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Journal of Exercise Science and Fitness logoLink to Journal of Exercise Science and Fitness
. 2023 Aug 30;21(4):395–404. doi: 10.1016/j.jesf.2023.08.003

Effects of low-versus high-volume high-intensity interval training on glycemic control and quality of life in obese women with type 2 diabetes. A randomized controlled trial

Ahmad Mahdi Ahmad a,∗, Asmaa Mohamed Mahmoud b, Zahra Hassan Serry a, Mohamed Mady Mohamed c, Heba Ali Abd Elghaffar a
PMCID: PMC10632101  PMID: 37954548

Abstract

Background/objective

Comparison between different training volumes of high-intensity interval training (HIIT) is understudied in type 2 diabetes. This study aimed to compare the effects of low- and high-volume HIIT on glycemic control, blood lipids, blood pressure, anthropometric adiposity measures, cardiorespiratory fitness, and health-related quality of life (HRQoL) in women with type 2 diabetes.

Methods

Seventy-two obese women with type 2 diabetes aged 36–55 were randomly assigned to a low-volume HIIT group (i.e., 2 × 4-min high-intensity treadmill exercise at 85%–90% of peak heart rate, with a 3-min active recovery interval in between), a high-volume HIIT group (i.e., 4 × 4-min high-intensity treadmill exercise at 85%–90% of peak heart rate, with three 3-min active recovery intervals in between), and a non-exercising control group. Patients in HIIT groups exercised three days a week for 12 weeks. All patients received oral hypoglycemic medications with no calorie restrictions. The outcome measures were glycosylated hemoglobin (HbA1c), fasting blood glucose (FBG), 2-hour postprandial blood glucose (2-hr PPBG), total cholesterol (TC), triglyceride (TG), low-density lipoprotein (LDL), high-density lipoprotein (HDL), systolic blood pressure (SBP), diastolic blood pressure (DBP), body mass index (BMI), waist circumference (WC), waist-to-hip ratio, time to maximal exhaustion determined from a maximal treadmill exercise test (i.e., a measure of cardiorespiratory fitness), and HRQoL assessed by the 12-item Short Form (SF-12) Health Survey.

Results

The low- and high-volume HIIT groups showed significant improvements in all outcome measures compared to the baseline and the non-exercising group (P < 0.05), except for DBP in the low-volume HIIT group (p > 0.05). Also, both low- and high-volume HIIT groups showed similar improvements in TC, HDL, SBP, DBP, BMI, WC, waist-to-hip ratio, and the SF-12 scores, with no significant between-groups difference (p > 0.05). The high-volume HIIT group, however, showed more significant improvements in HbA1c, FBG, 2-hr PPBG, TG, LDL, and treadmill time to maximal exhaustion than the low-volume HIIT group (p < 0.05). The non-exercising group showed non-significant changes in all outcome measures (p > 0.05).

Conclusion

Low-volume HIIT could be equally effective as high-volume HIIT for improving TC, HDL, blood pressure, anthropometric adiposity measures, and HRQoL in obese women with type 2 diabetes. Nevertheless, high-volume HIIT could have a greater impact on glycemic control, TG, LDL, and cardiorespiratory fitness in these patients.

Trial registration

ClinicalTrials.gov, NCT05110404.

Keywords: Low-volume HIIT, High-volume HIIT, Type 2 diabetes, Glycosylated hemoglobin, Blood lipids, Cardiorespiratory fitness

1. Introduction

About 537 million adults worldwide live with diabetes with 3 in 4 diabetic adults living in low- and middle-income countries.1 Obesity is a major risk factor for the incidence of type 2 diabetes, and this association is stronger in women compared to men.2 An interplay between obesity, type 2 diabetes mellitus, and cardiovascular diseases has been suggested.3 Sex differences in type 2 diabetes demonstrate a higher relative risk of diabetic cardiovascular complications in women than in men.4 The risk of cardiovascular disease (CVD) in patients with type 2 diabetes is more than twice as high as that of those without diabetes.5 Physical inactivity and low cardiorespiratory fitness increase the risk of developing CVD and high mortality rates in people with type 2 diabetes.6 Lower health-related quality of life (HRQoL) is also a common finding in patients with type 2 diabetes.7 The management strategies to prevent CVD in type 2 diabetes target glycemic control, blood pressure control, low levels of serum lipids,5,8 and increased physical activity/fitness level.6 The ultimate goal of the management plan is to optimize HRQoL in these patients.9 Exercise is a first-line lifestyle strategy in the management of type 2 diabetes,10 and can help protect against CVD,11 increase cardiorespiratory endurance,12 and ultimately improve HRQoL.13

Compelling evidence supports the cardiometabolic benefits of high-intensity interval training (HIIT).14 Recently, exercise-based international guidelines for managing type 2 diabetes have recognized HIIT as an alternative to traditional endurance training with the advantage of being more time-efficient.12 HIIT comprises high-intensity exercise bouts interspersed with rest intervals.15 It can be further sub-categorized into high-volume HIIT, during which the total time of the high-intensity workout is more than 15 minutes, and low-volume HIIT, during which the total time of the high-intensity workout is less than 15 minutes, excluding the total duration of the rest intervals.15 There is a paucity of studies that compare low-volume and high-volume HIIT in type 2 diabetes. Only one study has compared low- and high-volume HIIT in individuals having type 2 diabetes and found that both low- and high-volume HIIT were equally effective in reducing metabolic syndrome severity.16 Another study in prediabetes has compared the two HIIT subtypes in combination with a low-calorie diet and found that high-volume HIIT was more effective than low-volume HIIT in reducing glycosylated hemoglobin (HbA1c) and fasting blood glucose (FBG) levels.17 Neither of these studies has assessed a self-reported patient outcome, such as HRQoL. The use of HRQoL as a measurable outcome in the patient population has increased in recent decades. This could have occurred after the traditional disease-focused biomedical approach was shifted to a more holistic patient-centered biopsychosocial model.18 In type 2 diabetes, since poor HRQoL was associated with a higher mortality rate,19 evaluation of HRQoL can have important clinical implications.

