Effects of cardiorespiratory fitness on aging: glucose trajectory in a cohort of healthy men

Abstract

Purpose

We modeled the age-related trajectory of glucose and determined whether cardiorespiratory fitness altered the trajectory in a cohort of men from the Aerobics Center Longitudinal Study.

Methods

A total of 10,092 men free of diagnosed diabetes, cardiovascular disease, and cancer, ages 20 to 90 years, completed from 2 to 21 health examinations between 1977 and 2005. Cardiorespiratory fitness was measured by a maximal treadmill exercise test and normalized for age. The covariates included waist circumference, hypertension, elevated cholesterol, smoking behavior, and physical activity.

Results

Linear mixed models regression analysis showed that fasting glucose increased at a linear rate with aging. Glucose increased at a yearly rate of 0.17 mg/dL (95% confidence interval: 0.16–0.19). Fitness had little influence on the aging glucose trajectory below age 35, but significantly influenced the trend after age 35 (P for interaction < .001). The aging-related glucose increases in low-fitness men (0.25 mg/dL per year) was greater than average-fitness (0.15 mg/dL per year) and high-fitness (0.13 mg/dL per year) men.

Conclusions

The aging-related fasting glucose increases in low-fitness men was nearly double that of high-fitness men. Our results may suggest that it is possible to delay the age-related glucose impairment through increasing one's fitness level.

Introduction

With the U.S. population aging, chronic noncommunicable diseases such as heart disease and diabetes are becoming major public health problems. In 2010, an estimated 25.8 million Americans had type 2 diabetes, and another 79 million of them had blood glucose readings that placed them at risk for type 2 diabetes [1]. In studies, investigators have shown that advancing age is associated with impaired glucose processing [2], [3], [4], [5], [6], [7], [8]. Increased levels of fasting glucose are not only one of the most important predictors of subsequent diabetes [9], [10], [11] but also are associated with various cardiovascular disease outcomes [12], [13], [14], [15], [16]. Substantial evidence suggests that changes in diet and physical activity are among the major causes of age-related glucose impairment [6].

Previous authors have linked physical activity with the incidence of type 2 diabetes [17]. In a few studies, researchers have reported associations between cardiorespiratory fitness (hereafter as “fitness”), an objective measure of habitual physical activity, and type 2 diabetes [18], [19], [20]. The 15-year longitudinal results of the Coronary Artery Risk Development in Young Adults (CARDIA) study showed that a person's fitness measured with a maximal treadmill test was inversely associated with his or her developing type 2 diabetes and metabolic syndrome [18]. The investigators reported that risk of developing type 2 diabetes was double for low-fitness compared with high-fitness young adult men and women (i.e., those who have attained low/high levels of physical fitness). Improving one's fitness over 7 years significantly reduced the risk of developing type 2 diabetes and metabolic syndrome. One treatment goal for type 2 diabetes is to maintain an optimal glucose level [21]. Being physically active is viewed as an important lifestyle determinant in maintaining a healthy glucose level [22]. To date, longitudinal studies on effects of age-associated glucose changes with accurate measures of fitness are lacking. Defining the age-related glucose trajectory and identifying the factors that alter the trajectory is an important public health topic.

The Aerobics Center Longitudinal Study (ACLS) includes a large number of individuals who vary in age and have serial health examinations that include objectively measured fitness, percent body fat, and fasting plasma glucose (hereafter as “glucose”). Findings from our previous research document that elevated glucose is associated with both obesity and low fitness in men [20]. The influence of aging and fitness on the longitudinal change of fasting glucose is complex and not clearly defined because the level of fitness decreases with age [23], [24] and percent body fat increases with age [25]. The purposes of the current study were to 1) define the longitudinal, aging glucose trajectory; and 2) determine whether fitness altered the trajectory independent of percent body fat.