Yoga for Management of Type 2 Diabetes: A Review for Clinicians

Methods

Electronic searches of MEDLINE, PubMed, Google Scholar, CINAHL, Academic Search Premier, Health Source: Academic edition, SportDiscus, and AltHealth Watch were performed for experimental studies through December 2015. Figure 1 details the data extraction process. Intervention trials targeting the treatment or control of type 2 diabetes mellitus using primarily yoga asana (postures) were included in this review. Studies that focused only on breathing (pranayama) or meditation as the yoga intervention were not included. Additionally, studies that included nonexperimental study designs, no quantitative data, were written in a non-English language, and included individuals less than 18 years old were excluded. Studies that included individuals without a designated type 2 diabetes diagnosis (i.e., metabolic syndrome, prediabetes, gestational diabetes, cardiovascular disease) were not included in this review. Articles that did not include biochemical, physiological, anthropometric, or clinical outcomes were also excluded.

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Outcomes measured in the studies reviewed included fasting blood glucose (FBG), postprandial blood glucose (PPBG), HbA1c, and anthropometric measures.

Summary of Findings

Among the 19 identified studies (see Table 1), 6 used a randomized controlled design (40,41,46,47,51,58), 9 used a quasi-experimental design (5,6,16,25,31,34,42,43,52), and 4 used a pre-post design (19,33,55,59). The majority of studies did not specifically state a style of yoga or specifically which postures were chosen, making it difficult to indicate if any specific style of yoga or yoga postures were more effective than others. The studies selected for this review (Table 1) used asana as the primary intervention technique.

TABLE 1. Summary of Study Characteristics and Findings.

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TABLE 1. Continued.

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TABLE 1. Continued.

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The duration of yoga interventions ranged from 7 d (55) to multiple sessions over a 9-month intervention (40). There was a notable absence of long-term intervention designs. For the majority of interventions, outcome assessments were conducted between 3 to 6 months. To accurately assess the long-term effect of yoga interventions on type 2 diabetes symptoms, extended follow-up assessments should be measured.

Fasting Blood Glucose/Postprandial Blood Glucose

The studies reviewed reported significant decreases in FBG following the yoga interventions (5,16,19,25,31,33,34,35,40,41,42,47,52,58,59). Participants were prescribed oral antihyperglycemic agents and/or insulin in some of the studies (5,6,16,34,35). Postprandial blood glucose (PPBG) was significantly reduced in numerous studies (5,16,19,25,33,41,42). Significant reductions in FBG and PPBG were achieved when the yoga intervention included at least 3 sessions per week, at least 30 min per session (up to 90 min), and lasted at least 2 weeks. These design qualities resulted in FBG reductions of 10 to 50+ mg.dL⁻¹ (5,16,19,25,31,33,40,41,42,47,52,58,59). One study reported significant, progressive reductions in FBG each month over 6 months in patients not yet prescribed oral medications (1 h, daily practice) (42). In a larger trial (n = 231) with participants randomized to yoga, conventional exercise, or control, FBG was reduced by 30% after 6 months of yoga practice and 27% after conventional exercise, compared to a 7% reduction in the control group receiving standard care (22). This preliminary pilot data demonstrates reductions in FBG equivalent and potentially greater to that of oral medications and conventional exercise (42).

Reductions in PPBG ranged from 17 to 110 mg·dL⁻¹ across the previous studies. When yoga was performed daily, the greatest reductions in FBG and PPBG were observed. A minimum of 30 min per day was an effective duration to reduce FBG and PPBG on at least 5 d of the week. Daily sessions appear to result in the greatest reduction in FBG and PPBG. Although long-term outcomes, adherence, and sustainability have not been studied, these preliminary findings demonstrate reductions in FBG and PPBG in the short term. The current literature could support the recommendation for participation in regular yoga-asana practice soon following diagnosis to help control glucose, prevent the need for oral medications or insulin, improve glucose control in conjunction with oral medications and insulin, and possibly prevent the conversion to overt diabetes in those with prediabetes. The lack of adverse events related to yoga participation provides additional confidence when recommending yoga as a therapeutic modality.

These findings are consistent with known physiological responses of glucose to activity as exercise enhances glucose uptake even when insulin resistance is present (17). The effect of exercise on glucose uptake lasts up to 48 h. Therefore, asana-based yoga should be recommended daily, or at least every other day, to achieve the desired effect on blood glucose (17,39,61).

Hemoglobin A1c

Multiple studies have demonstrated reductions in HbA1c following a yoga-asana intervention (5,25,41,42,43,51,58, 59). Current recommendations are to achieve glucose control represented by a HbA1c of 7% or lower, as this has been shown to result in reduced risk of developing chronic complications associated with diabetes such as kidney disease, retinopathy, microalbuminuria, stroke, and heart disease (30). Yoga interventions have produced significant reductions in HbA1c (approximately 1% on average), in many of the studies, reducing measures below the recommended control value of 7% (6,27). Studies that reported significant reductions in HbA1c were those with supervised sessions lasting 45–90 min in duration, 3 to 6 d per week, or daily for at least 45 d and up to 12 weeks (5,25,41,43,51, 58,59).

