IngredientsThe series of cytoplasmic reactions known as the pentose phosphate pathway are also called the hexose monophosphate (HMP) shunt (or cycle) or the phosphogluconate pathway. Additional synonyms found throughout the scientific literature include the pentose phosphate cycle, the pentose shunt, and — in historical literature — the Warburg–Dickens pathway.
"Pentose" in the name refers to the five-carbon sugars produced during the process, and "phosphate" refers to the phosphorylation of intermediates throughout the pathway. The pathway is not a single substance consumed as a dietary supplement in the conventional sense. Rather, it is an endogenous metabolic route whose key intermediate molecules — most prominently ribose 5-phosphate (R5P), NADPH, ribulose 5-phosphate, 6-phosphogluconate, sedoheptulose 7-phosphate, and erythrose 4-phosphate — are the subjects of nutritional, pharmacological, and clinical investigation. The terms "pentose phosphate" and "pentose phosphate pathway" therefore describe both the biochemical network and the class of phosphorylated five-carbon sugar intermediates it generates.
The pentose phosphate pathway (PPP) is defined as a metabolic pathway that occurs in the cytosol, running parallel to glycolysis, and is responsible for providing precursors for nucleotide and amino acid synthesis, maintaining carbon homeostasis, and generating reducing molecules like NADPH, which is essential for redox balance and antioxidant production.
The PPP is not an exotic botanical extract or isolated phytochemical. It is a universal cellular pathway present in virtually every living organism, including humans. The growing interest in modulating this pathway through dietary and supplemental means — such as D-ribose, benfotiamine (a thiamine derivative), and antioxidant compounds — has generated a substantial body of biomedical research that constitutes the core of the clinical evidence reviewed in this article.
The first evidence of the pentose phosphate pathway emerged in the 1930s through the work of Otto Warburg, who received the Nobel Prize in Physiology or Medicine in 1931 for his work on cellular metabolism and respiratory enzymes. Warburg and his collaborator Erwin Christian discovered NADP during studies on the oxidation of glucose 6-phosphate to 6-phosphogluconate.
This work revealed the existence of a second coenzyme, termed triphosphopyridine nucleotide TPN (now widely known as NADP+), that is required for the oxidation of glucose 6-phosphate to 6-phosphogluconate, by an enzyme which was purified from yeast and erythrocytes and named Zwischenferment ["intermediate enzyme," now glucose 6-phosphate dehydrogenase (G6PDH)].
It is also noteworthy that the British biochemist Frank Dickens (Courtauld Institute, UK) had been making pioneering investigations on this topic since 1936. However, a solution of the problem was made in the USA, where metabolic biochemistry and enzymology were flourishing in the 1950s. The dominant contributions came from the laboratories of Bernard Horecker, Ephraim Racker, and Seymour Cohen.
The pathway was fully elucidated only in the 1950s, thanks to the work of several researchers, notably Efraim Racker, Bernard Horecker, Frank Dickens, and Fritz Lipmann. Transketolase was discovered independently in 1953 by Horecker and Racker, and named by Racker; it catalyzes the transfer of a two-carbon unit from a ketose donor to an aldose acceptor.
A comprehensive review of seminal work that led to the discovery and description of the pathway — work that dates back now for 80 years — addresses genetic and metabolic mechanisms that regulate its activity and discusses these biochemical principles in the context of PPP deficiencies causing metabolic disease and the role of this pathway in biotechnology, bacterial and parasite infections, neurons, stem cell potency, and cancer metabolism.
The PPP is divided into two sequential phases: the oxidative phase and the non-oxidative phase. The oxidative phase is termed such because it involves the oxidation of glucose-6-phosphate (G6P). It begins with the enzyme glucose-6-phosphate dehydrogenase (G6PD), which oxidizes G6P to produce 6-phosphogluconolactone. In this step, NADP+ is reduced to NADPH, yielding the first of two NADPH molecules produced in this phase. This reaction is the rate-limiting step of the pathway.
The G6PD reaction is followed by the enzyme lactonase, which hydrolyzes 6-phosphogluconolactone to 6-phosphogluconate. Finally, 6-phosphogluconate is oxidized by 6-phosphogluconate dehydrogenase to produce ribulose-5-phosphate, a second molecule of NADPH, and one molecule of CO2.
Whereas the oxidative PPP is considered unidirectional, the non-oxidative branch can supply glycolysis with intermediates derived from ribose 5-phosphate and vice versa, depending on the biochemical demand. Reactions of the non-oxidative PPP occur virtually ubiquitously and maintain a central metabolic role in providing the RNA backbone precursors ribose 5-phosphate and erythrose 4-phosphate as precursors for aromatic amino acids.
