The bioavailability of carbohydrates, the rate and extent to which they are absorbed and utilized, is intrinsically linked to their structural hierarchy and the food matrix in which they reside (9). Dietary carbohydrates range from simple monosaccharides to complex polysaccharides. Monosaccharides, such as glucose and fructose are present in their free form in honey, fruits and corn syrups, providing immediate bioavailability. Disaccharides, such as the lactose found in mammalian milk or the sucrose concentrated in sugar beets and cane, require minimal enzymatic cleavage prior to absorption (39). While animal-derived foods are generally carbohydrate-poor, dairy products are a unique exception, providing the essential disaccharide lactose, which accounts for almost 40% of the energy in human breast milk and is vital for neonatal development (40,41).

Contemporary nutritional science emphasizes that the health impact of these sugars is heavily dictated by their source. For instance, while refined sucrose and fructose in high-fructose corn syrup are associated with rapid glycemic spikes, the same sugars found within the fibrous matrix of whole fruits such as grapes or kiwi exhibit different kinetic profiles (42,43). As the structural complexity of polysaccharides, such as the starches found in legumes, tubers and cereal grains, increases, the pace of digestion typically slows. However, modern processing techniques have challenged the traditional dichotomy between simple and complex. Current evidence indicates that highly processed complex carbohydrates, such as white bread or instant rice, can have a glycemic index (GI) equal to or higher than that of simple table sugar, leading to significant insulin demand and increasing the risk of metabolic syndromes, obesity and ovulatory infertility (44). The role of carbohydrates in the diet is presented in Table I (41,44-61). Table I provides an overview of dietary carbohydrates, which are vital macronutrients; their types, quantity and metabolic fate critically influence health outcomes.

The interaction between dietary carbohydrates, the gut microbiome, and host health constitutes a dynamic and reciprocal relationship that is fundamental to modern nutritional frameworks. Dietary fibers and other indigestible carbohydrates serve as pivotal environmental factors influencing the composition and metabolic output of the gut microbial ecosystem (62). Subsequently, the gut microbiota converts these dietary elements into bioactive metabolites that affect host metabolism, immune function and even neurological processes (63). This relationship is increasingly recognized in the prevention and management of chronic diseases, including obesity, cardiometabolic disorders and certain types of cancer (62). The notion of carbohydrate quality currently includes not only its glycemic effects, but also its capacity to support a beneficial gut microbiota, thereby enhancing overall health (9,11). This integrated viewpoint emphasizes that the simplistic perception of carbohydrates as mere caloric sources is outdated; they should be regarded as information-rich molecules whose chemical language is deciphered by both human enzymes and the trillions of microbial entities in the gut, ultimately defining their significant biological roles in food and nutrition sciences (34,64).

Carbohydrates are the main source of oxidative fuel for the human body, providing with ~4 kcal/g (17 kJ/g) of energy. Glucose is particularly critical as it is the only metabolic substrate for the central nervous system and red blood cells, which lack the enzymes to efficiently oxidize other fuels under normal physiological conditions (65,66). In the fed state, dietary carbohydrates maintain blood glucose homeostasis and replenish glycogen stores in the liver and skeletal muscle. Under restricted exogenous intake, the body demonstrates metabolic flexibility by activating gluconeogenesis and ketogenesis to maintain brain function. This change indicates that the quality of carbohydrates is critical for overall health. A balanced intake (usually 45-65% of daily calories) maintains a stable metabolism; however, consuming high-GI foods continuously break down these homeostatic mechanisms (67-70).

Non-digestible polysaccharides, or dietary fiber, have become an integral part of modern preventive medicine. Unlike starch, which is broken down by enzymes in the small intestine, fibers such as cellulose and hemicellulose remain whole until they reach the colon. There, they add bulk to the stool and serve as fermentable substrates for the gut microbiota (71). The SCFAs produced by this process serve as an alternative energy source for colonocytes and exert systemic anti-inflammatory effects (72). Current clinical guidelines suggest that individuals should consume ~30 g of fiber per day, which is linked to lower rates of coronary heart disease, type 2 diabetes and colorectal cancer (73). This indicates that carbohydrate chemistry can affect long-term health outcomes.