Anthropometric Measures

A weight loss threshold of >5% has been previously reported to achieve beneficial metabolic outcomes, such as improved HbA1c, beneficial lipid changes, and reduced blood pressure, in patients with type 2 diabetes (21). A recent review reported body weight reductions following asana-based interventions in the range of 3–7% (5,50). In a group of adults with type 2 diabetes (n = 24), Tikhe et al. (55) reported a 1.06% decrease in weight (72.3 ± 13.3 kg to 71.5 ± 12.7 kg, p < 0.001) following a 7-d yoga intervention. Similarly, following a daily 60-min yoga class for 3 months, Shantakumari et al. (47) observed a reduction of approximately 2 kg (p < 0.001) in participants that combined yoga with an oral hypoglycemic drug in comparison to a control group receiving only medication. Weight was significantly reduced following a 45-d yoga-asana intervention, performed daily for 45 min, among middle-aged adults (50). The yoga-asanas were combined with standard oral hypoglycemic medications, which led to a significant reduction in weight and a nonsignificant reduction in body mass index (BMI), while the control group (medications only) had a significant increase in weight and a nonsignificant increase in BMI (3 kg weight loss, p < 0.05; 1 kg weight gain, p < 0.05) (50).

Similarly, significant reductions in weight were reported following a 3-month daily yoga-asana intervention compared to a control group (yoga and oral meds 66 ± 5 to 62 ± 5, p < 0.001; yoga, oral meds, and insulin 64 ± 5 to 62 ± 4.4, p < 0.001; control, 69 ± 4 to 65 ± 5) (5). Additionally, 1 study reported a significant reduction in waist-to-hip ratio (p < 0.05) (46) following a 3-month daily yoga intervention. Despite the short duration and small sample size of many studies, incorporating yoga-asanas has been shown to produce significant reductions in body weight independently and in conjunction with hypoglycemic medication use (6,47,50,54).

These reductions provide preliminary evidence-based suggestions for duration and frequency of yoga for reducing blood glucose measures, HbA1c, and anthropometric measures. The reductions reported are similar to those seen in traditional moderate-intensity exercise programs (13). Design varied greatly across studies, which reduces the ability to develop specific recommendations for this population. Importantly no adverse events were reported in any of the studies. Control of blood glucose is important for preventing and reducing diabetes-related complications such as eye, kidney, nerve, and cardiovascular diseases (30,53). Intensity of the yoga exercise was not measured; however, other studies among healthy individuals indicate that the intensity of asana-based yoga varies significantly across studies (10 to 27% of VO₂max) (56). Direct measurement of metabolic equivalent of task (MET) levels of asanas/postures were >2 METs depending on the style of yoga, which is considered low-intensity activity (56). Despite the low intensity of yoga-asanas, the previously mentioned studies still demonstrated positive and significant reductions in FBG, PPBG, and HbA1c. Although intensity was not directly measured in the majority of the current literature, significant reductions in FBG and PPBG were observed, suggesting that the low level of exercise intensity generated sufficient muscular contraction to promote increased glucose uptake and clearance. Further examination of the mechanisms underlying the metabolic changes associated with low-intensity yogic exercise is needed.

Future Research and Conclusions

Although the number of studies in this area has increased in recent years, there is a significant need for more systematic, methodologically robust interventions examining the efficacy of yoga as a viable treatment option for type 2 diabetes. Common limitations identified in the previous reviews included small sample size, nontheoretical frameworks, lack of program fidelity evaluation, significant variability in research design, and inconsistency in the populations studied (i.e., broad range of disease profiles and co-occurring illnesses). Furthermore, variation in style of yoga, dosage, and intensity; lack of definitions for yoga; and limited details related to the intervention protocol made comparison across studies challenging. Future studies must address these limitations and focus on the reliable quantification of benefits needed to guide clinical practice and policy.

Given the global burden of type 2 diabetes mellitus and the evidence of ethnic disparities associated with type 2 diabetes mellitus in Western countries (26,38), it may be beneficial for future research to test intervention protocols designed to manage symptoms associated with type 2 diabetes in other countries. There is also a need for adequately powered randomized trials examining the metabolic outcomes of habitual yoga practice. Future research of yoga interventions and adults with type 2 diabetes should include anthropometric measures to evaluate the effect of the intervention on weight loss and type 2 diabetes-related metabolic outcomes, including measures of insulin resistance. Finally, because type 2 diabetes is a lifelong disease, more interventions are needed that include a long-term intervention design. This would provide more accurate yoga recommendations for sustainable lifestyle, long-term behavioral changes, and the feasibility of maintaining long-term adherence to a yoga program.