Each glucose that goes through the PPP can generate two NADPH molecules and one ribose-5-phosphate molecule. For every six molecules of glucose-6-phosphate that enter the pathway, five molecules of glucose-6-phosphate + 6CO2 are produced. Thus, there is a net loss of one carbon from each glucose-6-phosphate that enters the cycle.
The two branches of the PPP can function together in three different modes based on cellular requirements: pentose insufficiency, pentose overflow, and pentose cycling. In pentose-insufficiency mode, the non-oxPPP pathway produces ribose 5-phosphate, as it is insufficiently supplied by the oxPPP pathway. In pentose-overflow mode, the non-oxPPP pathway consumes excess ribose 5-phosphate produced by the oxPPP pathway and feeds it back into glycolysis. In pentose-cycling mode, glycolytic intermediates made from excess ribose 5-phosphate are redirected up the glycolysis pathway to regenerate glucose 6-phosphate.
NADPH is a molecule with several important roles in the cell, and the pentose phosphate pathway is its primary source. NADPH provides the reducing power required in biosynthetic reactions. NADPH is involved in redox homoeostasis as well as in promoting biosynthetic processes, such as the synthesis of tetrahydrofolate, deoxyribonucleotides, proline, fatty acids, and cholesterol.
In the synthesis of fatty acids and cholesterol, NADPH donates the electrons needed to build these molecules. Fatty acids are structural components of cell membranes, while cholesterol is a precursor for steroid hormones such as estrogen and testosterone. These biosynthetic processes depend on a continuous supply of NADPH.
In the liver, NADPH supports detoxification. The liver processes a wide range of toxins, and NADPH is required to power the cytochrome P450 enzyme system, which breaks down drugs, alcohol, and other harmful compounds.
PPP-derived NADPH also supports purposeful cellular generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) for signalling and pathogen killing.
The pentose phosphate pathway is a glucose-oxidizing pathway that runs in parallel to upper glycolysis to produce ribose 5-phosphate and NADPH. Ribose 5-phosphate is used for nucleotide synthesis. It is the direct biosynthetic precursor to all purine and pyrimidine nucleotides incorporated into DNA and RNA, making it indispensable for cell division and repair.
The pentose phosphate pathway, a form of glucose oxidative decomposition, is the main pathway for the synthesis of phosphoribose and reduced nicotinamide adenine dinucleotide phosphate, which plays an important antioxidant role in the body. Abnormal pentose phosphate metabolism may indicate a disorder in the normal antioxidant stress function. Deoxyribose 5-phosphate and D-ribose jointly synthesize D-ribose 5-phosphate, which plays an important role in energy production.
Glucose-6-phosphate dehydrogenase (G6PD) is the key enzyme that catalyzes the first reaction — the oxidation of glucose-6-phosphate to 6-phosphogluconolactone — in the pentose phosphate pathway, thereby providing reducing energy to all cells by maintaining the level of the reduced coenzyme nicotinamide adenine dinucleotide phosphate (NADPH).
The non-oxidative part of the PPP is controlled by transketolase enzyme reactions. The non-oxidative part of the PPP is controlled by thiamine- (vitamin B1) dependent transketolase enzyme reactions. This thiamine dependency is of particular importance for the pharmacological strategy of using benfotiamine (a fat-soluble thiamine analogue) to activate transketolase and thus boost PPP flux in conditions of diabetic hyperglycemia.
Fructose-6-phosphate and glyceraldehyde-3-phosphate are also generated in this pathway, connecting the PPP directly to glycolysis. The pentose phosphate pathway produces metabolites and cofactors necessary for the synthesis of GSH, lipids and ATP, for the maintenance of the glycolytic flux, for detoxification processes, and for DNA duplication.
Within red blood cells, NADPH plays an indirect but essential role in protecting cellular structures from injury caused by oxygen-free radicals. Acting as a substrate for glutathione reductase, NADPH supports the production of reduced glutathione, which converts hydrogen peroxide into water. This reaction shields cellular components, particularly the red cell membrane, from oxidative damage.
Increased flux through the PPP could result in an increased supply of NADPH to the GSH redox cycle, thereby increasing the antioxidant capacity of the cell.
The PPP is important to maintain carbon homoeostasis, to provide precursors for nucleotide and amino acid biosynthesis, to provide reducing molecules for anabolism, and to defeat oxidative stress.
Under hyperglycemia, it is thought that the PPP could be protective by diverting these excess glycolytic metabolites away from the hexosamine, AGE, and PKC pathways and towards the production of less damaging endpoints.
