and reorganization. We emphasize that glucose and fatty acids supporting the production of KBs are provided in complex food matrices, such as breast milk, and understanding how their availability impacts the brain will be key to promote adequate nutrition to support brain metabolism and, therefore, optimal brain development.
IntroductionThe ability of hum ans to perform complex mental activities, including thinking, reasoning, remembering, problem - solving, decision- making, and learning new information, depends on the ability of the brain to adapt to its environment and alter
its functional and structural organization [ 1– 4] . This is often referred to as brain, neuron, or synapse plasticity [ 5] . More- over, the brain organization is incredibly complex: it is estimated that the human brain contains more than 200 billion neurons and non- neuron cells, 1 quadrillion of connections, 100 km of nerve fibers, and 600 km of blood vessels [ 6, 7] . In order to maintain such dynamic abilities and sustain the functioning of this complex architecture, outstanding energy sup- ply is required. Indeed, the adult brain, accounting for a m ere 2% of body weight, is estimated to be responsible for 20% of oxygen (O ) consumption and 20– 25% of glucose utilization [ 8, 9] . In comparison, adult vertebrate brains, with the exception of primates, use 2– 8% of total energy at resting state [ 10] .
While adult brain energy demands are astonishing, the energy requisite during early life is even higher and essential to sup- port the rapid development of the brain with a growth burst starting around the 5th gestational month and continuing postnatally, increasing the brain’s weight from ∼ 27% of its adult weight at birth to ∼ 80% by age 2 years [ 11, 12] . In addition to its enormous demand of energy, the dramatic brain size expansion that happens during the first years of life re- quires specific nutrients, such as lipids, proteins, and micro- nutrients, which are not only the building blocks of brain structures but also support brain and cognitive functions during the rest of the lifespan [ 3, 13, 14].
In normal conditions, the main source of energy for the brain is glucose that is utilized for the generation of energy in the form of adenosine triphosphate (ATP) from either glycolysis or oxidative phosphorylation, the latter being 15 times m ore efficient to generate energy [ 15– 17] . Nevertheless, the particularly high energy needs of the developing hum an brain seem not to be supported by the sole consumption of glucose. Indeed, it has recently been suggested that ketones (β- hydroxybutyrate, acetoacetate, and acetone) derived from the oxidation of newborn body fat by the liver, may provide an important additional fuel substrate for the developing brain through oxidative phosphorylation as well [ 2, 17].
The adult brain, accounting for a mere 2% of body weight, is estimated to be responsible for 20% of oxygen (O2) consumption and 20– 25% of glucose utilization.
While glucose and ketones have been traditionally considered for their canonical role in mamalian energy metabolism , recent studies showed that they play additional roles associated with brain structure development and function [12, 17– 20] . For example, in an adult brain, 10– 12% of glucose is metabolized through glycolysis to produce lactate, despite oxygen being available for oxidative phosphorylation, a phenomenon called “aerobic” glycolysis or the “Warburg effect”  . Aerobic glycolysis remains a prevalent metabolic pathway in the brain all along the lifespan and especially dur- ing brain development. It is especially crucial for the biosynthesis of cell constituents (e.g., lipids) that support key developmental processes, including synapse formation/elimination and myelination [19, 22] . On the other hand, ketones seem to be the substrate for the synthesis of certain macro- molecules, such as cholesterol and fatty acids, which represent around 50% of the gray matter of the brain  . Recent observations also underline the importance of ketones as key signalling and epigenetic mediators, suggesting that they m ay influence gene expression involved in brain plasticity and re- organization during brain development [23– 25] . Therefore, glucose and other fuel substrates including ketone bodies (KBs) m ay be used for other purposes than to fulfil the energy demands only and m ay play a broader role during brain development.
In this chapter, we first describe the key physiological processes underlying brain development and how brain metabolism may be necessary to support them . We will especially focus on describing how glucose and KBs support not only the energetic but also anabolic demands of the brain during development. Of course, the brain relies on m any other nutrients for its proper development, function, and maintenance, which will be discussed by a subsequent chapter in this volume and has been previously discussed in excellent re- views [ 3, 13, 17, 26] .
