Metabolic effects of physical training

Assessment of habitual physical activity in adolescents with type 1 diabetes

OBJECTIVE

To evaluate habitual physical activity in a cohort of adolescents with type 1 diabetes in relation to similarly aged control subjects.

METHODS

A cross-sectional case control study of 54 healthy adolescents and 66 patients with type 1 diabetes, 14 to 18 years of age, was conducted. Subjects were surveyed using the Habitual Activity Estimation Scale, a validated self-report instrument to assess activity levels in teens. Subjects' time was classified into categories ranging from inactive (lying down, resting) to very active (increased heart rate and diaphoresis). Active time, described in relative (%) and absolute hours per day was determined for each individual. Age, sex, weight, height and body mass index were recorded for all participants, and the charts of subjects with type 1 diabetes were reviewed for most recent levels of glycated hemoglobin, low-density lipoproteins, high-density lipoproteins, total cholesterol, triglycerides and blood pressure. A regression analysis was performed to determine factors associated with hours spent being active.

RESULTS

Subjects with type 1 diabetes spent similar hours being very active (3.4 hours vs. 3.5 hours, p=0.49) but reported more time being inactive than controls (2.0 hours vs. 1.3 hours, p=0.002). In both groups, female gender was associated with more hours spent being active. Metabolic control as assessed by glycated hemoglobin worsened with activity. In the group with type 1 diabetes, more hours spent being active were associated with lower systolic blood pressure, lower serum triglyceride levels, lower total cholesterol and higher high-density lipoproteins, whereas inactivity correlated with higher low-density lipoproteins and total cholesterol.

CONCLUSIONS

Adolescents with type 1 diabetes reported significantly more time being inactive than did healthy controls. In patients with type 1 diabetes, activity was associated with improved cardiovascular risk profile.

Exercise training-induced adaptations associated with increases in skeletal muscle glycogen content

Chronic exercise training results in numerous skeletal muscle adaptations, including increases in insulin sensitivity and glycogen content. To understand the mechanism leading to increased muscle glycogen, we studied the effects of exercise training on glycogen regulatory proteins in rat skeletal muscle. Female Sprague Dawley rats performed voluntary wheel running for 1, 4 or 7 weeks. After 7 weeks of training, insulin-stimulated glucose uptake was increased in epitrochlearis muscle. As compared with sedentary control rats, muscle glycogen did not change after 1 week of training, but increased significantly after 4 and 7 weeks. The increases in muscle glycogen were accompanied by elevated glycogen synthase activity and protein expression. To assess the regulation of glycogen synthase, we examined its major activator, protein phosphatase 1 (PP1), and its major deactivator, glycogen synthase kinase (GSK)-3. Consistent with glycogen synthase activity, PP1 activity was unchanged after 1 week of training but significantly increased after 4 and 7 weeks of training. Protein expression of R(GL)(G(M)), another regulatory PP1 subunit, significantly decreased after 4 and 7 weeks of training. Unlike PP1 activity, GSK-3 phosphorylation did not follow the pattern of glycogen synthase activity. The ~ 40% decrease in GSK-3α phosphorylation after 1 week of exercise training persisted until 7 weeks, and may function as a negative feedback mechanism in response to elevated glycogen. Our findings suggest that exercise training-induced increases in muscle glycogen content could be regulated by multiple mechanisms, including enhanced insulin sensitivity, glycogen synthase expression, allosteric activation of glycogen synthase, and PP1 activity.

Physical fitness, dietary intake, and metabolic control in adolescents with type 1 diabetes

PURPOSE

We examined whether physical fitness and dietary intake predicted better glycemic control and lipid profile in adolescents with type 1 diabetes mellitus (T1DM).

METHODS

The sample consisted of 109 adolescents with T1DM [age, 15.3 +/- 1.9 yr; diabetes duration, 6.2 +/- 3.7 yr; body mass index (BMI), 23.3 +/- 4.0 kg/m(2); and hemoglobin A1c (HbA1c), 8.7 +/- 1.6%]. Stepwise regression analyses were performed with the following independent variables: age, sex, duration of diabetes, BMI, Tanner stage, physical fitness, and average carbohydrate and total and saturated fat intake, and the following dependent variables: total cholesterol, low-density lipoprotein cholesterol (LDL-c), high-density lipoprotein cholesterol (HDL-c), triglycerides, and HbA1c. Physical fitness was measured by peak oxygen consumption (VO(2(peak))) during progressive cycle ergometry, and 3-day food intake was analyzed using Nutritionist Pro software.