The PPP can be transiently activated in response to oxidative stress or during the phagocyte oxidative burst to meet urgent NADPH demands.
The PPP is most active in tissues with high biosynthetic or antioxidant demands. Red blood cells do not contain a nucleus, so the pentose phosphate pathway is their only source of NADPH. Therefore, RBCs are more susceptible to oxidative damage than any other cells.
Brains and immune cells display high activity of glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme of the PPP. G6PD displays high activity in immune cells and participates in the modulation of oxidative stress and inflammatory reactions.
New compelling evidence indicates that the pentose phosphate pathway profoundly regulates lineage development in the immune system, influenced by genetic and environmental factors during metabolic stress underlying the development of autoimmunity. The PPP provides two unique metabolites: ribose 5-phosphate for nucleotide biosynthesis in support of cell proliferation and NADPH for protection against oxidative stress.
This is the area of PPP science with the strongest, most established clinical evidence, based on decades of human and population-level research.
Glucose-6-phosphate dehydrogenase deficiency, the most common enzyme deficiency worldwide, causes a spectrum of disease including neonatal hyperbilirubinemia, acute hemolysis, and chronic hemolysis. Persons with this condition may also be asymptomatic. This X-linked inherited disorder most commonly affects persons of African, Asian, Mediterranean, or Middle-Eastern descent. Approximately 400 million people are affected worldwide.
Because erythrocytes lack nuclei and repair mechanisms, a deficiency in G6PD can lead to acute hemolytic anemia during oxidative challenges such as infections, ingestion of fava beans, or exposure to certain drugs, including antimalarials and sulfonamides.
Prevalence of the deficiency is correlated with the geographic distribution of malaria, which has led to the theory that carriers of G6PD deficiency may incur partial protection against malarial infection.
In persons with G6PD deficiency, oxidative stresses can denature hemoglobin and cause intravascular hemolysis. Denatured hemoglobin can be visualized as Heinz bodies in peripheral blood smears processed with supravital staining.
Evidence strength: Very strong; based on large-scale epidemiological data, mechanistic clinical studies, and decades of observational evidence. The relationship between PPP dysfunction (G6PD deficiency) and hemolytic disease is one of the best-characterized genotype–phenotype associations in human medicine.
One of the most clinically investigated strategies for pharmacologically boosting PPP activity involves benfotiamine, a synthetic fat-soluble derivative of thiamine (vitamin B1) that activates transketolase, the rate-controlling enzyme of the non-oxidative branch.
Shunting of glycolytic intermediates into the pentose phosphate pathway via transketolase activation by benfotiamine has been suggested to protect from hyperglycemia-induced microvascular damage, but the long-term effects of benfotiamine on diabetic sensorimotor polyneuropathy (DSPN) remain unclear.
In patients with diabetes and DSPN, the efficacy and safety of benfotiamine have been investigated in four randomized, double-blind clinical trials. Their duration was 3–12 weeks. Various daily dosages and endpoints were studied. The Benfotiamine in Diabetic Polyneuropathy (BENDIP) trial showed a significant improvement of neuropathic symptoms at 6 weeks with a dosing scheme of 300 mg twice daily, but not 300 mg/day. The Benfotiamine in Diabetic Polyneuropathy (BEDIP) trial reported an improvement in neuropathic symptoms and signs at 3 weeks with the dose of 100 mg four times per day.
Randomized, placebo-controlled trials demonstrated that benfotiamine supplementation — at doses ranging from 400 to 1,050 mg/day — significantly improved neuropathic symptoms, endothelial function, and markers of oxidative stress.
The long-term BOND trial (a phase II randomized, double-blind, placebo-controlled trial) examined benfotiamine at 300 mg twice daily over 12 months in 57 participants with type 2 diabetes and mild-to-moderate DSPN. The changes from baseline to 12 months in corneal nerve fiber length (CNFL) did not differ between the two groups. The corresponding changes in the secondary morphometric, functional, and clinical neuropathic outcomes as well as quality of life were also similar in the two groups. Only the Neuropathy Symptom Score tended to improve after benfotiamine treatment (p=0.098 vs placebo). Benfotiamine treatment increased the concentrations of all six thiamine analytes studied. Safety analysis showed no relevant differences between the groups in the rates of adverse events.
Evidence strength: Moderate but mixed. Short-term trials (3–6 weeks) have shown symptomatic improvements in diabetic neuropathy, but the longer-term BOND trial did not meet its primary neurological endpoint. Benfotiamine is not included in standard pharmacotherapy guidelines for diabetic neuropathy. Evidence for a PPP-mediated protective mechanism is mechanistically well-grounded but has not been conclusively validated in long-term human trials.
