Study aims to determine whether exercise training at two distinct times of day would have differing effects on 24-hour glucose levels in men with type 2 diabetes.
When a person is diagnosed with prediabetes or type 2 diabetes, the typical response from the medical community is to increase or begin to get serious about exercise, but the best time to exercise is rarely offered.
Exercise is recommended for the prevention and treatment of type 2 diabetes. Low-volume high-intensity interval training (HIIT) remodels skeletal muscle and the cardiorespiratory system to a similar extent as continuous moderate-intensity training, with reduced time commitment and exercise volume. In addition, glucose tolerance, insulin sensitivity, and muscle oxidative capacity show circadian oscillations. Interactions between these factors could lead to divergent physiological adaptations to exercise training at different times of day. By determining the effects of morning and afternoon HIIT on 24 h glucose in men with type 2 diabetes in ‘free-living’ conditions, researchers tested the hypothesis that time of day would influence the metabolic response to exercise.
This study was done using a CGM to see what time is the most effective to achieve the most beneficial effects on health. Eleven men with type 2 diabetes underwent a randomized crossover trial. Inclusion criteria were 45–68 years of age and BMI between 23 and 33 kg/m2. Exclusion criteria were insulin treatment and presence of another systemic illness. Researchers were not blinded to the group assignment. The trial involved 2 weeks of either morning or afternoon high-intensity interval training (HIIT) (three sessions/week), followed by a 2-week washout period and a subsequent period of the opposite training regimen.
Supervised exercise training was performed at a gym in Stockholm. At a pilot test, each individual cycled at 75 rpm with a load that forced a rest after 1 minute. This was repeated six times with a 1-minute rest in between. The load reached for each 1-minute pulse was recorded, and a mean of the six pulses was used for all HIIT sessions. HIIT sessions were performed on a cycle ergometer, commencing with a 7-minute warm-up, followed by six 1-minute pulses at the predetermined load (usually above 220 W, range 180–350 W) and with a pedaling rate of 75 rpm. Each pulse was followed by a 1-minute recovery period (75 rpm with minimal load). Participants were instructed to have a light breakfast before (approximately 1 hour ahead of) the morning sessions, and a standardized snack (sandwich and juice) was offered ad libitum around 30 minutes after the exercise. Afternoon sessions took place after lunch (approximately 3 hours later) and were followed by dinner around 30 minutes thereafter.
The results showed that morning high-intensity interval training (HIIT) increased CGM-based glucose concentration (6.9 ± 0.4 mmol/l; mean ± SEM for the exercise days during week 1) compared with either the pre-training period (6.4 ± 0.3 mmol/l) or afternoon high-intensity interval training (HIIT) (6.2 ± 0.3 mmol/l for the exercise days during week 1). Conversely, afternoon high-intensity interval training (HIIT) reduced the CGM-based glucose concentration compared with either the pre-training period or morning HIIT. Afternoon HIIT was associated with elevated thyroid-stimulating hormone (TSH; 1.9 ± 0.2 mU/l) and reduced T4 (15.8 ± 0.7 pmol/l) concentrations compared with pre-training (1.4 ± 0.2 mU/l for TSH; 16.8 ± 0.6 pmol/l for T4). TSH was also elevated after morning HIIT (1.7 ± 0.2 mU/l), whereas T4 concentrations were unaltered.
From the result, it was concluded that afternoon high-intensity interval training ( HIIT) was more efficacious than morning HIIT at improving glucose in men with type 2 diabetes. Strikingly, morning HIIT had an acute, deleterious effect, increasing glucose. However, studies of longer training regimens are warranted to establish the persistence of this adverse effect. The data highlight the importance of optimizing the timing of exercise when prescribing it as treatment for type 2 diabetes.
- The data highlights the importance of optimizing the timing of exercise when prescribing HIIT as a treatment for type 2 diabetes.
- Morning HIIT had an acute deleterious effect on glucose values in men with type 2 diabetes
- High-intensity interval training (HIIT) has a beneficial effect on glucose concentration in individuals with type 2 diabetes.
- Most importantly, afternoon exercise is more efficacious than morning exercise at improving glucose levels in individuals with type 2 diabetes.
Comment: Dr. Richard K. Bernstein on patients with type 1 and the dawn phenomenon:
“Several of my type 1 patients must take additional rapid-acting insulin when they exercise in the morning, but not when they exercise in the afternoon. This is a dramatic example of how the dawn phenomenon reduces even injected serum insulin levels. In the afternoon, these patients’ blood sugar drops with exercise, but in the morning it actually goes up if they do not first inject some rapid-acting insulin.”