Type 2 diabetes mellitus (T2DM) is a chronic metabolic disease characterized by hyperglycemia resulting from insulin resistance and relative insulin deficiency.(1,2) Globally, T2DM prevalence is rising, affecting younger adults and increasing the burden of cardiovascular disease, particularly in developing countries like India(3). While the long-term complications of diabetes are well recognized, growing evidence suggests that cardiovascular and metabolic dysfunction begins early in the course of the disease(4,5).

Peak oxygen uptake (Peak VO₂) reflects the integrated response of the cardiovascular, pulmonary, and muscular systems during exercise(6,7) and is considered the gold standard measure of aerobic fitness(8). Reduced Peak VO₂ is associated with increased cardiovascular risk and mortality.(9)

Exercise testing using bicycle ergometry is a safe and reproducible method to evaluate exercise capacity(10). Previous studies predominantly focused on long-standing diabetes or patients with complications. There is limited data evaluating VO₂ peak in early-stage diabetes (<5 years duration). Identifying reduced functional capacity early could guide preventive strategies, including tailored aerobic exercise programs and lifestyle interventions.

This study aimed to assess VO₂ peak during bicycle ergometry in early diabetics and compare it with healthy controls to determine early impairment in aerobic capacity. And also aimed to assess the correlation between peak VO2 and duration of diabetes.

MATERIALS AND METHODS

This cross-sectional study was conducted in the Department of Physiology, G. R. Medical College, Gwalior. A total of 128 subjects were enrolled, including 64 patients with type 2 diabetes mellitus of less than five years’ duration and 64 age- and sex-matched healthy controls. Diabetic subjects were recruited from the Medicine Outpatient Department of Jaya Arogya Hospital, Gwalior.

The study group comprised patients aged 30–60 years with type 2 diabetes mellitus and no history of cardiovascular disease. The control group included healthy individuals without any chronic illness. Exclusion criteria were refusal to participate, body mass index (BMI) >30 kg/m², resting blood pressure >140/90 mmHg, abnormal electrocardiographic findings, history of cardiovascular, respiratory, hepatic, or renal disease, and presence of endocrine disorders other than diabetes mellitus.

A detailed medical history was obtained from all participants, followed by general physical examination and anthropometric measurements. Written informed consent was obtained prior to participation

Cardiopulmonary exercise testing (CPET) was performed using a GASSYS3 BIOPAC gas analyzer. After application of a tight-fitting face mask, resting oxygen consumption (VO₂), carbon dioxide production (VCO₂), respiratory exchange ratio (RER), and minute ventilation (VE) were recorded. Participants performed a 2-minute warm-up on a cycle ergometer, followed by rest until heart rate returned to baseline.

Submaximal, symptom-limited CPET was conducted using a constant work-rate protocol as described by Mezzani et al(11). Exercise was continued until approximately 75% of age-predicted maximum heart rate was achieved or till volitional fatigue.

Heart rate and blood pressure were recorded at rest and immediately after exercise. Peak VO₂, peak VCO₂, peak RER, and VE were documented.

OBSERVATION TABLES

Table 1: Anthropometric Parameters of Diabetic and Healthy Group

*=p<0.05 = Statistically Significant**=p<0.01 = Statistically Highly Significant

***=p<0.001 = Statistically Very Highly Significant

Table 2: Comparison of investigations between Diabetic Group and Healthy Group

Table 3: Comparison of haemodynamic parameters of Diabetic Group & Healthy Group at rest

Table 4: Comparison of haemodynamic parameters of Diabetic and Healthy group at end of exercise

5: Comparison of Cardiopulmonary Exercise Test Parameters between Diabetics and Healthy groups at rest

Table 6: Comparison of Cardiopulmonary Exercise Test Parameters between Diabetics and Healthy groups at end of exercise

Table7: Correlation of Cardiopulmonary exercise test with Duration of Diabetes Mellitus

*- signifies significant difference having p-value<0.05.

Statistical Analysis

Data were expressed as mean ± standard deviation. Student’s unpaired t-test was used for comparison between groups. A p-value <0.05 was considered statistically significant.

RESULTS

Anthropometric measurements and hemodynamic parameters at rest were comparable between the two groups (p>0.05). Even early(less than 5 years) diabetic subjects demonstrated significantly lower Peak VO₂ values, along with reduced exercise duration and early onset of fatigue.