The pentose phosphate pathway is a major glucose metabolic shunt that is upregulated in cancer cells. The PPP comprises an oxidative and a nonoxidative phase and is essential for nucleotide synthesis of rapidly dividing cells. The PPP also generates nicotinamide adenine dinucleotide phosphate, which is required for reductive metabolism and to counteract oxidative stress in tumor cells.
To meet biosynthetic demands, cancer cells are metabolically reprogrammed to direct glucose flux into the PPP. Indeed, growing evidence suggests that, similar to glycolysis, higher PPP flux is present in many human cancers. Further studies have also revealed that alterations of the PPP significantly contribute to tumor growth and survival under certain stress conditions.
During oxidative stress, cancer cells will shut down the glycolytic pathway and thus increase glucose flux through the PPP to produce more NADPH for antioxidant defense.
Researchers have detected upregulation of a mutated transketolase transcript (TKTL1) in human malignancies, whereas transketolase (TKT) and transketolase-like-2 (TKTL2) transcripts were not upregulated. Strong TKTL1 protein expression was correlated to invasive colon and urothelial tumors and to poor patient outcome.
Reduction of PPP activity decreases cancer cells' proliferation, with a profound effect in Warburg-phenotype cancer cells. The crucial role of the PPP in sustaining cancer cells' proliferation was confirmed using siRNAs against glucose-6-phosphate dehydrogenase, the first and rate-limiting enzyme of the PPP.
Exploiting the metabolic vulnerability of the PPP offers potential novel therapeutic opportunities and improves patients' response to cancer therapy.
Evidence strength: Predominantly preclinical (cell lines and animal models) with strong mechanistic rationale. Clinical translation of PPP inhibition as an anticancer strategy remains at an early stage. No approved therapeutic agent specifically targeting PPP enzymes was available based on current evidence. This is an active area of investigation, not an established clinical application.
Metabolic dysfunction and neuroinflammation are increasingly implicated in Parkinson's disease (PD). The pentose phosphate pathway converts glucose-6-phosphate into pentoses and generates ribose-5-phosphate and NADPH thereby governing anabolic biosynthesis and redox homeostasis.
A postmortem study reveals dysregulation of G6PD enzyme in brains of PD patients. A postmortem study of PD brains has detected an increase in NADPH production in the putamen (a brain region affected in PD) of late-stage cases; unexpectedly, the putamen of early-stage PD and the cerebellum of early- and late-stage PD display a reduction in G6PD.
Research findings indicated that G6PD-mediated PPP dysfunction and neuroinflammation exacerbated each other, mediating chronic dopaminergic neurodegeneration and locomotor impairment. Insight into the metabolic–inflammatory interface suggests that G6PD and NOX2 are potential therapeutic targets for PD.
Evidence strength: Primarily preclinical (animal models and postmortem human tissue). No randomized human clinical trials targeting the PPP for Parkinson's disease have been reported in the literature reviewed. Evidence is preliminary and mechanistic.
Expanding evidence implicates CD8+ antineural T cells in the neurodegeneration that underlies irreversible clinical progression in multiple sclerosis. CD8+ T cells from patients with MS exhibit increased engagement of the pentose phosphate pathway.
New compelling evidence indicates that the pentose phosphate pathway profoundly regulates lineage development in the immune system, influenced by genetic and environmental factors during metabolic stress underlying the development of autoimmunity.
Pharmacological inhibition of the PPP may be a powerful direct approach or adjunct therapy in patients with rapidly progressive immune-mediated neurodegenerative diseases mediated by autoimmune CD8+ T cells.
Evidence strength: Early/exploratory. Evidence derives from human immune cell profiling and animal model studies. Clinical trials specifically targeting the PPP in MS are described as not yet having been evaluated. Evidence is mechanistic and hypothesis-generating.
Diabetic hearts showed marked reduction in CPC abundance and proliferation when compared with controls. Sca-1pos CPCs isolated from hearts of diabetic mice displayed reduced activity of key enzymes of the pentose phosphate pathway, glucose-6-phosphate dehydrogenase (G6PD), and transketolase, increased levels of superoxide and advanced glucose end-products (AGE), and inhibition of the Akt/Pim-1/Bcl-2 signalling pathway.
In vivo and in vitro supplementation with benfotiamine reactivates the pentose phosphate pathway and rescues CPC availability and function.
Evidence strength: Preclinical (mouse models and human cell cultures). Findings are mechanistically important but have not been reproduced in human clinical cardiac trials.
D-ribose stimulates NADPH production via the pentose phosphate pathway (PPP), which supplies reducing power in the body.