Despite the extensive understanding of carbohydrate metabolism, a significant knowledge gap persists regarding inter-individual variability in glycemic response. While tools such as the GI and glycemic load provide a standardized framework for ranking foods, contemporary big data nutrition studies (74) have revealed that two individuals consuming the identical carbohydrate source can exhibit vastly different postprandial glucose excursions. This variability is deemed to be driven by a complex interplay between host genetics, sleep patterns, physical activity, and most crucially the unique composition of the gut microbiome of an individual (74,75).

Furthermore, a vigorous scientific debate remains unresolved concerning the carbohydrate-insulin model (CIM) of obesity. Proponents of the CIM argue that high-carbohydrate diets drive obesity by promoting insulin secretion, which partitions energy toward fat storage rather than oxidation, whereas critics maintain that total energy balance (calories in vs. calories out) remains the primary driver (76,77). Resolving these questions is essential for moving beyond one-size-fits-all dietary guidelines and toward personalized nutritional interventions that can effectively combat the global rise in metabolic disease.

Authoritative international dietary guidelines consistently identify refined carbohydrates as a key dietary component to limit due to their well-established association with chronic disease risk. The World Health Organization (WHO), in its global recommendations, explicitly advises that the intake of free sugars, which encompass added sugars and those naturally present in honey, syrups and fruit juices, should be limited to <10% of total energy intake for both adults and children, with a further conditional recommendation to reduce this to <5% for additional health benefits (78,79). This guidance is grounded in extensive epidemiological evidence linking a high sugar consumption to weight gain, dental caries and metabolic dysfunction. Similarly, the Dietary Guidelines for Americans (DGA), jointly issued by the US Department of Health and Human Services and the US Department of Agriculture, emphasize minimizing the intake of refined grains and added sugars, recommending that <10% of daily calories should be derived from added sugars and that at least half of all grain consumption should be whole grains (80). The European Food Safety Authority (EFSA) and national bodies such as the UK Scientific Advisory Committee on Nutrition (SACN) echo these principles, advocating for a shift from refined to whole-grain carbohydrate sources to improve long-term health outcomes (81).

The scientific foundation for these guidelines rests on a robust body of epidemiological research. The landmark Prospective Urban Rural Epidemiology (PURE) study, which followed >135,000 individuals across 18 countries, provided critical population-level evidence that high carbohydrate intake, particularly from refined sources, is associated with an increased total mortality (82). This finding challenged older paradigms that focused primarily on fat reduction and highlighted the importance of carbohydrate quality over mere quantity. Further analysis from the PURE cohort specifically linked the higher consumption of refined grains (median intake of 350 g/day in the highest quintile) to a significantly elevated risk of developing major cardiovascular events and total mortality. By contrast, whole grain intake exhibited neutral or protective associations (83). These results underscore the differential health impacts of carbohydrate subtypes, a nuance increasingly emphasized in modern nutritional science.

Longitudinal cohort studies from the USA have provided granular insights into specific refined carbohydrate sources. Data from the Nurses' Health Study and the Health Professionals Follow-Up Study (84) demonstrate a clear dose-response association between the consumption of sugar-sweetened beverages (SSBs) and the incidence of type 2 diabetes and cardiovascular disease. A meta-analysis of prospective cohort studies confirmed that each daily serving of SSBs is associated with an 18% higher risk of coronary heart disease (85). Similarly, a previous systematic review and meta-analysis found that the higher intake of refined carbohydrates predicts a significantly increased risk of developing type 2 diabetes, with the association persisting even after adjustment for body mass index and other confounders (86). The mechanisms underlying these associations are multifactorial: Refined carbohydrates drive rapid postprandial glycemia and hyperinsulinemia, promote hepatic DNL leading to dyslipidemia (elevated triglycerides and reduced high-density lipoprotein cholesterol) and induce systemic inflammation, all key pathways in the development of cardiometabolic diseases (87).

The concept of ultra-processed foods (UPFs), as defined by the NOVA classification system, has further refined the understanding of the risks associated with the consumption of refined carbohydrates. UPFs are industrial formulations that typically contain high levels of refined starches, added sugars and unhealthy fats, while being low in fiber and phytonutrients. A recent umbrella review of epidemiological meta-analyses concluded that a higher consumption of UPFs is consistently associated with increased risks of obesity, type 2 diabetes, cardiovascular disease and all-cause mortality (88). The Framingham Offspring Study reported that for every 10% increase in the proportion of UPFs in the diet, there was a 12% higher risk of overall cardiovascular disease and a 13% higher risk of cerebrovascular disease (89). These findings suggest that the health risks of consuming refined carbohydrates may be amplified in the context of ultra-processing, where the food matrix is degraded and hyper-palatable combinations override natural satiety signals, leading to excess energy intake and metabolic stress (87).

