Decreased Blood Glucose and Lactate: Is a Useful Indicator of Recovery Ability in Athletes?

Abstract

During low-intensity exercise stages of the lactate threshold test, blood lactate concentrations gradually diminish due to the predominant utilization of total fat oxidation. However, it is unclear why blood glucose is also reduced in well-trained athletes who also exhibit decreased lactate concentrations. This review focuses on decreased glucose and lactate concentrations at low-exercise intensity performed in well-trained athletes. During low-intensity exercise, the accrued resting lactate may predominantly be transported via blood from the muscle cell to the liver/kidney. Accordingly, there is increased hepatic blood flow with relatively more hepatic glucose output than skeletal muscle glucose output. Hepatic lactate uptake and lactate output of skeletal muscle during recovery time remained similar which may support a predominant Cori cycle (re-synthesis). However, this pathway may be insufficient to produce the necessary glucose level because of the low concentration of lactate and the large energy source from fat. Furthermore, fatty acid oxidation activates key enzymes and hormonal responses of gluconeogenesis while glycolysis-related enzymes such as pyruvate dehydrogenase are allosterically inhibited. Decreased blood lactate and glucose in low-intensity exercise stages may be an indicator of recovery ability in well-trained athletes. Athletes of intermittent sports may need this recovery ability to successfully perform during competition.

Effects of Individualized Low-Intensity Exercise and Its Duration on Recovery Ability in Adults

“…Sports scientists and clinical physicians have used lactate threshold (LT) tests for over fifty years due to their application as a useful metric for recommendations on individualized exercise intensity in elite athletes and cardiac patients [ 3 , 4 ]. A rightward shift of the exponential lactate curve is generally interpreted as improved endurance performance [ 5 , 6 , 7 , 8 , 9 ]. Furthermore, the blood lactate level during exercise is a more sensitive biomarker of endurance performance than maximal oxygen uptake (VO 2max ) [ 10 ].…”

“…Validated LT concepts such as anaerobic, aerobic–anaerobic threshold, maximal lactate steady state (MLSS), and the onset of blood lactate accumulation (OBLA; 4 mmol·L −1 ) were developed and refined by several groups [ 5 , 12 , 13 , 14 , 15 , 16 ]. However, some of these concepts have been controversial [ 9 , 14 ]. Despite several generally used definitions of the aerobic–anaerobic transition, this biochemical phenomenon does not actually exist because the glycolytic pathway is accelerated to supply adenosine triphosphate (ATP) for active skeletal muscles during increments in intense exercise [ 9 , 14 , 17 ].…”

“…However, some of these concepts have been controversial [ 9 , 14 ]. Despite several generally used definitions of the aerobic–anaerobic transition, this biochemical phenomenon does not actually exist because the glycolytic pathway is accelerated to supply adenosine triphosphate (ATP) for active skeletal muscles during increments in intense exercise [ 9 , 14 , 17 ].…”

“…Nowadays, the blood lactate level of the LT test is used to differentiate exercise intensity areas, which include low-intensity (<2 mmol·L −1 ; zone 1; LIE), moderate-intensity (2–4 mmol·L −1 ; zone 2; MIE), and high-intensity exercise (>4 mmol·L −1 ; zone 3; HIE) [ 9 , 15 , 18 , 19 , 20 ]. HIE induces dramatic effects on maximal aerobic performance, which is achievable through relatively short exercise times (volume).…”

“…However, HIE may be unsuitable for exercise beginners and/or the general population with relatively poor endurance performance because it causes faster depletion of carbohydrates, stress on skeletal muscle, high risk of muscle injury, and dysbalance of hormones [ 21 , 22 , 23 ]. During MIE, carbohydrates and fat are used as the main energy sources and represent the predominant energy source under aerobic conditions [ 9 , 24 , 25 ]. Therefore, high exercise volumes at MIE are difficult for exercise beginners and the general population [ 9 , 24 , 25 ].…”