Fig. 1. Timelines of neurodevelopment processes and glucose metabolism changes from conception to adulthood. The figure represents the key neurodevelopmental processes that are occurring during brain development and the changes in glucose metabolism associated with them. The top of the figure represents the major periods of human development expressed in days (d), postconceptional weeks (pcw), and years (y). The bars associated with each neurodevelopmental cellular process represent the approximative peak of the developmental period for each of them. The bottom of the figure represents the changes in glucose uptake (black line) and oxygen consumption (gray line) along time that peak around postnatal day 5. The glucose uptake is higher than the oxygen consumption, suggesting that a significant amount of glucose is metabolized via aerobic glycolysis (arrow) and parallels the rise in synapse formation and myelination. The figure has been adapted from [3, 12, 43] .
Main Physiological and Cellular Processes Underpinning Brain DevelopmentThe brain is one of the organs of the hum an body to develop the earliest, starting in utero during the 3rd gestational week, and it completes its development during the second and third decades of life, therefore being the organ with the longest development and maturation time [ 7, 22] . Once the primary organization of the brain is achieved, consisting of defining its main different regions during the embryonic period, key cellular processes emerge and proceed in developmentally overlapping waves (F ig. 1) . The generation of neurons (neuro- genesis) and their migration are initiated during the 8th gestational week, and the repertoire of neurons found in the adult neocortex is largely established before birth. Glia cell proliferation that follows neurogenesis peaks around birth, and it includes the generation of oligodendrocytes, supporting myelination and astrocytes, which have been shown to be involved in m any physiological processes in the brain, especially in the modulation of information processing, synaptic transmission, and energy dynamics [ 27– 29] . While regional variation exists, proliferation, migration, and differentiation of oligodendrocytes and astrocytes continue throughout the first 3 postnatal years, which coincides with the peak of synapse formation and neural network reorganization [ 7] . This reinforces the idea that oligodendrocytes and astrocytes play a crucial role in brain connectivity development and maturation all along childhood and adolescence.
While astrocytes and oligodendrocytes are generated and differentiated from neural progenitor cells, microglia, which are the resident macrophages of the brain involved in innate immunity, neuroprotection, synaptic pruning, and phagocytosis of cellular debris, originate from macrophages present in the yolk sac and migrate and colonize the brain during gestational week 4.5 [ 30, 31] . Interestingly, while neural progenitors are actively dividing and generate the first neurons early during embryogenesis, astrocytes and oligodendrocytes will appear only at late embryonic time points, as mentioned earlier. Con- sequently, early microglia colonization not only precedes the peak of neurogenesis and neuronal migration but constitutes the main glial population during a large part of fetal life, suggesting that microglia are involved in early brain development. Indeed, recent studies demonstrate that microglia contribute to the regulation of neuronal numbers and migrations and actively contribute to activity- dependent synaptic reorganization during neural network establishment [ 32] .
Development of neuron arborization consisting of axon and dendrite outgrowth followed by synapse formation is the key cellular process associated with the functional maturation of the brain after neuron migration. Indeed, from mid- gestation until the third postnatal year, immature neurons are initiating a protracted period of axon outgrowth and dendrite arborization, accompanied by the formation of synaptic junctions that ensure connectivity between neurons and lead to the formation of neuronal networks [ 33, 34] . Importantly, an overproduction of synapses will take place during the first 2 postnatal years with a burst of synapse growth between 3 and 15 months, depending on the brain areas, followed by a discrete period of synaptic pruning that typically starts during childhood and persists towards adolescence [ 35– 37] , although the visual area has been reported to undergo pruning as early as 3 months of age.
While neuronal networks are being built, oligodendrocytes generated myelin sheets are wrapped around axons, which act as insulators and lead to a dram atic increase in axonal conduction velocity and, therefore, information transmission [ 29] . Myelination starts during m id- gestation in the hum an brain, is a long process that dramatically accelerates during the first 2 postnatal years, and reaches its full maturity during the second to third decade of life [ 29, 38] . It also plays a key role in the maturation of brain networks, coordinated information processing, and ultimately cognitive performance in infants, children, and adults.