Despite this strong consensus, some nuances and ongoing debates exist. For instance, not all refined carbohydrate sources carry equal risk; the consumption of white rice in Asian populations exhibits complex associations that may be modified by the overall dietary pattern and glycemic load (83). Furthermore, the focus is shifting from isolated nutrients to holistic dietary patterns. The Mediterranean diet, rich in whole grains, legumes, fruits and vegetables, has been shown in randomized trials, such as the PREDIMED study, to significantly reduce cardiovascular events, demonstrating that the adverse effects of the consumption of refined carbohydrates can be mitigated by replacing them with high-quality carbohydrate sources within a balanced dietary pattern (90). Overall, authoritative guidelines and contemporary epidemiological evidence converge on a clear message that reducing the intake of refined carbohydrates, particularly in the form of added sugars and UPFs, is a critical public health strategy for preventing chronic diseases globally.

Glycolysis is a catalytic process that converts glucose into pyruvate via 10 enzyme steps. These highly regulated processes of glycolysis have two stages: The energy input for Step 1 requires two ATPs per glucose molecule, followed by the energy release as four ATPs, two for each glyceraldehyde molecule, which is described as Step 2(90). Glycolysis, a process in living cells, can also be used in the citric acid cycle or fermentation, and is the primary source of acetyl coenzyme A (acetyl-CoA), the primary energy source. The structures of glycolysis intermediates are depicted in Fig. 2 (92).

The initial stage of glycolysis requires energy in the form of ATP. ATP phosphorylates D-glucose at the six carbons via the enzyme hexokinase to produce D-glucose-6-phosphate (G-6-P). This regulatory process is inhibited by the presence of glucose-6-phosphate. Phosphoglucoisomerase (class: Isomerase) then converts it to D-fructose-6-phosphate (F-6-P), which is phosphorylated again by ATP, this time at the one-carbon position, by the enzyme phosphofructokinase (class: Transferase) to produce D-fructose-1,6-bisphosphate. Owing to its high G value, this is the committed stage of glycolysis. The enzyme, fructose bisphosphate aldolase, then cleaves D-fructose-1,6-bisphosphate into two three-carbon molecules, dihydroxyacetone phosphate (DHAP) and D-glyceraldehyde-3-phosphate (G-3-P). DHAP is transformed into G-3-P as the following step in glycolysis requires the molecule G-3-P by the enzyme triose phosphate isomerase (93).

The second stage involves producing four ATP molecules for each glucose molecule. The glyceraldehyde-3-phosphate dehydrogenase enzyme phosphorylates G-3-P at the one carbon to produce the high-energy molecule, 1,3-bisphosphoglycerate (BPG). Phosphoglycerate kinase then phosphorylates adenosine diphosphate (ADP) at the expense of BPG to produce ATP and 3-phosphoglycerate. Phosphoglycerate mutase then converts 3-phosphoglycerate to 2-phosphoglycerate to produce another high-energy molecule. Enolase then converts 2-phosphoglycerate to phosphoenolpyruvate, releasing water, potassium and manganese ions. Pyruvate kinase phosphorylates ADP to produce ATP and pyruvate, and cellular energy controls this reaction. High energy levels indicate inhibited pyruvate kinase. Stage 2 occurs twice due to glucose division (93).

In parallel, G-6-P can be diverted into the hexose monophosphate shunt (pentose phosphate pathway). Unlike glycolysis, this pathway is not primarily concerned with ATP production. Instead, it serves two vital biosynthetic roles: Generating nicotinamide adenine dinucleotide phosphate, which provides reducing power for fatty acid synthesis and antioxidant defense (reducing glutathione), and producing ribose-5-phosphate, a precursor for nucleotide synthesis (94). In the context of nutrition, the balance between these two pathways is essential; for instance, a diet chronically high in refined sugars can over-activate these shunt pathways, potentially fueling DNL and contributing to metabolic dysfunction (95).

