Diabetes is a serious metabolic health problem that greatly impacts the way nurses and other healthcare professionals provide care for many patients. An estimated 30.3 million individuals or 9.4% of the U.S. population have diabetes, with another 7.2 million cases (23.8% of the population) unreported (Centers for Disease Control and Prevention [CDC], 2018). Physical inactivity significantly contributes to the development of type 2 diabetes and is one of the most modifiable risk factors (Guthold, Stevens, Riley, & Bolt, 2018; Ross et al., 2016). It is well established that physical activity and regular exercise improve quality of life and prevent the progression to the development of prediabetes and type 2 diabetes (Colberg et al., 2016; Physical Activity Guidelines Committee, 2018; World Health Organization [WHO], 2018; Sallis et al., 2016; International Society for Physical Activity and Health [ISPAH], 2017), however despite this knowledge, only an estimated 31-37% of individuals with type 2 diabetes engage in adequate quantities of exercise (Sluik et al., 2012).
The American Diabetes Association (ADA, 2018) recommends individuals with type 2 diabetes participate in at least 150 minutes of moderate to vigorous intensity aerobic activity across 3 days per week (younger adults or more fit individuals may participate in vigorous or interval intensity activity for 75 or more minutes per week), with no more than 2 consecutive days without activity. The ADA also advises participating in resistance training two to three nonconsecutive days per week, as well as flexibility and balance training two to three times per week for older adults. While these guidelines exist, questions remain regarding the individual benefits of exercise performed at various intensities on glucose levels and glycemic control (HbA1c), as well as what parameters constitute each intensity level across exercise studies. Additionally, not all individuals may be able to participate in moderate to vigorous exercise regimens on a consistent basis, as suggested per ADA guidelines (ADA, 2018). These disparities in knowledge and implementation provide barriers to more widespread and effective realization of exercise prescriptions for clients with type 2 diabetes.
This review of the literature seeks to examine the outcomes of past studies that have been conducted to suggest better unification of exercise-intensity related clinical studies in future research. These findings will help produce a more cohesive and comparable atmosphere for future studies, and more specific conclusions to be drawn across study comparisons. Ultimately, this will provide insight into the development of personalized and efficacious exercise prescriptions, which in turn will facilitate greater healthcare professional knowledge, increased nursing and provider educational capacity, and more consistent integration of exercise as a treatment modality into patient lifestyle habits.
Methods:
We performed a systematic review of the literature to analyze methodology and metabolic outcomes across studies assessing effects of exercise intensity in individuals with type 2 diabetes. For purpose of analysis, we categorized each intensity intervention according to absolute measures of intensity consistent with the American College of Sports Medicine [ACSM] (2017), which included MET, percent VO2max, percent of heart rate reserved, percent of maximum heart rate, rating of perceived exertion (per Borg Scale), and percent of one repetition maximum: Category 1 corresponds with very light exercise; Category 2, light exercise; Category 3, moderate exercise; Category 4, vigorous exercise, and Category 5, near-maximal to maximal exercise. Articles were retrieved from PubMed and CINHAL using the subject headings type 2 diabetes mellitus, exercise, insulin resistance, insulin sensitivity, blood glucose. Selected articles were limited to randomized controlled trials involving adults, published in English between 1984 to 2018. Ultimately, 29 articles met inclusion criteria.
Results:
Six studies included maximum-intensity or sprint interval (Category 5) intervention groups, eighteen included high-intensity (Category 4) interventions, seventeen included moderate-intensity (Category 3) interventions, and one study included a low-intensity (Category 2) exercise intervention group. Thirteen studies included a no-exercise control group, twelve studies compared Category 4 with Category 3 interventions, and one study compared Category 5 with Category 4 intervention. Additionally, five studies evaluated the effect of interval versus continuous exercise and six utilized resistance training as either an exercise intervention, or component of their primary intervention. There was considerable variation in the names used for different categorizations of exercise. For example, high intensity groups (Category 4) were classified by six different intervention names: high intensity, high intensity interval training, high intensity intermittent training, high intensity interval exercise, reduced-exertion high-intensity interval training, and high intensity aerobic training; moderate intensity exercise (Category 3) was classified by four different names; moderate intensity training, moderate intensity continuous exercise, moderate intensity exercise, and moderate intensity walking.
There was considerable variety in results across the 29 articles; more than 50 outcome measurements were reported, none of which were present across all studies. Results reported most frequently noted included HbA1c (reported in 20 articles), fasting glucose (21 articles), fasting insulin (8 articles), and HOMA-IR (11 articles).
Another notable trend was the diversity in categorization of intensity as low, moderate, and high in the study methodology. Six studies verbally categorized groups as low intensity, however these groups fell into the moderate intensity when categorized per ACSM definitions (Balducci et al., 2012; Da Silva et al., 2012; Li et al., 2012; Yang et al., 2017; Usui et al., 1998; Hazley, Ingle, Tsakirides, Carroll, & Nagi, 2010). Two studies also included moderate intensity groups per author categorization that included ACSM low-intensity measurements in their range (Taylor, Fletcher, Mathis, & Cade, 2014; Usui et al., 1998). One study reported the parameters for their high-intensity group, but did not do so for their moderate-intensity (Hollekim-Strand et al., 2014), and another likewise did not report parameters used to achieve its participants’ moderate intensity goal (Honkola, Forsén, & Eriksson, 1997). This variation and lack of delineation in the categorization of exercise groups makes it difficult to compare across groups without implementation of a scoring system as was used in this study.
Conclusions:
The variability in methodology, exercise classification, and measurement of results contributed to inconsistent outcomes and increased difficulty in comparisons across studies. These disparities create barriers towards the formation of more specific conclusions within and across studies, which may hinder implementation of optimal exercise interventions in type 2 diabetic care.
The most important concern for individuals with T2DM is whether training leads to improvements in glucose tolerance or insulin sensitivity (such as HOMA-IR) on a long-term basis. Study outcomes from aerobic and resistance exercise training in T2DM are variable and may include improved glucose control due to adaptations in skeletal muscle, adipose tissue, and liver function associated with enhanced insulin action (Bacchi et a., 2012; Hallsworth, Fattakhova, Hollingsworth, 2011), however weight loss may be necessary in order to maximize beneficial adaptation in glycemic control (Solomon). Exercise training influences glucose tolerance directly by improving insulin sensitivity, or indirectly by preventing obesity or decreasing centrally located fat as a result of weight loss (Lidegaard et al., 2015).
We recommend all future T2D studies analyzing the metabolic effects of exercise intensity include HbA1c, fasting glucose, fasting insulin, and body composition changes to enhance comparison capacities across studies. Additionally, adherence to a core intensity rating system such as that proposed by ACSM should be utilized to likewise enhance comparisons and ensure accuracy of intensity recommendations.