A recent systematic review has reported cardiometabolic benefits from low-volume HIIT in type 2 diabetes and called for further research to confirm these benefits.20 To verify such benefits, confirm other results from previous studies, and assess a new outcome, the current study mainly aimed to compare low-volume HIIT to high-volume HIIT for HbA1c, FBG, 2-hour postprandial blood glucose (2-hr PPBG), lipid profile, blood pressure, anthropometric adiposity measures, cardiorespiratory fitness, and HRQoL in obese women with type 2 diabetes. We hypothesized that high-volume HIIT could have a greater effect on glycemic control than low-volume HIIT in type 2 diabetics based on previous data on prediabetics.17 The results of the present study can help provide new insights into the optimal volume of HIIT needed to elicit clinical and self-reported benefits for obese women with type 2 diabetes.

2. Methods

This study is reported following the CONSORT 2010 Statement Guidelines for reporting randomized controlled trials.21

2.1. Study design, settings, and ethical considerations

This study is a randomized, controlled, parallel-group intervention study. It was conducted at the Physiotherapy Department of Fayoum University Hospital from December 2021 to April 2022. The protocol of this study obtained ethical approval from the Ethics Committee of Human Scientific Research of the Faculty of Physical Therapy at Cairo University (No: P.T.REC/012/002900). This study has followed the principles of the Helsinki Declaration. All patients provided informed written consent before participating in the study.

2.2. Randomization and concealed allocation

Simple randomization was used in this study, using a randomization table designed by a computer software program with an allocation ratio of 1:1:1. Sequentially numbered opaque sealed envelopes were used to conceal the allocation sequence. Neither the allocator nor the participants were aware of the upcoming allocation.

2.3. Implementation and blinding

A researcher not involved in study interventions generated the randomization sequence. A researcher involved in the research setting enrolled participants and assigned them to groups. After patient assignment, the assessor of blood biochemicals (i.e., blood glucose measures and blood lipids) and the physician prescribing medications were blinded to patient allocation. For the assessment of HRQoL, software was utilized to score the SF-12 Health Survey. The assessor of the rest of the outcomes was unblinded to patient allocation due to practical reasons. The physiotherapist conducting the exercise interventions was not blinded to the interventions due to the necessity of knowing the exercise prescription for each HIIT protocol. For the same reason, it was not possible to make the participants receiving the exercise interventions blind to the interventions.

2.4. Sample size calculation

Since there was no published data on the effect of low-volume HIIT compared to high-volume HIIT on glycemic control in type 2 diabetes, the sample size was calculated for HbA1c based on previous data on prediabetes.17 The power of this study was set at 80%, and the level of significance was set at a p-value of <0.05. The sample size (n) was calculated as follows: n = 2 SD2 (Zα/2 + Zβ)2 / d2.22 Where: Zα/2 = 1.96 for 2-tailed results at p < 0.05; Zβ = 0.84 for a power of 80%; SD, Standard Deviation (pooled SD of HbA1c for high- and low-volume HIIT groups) = 0.4717; d, expected effect size = mean change of HbA1c in high-volume HIIT group – mean change of HbA1c in low-volume HIIT group = 1.27–0.87 = 0.4 (%).17 Accordingly, n = 2 (0.47)2 × (1.96 + 0.84)2 ÷ (0.4)2 = 21.6. Thus, the minimal sample size was estimated as 22 patients per group. However, to account for a 10% drop-out rate, we recruited 24 patients per group.

2.5. Subjects

Seventy-two women with type 2 diabetes were recruited for this study by referral from a physician. Eligibility criteria were a diagnosis of type 2 diabetes mellitus established by a physician as HbA1c ≥6.5%, disease duration of at least one year and less than five years, women aged from 36 to 55, body mass index of 30-39.9 kg/m2, and treatment with oral hypoglycemic medications. Exclusion criteria were poorly controlled diabetes (HbA1c > 9%), uncontrolled blood pressure, cardiovascular or chest diseases, diabetic complications (diabetic foot, retinopathy, nephropathy, and peripheral neuropathy), pregnancy, insulin therapy, smokers, anemia, post-COVID-19 syndrome, contraindications to maximal exercise testing, and musculoskeletal or neurological limitations to exercise. Eligible subjects were assigned randomly into three groups: a low-volume HIIT group (n1 = 24), a high-volume HIIT group (n2 = 24), and a non-exercising control group (n3 = 24). All groups received the usual medical care. The flow of subjects throughout the study can be seen in Fig. 1.

Fig. 1.

Fig. 1

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Flowchart of the study.

2.6. Evaluation

2.6.1. History taking and clinical examination

At the beginning of the study, an experienced endocrinologist conducted a comprehensive medical history taking and clinical evaluation for patient selection and documentation of patients’ baseline demographic and clinical characteristics.

2.6.2. Primary outcomes (blood glucose measures)

Venous blood samples were withdrawn from all patients at baseline and after 12 weeks for analysis of HbA1c, FBG (i.e., after about 8 hr of fasting), and 2-hr PPBG. HbA1c was analyzed by the Fast Ion-Exchange Resin Separation method.23 FBG and 2-hr PPBG were analyzed by the enzymatic colorimetric method.24 The post-training plasma glucose sampling was done at least 48 hr after the last exercise session to avoid the acute effect of exercise.

2.6.3. Secondary outcomes

2.6.3.1. Blood lipid analysis

Fasting lipid profile was evaluated before and after 12 weeks. The lipid panel included total cholesterol (TC), triglycerides (TG), and high-density lipoprotein (HDL). The low-density lipoprotein (LDL) was calculated using Friedland's formula. Lipid analysis was conducted using an automated analyzer (Microlab 300, ELITECH, Netherlands).

2.6.3.2. Blood pressure measurement

The systolic blood pressure (SBP) and diastolic blood pressure (DBP) were measured using a mercury sphygmomanometer in the left arm. Measurements were taken in a quiet room while patients were in the sitting position and complete rest for at least 15 minutes. The measuring equipment was kept at the level of the heart. For accuracy of measurement, blood pressure measurements were repeated three times, and the average value was taken. Blood pressure measurements were performed before and after 12 weeks.

2.6.3.3. Anthropometric measures

Anthropometric adiposity measures were taken at baseline and after 12 weeks and included body mass index (BMI), waist circumference (WC), and waist-to-hip ratio. The BMI was calculated by dividing body weight in kilograms by height in meters squared.25 The WC was measured in cm using a flexible non-stretch measuring tape at the midpoint between the lowest rib and the superior border of the iliac crest.26 Patients were instructed to avoid eating before the measurement. The measurement was taken from a standing position at the end of normal expiration, and patients were instructed not to contract their abdomen during the measurement. Hip circumference was measured by a flexible non-extensible measuring tape from a standing position at the widest level of the iliac crest.27 Then, the waist-to-hip ratio was calculated by dividing the WC by the hip circumference. The measurement errors were eliminated by taking the measurements on bare skin and avoiding pulling the tape too tight or loose. Besides, three measurements were taken, and the average value was recorded.