The citric acid cycle is the ultimate oxidative process in which carbs, lipids and amino acids interact. It is the most significant metabolic pathway for the energy source of the body. TCA is the most essential core metabolic pathway, connecting nearly all metabolic processes. The Krebs cycle (Fig. 3) is an aerobic biodegradation process that begins with acetyl-CoA and ends with CO₂. Patients with diabetes have lower oxaloacetic acid and Krebs cycle activity due to reduced glucose and pyruvate levels (96).

At higher energy levels, the citrate synthesis rate increases, while the flux through the Krebs cycle decreases. This excess citrate is subsequently transported to the cytosol and degraded to acetyl-CoA. Acetyl-CoA can then be used to produce fatty acids and cholesterol. Citrate has a positive regulatory effect on lipogenic enzymes (e.g., acetyl-CoA carboxylase) and a negative regulatory effect on phosphofructokinase-1 (a glycolytic enzyme). Citrate is also necessary for ketogenesis in non-hepatic tissues (96).

The α-ketoglutarate dehydrogenase complex converts α-KG to succinyl coenzyme A (succinyl-CoA) through multiple steps (Fig. 3). Succinyl-CoA is a highly energetic molecule that generates the only guanosine triphosphate molecule in this pathway when succinate thiokinase is present (96). Nicotinamide adenine dinucleotide (NADH₂) and flavin adenine dinucleotide (FADH₂) are the energy molecules created during this oxidative decarboxylation cycle. These molecules release energy during the oxidative phosphorylation reaction; thus, their power is retained in ATP as high-energy phosphate (97). Contemporary biochemical evidence suggests that the total aerobic yield from a single glucose molecule is ~30-32 ATP. This efficiency highlights why carbohydrates remain the preferred primary energy substrate of the human body. However, this high-efficiency system is also sensitive to nutrient overload. When carbohydrate intake exceeds the capacity for oxidation and glycogen storage, the excess acetyl-CoA is redirected toward triglyceride synthesis, reinforcing the link between high-glycemic diets and adipose tissue expansion (98).

One glucose molecule is transformed into 38 ATP molecules during cellular respiration. One glucose molecule is broken up to produce two molecules of pyruvate, producing eight ATP directly from anaerobic glycolysis. Each three-carbon pyruvate yields one NADH₂ (equivalent to three ATP) during the pyruvate decarboxylation reaction. As a result, each six-carbon glucose molecule in this process generates six ATP. The total energy produced by each three-carbon pyruvate in the Krebs cycle is 12 ATP (91).

This quantity is generated by isocitrate dehydrogenase (+NADH₂ 3ATP), alpha-ketoglutarate dehydrogenase (+NADH₂=3ATP), malate dehydrogenase (+NADH₂=3 ATP), succinate dehydrogenase (+FADH₂=2 ATP) and succinate thiokinase (+GTP). As each six-carbon glucose molecule generates two three-carbon pyruvate molecules, the total quantity produced by one glucose molecule is 24 ATP. The glycolysis 8 ATP, the pyruvate dehydrogenase complex 6 ATP, and the Krebs cycle 24 ATP combine to yield 38 ATP (91).

The systemic health impact of carbohydrate metabolism is best observed through the glycemic response, the postprandial rise and fall of blood glucose. This response is governed by a delicate hormonal ‘tug-of-war’ between insulin and glucagon (99). High-bioavailability carbohydrates (simple sugars and refined starches) elicit a rapid insulin spike, which promotes glucose uptake via GLUT4 transporters in muscle and adipose tissue, while simultaneously suppressing lipolysis (99,100).

The connection here involves metabolic flexibility. In a healthy individual, the body can switch seamlessly between carbohydrate oxidation in the fed state and fat oxidation in the fasted state. However, a diet dominated by high-GI foods can lead to chronic hyperinsulinemia and eventual insulin resistance. This state represents a breakdown in systemic energy balance: Cells become starved of glucose despite elevated blood glucose levels, leading to increased hunger and a cycle of metabolic stress (44). Understanding these pathways emphasizes that the nutritional value of a carbohydrate is defined not only by its caloric content, but by its kinetic interaction with the body's metabolic machinery.

