Effect of diet on type 2 diabetes mellitus: A review

Abstract

Globally, type 2 diabetes mellitus (T2DM) is considered as one of the most common diseases. The etiology of T2DM is complex and is associated with irreversible risk factors such as age, genetic, race, and ethnicity and reversible factors such as diet, physical activity and smoking. The objectives of this review are to examine various studies to explore relationship of T2DM with different dietary habits/patterns and practices and its complications. Dietary habits and sedentary lifestyle are the major factors for rapidly rising incidence of DM among developing countries. In type 2 diabetics, recently, elevated HbA1c level has also been considered as one of the leading risk factors for developing microvascular and macrovascular complications. Improvement in the elevated HbA1c level can be achieved through diet management; thus, the patients could be prevented from developing the diabetes complications. Awareness about diabetes complications and consequent improvement in dietary knowledge, attitude, and practices lead to better control of the disease. The stakeholders (health-care providers, health facilities, agencies involved in diabetes care, etc.) should encourage patients to understand the importance of diet which may help in disease management, appropriate self-care and better quality of life.

Introduction

Diabetes mellitus (DM) was first recognized as a disease around 3000 years ago by the ancient Egyptians and Indians, illustrating some clinical features very similar to what we now know as diabetes.1 DM is a combination of two words, “diabetes” Greek word derivative, means siphon - to pass through and the Latin word “mellitus” means honeyed or sweet. In 1776, excess sugar in blood and urine was first confirmed in Great Britain.2,3 With the passage of time, a widespread knowledge of diabetes along with detailed etiology and pathogenesis has been achieved. DM is defined as “a metabolic disorder characterized by hyperglycemia resulting from either the deficiency in insulin secretion or the action of insulin.” The poorly controlled DM can lead to damage various organs, especially the eyes, kidney, nerves, and cardiovascular system.4 DM can be of three major types, based on etiology and clinical features. These are DM type 1 (T1DM), DM type 2 (T2DM), and gestational DM (GDM). In T1DM, there is absolute insulin deficiency due to the destruction of β cells in the pancreas by a cellular mediated autoimmune process. In T2DM, there is insulin resistance and relative insulin deficiency. GDM is any degree of glucose intolerance that is recognized during pregnancy. DM can arise from other diseases or due to drugs such as genetic syndromes, surgery, malnutrition, infections, and corticosteroids intake.5-7

T2DM factors which can be irreversible such as age, genetic, race, and ethnicity or revisable such as diet, physical activity and smoking.8, 9

Epidemiology

Globally, T2DM is at present one of the most common diseases and its levels are progressively on the rise. It has been evaluated that around 366 million people worldwide or 8.3% in the age group of 20-79 years had T2DM in 2011. This figure is expected to rise to 552 million (9.9%) by 2030.10 This disease is associated with severe complications which affect patient’s health, productivity, and quality of life. More than 50% of people with diabetes die of cardiovascular disease (CVD) (primarily heart disease and stroke) and is a sole cause of end stage renal disease which requires either dialysis or kidney transplantation. It is also a major cause of blindness due to retinal damage in adult age group referred to as diabetic retinopathy (DR). People with T2DM have an increased risk of lower limb amputation that may be 25 times greater than those without the disease. This disease caused around 4.6 million deaths in the age-group of 20-79 years in 2011.11

Physical Activity and Lifestyle

A large number of cross-sectional as well as prospective and retrospective studies have found significant association between physical inactivity and T2DM.12 A prospective study was carried out among more than thousand nondiabetic individuals from the high-risk population of Pima Indians. During an average follow-up period of 6-year, it was found that the diabetes incidence rate remained higher in less active men and women from all BMI groups.13 The existing evidence suggests a number of possible biological pathways for the protective effect of physical activity on the development of T2DM. First, it has been suggested that physical activity increases sensitivity to insulin. In a comprehensive report published by Health and Human Services, USA, 2015 reported that physical activity enormously improved abnormal glucose tolerance when caused by insulin resistance primarily than when it was caused by deficient amounts of circulating insulin.14 Second, physical activity is likely to be most beneficial in preventing the progression of T2DM during the initial stages, before insulin therapy is required. The protective mechanism of physical activity appears to have a synergistic effect with insulin. During a single prolonged session of physical activity, contracting skeletal muscle enhances glucose uptake into the cells. This effect increases blood flow in the muscle and enhances glucose transport into the muscle cell.15 Third, physical activity has also been found to reduce intra-abdominal fat, which is a known risk factor for insulin resistance. In certain other studies, physical activity has been inversely associated with intra-abdominal fat distribution and can reduce body fat stores.16 Lifestyle and environmental factors are reported to be the main causes of extreme increase in the incidence of T2DM.17