2.6.3.4. Maximal treadmill exercise test

A symptom-limited maximal treadmill exercise test, using the standardized modified Bruce protocol,28 was carried out for all patients in the present study at baseline and after 12 weeks. The reasons for exercise testing were (i) to identify any inappropriate signs or symptoms in response to higher intensity of exercise (i.e., safety evaluation before HIIT)28; (ii) to record the baseline peak heart rate (peak HR) of each patient needed for exercise prescription28; (iii) to record treadmill time to maximal exhaustion attained during the exercise test as a measure of cardiorespiratory fitness before and after the interventions.29 Before the test, all participants were asked to refrain from strenuous exercise for at least 48 hr, caffeine consumption for at least 24 hr, and a meal for at least 2 hr. Patients were also instructed to take their usual medication, ensure good hydration, and wear proper footwear and clothes before the test. During the test, signs and symptoms of exercise intolerance were monitored. Also, the heart rate was continuously monitored by a fingertip pulse oximeter (YK-80C, Xuzhou Yongkang, China) during the test. All patients underwent the exercise test safely and terminated it because of maximal exhaustion. Upon test termination, the peak HR and treadmill time to maximal exhaustion were recorded. The peak HR of each patient was used to determine the exercise HR (i.e., % peak HR) on an individual basis. Treadmill time to maximal exhaustion was used in the present study as a measure of cardiorespiratory fitness, as previously used.29

2.6.3.5. Health-related quality of life (HRQoL)

The Arabic version of the 12-item Short Form (SF-12) Health Survey was used in the present study as a reliable and valid tool to evaluate HRQoL.30 It was previously translated and validated from its original English version.31 In-person interviews with patients were conducted to administer the SF-12 at baseline and 12 weeks later. The physical component summary (PCS) and the mental component summary (MCS) are two measures that the SF-12 produces from its 12 items.31 The PCS summarizes physical functioning, role-physical, bodily pain, and general health scales and relates to total physical health.31 The MCS summarizes vitality, social functioning, role-emotional, and mental health measures and relates to total mental health.31 The scoring of the SF-12 was done using online software at https://orthotoolkit.com/sf-12/. Higher scores of PCS and MCS denote better physical and mental HRQoL, respectively.31

2.7. Interventions

2.7.1. Pharmacological treatment

All patients in the three groups received oral antidiabetic and antihyperlipidemic medications prescribed by an experienced endocrinologist. These include Metformin, Glibenclamide, Simvastatin, Fenofibrate, and Statins. No antihypertensive medications were prescribed.

2.7.2. Dietary instructions

General instructions on healthy food intake were provided to all patients at the beginning of the study through face-to-face interviews. No calorie-restricted dietary regimens were introduced to patients in the present study.

2.7.3. Exercise interventions

Patients in the low- and high-volume HIIT groups performed walking and jogging workouts on a treadmill for three sessions per week and 12 weeks. The treadmill speed and inclination were either increased or decreased, to make patients reach their targeted exercise HR during the high-intensity and recovery intervals, respectively. The targeted HR for each patient was monitored throughout the exercise session by a fingertip pulse oximeter (YK–80C, Xuzhou Yongkang, China). An experienced physiotherapist delivered the HIIT for each patient individually through face-to-face exercise sessions.

2.7.3.1. Low-volume HIIT

Patients in this group began the exercise session with a warm-up phase at 65%–70% of peak HR for 5 minutes (min). Then, modified from Tjønna et al.,32 patients exercised on a treadmill for two intervals of 4 min each (2 × 4 min), at an intensity corresponding to 85%–90% of peak HR, with a 3-min active recovery interval in between, at 65%–75% of peak HR. Finally, a 3-min cool-down period was permitted. The total exercise session duration was 19 min (i.e., including 8 min of high-intensity workout).

2.7.3.2. High-volume HIIT

Similarly, as above, patients in this group started each session with a 5-min warm-up at 65–70% of peak HR and ended with a 3-min cool-down period. However, this group exercised for four intervals of 4 min each (4 × 4 min), at an intensity corresponding to 85%–90% of peak HR, with three 3-min active recovery intervals in between, at 65%–75% of peak HR.33 The total exercise session duration was 33 min (i.e., including 16 min of high-intensity workout).

2.8. Statistical analysis

Data were screened for normality by the Shapiro-Wilk test and for homogeneity of variance by Levene's test. The data showed a normal distribution; therefore, parametric statistics were used. Quantitative descriptive statistics were used to present the data as means and standard deviations. One-way analysis of variance (ANOVA) test was used to compare demographic and anthropometric data between the three groups at baseline. A mixed-design multivariate analysis of variance (MANOVA) was used to compare the outcome measures within and between groups. The Bonferroni correction test was used for pairwise comparisons of the outcome measures when the p-value from the MANOVA test was significant. The significance level was set at p < 0.05 in all statistical tests. Percent mean changes from the baseline were calculated for the outcome measures. Between-groups mean difference (MD) and 95% confidence interval (CI) were computed post-intervention. The statistical analysis was conducted using the Statistical Package for the Social Sciences (SPSS) statistics software program version 25 for Windows (SPSS, Inc., Chicago, IL).

3. Results

3.1. Baseline characteristics

There were no significant differences in the ages and anthropometric characteristics of patients or diabetes duration between groups at baseline (p > 0.05) (Table 1). Likewise, no significant differences were found between groups in the means of the outcome measures at baseline (p > 0.05), as shown in Table 2, Table 3.

Table 1.

Baseline characteristics.

Variables Low-volume HIIT group (n1= 24) High-volume HIIT group (n2= 24) Non-exercising group (n3= 24) p-value
Age (years) 42.96 ± 5.87 43.29 ± 6.20 42.46 ± 5.57 0.886
Body weight (kg) 88.75 ± 8.32 87.54 ± 9.82 86.97 ± 8.74 0.979
Height (cm) 161.15 ± 4.72 160.52 ± 5.74 159.63 ± 4.76 0.785
BMI (kg/m2) 34.22 ± 3.30 34.94 ± 3.00 34.08 ± 2.59 0.586
Diabetes duration (years) 2.90 ± 1.49 2.96 ± 1.50 2.88 ± 1.42 0.949
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Data are expressed as Means ± SD. HIIT: High-intensity interval training. p-value from the one-way ANOVA test. BMI: body mass index.