While glucose and fructose are structural isomers sharing the same caloric density, their metabolic destinies in the human body are fundamentally different due to divergent hepatic processing (101). Glucose metabolism is systemic; although the liver is a primary site for its regulation, glucose is utilized by nearly every cell in the body, and its entry into glycolysis is strictly governed by the energy status of the cell. By contrast, fructose is almost exclusively metabolized by the liver, and its entry into central metabolic pathways bypasses the most critical regulatory checkpoints (101,102).

The primary kinetic distinction lies in the first steps of phosphorylation. Glucose is converted to G-6-P by hexokinase or glucokinase. Fructose, however, is phosphorylated by fructokinase (ketohexokinase or KHK) to form fructose-1-phosphate (F-1-P). This enzyme is not inhibited by ATP or citrate. Furthermore, F-1-P is cleaved by aldolase B into DHAP and glyceraldehyde. These triose phosphates enter the glycolytic sequence downstream of the phosphofructokinase-1 (PFK-1) regulatory step (103,104). Essentially, the excess of fructose in the metabolic machinery provides a continuous supply of carbon precursors, such as acetyl-CoA, regardless of the actual energy requirements of the cell (105). Additionally, the subsequent conversion of fructose-6-phosphate to fructose-1,6-bisphosphate by PFK-1 is the rate-limiting step of glycolysis. When cellular ATP levels are high, PFK-1 is inhibited, decelerating glucose oxidation to prevent an unnecessary surplus of metabolic intermediates (103).

This unregulated flux has profound implications for metabolic health. As the liver is suddenly presented with an abundance of acetyl-CoA that it cannot oxidize in the TCA cycle, it must divert these carbons toward DNL. This leads to the synthesis of palmitate and other fatty acids, contributing to the accumulation of intrahepatic fat (non-alcoholic fatty liver disease, NAFLD) and the secretion of very-low-density lipoprotein, which raises blood triglyceride levels (106).

Furthermore, the rapid, uncontrolled phosphorylation of fructose by KHK leads to transient intracellular ATP depletion. As ATP is rapidly consumed to phosphorylate fructose without the brake of feedback inhibition, the resulting rise in AMP triggers the purine degradation pathway, leading to the production of uric acid. Elevated uric acid is not only a risk factor for gout, but also functions as a pro-oxidant within the hepatocyte, potentially inducing mitochondrial oxidative stress and further impairing insulin sensitivity (107,108). This kinetic reality explains why high-fructose diets, particularly those rich in refined syrups, are uniquely linked to metabolic syndrome, independent of total caloric intake.

Resistant starches and oligosaccharides represent two critical classes of non-digestible carbohydrates that exert profound influence on gut microbial ecology and, consequently, on the risk of developing metabolic diseases, such as obesity, type 2 diabetes and cardiovascular disorders. Their biological significance stems not from caloric contribution, since they largely escape digestion in the small intestine, but from their role as selective substrates for beneficial gut commensals, thereby modulating microbial composition, metabolic output and host physiology (15).

Resistant starch is a form of starch that resists enzymatic hydrolysis by human amylases and reaches the colon intact, where it serves as a fermentable substrate for specific members of the gut microbiota. Primary degraders of resistant starch include Ruminococcus bromii and Bifidobacterium adolescentis, with emerging evidence identifying additional specialists such as Ruminococcoides bili (109). These bacteria initiate the breakdown of complex resistant starch granules through specialized carbohydrate-active enzymes (CAZymes), enabling secondary fermenters to access the resulting oligosaccharides and monosaccharides. This hierarchical degradation process fosters cross-feeding networks, thereby enhancing microbial diversity and stability. The fermentation of resistant starch predominantly yields SCFAs, notably butyrate, acetate and propionate, which serve as key mediators of metabolic health. Butyrate, in particular, is the preferred energy source for colonocytes, strengthens the gut barrier by upregulating tight junction proteins, and exerts anti-inflammatory effects by inhibiting histone deacetylases and activating G-protein-coupled receptors (e.g., GPR109A). Clinical and preclinical studies consistently link higher a intake of resistant starch with improved insulin sensitivity, reduced adiposity and lower systemic inflammation, all of which are protective against cardiometabolic diseases (24,62).