Patient’s Knowledge Regarding DM

Among the patients, diabetes awareness and management are still the major challenges faced by stakeholders worldwide. Poor knowledge related to diabetes is reported in many studies from the developing countries.18 Some studies have suggested that the occurrence of diabetes is different in various ethnic groups.19 Knowledge is a requirement to achieve better compliance with medical therapy.20 According to a study conducted by Mohammadi21 patient’s knowledge and self-care management regarding DM was not sufficient. Low awareness of DM affects the outcome of diabetes. Another study conducted in Slovakia by Magurová22 compared two groups of patients (those who received diabetes education and those who did not). The results indicated that receiving diabetes education significantly increased awareness about the disease in patients (p < 0.001). The study further concluded that having diabetes knowledge can notably improve patient’s quality of life and lessen the burden on their family. Dussa23 conducted a cross-sectional study on assessment of diabetes awareness in India. The study concluded that level of diabetes awareness among patients and general population was low. Another study conducted in India by Shah24 reported that 63% of T2DM patients did not know what DM is and the majority were also unaware about its complications.

According to the study conducted by Bani25 in Saudi Arabia, majority of the patients 97.3% males and 93.1% females were unaware about the importance of monitoring diabetes, with no significant gender difference. Diabetes knowledge, attitude, and practice were also studied in Qatari type 2 diabetics. The patients’ knowledge regarding diabetes was very poor, and their knowledge regarding the effect of diabetes on feet was also not appreciable.26 Results from a study conducted in Najran, Saudi Arabia27 reported that almost half of the patients did not have adequate knowledge regarding diabetes disease. Males in this study had more knowledge regarding diabetes than female patients. Diabetes knowledge among self-reported diabetic female teachers was studied in Al-Khobar, Saudi Arabia.28 The study concluded that diabetes knowledge among diabetic female teachers was very poor. It was further suggested that awareness and education about diabetes should be urgently given to sample patients. The knowledge of diabetes provides the information about eating attitude, workout, weight monitoring, blood glucose levels, and use of medication, eye care, foot care, and control of diabetes complications.29

Relation between Diet and Type 2 DM

The role of diet in the etiology of T2DM was proposed by Indians as mentioned earlier, who observed that the disease was almost confined to rich people who consumed oil, flour, and sugar in excessive amounts.30 During the First and Second World Wars, declines in the diabetes mortality rates were documented due to food shortage and famines in the involved countries such as Germany and other European countries. In Berlin, diabetes mortality rate declined from 23.1/100,000 in 1914 to 10.9 in 1919. In contrast, there was no change in diabetes mortality rate in other countries with no shortage of food at the same time period such as Japan and North American countries.31 Whereas few studies have found strong association of T2DM with high intake of carbohydrates and fats. Many studies have reported a positive association between high intake of sugars and development of T2DM.32 In a study, Ludwig33 investigated more than 500 ethnically diverse schoolchildren for 19 months. It was found that for each additional serving of carbonated drinks consumed, frequency of obesity increased, after adjusting for different parameters such as dietary, demographic, anthropometric, and lifestyle.

A study was conducted which included the diabetic patients with differing degrees of glycemic control. There were no differences in the mean daily plasma glucose levels or diurnal glucose profiles. As with carbohydrates, the association between dietary fats and T2DM was also inconsistent.34 Many of prospective studies have found relations between fat intake and subsequent risk of developing T2DM. In a diabetes study, conducted at San Louis Valley, a more than thousand subjects without a prior diagnosis of diabetes were prospectively investigated for 4 years. In that study, the researchers found an association between fat intake, T2DM and impaired glucose tolerance.35,36 Another study observed the relationship of the various diet components among two groups of women, including fat, fiber plus sucrose, and the risk of T2DM. After adjustment, no associations were found between intakes of fat, sucrose, carbohydrate or fiber and risk of diabetes in both groups.37

Recently, evidence suggested a link between the intake of soft drinks with obesity and diabetes, resulting from large amounts of high fructose corn syrup used in the manufacturing of soft drinks, which raises blood glucose levels and BMI to the dangerous levels.38 It was also stated by Assy39 that diet soft drinks contain glycated chemicals that markedly augment insulin resistance. Food intake has been strongly linked with obesity, not only related to the volume of food but also in terms of the composition and quality of diet.40 High intake of red meat, sweets and fried foods, contribute to the increased the risk of insulin resistance and T2DM.41 In contrast, an inverse correlation was observed between intake of vegetables and T2DM. Consumption of fruits and vegetables may protect the development of T2DM, as they are rich in nutrients, fiber and antioxidants which are considered as protective barrier against the diseases.42 Recently, in Japanese women, a report revealed that elevated intake of white rice was associated with an increased risk of T2DM.43 This demands an urgent need for changing lifestyle among general population and further increase the awareness of healthy diet patterns in all groups.