Table 2.

Results of blood glucose measures and blood lipids before and after the interventions.

Outcome measures Low-volume HIIT group (n1= 24) High-volume HIIT group (n2= 24) Non-exercising group (n3= 24) p-valueb
HbA1c(%) Pre 8.15 ± 0.52 8.15 ± 0.56 8.14 ± 0.55 0.998
Post 7.12 ± 0.49 6.65 ± 0.17 8.19 ± 0.50 <0.001**
p-valuea <0.001* <0.001* 0.753
% mean change ↓12.64% ↓18.40% ↑0.61%
FBG (mg/dL) Pre 153.50 ± 15.15 153.46 ± 16.21 152.92 ± 16.21 0.987
Post 139.04 ± 12.41 129.71 ± 4.99 154.83 ± 14.39 <0.001**
p-valuea 0.001* <0.001* 0.639
% mean change ↓9.42% ↓15.48% ↑1.25%
2-hr PPBG (mg/dL) Pre 221.54 ± 15.69 220.58 ± 15.45 221.29 ± 15.85 0.971
Post 191.46 ± 13.37 183.83 ± 4.85 222.46 ± 13.92 <0.001**
p-valuea <0.001* <0.001* 0.776
% mean change ↓13.58% ↓16.66% ↑0.53%
TC (mg/dL) Pre 225.21 ± 11.37 226.42 ± 10.96 223.21 ± 16.37 0.679
Post 212.21 ± 11.73 206.42 ± 10.97 228.83 ± 13.14 0.018**
p-valuea <0.001* <0.001* 0.128
% mean change ↓5.77% ↓8.83% ↑2.52%
TG (mg/dL) Pre 169.25 ± 14.62 168.79 ± 15.75 171.12 ± 16.21 0.865
Post 158.29 ± 16.07 150.67 ± 16.69 174.75 ± 16.18 0.001**
p-valuea 0.018* <0.001* 0.430
% mean change ↓6.48% ↓10.74% ↑2.12%
HDL (mg/dL) Pre 41.00 ± 4.58 41.58 ± 518 41.71 ± 4.92 0.490
Post 42.31 ± 4.93 43.29 ± 5.34 41.13 ± 5.06 0.013**
p-valuea 0.046* 0.002* 0.989
% mean change ↑3.20% ↑4.11% ↓0.01%
LDL (mg/dL) Pre 150.29 ± 10.45 151.17 ± 11.47 149.46 ± 12.18 0.883
Post 138.63 ± 10.78 132.25 ± 11.28 151.75 ± 12.07 <0.001**
p-valuea 0.045* 0.037* 0.504
% mean change ↓7.76% ↓12.52% ↑1.53%
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Data are expressed as Means ± SD and percent mean changes from baseline. HIIT: High-intensity interval training. p-valuea: p-value from within-group comparison based on MANOVA test. * significant within-group difference (p-value <0.05); p-valueb: p-value from between-groups comparison based on MANOVA test. ** significant between-groups difference (p-value <0.05). HbA1c: glycosylated hemoglobin; FBG: fasting blood glucose; 2-hr PPBG: 2-hour post-prandial blood glucose; TC: Total cholesterol; TG: Triglycerides; HDL: high-density lipoprotein; LDL: low-density lipoprotein.

Table 3.

Results of blood pressure, anthropometry, treadmill time to maximal exhaustion, and SF-12 Health Survey before and after the interventions.

Outcome measures Low-volume HIIT group (n1= 24) High-volume HIIT group (n2= 24) Non-exercising group (n3= 24) p-valueb
SBP (mmHg) Pre 125.46 ± 3.65 125.33 ± 4.16 125.50 ± 3.63 0.988
Post 121.67 ± 3.89 120.88 ± 3.75 125.79 ± 3.61 <0.001**
p-valuea 0.001* <0.001* 0.790
% mean change ↓3.02% ↓3.55% ↑0.23%
DBP (mmHg) Pre 82.38 ± 4.03 82.71 ± 4.01 81.58 ± 3.90 0.627
Post 80.17 ± 4.47 79.17 ± 4.32 82.29 ± 4.03 0.031**
p-valuea 0.066 0.004* 0.554
% mean change ↓2.68% ↓4.28% ↑0.87%
BMI (kg/m2) Pre 34.22 ± 3.30 34.94 ± 3.00 34.08 ± 2.59 0.950
Post 32.13 ± 3.33 31.96 ± 3.14 34.54 ± 2.60 0.003**
p-valuea 0.018* 0.026* 0.599
% mean change ↓6.11% ↓8.53% ↑1.35%
WC (cm) Pre 100.41 ± 7.33 100.63 ± 7.32 101.40 ± 6.14 0.875
Post 95.58 ± 7.76 93.00 ± 7.50 102.03 ± 6.15 0.001**
p-valuea 0.003* 0.001* 0.889
% mean change ↓4.81% ↓7.58% ↑0.62%
Waist-to-hip ratio Pre 0.85 ± 0.05 0.86 ± 0.04 0.86 ± 0.04 0.904
Post 0.81 ± 0.05 0.80 ± 0.04 0.87 ± 0.04 0.002**
p-valuea 0.043* 0.031* 0.998
% mean change ↓4.71% ↓6.98% ↑1.16%
Treadmill time to maximal exhaustion (min) Pre 15:37 ± 1:09 15:35 ± 1:04 15:36 ± 1:20 0.997
Post 16:45 ± 1:09 17:50 ± 1:00 15:52 ± 1:20 <0.001**
p-valuea 0.012* <0.001* 0.535
% mean change ↑7.03% ↑14.01% ↑1.04%
PCS score of the SF-12 Health Survey Pre 42.03 ± 5.35 41.41 ± 6.71 42.53 ± 6.57 0.818
Post 48.69 ± 5.49 48.81 ± 6.51 42.23 ± 5.82 <0.001**
p-valuea <0.001* <0.001* 0.868
% mean change ↑15.85% ↑17.87% ↓0.71%
MCS score of the SF-12 Health Survey Pre 46.79 ± 6.33 46.50 ± 7.42 46.29 ± 5.29 0.211
Post 50.41 ± 6.46 54.45 ± 4.17 44.21 ± 3.21 0.008**
p-valuea 0.044* <0.001* 0.851
% mean change ↑7.74% ↑17.10% ↓4.49%
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Data are expressed as Means ± SD and percent mean changes from baseline. HIIT: High-intensity interval training. p-valuea: p-value from within-group comparison based on MANOVA test. * significant within-group difference (p-value<0.05); p-valueb: p-value from between-groups comparison based on MANOVA test. ** significant between-groups difference (p-value <0.05). SBP: systolic blood pressure; DBP: diastolic blood pressure; BMI: body mass index; WC: waist circumference; SF-12: 12-item Short Form; PCS: Physical Component Summary; MCS: Mental Component Summary.