Oligosaccharides, such as fructooligosaccharides (FOS), galactooligosaccharides and human milk oligosaccharides, function similarly as prebiotics by selectively stimulating the growth and activity of beneficial bacteria, especially Bifidobacteria and certain Lactobacilli. Their structural complexity, defined by glycosidic linkage types [e.g., β(2→1) in inulin-type FOS] and the degree of polymerization, determines their resistance to host enzymes and specificity for microbial utilization. Research profiling the metabolic capacities of 17 gut commensal strains has revealed marked strain-specific differences in oligosaccharide fermentation, underscoring that prebiotic efficacy is contingent on both substrate structure and the recipient's microbial repertoire (110). This individualized response explains the variable outcomes observed in dietary interventions and highlights the need for personalized nutrition approaches. The SCFAs produced from oligosaccharide fermentation regulate host metabolism through multiple pathways: propionate reduces hepatic lipogenesis and gluconeogenesis, while acetate influences appetite regulation via central nervous system signaling (62). Moreover, oligosaccharides can directly inhibit pathogen adhesion by acting as decoy receptors, thereby enhancing gut defense mechanisms.

The interplay between these carbohydrates and the gut microbiome has significant implications for the prevention of chronic disease. Dietary fibers, including RS and oligosaccharides, are associated with a reduced risk of obesity, type 2 diabetes and types of certain cancer, largely through microbiota-dependent mechanisms (62,111). For instance, low-carbohydrate diets that inadvertently reduce fiber intake may diminish SCFA production, potentially compromising gut barrier integrity and promoting endotoxemia, a known driver of insulin resistance (111). Conversely, the strategic inclusion of high-quality, fermentable carbohydrates supports a resilient microbial ecosystem that buffers against dysbiosis induced by Western diets high in refined sugars and saturated fats (112). Notably, the concept of ‘carbohydrate quality’ now supersedes mere quantity; slowly digestible or non-digestible forms such as resistant starch and oligosaccharides are favored for their ability to modulate postprandial glycemia and sustain microbial health without contributing to hyperinsulinemia or DNL (24,112). Hence, resistant starches and oligosaccharides function as keystone dietary components that shape a health-promoting gut microbiota, drive beneficial metabolic signaling via SCFAs, and mitigate risk factors for major chronic diseases, positioning them at the nexus of food science, microbiome research and precision nutrition.

Despite these advances however, challenges remain in translating findings into universal dietary recommendations due to inter-individual variability in gut microbiota composition, host genetics and baseline metabolic status. Furthermore, the structural heterogeneity of dietary carbohydrates complicates analytical characterization and standardization. Emerging glycomic platforms combining chromatography and mass spectrometry are beginning to address these gaps by enabling precise mapping of carbohydrate structures to microbial responses (34).

Carbohydrates have not only been a major source of food and nutrition but also have excellent pharmaceutical applications. Carbohydrates derived from natural sources have been studied as therapeutic agents for a number of years. Among all carbohydrate-based drugs developed between 2000 and 2021, the majority have been derived from naturally occurring carbohydrate moieties, of which 37% are nucleosides. Furthermore, 60% of all carbohydrate-based drugs have broad applications in the preparation of antiviral, antibacterial/antiparasitic, anticancer and antidiabetic drugs (156). For instance, saponins, a diverse family of glycosides having brilliant adjuvant activity, such as Quillaja brasiliensis saponins (triterpene glycosides). Its source is the Chilean tree Quillaja saponaria bark. It has been reported to have >20 water-soluble Quillaja saponaria saponins, from which several led to the eliciting of humoral and cell-mediated responses (158). Similarly, in Asian countries, Coriolus versicolor polysaccharides (polysaccharides Krestin and Lentinan) have been used for >40 years as antitumor adjuvants (159). Pectin, a family of polysaccharides, is a fine source of dietary fiber. It has been reported to exhibit inhibitory activity against various cancer cell lines (160-164).

Structures of naturally occurring carbohydrates are often very complex due to their complex biosynthetic pathway. Hence, isolating structurally defined pure compounds from natural sources becomes cumbersome. To better address demands in glycobiology, synthetic carbohydrate derivatives or glycoconjugates comprise a fascinating alternative. Synthetic or non-natural glycoconjugates can aid in understanding structure-activity associations in biological responses and mimic the functions of native saccharide derivatives (165).