The high energy requirement of the brain is fulfilled by a constant transport of nutrients into the brain through the blood- brain barrier (BBB). The BBB tightly controls the pas- sage of selected substances in and out of the brain, provides protection against external potentially toxic agents, and is critical to maintain brain homeostasis and, thus, proper brain function [ 39, 40] . The BBB includes 3 m ajor cellular com po- nents: endothelial cells constituting the wall of blood vessels, pericytes that stabilize the BBB and are critical to maintain its integrity, and astrocytes that extend cellular processes whose endfeet ensheath the blood vessels and play a vital role in the transport of nutrients into neuronal cells [ 39] . The development and differentiation of the BBB is supposed to start in the very young embryo [ 41, 42] . Formation of blood vessels by endothelial cells is quickly accompanied by the recruitment of pericytes and astrocytes that will “seal” the BBB in order to isolate the brain from the external environment and control transport of substances. At the tim e of birth, the pattern of brain vasculature is very similar to what it will become in the adult brain [ 40] .
The high energy requirement of the brain is fulﬁlled by a constant transport of nutrients into the brain through the blood-brain barrier.
Recently, significant efforts have been made to under- stand how genes orchestrate the physiological and cellular processes described above. Gene expression analysis revealed that the brain transcriptome is segregated in clusters that are spatially and temporally organized and parallel the structural and functional developmental aspects of the brain. For example, clusters of genes associated with neuronal fate specification are mainly expressed embryonically and early fetally, while genetic clusters controlling synapse formation and function are highly expressed during early childhood  . Interestingly, it has also been shown that gene expression associated with mitochondria closely follows synapse density, suggesting that the proper development and maturation of brain connectivity is highly linked to energy availability [12, 43] .
In summary, the hum an brain undergoes a rapid growth from the 4th gestational week to the 3rd postnatal year. Subsequently, the rate of growth slows down and the brain is subjected to a significant reorganization, which is dominated by synaptic pruning and myelination that extends throughout the third decade of life. It is important to keep in m ind that brain development is not structurally and functionally homogeneous with age. Indeed, associative regions of the neocortex and especially prefrontal cortex are slower to mature than motor and sensory cortices, for example. Therefore, while being less important than during the prenatal or early postnatal periods, brain growth and especially brain reorganization at cellular and molecular levels continue beyond childhood to early adulthood. Refinement of neuronal networks is indeed thought to be critical for the functional specification of brain regions and crucial for the development of higher cognitive functions and behavior.
Energetic and Anabolic Demands during Human Brain DevelopmentAs briefly mentioned earlier, energy demands for vertebrate species correspond to 2– 8% of the total energy provided by basal metabolism , while the hum an adult brain requires as much as 20– 25% of it  . The energy demand during brain development is even m ore striking; it has been estimated that the newborn hum an brain, which represents about 13% of lean body weight, is consuming around 60% of the body’s daily requirement [ 12, 15, 17, 19, 20, 44] . This dramatic energetic demand persists and is even increased during child- hood; while a child brain at age 10 years accounts for 5– 10% of the body m ass, it approximately consumes 50% of the total basal metabolic rate of the hum an body [ 3, 12] .