Dietary Knowledge of Type 2 Diabetics

American Diabetes Association has defined self-dietary management as the key step in providing the diabetics, the knowledge and skill in relation with treatment, nutritional aspects, medications and complications. A study showed that the dietary knowledge of the targeted group who were at high risk of developing T2DM was poor. Red meat and fried food were consumed more by males as compared to females. The percent of males to females in daily rice consumption was significantly high.44

In recent times in Saudi Arabia, food choices, size of portions and sedentary lifestyle have increased dramatically that resulted in high risk of obesity. Unfortunately, many Saudis are becoming more obese because of the convenience of fast foods, and this adds to the scary diabetes statistics.45 On the other hand, Saudis drink too many high-sugar drinks. In addition, Backman46 reported dietary knowledge to be a significant factor that influences dietary behaviors. In another study conducted by Savoca and Miller47 stated that patients’ food selection and dietary behaviors may be influenced by the strong knowledge about diabetic diet recommendations. Significant positive relationship was observed between knowledge regarding diabetic diet and the amount of calorie needs (r = 0.27, p < 0.05).48 The study concluded that knowledge regarding diabetic diet is essential and is needed to achieve better dietary behaviors. Results of study conducted in Saudi Arabia25 reported that more than half of the diabetic patients denied modifying their dietary pattern, reduction in weight and perform exercise.

National Center for Health Statistics reported that socioeconomic status plays an important role in the development of T2DM; where it was known as a disease of the rich.49 On the contrary, the same reference reported that T2DM was more prevalent in lower income level and in those with less education. The differences may be due to the type of food consumed. Nutritionists advised that nutrition is very important in managing diabetes, not only type but also quantity of food which influences blood sugar. Meals should be consumed at regular times with low fat and high fiber contents including a limited amount of carbohydrates. It was observed that daily consumption of protein, fat and energy intake by Saudi residents were higher than what is recommended by the International Nutritional Organization.50

Attitude of Type 2 Diabetics Toward Food

DM can be controlled through improvement in patient’s dietary knowledge, attitudes, and practices. These factors are considered as an integral part of comprehensive diabetes care.51 Although the prevalence of DM is high in gulf countries, patients are still deficient in understanding the importance of diet in diabetes management.52 Studies have shown that assessing patients’ dietary attitude may have a considerable benefit toward treatment compliance and decrease the occurrence rate of complications as well.52 A study conducted in Egypt reported that the attitude of the patients toward food, compliance to treatment, food control with and without drug use and foot care was inadequate.53 Another study presented that one-third of the diabetic patients were aware about the importance of diet planning, and limiting cholesterol intake to prevent CVD. Various studies have documented increased prevalence of eating disorders and eating disorder symptoms in T2DM patients. Most of these studies have discussed about the binge eating disorder, due to its strong correlation with obesity, a condition that leads to T2DM.53 Furthermore, a study revealed that the weight gain among diabetic patients was associated with the eating disorder due to psychological distress.54 In another study that examined eating disorder-related symptoms in T2DM patients, suggested that the dieting-bingeing sequence can be applied to diabetics, especially obese diabetic patients.55 Unhealthy eating habits and physical inactivity are the leading causes of diabetes. Failure to follow a strict diet plan and workout, along with prescribed medication are leading causes of complications among patients of T2DM.56 Previous studies57 conducted in Saudi Arabia have reported that diabetic patients do not regard the advice given by their physicians regularly regarding diet planning, diet modification and exercise.

Dietary Practices of Type 2 Diabetics

Diabetic’s dietary practices are mainly influenced by cultural backgrounds. Concerning each of the dimensions of dietary practices, there were significant positive relationships between knowledge regarding diabetic diet and dietary practices. Knowledge was a salient factor related to dietary behaviors control.46 Moreover, patients’ knowledge on a recommended diet indicates their understanding of dietary guidelines which influenced their food selection and eating patterns.47 The association between dietary knowledge and dietary practices among T2DM patients in the previous studies were inconsistent. Another study revealed that there was no relationship between dietary knowledge and compliance of dietary practices.58 On the other hand, the same study found that a high dietary knowledge score was associated with following dietary recommendations and knowledgeable patients performed self-management activities in a better way. Dietary knowledge significantly influences dietary practices. In Indonesia, a study was conducted to measure dietary practices among diabetic patients, which elaborated that the Indonesian people, preferred to consume high-fat foods which lead to an increased risk of CVD.59 The trend of skipping breakfast has dramatically increased over the past 10 years in children, adolescents, and adults.60,61 There is increasing evidence that skipping breakfast is related with overweight and other health issues.62 In addition, frequent eating or snacking may also increase the body weight and risk of metabolic diseases.63,64 Rimm65 demarcated western and prudent dietary patterns. The prudent dietary pattern was characterized by increased consumption of fish, poultry, various vegetables and fruits whereas; the western dietary pattern was characterized by an increased consumption of processed and red meat, chips, dairy products, refined grains, and sweets and desserts. These patterns were previously associated with T2DM risk. The glycemic index is an indicator of the postprandial blood glucose response to food per gram of carbohydrate compared with a reference food such as white bread or glucose. Hence, the glycemic load represents both the quality and quantity of the carbohydrates consumed.66-69 Another study conducted in Lebanon demonstrated direct correlation of the refined grains and desserts and fast food patterns with T2DM, however, in the same study an inverse correlation was observed between the traditional food pattern and T2DM among Lebanese adults.70