3.2. Blood glucose measures

The low-volume HIIT group showed significant reductions in HbA1c by 12.64%, FBG by 9.42%, and 2-hr PPBG by 13.58% compared to the baseline (p < 0.001) (Table 2) and the non-exercising group (p < 0.001) (Table 4). Likewise, the high-volume HIIT group showed significant reductions in HbA1c by 18.40%, FBG by 15.48%, and 2-hr PPBG by 16.66% compared to the baseline (p < 0.001) (Table 2) and the non-exercising group (p < 0.001) (Table 4). However, the high-volume HIIT group showed more significant reductions in HbA1c (MD, 0.47%; 95% CI, 0.25 to 0.68; p = 0.039), FBG (MD, 9.33 mg/dL; 95% CI, 3.83 to 14.82; p = 0.009), and 2-hr PPBG (MD, 7.63 mg/dL; 95% CI, 1.78 to 13.47; p = 0.031) than the low-volume HIIT group after the study (Table 4). The non-exercising group showed non-significant changes in blood glucose measures (p > 0.05) (Table 2).

Table 4.

Results of the pairwise comparisons between groups post-intervention.

Outcome measures Pairwise comparisons
Low-volume HIIT versus high-volume HIIT
 Low-volume HIIT versus not exercising
 High-volume HIIT versus not exercising
MD, 95% CI p-value MD, 95% CI p-value MD, 95% CI p-value
HbA1c(%) 0.47, [0.25, 0.68] 0.039† 1.07, [0.78, 1.35] <0.001† 1.54, [1.32, 1.75] <0.001†
FBG (mg/dL) 9.33, [3.83, 14.82] 0.009† 15.79, [7.98, 23.59] <0.001† 25.12, [18.86, 31.37] <0.001†
2-hr PPBG (mg/dL) 7.63, [1.78, 13.47] 0.031† 31.0, [23.06, 38.93] <0.001† 38.63, [32.57, 44.68] <0.001†
TC (mg/dL) 5.79, [-0.80, 12.38] 0.364 16.62, [9.38, 23.85] <0.001† 22.41, [15.37, 29.44] <0.001†
TG (mg/dL) 7.62, [-1.89, 17.13] 0.029† 16.46, [7.09, 25.82] 0.001† 24.0, [14.52, 33.63] <0.001†
HDL (mg/dL) 0.98, [-2.00, 3.96] >0.999 1.18, [-1.72, 4.08] 0.032† 2.16, [-0.86, 5.18] 0.001†
LDL (mg/dL) 6.38, [-0.03, 12.79] 0.027† 13.12, [6.47, 19.76] <0.001† 19.50, [12.71, 26.28] <0.001†
SBP (mmHg) 0.79, [-1.43, 3.01] >0.999 4.12, [1.93, 6.30] 0.001† 4.91, [2.77, 7.04] <0.001†
DBP (mmHg) 1.00, [-1.55, 3.55] >0.999 2.12, [-0.35, 4.59] 0.232 3.12, [0.69, 5.54] 0.030†
BMI (kg/m2) 0.17, [-1.71, 2.05] >0.999 2.41, [0.67, 4.14] 0.020† 2.58, [0.90, 4.25] 0.031†
WC (cm) 2.58, [-1.85, 7.01] >0.999 6.45, [2.38, 10.51] 0.001† 9.03, [5.04, 13.01] <0.001†
Waist-to-hip ratio 0.01, [-0.01, 0.03] >0.999 0.06, [0.03, 0.08] 0.032† 0.07, [0.04, 0.09] 0.035†
Treadmill time to maximal exhaustion (min) 1.05, [0.44, 1.65] 0.005† 0.93, [0.26, 1.59] 0.045† 1.98, [1.33, 2.62] <0.001†
PCS score of the SF-12 Health Survey 0.12, [-3.37, 3.61] >0.999 6.46, [3.17, 9.74] 0.001† 6.58, [2.99, 10.16] 0.001†
MCS score of the SF-12 Health Survey 4.04, [0.88, 7.19] 0.075 6.20, [3.23, 9.16] 0.024† 10.24, [8.07, 12.40] 0.009†
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MD: Mean difference between groups post-intervention. HbA1c: glycosylated hemoglobin; FBG: fasting blood glucose; 2-hr PPBG: 2-hour post-prandial blood glucose. TC: Total cholesterol. TG: Triglycerides. HDL: high-density lipoprotein; LDL: low-density lipoprotein; SBP: systolic blood pressure. DBP: diastolic blood pressure. BMI: body mass index. WC: waist circumference; SF-12: 12-item Short Form; PCS: Physical component summary; MCS: Mental component summary.† significant between-groups difference based on the Bonferroni correction test (p-value <0.05) .

3.3. Blood lipids

The low-volume HIIT group showed significant reductions in TC by 5.77% (p < 0.001), TG by 6.48% (p = 0.018), and LDL by 7.76% (p = 0.045), as well as a significant increase in HDL by 3.20% (p = 0.046) compared to the baseline (Table 2) and the non-exercising group (p < 0.05) (Table 4). Likewise, the high-volume HIIT group showed significant reductions in TC by 8.83% (p < 0.001), TG by 10.74% (p < 0.001), and LDL by 12.52% (p = 0.037), as well as a significant increase in HDL by 4.11% (p = 0.002) compared to the baseline (Table 2) and the non-exercising group (p < 0.05) (Table 4). However, the high-volume HIIT group showed more significant reductions only in TG (MD, 7.62 mg/dL; 95% CI, −1.89 to 17.13; p = 0.029) and LDL (MD, 6.38 mg/dL; 95% CI, −0.03 to 12.79; p = 0.027) compared to the low-volume HIIT group (Table 4). Both low- and high-volume HIIT groups showed similar improvements in TC (MD, 5.79 mg/dL; 95% CI, −0.80 to 12.38; p = 0.364) and HDL (MD, 0.98 mg/dL; 95% CI, −2.00 to 3.96; p > 0.999) after the interventions (Table 4). The non-exercising group showed non-significant changes in blood lipids (p > 0.05) (Table 2).

