Hyperthermia accelerates dehydration and can lead to a glycolysis malfunction. Therefore, to deeply understand the relationship between dehydration and hyperthermia during exercise, as well as in the recovery time, there might be important factors to improve athletic performance. A systematic review was carried out in different databases using the words “hydration” OR “dehydration” AND “glycogen” OR “glycogenesis” OR “glycogenolysis” AND “muscle” OR “muscle metabolism” OR “cardiovascular system” and adding them to the “topic section” in Web of Science, to “Title/Abstract” in PubMed and to “Abstract” in SPORTDiscus. A total of 18 studies were included in the review and 13 in the meta-analysis. The free statistical software Jamovi was used to run the meta-analysis (version 1.6.15). A total sample of 158 people was included in the qualitative analysis, with a mean age of 23.5 years. Ten studies compared muscle glycogen content after hydration vs. remaining dehydrated (SMD −4.77 to 3.71, positive 80% of estimates, \hat{\mu} = 0.79 (95% CI: −0.54 to 2.12), z = 1.17, p = 0.24, Q-test (Q(9) = 66.38, p < 0.0001, tau2 = 4.14, I2 = 91.88%). Four studies examined the effect of temperature on postexercise muscle glycogen content (SMD −3.14 to −0.63, 100% of estimates being negative, \hat{\mu} = −1.52 (95% CI: −2.52 to −0.53), (z = −3.00, p = 0.003, Q-test (Q(3) = 8.40, p = 0.038, tau2 = 0.68, I2 = 66.81%). In conclusion, both hyperthermia and dehydration may contribute to elevated glycogenolysis during exercise and poor glycogen resynthesis during recovery. Although core and muscle hyperthermia are the key factors in glycogen impairments, they are also directly related to dehydration.
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