A randomized, triple-blind, placebo-controlled, crossover pilot clinical trial assessed a combination of nicotinamide with D-ribose (RiaGev) for NAD+ metabolome enhancement. This study investigated the efficacy and safety of RiaGev via evaluation of the NAD+ metabolome and diverse health-related parameters in healthy adults (age 35–65 years), a group selected because many health problems that directly affect healthy aging occur during this period. Supplementing with 1,520 mg RiaGev twice daily for 7 days significantly increased the NAD+ metabolome in blood, especially NADP+ by 27% compared to the placebo group (p = 0.033) and over the baseline (p = 0.007). Increases in glutathione and high energy phosphates were also observed in the blood.
Evidence strength: Very preliminary. This is a small pilot crossover study of short duration (7 days). Results are biologically plausible but cannot be considered evidence of clinical benefit.
Research in Drosophila reported that a strain with reduced expression of ribose-5-phosphate isomerase (rpi) expresses higher levels of NADPH and exhibits increased resistance to oxidative stress and enhanced lifespan.
This manipulation was also found to ameliorate the effects of genetic manipulations aimed at creating a model for studying Huntington's disease by overexpression of polyglutamine in the eye, suggesting that modulating rpi levels could serve as a treatment for normal aging as well as for polyglutamine neurotoxicity.
Evidence strength: Preclinical only (invertebrate model). No human clinical data exist on directly manipulating PPP enzyme activity for longevity or aging outcomes.
A model has been proposed in which the proline-linked pentose-phosphate pathway is suggested to be critical for modulating protective antioxidant response pathways in diverse biological systems, including biochemical and cellular pathways important for human health. The proposed proline-linked pentose-phosphate pathway model provides a mechanism for understanding the mode of action of phenolic phytochemicals in modulating antioxidant pathways and provides avenues by which dietary approaches may manage oxidation-linked chronic and infectious diseases.
Evidence strength: Theoretical/mechanistic model. Not confirmed in human clinical trials. Evidence is largely in vitro and from model systems.
Because the PPP itself is not directly supplemented, clinical dosing in the relevant literature focuses on agents known to modulate the pathway. The following dosages are reported from cited sources and are presented for informational purposes only, exactly as stated in the scientific literature reviewed.
The most clinically significant safety consideration associated with the PPP concerns individuals with inherited G6PD deficiency, who cannot maintain adequate PPP flux for antioxidant defense. A deficiency in G6PD can lead to acute hemolytic anemia during oxidative challenges such as infections, ingestion of fava beans, or exposure to certain drugs, including antimalarials and sulfonamides.
The degree of G6PD deficiency determines the clinical expression of the disorder. Individuals with minimally reduced enzyme levels do not experience hemolysis. Others with a greater degree of deficiency have episodes of brisk hemolysis triggered by infections, taking drugs that increase oxidative stress, ingesting fava beans, or ketoacidosis.
In the BOND trial, benfotiamine treatment increased the concentrations of all six thiamine analytes studied (p≤0.003 vs placebo). Safety analysis showed no relevant differences between the groups in the rates of adverse events.
In patients with reduced RPI (ribose-5-phosphate isomerase) activity, the diminished production of NADPH may impair the liver's ability to detoxify drugs, leading to increased toxicity or adverse drug reactions. Conversely, patients with high RPI activity may metabolize drugs more rapidly, potentially reducing their therapeutic effectiveness.
Through NADPH, the PPP plays a critical role in suppressing oxidative stress, including in certain cancers, in which PPP inhibition may be therapeutically useful. This dual nature — where boosting PPP activity is beneficial in normal antioxidant physiology but potentially counterproductive in cancer contexts — underscores the importance of disease-context specificity when considering PPP modulation.
In high glucose conditions, glucose-6-phosphate dehydrogenase (G6PD) has been shown to be inhibited. Increased expression of G6PD in hepatocytes generates more NADPH for de novo lipogenesis (DNL), which promotes hepatic steatosis and insulin resistance. This suggests complex, tissue-dependent effects of PPP modulation in metabolic disease, where enhanced G6PD activity in the liver may be detrimental while reduced PPP in erythrocytes or neurons is harmful.
The PPP occupies a foundational role in cellular biochemistry, and its dysregulation is implicated across a wide range of human diseases. These functions require dynamic regulation of the PPP pathway that is achieved through hierarchical interactions between transcriptome, proteome, and metabolome. Consequently, the biochemistry and regulation of this pathway, while still unresolved in many cases, are archetypal for the dynamics of the metabolic network of the cell.
From a supplement and clinical intervention perspective, the evidence can be summarized as follows:
Health conditions that Pentose phosphate may help support.
Body systems that Pentose phosphate may help support.
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