Why are the energetic costs associated with brain function so high in humans and especially during brain development? In order to understand this, it is first necessary to identify the brain components and processes that cost energy. From an evolutionary point of view, a comparison of glucose and oxygen metabolic rates of the adult brain in awake mammals (rodents, macaque, baboon) suggests that the total metabolic cost is a simple linear function of the number of neurons present in the brain  . This is in agreement with a recent approximation of neural cellular energy demands which estimates that neurons consume 75– 80% of the energy produced, whereas the rest is used for glia- based processes [15, 46, 47] . Two main reasons may explain why neurons have high energetic demands: first, the generation of action potentials along the axons and synaptic transmission from neuron to neuron are based on electrochemical and cellular processes, such as ion fluxes, neurotransmitter release and re-uptake, and vesicle cycling, which are energetically costly [15,44, 46] . A signalling mechanism at the synapse has been suggested to be especially energy consuming; for example, it has been estimated that 80% of the energy in m hippo- cam pal axons is expended by postsynaptic potentials  . Second, the ability of the brain to change and adapt continuously along the lifespan is due to the constant remodeling of its architecture that culminates by the addition or the elimination of synapses to strengthen or weaken neuronal net- work activities accordingly. Constant synthesis of proteins, lipids, and amino acids is necessary to support the molecular modifications that underlie brain plasticity, which contributes to increasing brain energy expenditure [19, 46, 49, 50] . Rapid turnover of proteins and lipids is crucial to support dendritic spines and synapse modification, which are essential for learning and memory processes  . Nevertheless, it has been widely recognized that the majority of the energy in the adult brain is used to maintain its physiological baseline activity, including neuron and synaptic resting membrane potential, while changes in brain activity required to sustain specific cognitive tasks linked to synaptic plasticity result in an increase of energy demands by only 5% [18, 46, 52] . Importantly, brain development has significant additional energetic needs that are essential to support the constant and sustained synthesis of the molecular building blocks (proteins, lipids, and nucleic acids) underlying the rapid development and maturation of neuronal networks. In particular, at birth, the brain is about 25% of the weight of an adult brain, by age 2 years it is about 75% of its adult size, and around the age of 7 years, the hum an brain has reached its maximal size. Therefore, postnatal growth, especially during early childhood, happens rapidly and is not the result of the addition of new neurons, since neurogenesis mainly happens prenatally. In- stead, it is the development and the maturation of neurons already present at birth that account for the increase in brain biomass and energy demands, including axon growth, dendritic arborization elaboration, synaptic formation/elimination, and axon myelination [7, 18, 36, 37] . Interestingly, brain energy metabolism requirement follows the development and maturation of the brain, reaching its peak in energy demand per gram of tissue during postnatal year 2 and 3, especially when the rates of synapse formation and myelination are reaching their maximal intensity [3, 53] . Indeed, metabolic and especially anabolic demands are expected to in- crease with the addition of new synaptic connections and myelin wraps around axons. Therefore, there is a very close temporal and spatial relationship between the brain metabolic and anabolic needs and the cellular and physiological changes of the neural tissue through development and adulthood.
Fig. 2. Key biochemical pathway involved in glucose metabolism. Blood glucose is crossing the BBB in order to enter the brain. Glucose enters cells through glucose transporters (GLUTs) and is immediately phosphorylated to generate glucose- 6- phosphate (glucose- 6P). Glucose- 6P is used as the metabolic substrate for different biochemical pathways. First, it is converted into 2 molecules of pyruvate through glycolysis that generate ATP and NADH. Pyruvate is then either reduced in lactate, consuming one molecule of NADH, or is metabolized in acetyl- CoA. Lactate can be released in the extracellular space through monocarboxylate transporters and is used as a source of energy or as a biosynthetic substrate by neurons and oligodendrocytes. Second, acetyl- CoA is metabolized through the tricarboxylic acid (TCA) cycle and oxidative phosphorylation that produce ATP and CO 2 while consuming oxygen. The complete oxidation of glucose produces larger amounts of energy in the form of ATP in the mitochondria (30– 36 ATPs) compared to glycolysis (2 ATPs). Alternatively, acetyl- CoA is used for the synthesis of fatty acid and amino acids. Glucose- 6P can also be metabolized along the pentose phosphate pathway (PPP) and leads to the generation of ribulose- 5- phosphate (ribulose- 5- P, use in the synthesis of nucleic acid) and NADPH. NADPH is important to support fatty acid synthesis but also for the regulation of glutathione metabolism. Glutathione exists in the reduced form (GSH) or as a disulfide from (GSSG). The reduced form GSH is a source of reducing equivalent that can neutralize reactive oxygen species (ROS), such as hydroxyperoxides (ROOH). GSH is converted into GSSG, which is then recycled back to GSH by using NADPH as an electron donor. Glutathione is, therefore, a key antioxidant that protects cells against oxidative stress and is also critically involved in the control of cell redox homeostasis. Ultimately, nucleic acid, fatty acid, amino acid synthesis, and the control of redox homeostasis are providing the necessary energy and source of macromolecules that support neurodevelopment processes in brain development. The figure has been modified from  .
