In Korea, the prevalence of diabetes increases sharply from 2.5% in those aged 30~39 years to 13.0% in those aged 50~59 years, and 26.5% in those aged ≥70 years, showing an increasing trend as age increases [1]. The diabetes-related mortality rate was 17.1% in 2018; the condition ranked sixth in the most common causes of death in adults that year [2], and by 2050, the prevalence of diabetes is expected to increase by more than twice the current level [3]. Type 2 diabetes is a progressive disease caused by an impaired function of pancreatic beta cells and decreased insulin secretion. Treatment for type 2 diabetes is chosen according to the degree of disease progression [4]. In particular, patients with type 2 diabetes have to improve their lifestyle habits (e.g., diet, exercise, and weight control) and also take oral hypoglycemic agents [5]. Many patients are prescribed insulin injections due to impaired insulin secretion after several years, with insulin therapy becoming stronger over time [6]. Moreover, because there is a tendency for insulin secretion to decrease to approximately 25% of the normal level at 6 years after diagnosis, early insulin therapy is recommended to protect beta cells, maintain an appropriate level of glycated hemoglobin, and prevent cardiovascular complications [5].

Insulin therapy adherence is an important factor in diabetes self-management that influences the maintenance of an appropriate level of blood sugar, and the risk of complications and mortality [7]. However, most patients with diabetes show low insulin therapy adherence, and the major causes associated with this have been identified as patient-related factors such as the fear of needles (for insulin injection), becoming hypoglycemic, weight gain, and complications associated with insulin administration; system-related factors such as inadequate follow-up and lack of a support system; and drug-related factors such as complicated insulin therapy and frequent changes in dosage [8]. Additionally, patients may have negative attitudes that interfere with their insulin therapy adherence. They may express anxiety about starting insulin therapy or refuse to administer insulin. This phenomenon has been termed “psychological insulin resistance” [9]. The major causes of psychological insulin resistance include patients having a misperception that insulin administration means failed diabetes management, pain associated with injection, anxiety about complications associated with insulin administration, absence of family members who can help with insulin administration, and being misinformed about insulin therapy [10]. From a long-term perspective, successful insulin therapy requires better understanding of psychological insulin resistance in patients and identification of the related factors [11].

In addition to psychological insulin resistance, patients with type 2 diabetes also experience lifestyle changes associated with diabetes management, changes in social activities, and high emotional stress concerning diabetes complications [12]. This experience is referred to as “diabetes distress.” Diabetes distress represents a negative emotional response from the patient to their own lives as individuals learning to live with diabetes [13]. Diabetes distress can have the physiological effect of elevating blood sugar by activating cortisol, growth hormones, and endorphin [12]. This distress is also known to have negative emotional effects. For example, it can hinder self-care and therapy adherence by lowering the will to manage diabetes [14]. Consequently, it is important to reduce diabetes patients' negative attitudes toward disease therapy and their own lives, in order to increase therapy adherence. Moreover, in many patients, the negative response toward insulin therapy is based on the belief that they will lose control over their own lives once insulin administration begins. Therefore, the confidence that they can prevent hyperglycemia and complications associated with diabetes and maintain their well-being can help increase insulin therapy adherence [9]. “Self-efficacy” refers to the ability of individuals to achieve their desired outcome or effect. Individuals with high self-efficacy not only think and behave in a manner that supports their aims but also tend to put more effort into problem-solving, which enables them to better overcome crises [15]. “Patient self-efficacy” refers to the ability of a patient to manage disease-related factors; when self-efficacy is high, the patient has positive thoughts about overcoming his or her disease and puts greater effort into adapting to treatment [16].

Berg and Upchurch's [17] model of couples coping with chronic illness suggests that the stress and disease awareness of patients interacts with the stress and disease awareness of their spouses. Such evaluative factors have a mutual effect on the process of patient and spouse coping and adapting to the disease. Because the couple's relationship is one of high interdependency, the psychological insulin resistance and diabetes distress of the patient with type 2 diabetes affects that of the spouse. The self-efficacy of the spouse refers to his or her level of confidence in providing physical and/or emotional care to the patient [18]. Thus, spouses with high self-efficacy have lower negative emotions while caring for patients, which allows them to provide better care and affects the self-efficacy of the patient, helping the patient to have a more positive attitude toward the therapy process [19]. Consequently, the diabetes self-efficacy of a patient with type 2 diabetes and his or her spouse can increase therapeutic effect by reducing negative emotions regarding therapy and increasing adherence to therapy. However, most Korean studies on insulin therapy adherence in patients with type 2 diabetes have only considered the patient's perspective, and studies on psychological insulin resistance have been limited to concept analyses and the development of measurement tools.