Some of the advantages of these synthetic carbohydrate-based materials stem from their biocompatibility and intrinsic biological functionality, resulting in generally low toxicity and excellent biodegradability, which are crucial for clinical translation (166). Their most significant asset is their ability to engage in specific, high-affinity interactions with carbohydrate-binding proteins (lectins, selectins and siglecs) that are central to numerous disease processes, including cancer, inflammation and infection This allows for the rational design of therapeutics that can modulate immune responses; for instance, synthetic glycopolymers can be engineered to mimic the multivalent presentation of glycans on cell surfaces, amplifying binding avidity through the ‘glycocluster effect’ to either activate or suppress immune cells for applications in tumor immunotherapy (167,168). Furthermore, carbohydrate polymers, such as chitosan, alginate and hyaluronic acid serve as excellent scaffolds for targeted drug delivery, particularly for colon-specific release, as they can be degraded by colonic microflora enzymes, thereby minimizing systemic side-effects (169,170). The structural diversity of the sugar scaffold also provides multiple points for chemical modification, enabling fine-tuning of pharmacokinetic properties such as solubility and half-life (171).

Automated glycan assembly (AGA) has expedited access to synthetic glycans up to 50-mers (172), while other automated platforms based on electrochemical assembly (173), fluorous-assisted solution-phase (174) and HPLC-assisted synthesis (175) have been limited to a few examples not exceeding hexasaccharides (176). From the proof-of-concept using a modified peptide synthesizer in 2001 to the first commercial Glyconeer 2.1 synthesizer (177), AGA has been developed using glycans of mammalian, bacterial and plant origin as a precursor (178,179).

The AGA oligosaccharide synthesis workflow is designed to minimize the number of purification steps and manipulations (Fig. 5). Inside the reaction vessel of the synthesizer, a resin-bound linker serves as an anchor for sequentially attaching monosaccharide building blocks. In this manner, excess reagents can be washed away, and time-consuming intermediate purification steps can be avoided. Following synthesis, the resin-bound oligosaccharide is removed from the synthesizer and cleaved from the solid support. Analytical normal-phase high-performance liquid chromatography (NP-HPLC) and MALDI analysis of the crude product after cleavage are used to assess the success of the synthesis qualitatively. The protected glycan is purified using preparative NP-HPLC. Global deprotection removes all permanent protecting groups and following reverse-phase HPLC (RP-HPLC), the unprotected glycan is obtained (177,178,180,181). This enhanced accessibility has directly fueled progress in glycomimetic design, in which natural carbohydrate leads are modified to overcome inherent limitations such as poor metabolic stability and low bioavailability (182). Common strategies include replacing the endocyclic oxygen with carbon (carbasugars) or sulfur (thiosugars), or substituting the glycosidic oxygen with a methylene group to form C-glycosides, all of which confer resistance to enzymatic hydrolysis by glycosidases (183).

Cyclodextrin (CD)-based materials have garnered applications in the food and pharmaceutical industries since the 1980s. For example, Sugammadex (Bridion), a modified γ-CD, has been used in anesthesia since 2008 and also functions as an antidote to some curare-like muscle relaxants (184). The electrostatic interaction and complex formation of CD with drugs can significantly alter the pharmacokinetic profile of a drug, and CD inclusion complex-based nanofibers have great applications in drug discovery and delivery, as they tend to increase the bioavailability of the drugs (185-187). For instance, the oral delivery of insulin is difficult owing to its instability in the gastric conditions of patients. However, reports suggest that carboxymethyl-β-CD-grafted chitosan (CMCD-g-CS) leads to the more effective oral delivery of insulin at pH 7.4 due to increased insulin bioavailability after pH-triggered complexation with CMCD-g-CS (188).

Other saccharide derivatives, such as imine sugars, have been suggested to function as potent inhibitors to several glycosidases, such as glycogen phosphorylases, glycoside hydrolases and glycosyltransferases. Their inhibitory potency against these carbohydrate-processing enzymes is suggested to depend on structural and electronic similarity to the oxocarbenium transition state of the natural substrate at a physiological pH (189,190).

Additionally, research on carbohydrate-conjugated noble metal-based nanoparticles, such as Au, Ag and Pt, has seen tremendous growth over the past few decades due to their applications in bioimaging, diagnostic imaging, biosensing, gene/drug delivery (targeted/triggered/controlled), targeted therapeutics. Despite their inertness, they can form a stable covalent linkage via facile synthetic methods with compounds containing functional groups, such as thiols and disulfides (16).