D-Ribose is a five-carbon monosaccharide that is the essential substrate for the de novo synthesis of the purine nucleotides (ATP, ADP, AMP) and of the pyrimidine nucleotides (CTP, UTP,TTP) — the building blocks of the DNA, the RNA, and the ATP. The D-ribose is synthesised in the pentose phosphate pathway (also called the pentose phosphate shunt) from the glucose-6-phosphate, and it is used for the nucleotide synthesis in all proliferating cells, including the bone marrow cells, the intestinal epithelial cells, and the immune cells. In the skeletal muscle and the heart, however, the D-ribose synthesis is relatively slow (because the pentose phosphate pathway is less active in the muscle than in other tissues), and the D-ribose pool turns over slowly. This means that after the high-intensity exercise, when the ATP is rapidly depleted and must be regenerated, the muscle is dependent on the dietary D-ribose or on the exogenous supplementation to restore the ATP pool quickly. Without adequate D-ribose, the post-exertional ATP recovery is slow, the muscle fatigue persists, and the exercise tolerance is reduced — this is the primary mechanism of the D-ribose deficiency and the basis for the use of D-ribose as an ergogenic aid for the high-intensity exercise and for the recovery from the strenuous exercise.
D-Ribose and the Purine Nucleotide Synthesis
The purine nucleotides (ATP, ADP, AMP) are the universal energy currency of the cell, and their synthesis is essential for all cellular functions — including the muscle contraction, the nerve impulse conduction, the active transport, the biosynthesis, and the cell division. The de novo synthesis of the purine nucleotides is a complex, multi-step pathway that starts with the D-ribose-5-phosphate (R5P) and ends with the formation of the inosine monophosphate (IMP), which is then converted to the ATP, the ADP, and the AMP. The rate-limiting step of the purine nucleotide synthesis is the formation of the phosphoribosyl pyrophosphate (PRPP) from the R5P, and this step is dependent on the availability of the D-ribose. When the D-ribose is limiting (as it is in the skeletal muscle after the high-intensity exercise, when the ATP has been rapidly depleted), the purine nucleotide synthesis is impaired, and the ATP pool cannot be rapidly restored — this is the primary mechanism of the post-exertional fatigue and of the reduced exercise tolerance that are associated with the D-ribose deficiency.
The clinical importance of D-ribose for the post-exertional ATP recovery is underscored by the observation that the D-ribose supplementation accelerates the ATP recovery and reduces the post-exertional fatigue in the skeletal muscle. A study in 26 healthy subjects found that D-ribose supplementation at 100mg/kg (approximately 7g for a 70kg adult) taken 30 minutes before the high-intensity exercise significantly reduced the post-exertional fatigue (as measured by the isokinetic dynamometer and by the visual analogue scale for fatigue) and improved the exercise performance (as measured by the number of repetitions performed at 80% of the one-repetition maximum) compared to placebo. These findings are consistent with the hypothesis that the D-ribose supplementation accelerates the ATP recovery and reduces the post-exertional fatigue by providing the essential substrate for the purine nucleotide synthesis.
D-Ribose and the Cardiac Energy Recovery
The heart is one of the organs that is most dependent on the continuous supply of the ATP from the mitochondrial oxidative phosphorylation, and it has a very limited capacity for the anaerobic metabolism (because the cardiac muscle has few mitochondria relative to its mass). After the ischaemia-reperfusion injury (as occurs in the acute myocardial infarction and in the cardiac surgery), the cardiac ATP pool is severely depleted, and the recovery of the ATP is slow because the pentose phosphate pathway is relatively inactive in the cardiac muscle. The D-ribose supplementation has been studied as an intervention to accelerate the cardiac ATP recovery after the ischaemia-reperfusion injury — with some evidence from animal studies suggesting that the D-ribose supplementation accelerates the cardiac ATP recovery and reduces the infarct size. However, the evidence in humans is limited, and more research is needed to establish the optimal dose and the therapeutic window for the D-ribose in the cardiac ischaemia-reperfusion setting.
Practical Application
For general D-ribose supplementation, the evidence-based approach is to supplement with 5-10g of D-ribose daily (as the D-ribose powder, which is the most common supplemental form), taken 30-60 minutes before the exercise or immediately after the exercise for the post-exercise recovery. The D-ribose is generally well-tolerated with no significant adverse effects at doses up to 10g daily, though very high doses may produce the gastrointestinal symptoms (nausea, bloating). The D-ribose is particularly useful for the people who engage in the high-intensity exercise (sprinters, weight lifters, HIIT practitioners) and for the people who want to accelerate the post-exercise recovery. For comprehensive energy and exercise support, D-ribose pairs well with the creatine (which provides the rapid ATP regeneration through the phosphocreatine system), with the CoQ10 (which is required for the electron transport chain and for the ATP synthesis), with the L-carnitine (which is required for the fatty acid transport into the mitochondria), and with the beta-alanine (which buffers the acid that is produced during the high-intensity exercise and which extends the time to exhaustion).
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