The main purpose of the study was to analyze the associations between health-related physical fitness and fasting blood glucose in war veterans. We found significant inversed associations between fasting blood glucose and all physical fitness components, except for positive association with fat mass. If all physical fitness components were used simultaneously, fasting blood glucose accounted for 60% of the variance.
We the regression model, we found that fat mass was positively associated with fasting blood glucose, yet negative association between fat-free mass and fasting blood glucose was observed. Such results have been confirmed previously [41–44]. Specifically, a study by Kim & Park  showed that compared to high muscle and low-fat group, those participants categorized as having low muscle and high fat and high muscle and high fat were 1.90 and 2.30 more likely to develop metabolic syndrome. However, a follow-up study by Kim et al.  highlighted that only changes in total body fat mass were associated with development of Type 2 diabetes, while no significant associations between the changes in fat-free mass and Type 2 diabetes were observed. Finally, a large cohort study examining the associations between fat-free mass and insulin resistance concluded that fat-free mass was an independent predictor of insulin resistance .
Our findings of the associations between fasting blood glucose and cardiorespiratory fitness are in line with previous studies [24–28]. For example, a study by Loprinzi and Pariser  showed that cardiorespiratory fitness assessed through a submaximal treadmill-based test was significantly associated with impaired blood glucose control in in obese and inactive adults. Another cross-sectional study presented similar results, where a low maximal oxygen uptake was moderately associated with high fasting blood glucose (r=-0.34) . Similar associations have been obtained in previous longitudinal studies . During an average follow-up of 6 years, men in the low-fitness group (the least fit 20% of the cohort) at baseline were almost two times more likely to have impaired blood glucose, compared to with those in the high-fitness group (the most fit 40% of the cohort) . On the other hand, studies have also shown that muscular fitness may have beneficial effects on glycemic control . In a recent meta-analysis, groups who performed a muscular training lowered glycosylated hemoglobin (mean ES=-0.37, 95% CI -0.55 to -0.20, p < 0.01), compared to those who did not receive a treatment. Moreover, the same study showed that high-intensity muscular training groups had a slight tendency to improve glycemic control, irrespective of duration, frequency, and weekly volume .
The mechanism underlying the aforementioned associations in our context may be explained by acute and regular exercise training . For example, exercise may acutely increase muscle glucose transport , which can lead to reverse impaired insulin sensitivity and a reduced endogenous glucose production . Moreover, higher physical performance is associated with induced adaptations of in pancreatic β-cells, leading to a reduction in glucose-induced insulin secretion .
This study has a few strengths. First, the findings of the study were based on a large representative sample of war veterans aged 45–75 years. Second, we covered all aspects of physical fitness, including cardiorespiratory, muscular and flexibility fitness and body composition.
However, this study is not without limitations. By using a cross-sectional design, we cannot establish the causality of the association, where higher fasting blood glucose led to lower physical fitness values. Next, cardiorespiratory fitness was assessed by the 2-minute step test, which was only moderately correlated with the treadmill protocol and might underestimate the level of ‘real’ cardiorespiratory fitness. Finally, we did not adjust for genetical and environmental factors, which might be associated to both physical fitness and fasting blood glucose. Therefore, future research among war veterans should focus in exploring longitudinal associations between physical fitness and fasting blood glucose with additional adjustments, to establish bidirectional associations between physical fitness and fasting blood glucose.
In conclusion, this study confirms the findings of previous studies, where fat-free mass, cardiorespiratory and muscular fitness are negatively associated, and fat mass is positively associated with fasting blood glucose. Of all physical fitness components in the regression model, cardiorespiratory fitness exhibits the most protective effects against having high fasting blood glucose, followed by muscular dynamic endurance of upper extremities and fat-free mass. Therefore, randomized controlled trials consisted of both aerobic and resistance training protocols should be implemented within the rehabilitation centers.