Type 2 Diabetes Complications

DM is the fourth among the leading causes of global deaths due to complications. Annually, more than three million people die because of diabetes or its complications. Worldwide, this disease weighs down on health systems and also on patients and their families who have to face too much financial, social and emotional strains. Diabetic patients have an increased risk of developing complications such as stroke, myocardial infarction, and coronary artery disease. However, complications such as retinopathy, nephropathy, and neuropathy can have a distressing impact on patient’s quality of life and a significant increase in financial burden. The prevalence reported from studies conducted worldwide on the complications of T2DM showed varying rates. The prevalence of cataracts was 26-62%, retinopathy 17-50%, blindness 3%, nephropathy 17-28%, cardiovascular complications 10-22.5%, stroke 6-12%, neuropathy 19-42%, and foot problems 5-23%. Mortality from all causes was reported between 14% and 40%.71 In a study, researchers found that 15.8% incidence of DR is in the developing countries. The prevalence of DR reported from Saudi Arabia, Sri Lanka, and Brazil was 30%, 31.3%, and 35.4%, respectively; while in Kashmir it was 27% and in South Africa it was 40%. The prevalence of DR 26.1% was observed among 3000 diabetic patients from Pakistan; it was significantly higher than that what was reported in India (18%) and in Malaysia (14.9%).72-76 Studies conducted on diabetes complications in Saudi Arabia are very few and restricted. A 1992 study from Saudi Arabia showed that in T2DM patients; occurrence rate of cataract was 42.7%, neuropathy in 35.9% patients, retinopathy in 31.5% patients, hypertension in 25% patients, nephropathy in 17.8% patients, ischemic heart disease in 41.3% patients, stroke in 9.4% patients, and foot infections in 10.4% of the patients. However, this study reported complications for both types of diabetes.77

Relation between Dietary Practices and Diabetes Complications

Interventional studies showed that high carbohydrate and high monounsaturated fat diets improve insulin sensitivity, whereas glucose disposal dietary measures comprise the first line intervention for control of dyslipidemia in diabetic patients.78 Several dietary interventional studies recommended nutrition therapy and lifestyle changes as the initial treatment for dyslipidemia.79,80 Metabolic control can be considered as the cornerstone in diabetes management and its complications. Acquiring HbA1c target minimizes the risk for developing microvascular complications and may also protect CVD, particularly in newly diagnosed patients.81 Carbohydrate intake has a direct effect on postprandial glucose levels in people with diabetes and is the principal macronutrient of worry in glycemic management.82 In addition, an individual’s food choices and energy balance have an effect on body weight, blood pressure, and lipid levels directly. Through the mutual efforts, health-care professionals can help their patients in achieving health goals by individualizing their nutrition interventions and continuing the support for changes.83-85 A study suggested that intake of virgin olive oil diet in the Mediterranean area has a beneficial effect on the reduction of progression of T2DM retinopathy.86 Dietary habits are essential elements of individual cardiovascular and metabolic risk.87 Numerous health benefits have been observed to the Mediterranean diet over the last decades, which contains abundant intake of fruit and vegetables. The beneficial effects of using fish and olive oil have been reported to be associated with improved glucose metabolism and decreased risk of T2DM, obesity and CVD.88

Conclusion

The review of various studies suggests that T2DM patients require reinforcement of DM education including dietary management through stakeholders (health-care providers, health facilities, etc.) to encourage them to understand the disease management better, for more appropriate self-care and better quality of life. The overall purpose of treating T2DM is to help the patients from developing early end-organ complications which can be achieved through proper dietary management. The success of dietary management requires that the health professionals should have an orientation about the cultural beliefs, thoughts, family, and communal networks of the patients. As diabetes is a disease which continues for the lifetime, proper therapy methods with special emphasis on diet should be given by the healthcare providers in a way to control the disease, reduce the symptoms, and prevent the appearance of the complications. The patients should also have good knowledge about the disease and diet, for this purpose, the health-care providers must inform the patients to make changes in their nutritional habits and food preparations. Active and effective dietary education may prevent the onset of diabetes and its complications.