Glucose Requirements to Support Energy and Anabolic Demands during Brain DevelopmentMeasurements of cerebral metabolic rate for glucose (CMRGlc) and for oxygen (CMRO ), which is a measure of glucose and oxygen utilization in the brain, show a constant in- crease in their values during the first 2 years of life, reach approximately 2 and 1.5 times the adult value around 3– 5 years of life, respectively, and then gradually decrease to the aver- age adult value during the second decade of life (F ig. 1) [ 3, 54].
These observations suggest that energy demand increases during brain development, presumably due to an increase in synaptic transmission. Intriguingly, glucose utilization is in- creased to a greater extent com pared to brain oxygen utilization. Indeed, at birth, CMRGlc and CMRO measurement showed that the glucose consumption rate exceeds oxidative phosphorylation by around 34% [ 54, 55] . Moreover, Goyal et al. [ 20] reported that oxygen utilization during childhood ac- counts for approximately 70% of the total glucose consumption in a child brain. Since oxygen in the brain is utilized almost entirely for the oxidation of carbohydrate through oxidative phosphorylation to generate ATP, these results suggest that glucose m ay play additional functions to being an energetic substrate (F ig. 2) [ 3, 18, 56] . The preference of converting the glucose metabolite pyruvate produced through glycolysis into lactate or using it as a source of carbon for biosynthetic processes instead of converting it to ATP despite the availability of oxygen has been named aerobic glycolysis or the Warburg effect [ 21] . Aerobic glycolysis has been well described in tum or tissues that metabolize approximately 10- fold m ore glucose to lactate than norm al tissues, to provide substrates for biosynthesis of cell constituents and support cancer cell proliferation [ 57] . Nevertheless, since neurogenesis at birth is limited and restricted to specific brain areas, metabolic and anabolic demands supported by glucose are presumably due to the maturation of preexisting neurons, refinement of synaptic connectivity, glia proliferation, and rapid rise in axon myelination.
Glycolytic byproducts are a crucial source of carbons to produce glutathione, NADPH, and riboses along the pentose phosphate pathway (PPP), which are them selves essential for the synthesis of fatty acids and nucleotide sensitive, respectively, and to maintain oxidative stress homeostasis (F ig. 2) [ 58] . Biosynthesis of macro-molecules from glucose metabolites is critical to support key physiological processes behind
proper brain growth and maturation; it has been shown, for exam ple, that axon growth, synapse formation, and myelination rely critically on aerobic glycolysis [ 20, 59, 60] . Interestingly, aerobic glycolysis is predominant in the white matter com pared to the gray matter, and it has been shown that glycolytic byproducts, such as lactate, are especially important for myelin production by oligodendrocytes [ 61, 62] . While it has been assumed that most of the glucose is used for ion pumping to maintain synaptic activity, these findings highlight that glucose is critically involved in anabolic requirem ents be- yond energetic dem ands during neurodevelopm ent [ 18, 63, 64].
As discussed, aerobic glycolysis varies through the lifespan depending on regional and temporal metabolic and anabolic demands and seem s to be critical as well during early fetal brain development; measurement of glucose uptake in 12- to 21- week previable human fetuses demonstrated that around one- third of the total body glucose was consumed by the brain and only half of it was presumably oxidized [ 65] . Studies perform ed in preterm infants, when neurogenesis is still active, demonstrated a very low rate of oxygen consumption and suggested that 90% of glucose is dedicated to aerobic glycolysis [ 66, 67] . While studies during early stages of brain development are limited, these data indicate that the fetal brain is highly dependent on aerobic glycolysis, possibly due to the large requirement of denovo biosynthesis of lipids, amino and nucleic acids that are associated with neuron generation and proliferation (Fig. 2) .