RESULTS

Sex and VO(2(peak)) explained 19% of the variance for HbA1c. Age, VO(2(peak)), and saturated fat intake predicted 23% of the variance for total cholesterol, although only diabetes duration and saturated fat intake predicted LDL-c (11%). Duration of diabetes explained 5% of the variance in triglyceride levels, and there were no significant independent predictors for HDL-c.

CONCLUSIONS

Greater fitness levels predicted both better glycemic control and total cholesterol in adolescents with T1DM, whereas lower saturated fat affected total cholesterol but not glycemic control. These findings support the importance of physical fitness and diets of lower saturated fat for overall metabolic health in adolescents with T1DM.

Metabolic effects of voluntary wheel running in young and old Syrian golden hamsters

To explore the metabolic effects of high volume wheel running in the Syrian golden hamster, 6-week old (YOUNG) and 6-month old (OLD) male animals were randomly divided into sedentary (i.e., YOUNG-S or OLD-S) or running wheel (i.e., YOUNG-RW or OLD-RW) groups (n = 8/group). RW groups had 24-h access to activity wheels while S were housed in standard rodent cages. At the start of wheel exposure, the number of revolutions were similar in both groups, but by day 15 were nearly two-fold higher in the YOUNG vs. OLD. OLD ate more than YOUNG and wheel running increased food intake by approximately 50%. YOUNG-RW maintained the same total body mass as YOUNG-S, while OLD-RW had a transient weight loss of approximately 10 g. Perirenal fat mass was smaller in YOUNG- and OLD-RW groups compared with S groups (45% and 66%, respectively. Plantaris muscle cytochrome c oxidase activity was also approximately 2-fold higher in YOUNG-RW than in YOUNG-S hamsters but was similar between OLD-RW and OLD-S groups. Plasma leptin levels were approximately 60% lower in YOUNG-RW compared with YOUNG-S and correlated significantly with visceral fat pad mass (r2 = 0.58, p = 0.001). Corticosterone levels were lower in YOUNG-RW (13.0 +/- 0.36 ng/ml) than in YOUNG-S (16.4 +/- 0.83 ng/ml) hamsters and higher in OLD-RW (22.62 +/- 0.47 ng/ml) than in OLD-S (15.54 +/- 0.13 ng/ml) hamsters. These observations reveal that the hamster is a suitable model for accelerating the effects of exercise on body composition and metabolic alterations associated with training and that the training adaptations are more pronounced in younger compared with older hamsters, possibly as a result of the higher voluntary wheel activity in the former group.

Acute and chronic treatment of ob/ob and db/db mice with AICAR decreases blood glucose concentrations

The enzyme 5'AMP-activated protein kinase (AMPK) is activated by increases in intracellular AMP concentration through a complex interaction of phosphorylation and allosteric regulation. Actions of AMPK elucidated thus far suggest that AMPK may be a viable target for pharmacologic intervention in type II diabetes. Activation of AMPK is believed to mediate both the acute increase in skeletal muscle glucose uptake during exercise, as well as the adaptive responses to chronic exercise such as regulation of expression of components of the muscle glucose uptake system. In addition, AMPK is known to inhibit key enzymes involved in lipid and cholesterol synthesis, suggesting that activation of this kinase may also ameliorate dyslipidemia. To investigate the effects of AMPK activation in animal models of type II diabetes, db/db and ob/ob mice were administered 5-aminoimidazole-4-carboxamide 1-beta-ribofuranoside (AICAR) subcutaneously either acutely (single injection) or twice per day for 8 days (chronic treatment). Blood glucose was lowered transiently in both db/db and ob/ob mice by acute AICAR treatment, returning to basal levels approximately 3 h after AICAR administration. In response to chronic treatment, blood glucose (measured 18 h post-AICAR administration) was significantly decreased in both mouse models when compared to vehicle control groups, with morning blood glucose values on Day 8 being decreased approximately 30-35% in both mouse models. Chronic AICAR administration also resulted in an elevation of total Glut4 concentration in skeletal muscle from ob/ob mice, but not db/db mice. In contrast to the beneficial effects on glucose metabolism, AICAR treatment of db/db and ob/ob mice led to approximately a 2.5-3-fold increase in serum triglyceride levels compared to vehicle-treated controls. These data suggest that pharmacological activation of AMPK may enhance glucose uptake in individuals with type II diabetes, however, this benefit may be offset by the concomitant elevation in triglycerides.