The present study applied the actor-partner interdependence model (APIM) [20] to its analysis of variables related to the insulin therapy adherence of patients with type 2 diabetes that apply to both the patient and his or her spouse. When dyadic data are treated as individual data, the dynamics between couples cannot be examined. Even if the data are collected from both members of a couple, analysis of such interdependent data as independent data would violate the major hypothesis of inferential statistics that the data are independent. Accordingly, the standard error would appear lower than it is, and type I error may occur. The APIM [20], a model for dyadic data analysis, can be used to analyze the data of interdependent dyads. Psychological insulin resistance, diabetes distress, diabetes self-efficacy, and insulin therapy adherence of patients with type 2 diabetes and their spouse must be analyzed by dyadic analysis. The purpose of this study was to identify the effects of psychological insulin resistance, diabetes distress, and the diabetes self-efficacy of patients and their spouses on insulin therapy adherence of patients with type 2 diabetes using APIM.

The purpose of this study was to identify the effects of psychological insulin resistance, diabetes distress, and diabetes self-efficacy of patients and their spouses on the insulin therapy adherence of patients with type 2 diabetes.

First, the psychological insulin resistance of patients with type 2 diabetes showed actor and partner effects on diabetes self-efficacy. This was consistent with the model proposed by Berg and Upchurch [17], who claimed that stress and disease awareness in patients interact with stress and disease awareness perceived by their spouse, while such evaluative factors mutually affect the process of patient and spouse coping and adapting to the disease. Because patients with type 2 diabetes need to personally administer insulin therapy, which is more complicated than therapies for other diseases [8], psychological insulin resistance, which is a negative emotion, is believed to affect the diabetes self-efficacy needed for efficient self-management of the disease. In particular, patients often need to rely on their spouse to help with insulin therapy; the psychological insulin resistance of the patient affects the diabetes self-efficacy of the spouse. Moreover, the psychological insulin resistance of the spouse also affects the diabetes self-efficacy of the spouse. Ultimately, the psychological insulin resistance of patients with type 2 diabetes is not only a problem for the patient, but it is a negative emotion that can also occur in the spouse. Considering that diabetes self-efficacy plays an important role in increasing the drug and insulin therapy adherence of the patient [9], medical professionals should provide interventions that target the psychological insulin resistance in both the patient with type 2 diabetes and the spouse.

Second, the stress of patients with type 2 diabetes was found to affect the diabetes self-efficacy of the spouse. The disease awareness and stress of the patient interact with the perceived disease awareness and stress of the spouse [17]; thus, the diabetes-related stress experienced by the patient is believed to affect the diabetes self-efficacy of the spouse. In particular, the spouse of a patient with type 2 diabetes performs various diabetes management activities, such as insulin administration, dietary management, and skin care, in daily life together with the patient. Thus, negative emotions of the patient that are related to the disease can lower the level of confidence the spouse has in providing physical and/or emotional care to the patient [18]. However, self-efficacy of the spouse can be a driving force for performing patient care work, which, consequently, can lead to the patient having a more positive attitude toward the therapy process [19]. Therefore, medical professionals should develop and implement stress management programs for patients with diabetes, to increase the diabetes self-efficacy of the spouse. Such programs are likely to have positive therapeutic effects for patients with type 2 diabetes.

Third, the diabetes self-efficacy of the patient and spouse were identified as factors that affect the insulin therapy adherence of the patient. This was consistent with a previous study on patients with diabetes [24] that reported that diabetes self-management education that included family members had a positive effect on diabetes management and maintenance by having a positive effect on the self-management behavior and self-efficacy of the patient. In particular, diabetes self-efficacy can directly strengthen self-management and improve the diabetes therapy adherence of the patient to have a long-term effect of lowering glycated hemoglobin levels [25]; thus, it is necessary for medical professionals to include the spouse in diabetes programs and education for the patient to improve the insulin therapy adherence of patients with type 2 diabetes.

Based on such findings, it is important to consider both the patient and spouse when addressing the insulin therapy adherence of patients with type 2 diabetes. In particular, the psychological insulin resistance of the patient and insulin resistance of the spouse can affect the diabetes self-efficacy of the patient and spouse concerning the therapy process, which ultimately affects the patient's therapy adherence. Therefore, it is necessary for medical professionals to develop interventions that can lower psychological insulin resistance and increase the diabetes self-efficacy of both the patient and spouse. However, it is necessary to conduct future longitudinal studies for an in-depth analysis of the effects of psychological insulin resistance, stress, and the diabetes self-efficacy of patients and their spouses on insulin therapy adherence. This study did not analyze the insulin therapy adherence of patients by age group. In future studies, it is necessary to grasp the degree of insulin therapy adherence perceived by patients and caregivers by age group and to determine factors affecting insulin therapy adherence.

The present study was conducted to identify the effects of psychological insulin resistance, diabetes distress, and the diabetes self-efficacy of patients with type 2 diabetes and their spouse on the insulin therapy adherence of patients. The findings provide basic data for the development of programs that can enhance the insulin therapy adherence of patients with type 2 diabetes from the perspectives of both the patient and spouse.