The peak of CMRGlc happens around postnatal year 5 and stays elevated until 10 years of age [ 3, 54, 55] . While aging, glucose consumption slightly declines before reaching its adult life level during the second decade of life, and aerobic glycolysis decreases to one- third of its value in adulthood, representing 8– 10% of glucose utilization [ 55] . Nevertheless, aerobic glycolysis in some areas of the brain, such as the m e- dial and lateral parietal and prefrontal cortices, can contribute to as much as 20– 25% of glucose utilization [ 68, 69] . These brain areas integrate multi-modal sensory information and participate in complex cognitive functions, such as executive function and self- awareness, that necessitate a high level of synaptic plasticity and, therefore, a significantly high biomoecular turnover. It is, therefore, possible that during brain development glucose utilization is the key to not only provide energy through oxidative phosphorylation but also to support increase in biomass and macr-omolecule biosynthesis through aerobic glycolysis. Importantly, aerobic glycolysis seem s to be especially crucial prenatally to support neurogenesis and then postnatally to support mainly neuronal growth, synapse formation, and myelination and eventually the proper develop- m ent of neuronal networks underlying cognitive function. During brain m aturation, a gradual m etabolic switch between aerobic glycolysis and oxidative phosphorylation is happening. As neuronal networks mature, oxidative phosphorylation is predominant and maintains basal synaptic activity, while aerobic glycolysis is prevalent in areas where brain plasticity is engaged to sustain experience- dependent structural and functional changes that accompany higher cognitive functions (Fig. 2 ).
While aging, glucose consumption slightly declines before reaching its adult life level during the second decade of life, and aerobic glycolysis decreases to one- third of its value in adulthood.
Ketones as a Source of Energy for the Brain during DevelopmentGlucose is the primary metabolic substrate used by the brain to generate ATP in the central nervous system of adult mammals [ 8] . However, during brain development and maturation, the demand of energy and the high rate of macro-molecular biosynthesis exceed the availability of blood glucose. The most relevant additional fuels to support extra energetic needs of brain development are KBs, especially
β- hydroxybutyrate and acetoacetate [ 17, 70– 72] . KBs are short- chain fatty acids (SCFAs) derived mainly from liver β- oxidation of fatty acids that are available for the brain in direct proportion to their concentration in the blood [ 17, 73] . KBs are then oxidized through oxidative phosphorylation in the mitochondria of neuronal cells to generate ATP [ 17] . While the use of KBs for brain development is starting in utero, post- natal brain development also highly depends on KBs [ 65] . An interruption of continuous transplacental nutrient and energy supplies at birth necessitates the newborn to adapt quickly to the new metabolic environment and, especially, to move from continuous feeding to alternate periods of feeding and fasting  . This leads to a rapid metabolization of available substrates to produce energy, first, acutely from newborn reserve and, second, from alimentation in the form of milk from the m other [71, 74] . At birth, brain glucose reserves are very limited and could support brain needs for a few hours only. Metabolization and utilization of glucose from other organs, such as muscle, or tissue breakdown are not a viable long- term solution to support growth development. Interestingly, at birth, newborns are in a state of perm anent mild ketosis (0.1– 0.5 m M β- hydroxybutyrate), which is independent of feeding status or hypoglycem ia [ 75] . Moreover, the brain up- take of KBs is 4– 5 times faster in infants and children than in adults, which means that infant and child metabolism is pro- gram m ed to actively produce KBs from liver β- oxidation and that the brain is dependent on KBs to support its metabolic and anabolic needs. Indeed, it has been estimated that KBs may be able to replace for up to two- third of brain energy demands when glucose availability is low [ 17] .
It has now been recognized that fatty acids, especially medium - chain fatty acids (MCFAs) that constitute up to 10–20% of fatty acids contained in breast m ilk, are one of the m ain substrates used to produce SCFAs and maintain sustained ketosis in infants  . MCFAs are either directly converted into KBs by β- oxidation in the liver that will be taken up by the brain or they can be stored in adipose tissues and can be used later to support energy demands during a fasting period ( Fig. 3, 4 ). Moreover, it is estimated that hum an m ilk contains around 15– 17% SCFAs, which are highly ketogenic and may support brain energy and anabolic needs immediately [72, 76, 77].
Interestingly, body fat deposition during development is unique in hum ans; the hum an fetus starts to accumulate fat in subcutaneous adipose tissues during m id- gestation and has 500– 600 g of subcutaneous fat at birth, while most mammals have a very limited amount of adipose tissues and, therefore, inability to store either MCFAs or SCFAs [ 17].