3.4. Blood pressure

The low-volume HIIT group showed a significant reduction in SBP by 3.02% (p = 0.001) compared to the baseline (Table 3) and the non-exercising group (p = 0.001) (Table 4). Also, the low-volume HIIT group showed a non-significant reduction in DBP by 2.68% with a tendency towards statistical significance ( p = 0.066) compared to the baseline (Table 3), with a non-significant difference compared to the non-exercising group (p = 0.232) (Table 4). The high-volume HIIT group showed significant reductions in SBP by 3.55% (p < 0.001) and DBP by 4.28% (p = 0.004) compared to the baseline (Table 3) and the non-exercising group (p < 0.05) (Table 4). Both low- and high-volume HIIT groups showed similar reductions in SBP (MD, 0.79 mmHg; 95% CI, −1.43 to 3.01; p > 0.999) and DBP (MD, 1.00 mmHg; 95% CI, −1.55 to 3.55; p > 0.999) at the end of the interventions (Table 4). The non-exercising group showed non-significant changes in blood pressure (p > 0.05) (Table 3).

3.5. Anthropometric measures

The low-volume HIIT group showed significant reductions in BMI by 6.11% (p = 0.018), WC by 4.81% (p = 0.003), and waist-to-hip ratio by 4.71% (p = 0.043) compared to the baseline (Table 3) and the non-exercising group (p < 0.05) (Table 4). Likewise, the high-volume HIIT group showed significant reductions in BMI by 8.53% (p = 0.026), WC by 7.58% (p = 0.001), and waist-to-hip ratio by 6.98% (p = 0.031) compared to the baseline (Table 3) and the non-exercising group (p < 0.05) (Table 4). Both low- and high-volume HIIT groups showed similar reductions in BMI (MD, 0.17 kg/m2; 95% CI, −1.71 to 2.05; p > 0.999), WC (MD, 2.58 cm; 95% CI, −1.85 to 7.01; p > 0.999), and waist-to-hip ratio (MD, 0.01; 95% CI, −0.01 to 0.03; p > 0.999) after the interventions (Table 4). The non-exercising group showed non-significant changes in the anthropometric measures (p > 0.05) (Table 3).

3.6. Treadmill time to maximal exhaustion

The low-volume HIIT group showed a significant increase in treadmill time to maximal exhaustion by 7.03% (p = 0.012) compared to the baseline (Table 3) and the non-exercising group (p = 0.045) (Table 4). Also, the high-volume HIIT group showed a significant increase in treadmill time to maximal exhaustion by 14.01% (p < 0.001) compared to the baseline (Table 3) and the non-exercising group (p < 0.001) (Table 4). The high-volume HIIT group, however, showed a significantly higher increase in treadmill time to maximal exhaustion (MD, 1.05 min; 95% CI, 0.44 to 1.65; p = 0.005) than the low-volume HIIT group after the study (Table 4). The non-exercising group showed no significant change in this outcome (p > 0.05) (Table 3).

3.7. Quality of life (the SF-12 Health Survey)

The low-volume HIIT group showed significant increases in the PCS mean score by 15.85% (p < 0.001) and the MCS mean score by 7.74% (p = 0.044) compared to the baseline (Table 3) and the non-exercising group (p < 0.05) (Table 4). Likewise, the high-volume HIIT group showed significant increases in the PCS mean score by 17.87% (p < 0.001) and the MCS mean score by 17.10% (p < 0.001) compared to the baseline (Table 3) and the non-exercising group (p < 0.05) (Table 4). Both low- and high-volume HIIT groups showed similar improvements in the PCS score (MD, 0.12; 95% CI, −3.37 to 3.61; p > 0.999) and the MCS score (MD, 4.04; 95% CI, 0.88 to 7.19; p = 0.075) after the interventions (Table 4). The non-exercising group showed non-significant changes in the SF-12 scores (p > 0.05) (Table 3).

4. Discussion

The purpose of the present study was to compare the effects of low- and high-volume HIIT on glycemic measures, lipid profile, blood pressure, anthropometric adiposity measures, cardiorespiratory fitness, and HRQoL in obese women with type 2 diabetes. This study is one of the few randomized controlled studies comparing the benefits of low- and high-volume HIIT in type 2 diabetes. Also, it is the first study to assess HRQoL in response to different volumes of HIIT in type 2 diabetes. In agreement with our hypothesis, this study showed that high-volume HIIT can be more effective than low-volume HIIT for glycemic control in type 2 diabetic women. The main findings of the present study can be summarized as follows: (a) High-volume HIIT induced significantly more improvements in blood glucose measures (i.e., HbA1c, FBG, and 2-hr PPBG), TG, LDL, and cardiorespiratory fitness than low-volume HIIT; (b) Both low- and high-volume HIIT showed similar improvements in TC, HDL, SBP, DBP, anthropometric adiposity measures (i.e., BMI, WC, and waist-to-hip ratio), and HRQoL; (c) Both low- and high-volume HIIT showed significantly greater improvements in all outcome measures compared to the baseline and no exercise, except for DBP following low-volume HIIT.

