This study investigates the association between the triglyceride-glucose (TyG) index and impaired cardiovascular fitness (CVF) in a non-diabetic young population. Using data from the National Health and Nutrition Examination Survey (NHANES) 1999–2004, the study analyzed 3364 participants who completed a cardiovascular fitness examination. Impaired CVF was defined as low or moderate levels of maximal oxygen consumption (Vo2max), determined by sex- and age-specific criteria. The TyG index was calculated using the formula: Ln(TG(mg/dL) × FPG(mg/dL)/2). The study found a significant association between the TyG index and impaired CVF, with a per 1-unit increase in the TyG index associated with an odds ratio (OR) of 1.46 (95% CI 1.13–1.90). A dose-response relationship was observed, and a significant interaction between sex and the TyG index was found in the population aged <20 years. Males were more susceptible to impaired CVF under comparable TyG index conditions. The study also found that higher TyG index values were associated with lower cardiovascular fitness in non-diabetic young individuals. The TyG index was significantly correlated with impaired CVF in three weighted multivariable logistic models. The association was strongest in the highest quartile of the TyG index. Subgroup analysis revealed that the association between the TyG index and impaired CVF was significant in males and in individuals without metabolic syndrome. The study also found that the TyG index was associated with impaired CVF in individuals aged 20 years and older, but not in those with metabolic syndrome. The study concluded that individuals with higher TyG index values are more likely to experience impaired CVF in a non-diabetic young population. Additionally, sex may influence CVF, with males being more susceptible to impaired CVF under similar TyG index conditions. The study highlights the importance of monitoring the TyG index in young non-diabetic individuals to improve cardiovascular fitness and reduce the risk of cardiovascular disease.This study investigates the association between the triglyceride-glucose (TyG) index and impaired cardiovascular fitness (CVF) in a non-diabetic young population. Using data from the National Health and Nutrition Examination Survey (NHANES) 1999–2004, the study analyzed 3364 participants who completed a cardiovascular fitness examination. Impaired CVF was defined as low or moderate levels of maximal oxygen consumption (Vo2max), determined by sex- and age-specific criteria. The TyG index was calculated using the formula: Ln(TG(mg/dL) × FPG(mg/dL)/2). The study found a significant association between the TyG index and impaired CVF, with a per 1-unit increase in the TyG index associated with an odds ratio (OR) of 1.46 (95% CI 1.13–1.90). A dose-response relationship was observed, and a significant interaction between sex and the TyG index was found in the population aged <20 years. Males were more susceptible to impaired CVF under comparable TyG index conditions. The study also found that higher TyG index values were associated with lower cardiovascular fitness in non-diabetic young individuals. The TyG index was significantly correlated with impaired CVF in three weighted multivariable logistic models. The association was strongest in the highest quartile of the TyG index. Subgroup analysis revealed that the association between the TyG index and impaired CVF was significant in males and in individuals without metabolic syndrome. The study also found that the TyG index was associated with impaired CVF in individuals aged 20 years and older, but not in those with metabolic syndrome. The study concluded that individuals with higher TyG index values are more likely to experience impaired CVF in a non-diabetic young population. Additionally, sex may influence CVF, with males being more susceptible to impaired CVF under similar TyG index conditions. The study highlights the importance of monitoring the TyG index in young non-diabetic individuals to improve cardiovascular fitness and reduce the risk of cardiovascular disease.