What are the main advantages for the brain to use KBs during development? First, SCFAs are either immediately avail- able through the milk, or MCFAs stored within subcutaneous adipose tissues can be mobilized and metabolized to KBs. Second, the use of KBs as an alternative source of energy preserves glucose utilization for additional key metabolic path- ways, such as the PPP, as described earlier. Third, KBs are not only a high energetic substrate but are also used as anabolic metabolites. For example, cholesterol is the main carbon source for the synthesis of cholesterol, which represents 20% of total brain lipids. Cholesterol is not only indispensable to properly build the cell membrane, but it is also crucial to build axon yelination [ 78, 79].
Finally, the generation of KBs from MCFAs and SCFAs is very fast com pared to other fatty acids, since they directly reach the liver through the portal vein, by- passing the lym phatic system , and are β- oxidized into the m i-tochondria without the usual activation of the enzym e carni- tine palm itoyltransferase (Fig. 3).
Therefore, contrary to the adult brain, it appears that KBs are essential for brain development and maturation as they are not only an essential source of energy to complement glucose to entirely fulfil the brain metabolic needs but also, sim ilar to glucose, to support the anabolic demands associated with cell proliferation, growth, and maturation.
ConclusionFrom conception to the third year of life, brain size increases dram atically, leading to the formation and expansion of neuronal networks that will eventually be reorganized and re- shaped according to a variety of genetic and environmental factors [ 3] . Amongst the latter, nutrition is important for optimal brain development, since it provides glucose, KBs, and ketogenic fatty acids, which are the main metabolic substrates of the brain, during its development and maturation. Aerobic glycolysis and biosynthesis of macro-molecules from glucose seem s to be particularly important to support the establishment and maintenance of synaptic plasticity associated with higher cognitive functions. Utilization of KBs may, therefore, be used to “free” glucose for aerobic glycolysis, while sup- porting energy demands for synaptic transmission. In addition, KBs are also necessary for the synthesis of specific macro-molecules, such as cholesterol, as discussed earlier. Nevertheless, m any questions remain: first, it is still unknown what are the intrinsic and extrinsic factors that trigger aerobic glycolysis and oxidative phosphorylation in the brain, depending on its stage of development; second, it is not well understood how the balance between the use of glucose and KBs as energetic versus anabolic substrates is regulated. It will, there- fore, be crucial to elucidate the genetic, metabolic, and physiological processes during brain development that dictate brain metabolism changes. Such nutrients are normally provided in complex food matrices, such as breast m ilk. Under- standing fully how specific nutrients, especially in the context of food intake, interact together and affect brain metabolism (please see the chapter in this issue focusing on essential nutrients for early brain development: “Nutritional Factors in Fetal and Infant Brain Development” by Cheatham ) will help us to better define the minimal requirements to support it and promote brain development.
Disclosure StatementP.S. is an employee of Société des Produits Nestlé SA and the writing of this article was supported by Nestlé Nutrition Institute. The author declares no other conflicts of interest.
Fig. 3. Comparison of the absorption of medium- chain fatty acids (MCFAs) and long- chain fatty acid (LCFAs). MCFAs are rapidly absorbed from the gut and directly reach the liver through the portal vein. LCFAs are first integrated into chylomicrons and are primarily absorbed through the lymphatic system before reaching the target organs, including the liver, from the peripheral circulation. In the liver, MCFAs and LCFAs are converted into ketone bodies (KBs) (see Fig. 4) that are then released in the peripheral circulation before reaching the brain through the BBB. KBs are then converted to acetyl- CoA that is metabolized to produce either cholesterol in the smooth endoplasmicreticulum or energy in the form of ATP through the TCA cycle. The figure has been modified from  .
Fig. 4. Ketone body synthesis. Fatty acids are converted to acetyl- CoA through β- oxidation. Acetyl- CoA can then either enter the TCA cycle to generate ATP, or 2 molecules of acetyl- CoA are condensed in acetoacetyl- CoA. Acetoacetyl- CoA is then used to produce acetoacetate that can be used to produce either acetone or D- β-
hydroxybutyrate. The figure has been adapted from  .
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