The present study showed that high-volume HIIT is superior to low-volume HIIT for improving blood glucose measures in obese type 2 diabetic women. This finding followed RezkAllah and Takla,17 who found that high-volume HIIT was more beneficial in reducing HbA1c and FBG than low-volume HIIT in overweight prediabetics on a low-calorie diet. Contrary to our findings, Tjønna et al.,32 found that low- and high-volume HIIT induced similar improvements in FBG in healthy men. However, since FBG levels were within normal at baseline in their study,32 it is reasonable to suggest that there was no space for further reductions in blood glucose levels presumably induced by high-volume HIIT (i.e., a floor effect). Interestingly, the present study showed that the absolute mean changes in HbA1c were ↓1.03% (i.e., from 8.15 ± 0.52 to 7.12 ± 0.49%) in the low-volume HIIT group and ↓1.5% (i.e., from 8.15 ± 0.56 to 6.65 ± 0.17%) in the high-volume HIIT group. These reductions were not only statistically significant but also clinically meaningful, as each 1% reduction in mean HbA1c is associated with a 21% reduction in diabetes-related deaths, a 14% reduction in myocardial infarction incidence, and a 37% reduction in microvascular complications.34 Additionally, the high-volume HIIT group in the present study had a post-intervention HbA1c of 6.65 ± 0.17%, showing a better state of glycemic control. Possible mechanisms for enhanced glucose uptake by muscles during and following HIIT include a higher degree of recruitment of muscle fibers and use of muscle glycogen, improved insulin sensitivity, and insulin-independent glucose transporter 4 translocation.35

The present study also showed that high-volume HIIT was more effective for reducing TG and LDL than low-volume HIIT. Nevertheless, both low- and high-volume HIIT similarly improved TC and HDL. Also, both low- and high-volume HIIT led to significant improvements in all TC, TG, HDL, and LDL compared to the baseline and no exercise in this study. Contrary to our study, Ramos et al.16 found that triglycerides showed non-significant reductions compared to the baseline following both low-volume HIIT (1 × 4 min) and high-volume HIIT (4 × 4 min) in a subgroup of patients with type 2 diabetes. However, consistent with our findings, a recent systematic review reported that HIIT has positive effects on the control of blood lipids, especially in diabetic patients.36

Another main finding in the present study is that both low- and high-volume HIIT similarly improved systolic and diastolic blood pressure. This finding is in line with a previous finding by Tjønna et al.32 who found that both low- and high-volume HIIT induced similar improvements in systolic and diastolic blood pressure. A recent study by Soltani et al.37 has also shown that both low- and high-volume HIIT led to similar improvements in SBP in patients with stage I hypertension. On the other hand, Ramos et al.16 reported non-significant changes in SBP and DBP following high-volume HIIT (4 × 4 min) in type 2 diabetics. Of interest, the present study showed that the absolute mean changes in SBP were −3.79 mmHg (i.e., from 125.46 ± 3.65 to 121.67 ± 3.89 mmHg) in the low-volume HIIT group and −4.45 mmHg (i.e., from 125.33 ± 4.16 to 120.88 ± 3.75 mmHg) in the high-volume HIIT group. Whereas the absolute mean changes in DBP were −2.21 mmHg (i.e., from 82.38 ± 4.03 to 80.17 ± 4.47 mmHg) in the low-volume HIIT group and −3.54 mmHg (i.e., from 82.71 ± 4.01 to 79.17 ± 4.32 mmHg) in the high-volume HIIT group. These findings are consistent with the 2017 International Guidelines for High Blood Pressure in Adults,38 which reported that the usual impact of lifestyle interventions, such as structured exercise programs, on blood pressure is a 4–5 mmHg decrease in SBP and a 2–4 mmHg decrease in DBP. It needs to be highlighted that the reductions in SBP found in the present study following either low- or high-volume HIIT can be deemed clinically significant, as a 4-mmHg decrease in average systolic blood pressure was associated with a 5.8% reduction in stroke risk in Japanese women.39 Also, it was reported that a 5 mmHg reduction of systolic blood pressure can reduce the risk of major cardiovascular events by about 10%, even at blood pressure levels not considered for pharmacological treatment.40 The mechanism for HIIT-induced reduction in blood pressure can be partly explained by the higher shear stress on the vessel wall originating from the alternation between the high- and low-intensity bouts of the HIIT, which stimulates nitric oxide production with resultant vasodilation and lowering of blood pressure.41

This study has also shown that low- and high-volume HIIT similarly induced significant reductions in BMI, WC, and waist-to-hip ratio. Conversely, Ramos et al.,16 found that BMI and body fatness were not significantly changed following either low- or high-volume HIIT. Tjønna et al.,32 however, found that both HIIT subtypes led to similar reductions in BMI and trunk fat in overweight men. In addition, a meta-analysis showed that HIIT interventions resulted in abdominal fat loss.42 Furthermore, it has been shown that overweight women with type 2 diabetes can largely lose abdominal fat following HIIT.43 Possible mechanisms behind HIIT-induced fat loss include boosting fat oxidation during and after exercise, increasing the capacity of the whole body and skeletal muscle for fatty acid oxidation, and reducing appetite post-exercise.43

Noteworthy, our results for TG, BP, and BMI differ from what was reported by Ramos et al., 16 as previously mentioned. It could be assumed that the dietary instructions on healthy food given to patients in the present study might have been followed and played an additional role in improving these measures.

Another important finding in the present study is that cardiorespiratory fitness, assessed by treadmill time to maximal exhaustion, improved significantly following both HIIT protocols compared to the baseline and no exercise. This finding could be supported by a recent meta-analysis documenting that HIIT can lead to improvements in cardiorespiratory function in middle-aged adults.44 Also in this context, the present study showed that the improvement in cardiorespiratory fitness was higher following high-volume HIIT than low-volume HIIT. This observation could be explained on the basis that patients in the high-volume HIIT group performed high-intensity workout for a total of 16 minutes per session, which was twice the total duration of the high-intensity workout per session (i.e., 8 minutes) in the low-volume HIIT group. Because of this, patients in the high-volume HIIT group could have become more accustomed to the longer duration of high-intensity exercise and, accordingly, they could have had more ability to sustain the high incremental workloads of the exercise test than their counterparts in the low-volume HIIT group. This could explain the longer treadmill time to maximal exhaustion in the high-volume HIIT group. Contrary to our findings, Ramos et al.16 reported a non-significant difference between low- and high-volume HIIT in the improvement of cardiorespiratory endurance, as assessed by maximum oxygen consumption (VO2 max), despite a higher magnitude of changes following the latter type of training. It should, however, be pointed out that endurance performance can be improved without a concomitant increase in VO2 max.29 Thus, a possible reason for such discrepancy in the results between our study and theirs may be attributed to the difference in the measures used to assess cardiopulmonary fitness. Of note, given that low cardiorespiratory endurance is an independent predictor of mortality in patients with type 2 diabetes,45 the improvement in cardiorespiratory fitness following low- and high-volume HIIT in the present study, evidenced by a longer treadmill time to maximal exhaustion compared to the pretraining time, can be of clinical relevance.