DISCUSSION

This study evaluated cardiovascular fitness in normotensive patients with early type 2 diabetes mellitus using cardiopulmonary exercise testing (CPET) and compared the findings with healthy controls. Anthropometric parameters were comparable between the two groups, thereby minimizing the potential confounding effects of body composition on exercise capacity. As expected, fasting and random blood glucose levels were significantly higher in diabetic subjects, reflecting suboptimal glycemic control, while lipid profiles did not differ significantly between groups. Similar observations have been reported by DeFronzo and Ferrannini et al(12), who highlighted that metabolic abnormalities often precede overt cardiovascular manifestations in early diabetes.

Resting systolic and diastolic blood pressure, mean arterial pressure, and heart rate were comparable between diabetic and control subjects, confirming preserved resting hemodynamic status and normotension. Post-exercise increases in hemodynamic parameters were observed in both groups without significant intergroup differences. These findings are consistent with earlier studies by Regensteiner et al.  (13)and Fang et al.(14), who demonstrated preserved resting cardiovascular function in patients with early type 2 diabetes prior to the onset of overt cardiovascular disease.

Resting oxygen consumption (VO₂) did not differ significantly between groups, suggesting similar basal metabolic requirements. However, peak oxygen consumption (VO₂ peak) was significantly lower in diabetic subjects, indicating impaired cardiorespiratory fitness. VO₂ peak is a well-established indicator of aerobic capacity and cardiovascular health, and its reduction reflects early limitations in oxygen delivery and utilization. Similar reductions in VO₂ peak among patients with type 2 diabetes have been consistently reported by Regensteiner et al. (1) and Baldi and Snowling et al(15).

Several mechanisms may underlie the observed reduction in VO₂ peak. Chronic hyperglycemia contributes to endothelial dysfunction, impaired nitric oxide–mediated vasodilation, and microvascular remodeling, resulting in reduced skeletal muscle perfusion during exercise (Hamburg et al.,(16); Ceriello et al(17). In addition, mitochondrial dysfunction, decreased oxidative enzyme activity, and reduced skeletal muscle oxidative capacity have been demonstrated in patients with type 2 diabetes Kelley et al.,(18); Phielix and Mensink et al, (19). The accumulation of advanced glycation end products further compromises vascular compliance and oxygen diffusion Brownlee et al, (20).

The present findings align with previous investigations suggesting that diminished aerobic capacity may represent an early marker of subclinical cardiovascular dysfunction in type 2 diabetes. Studies by Poole et al.(21) and Ratchford et al. (22)have shown that impaired cardiac output, reduced peripheral oxygen extraction, and a diminished arteriovenous oxygen difference contribute to reduced exercise tolerance in diabetic individuals.

Peak respiratory exchange ratio (RER) was significantly higher in diabetic subjects, indicating an increased reliance on anaerobic metabolism during maximal exercise. This finding suggests reduced metabolic flexibility and an early shift toward glycolytic pathways. Impaired mitochondrial oxidative phosphorylation, altered substrate utilization, and suboptimal blood flow distribution to exercising muscles may account for this response Mogensen et al.,(23); Schrauwen and Hesselink et al, (24).

A significant negative correlation between diabetes duration and VO₂ peak was observed, indicating a progressive decline in cardiovascular fitness with increasing disease duration. Prolonged exposure to hyperglycemia promotes cumulative oxidative stress, endothelial injury, inflammation, and microvascular damage, which collectively impair cardiovascular and metabolic function (Laakso et al, (25); Stratton et al., (26)). These findings support earlier evidence linking longer duration of diabetes with increased cardiovascular risk and reduced functional capacity.

CONCLUSION

In conclusion, normotensive patients with early type 2 diabetes demonstrate significantly reduced aerobic capacity despite preserved resting hemodynamics and comparable anthropometric characteristics. CPET emerges as a sensitive tool for detecting early impairments in cardiovascular fitness in this population. Early identification of reduced VO₂ peak may facilitate timely lifestyle and therapeutic interventions aimed at improving glycemic control and cardiorespiratory fitness, thereby reducing future cardiovascular risk.

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