Both generic and diabetes-specific measures can be used to evaluate HRQoL in type 2 diabetes.46 The SF-12 Health Survey was used in the present study to assess HRQoL in response to interventions. The present study showed that the PCS and the MCS mean scores of the SF-12 were similarly improved in the low- and high-volume HIIT groups. Considering that cardiorespiratory fitness improved in both groups compared to pre-training levels, as previously mentioned, it is reasonable to assume that the tolerance to daily life activities could have also improved, resulting in enhanced physical HRQoL. Enhanced HRQoL in the present study could also be attributed to improved blood glucose measures in both HIIT groups compared to baseline values. This explanation could be supported by a previous report indicating that improving glycemic control reduces the negative impact of diabetes on HRQoL by slowing down the onset and/or progression of diabetes-related complications.46 Furthermore, since obesity has a clear relationship with reduced HRQoL,47 reducing obesity measures can lead to better HRQoL. Viewed in this way, the significant reductions in adiposity measures in the low- and high-volume HIIT groups could be possible reasons for the improved HRQoL reported by these groups in the present study. It has to be mentioned that poor HRQoL relates to higher cardiovascular mortality,19 and hence, the enhanced HRQoL reported in the current study may be associated with better cardiovascular health. Interestingly, although there was no statistically significant difference in the MCS between the low- and high-volume HIIT groups post-intervention in the present study, there was a trend towards significance in favor of the high-volume HIIT with a p-value of 0.075 (Table 4), suggesting that high-volume HIIT may be better for the mental HRQoL in type 2 diabetic women. In a similar context, Ahmad and Ali,48 evaluated the effect of 4 × 4 min high-volume HIIT on HRQoL, assessed by the Chronic Liver Disease Questionnaire (CLDQ) in middle-aged women with fatty liver disease and found that high-volume HIIT improved emotional function and worry domains of the CLDQ to a greater extent than moderate-intensity combined resistance and aerobic training.

Unsurprisingly, the present study found that traditional high-volume HIIT showed significant improvements in all outcome measures compared to the baseline and no exercise. In line with these findings, a recent meta-analysis documented that HIIT leads to significant reductions in FBG and Hb1Ac compared with no exercise.49 Also, it has been shown that long-term HIIT (≥12 weeks) can significantly improve SBP, DBP, waist circumference, and body fat percentage in overweight or obese individuals.50 Furthermore, HIIT has been shown to induce positive changes in blood lipids in type 2 diabetics.36 Interestingly, the present study found that low-volume HIIT also showed significant improvements in blood glucose measures, blood lipids, anthropometric adiposity measures, SBP but not DBP, cardiorespiratory fitness, and HRQoL compared to the baseline and no exercise. Conforming to these findings, Peng et al.,20 in their meta-analysis, reported that low-volume HIIT can induce a significantly greater reduction in FBG and a more significant increase in HDL than no exercise in type 2 diabetics. They also reported that low-volume HIIT can significantly lower HbA1c, TC, TG, LDL, SBP (i.e., but not DBP), and body mass index in type 2 diabetes.20 Lastly, it is important to point out that no unintended effects or harms were reported during or following training in either of the HIIT groups in the present study.

The practical implications of the present study can be that although high-volume HIIT could be better than low-volume HIIT for control of blood glucose, TG, and LDL levels and for enhancing cardiorespiratory fitness, low-volume HIIT could still be a potential exercise strategy to obtain similar clinical benefits in TC, HDL, BP, anthropometric adiposity measures, and HRQoL, but with less time, in type 2 diabetic women. Since the main barrier to exercising is lack of time, the more time-efficient 2 × 4 min low-volume HIIT can be an appealing exercise form for individuals who may have a tight schedule due to work or family commitments.

Finally, as with the majority of studies, this study has limitations, and its results should be viewed in light of them. The limitations of this study include a lack of assessment of cardiorespiratory fitness by VO2 max (i.e., the gold standard measure) due to the unavailability of the measurement unit. Also, double blinding was not feasible in the present study owing to practical reasons related to the nature of the interventions. In addition, the assessor was blinded only for the biochemical analysis of blood glucose measures and serum lipids. Furthermore, only obese women patients were included in the present study, which might affect the generalization of the findings. Moreover, there was a lack of dietary monitoring (i.e., general dietary instructions with no dietary records). Further limitations could be a lack of control for possible hormonal influences in the studied women (i.e., possible effects of menopause, menstrual phases, or use of contraceptive pills). The present study also did not account for the dosage of oral antidiabetic and antihyperlipidemic medications in its data analysis. Nevertheless, despite its limitations, the present study has several strengths. This study is one of the few studies looking at the different responses to low and high doses of HIIT in type 2 diabetes concerning cardiometabolic outcomes and cardiopulmonary fitness. In addition, the present study is the first to assess HRQoL in response to two different subtypes of HIIT (i.e., low- and high-volume HIIT) in type 2 diabetes. Furthermore, the present study included a diversity of outcome measures that helped establish a comprehensive patient assessment at baseline and in response to the interventions. Moreover, the findings of this study can be of clinical relevance to researchers, clinicians, and physiotherapists involved in diabetes care.

5. Conclusion

Low-volume HIIT could be equally effective as high-volume HIIT for improving TC and HDL, systolic and diastolic blood pressure, anthropometric adiposity measures, and HRQoL in middle-aged obese women with type 2 diabetes on oral antidiabetic medication. However, high-volume HIIT could be better than low-volume HIIT for glycemic control, TG and LDL control, and cardiorespiratory fitness in these patients. The findings of this study may be of clinical significance for those interested in lifestyle therapy/exercise interventions for type 2 diabetes. Nevertheless, future randomized-controlled studies addressing the current study's limitations are needed to verify our findings.

CRediT author statement

Ahmad Mahdi Ahmad: Conceptualization, Methodology, Validation, Formal analysis, Writing - Original Draft Preparation, Writing - Review & editing, Visualization, Supervision, Project Administration. Asmaa Mohamed Mahmoud: Methodology, Investigation, Resources, Writing-Original Draft Preparation, Project Administration. Zahra Serry: Supervision. Mohamed Mady: Supervision. Heba Ali: Revision.

Declaration of competing interest

The authors declare no conflict of interest.

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