J Kayode, A Sola, A Adelani, A Adeyinka, O Kolawole, O Bashiru
carbohydrate, definition, diabetes, food processing, glycaemic index, nutrition
J Kayode, A Sola, A Adelani, A Adeyinka, O Kolawole, O Bashiru. The Role Of Carbohydrate In Diabetic Nutrition: A Review. The Internet Journal of Laboratory Medicine. 2008 Volume 3 Number 2.
This review looks at the definition of carbohydrate and its classification; the type and effect of food processing that affect blood glucose and insulin response to carbohydrate. The glycaemic index of carbohydrate food and its role in nutritional management of diabetes was emphasized. The goals of MNT in diabetes management were briefly mentioned.
Carbohydrates play a major role in human diets, comprising some 40-75% of energy intake. Their most important nutritional property is digestibility. In an attempt to prepare food tables for diabetic diet, McCance and Lawrence in 1929 1 divided dietary carbohydrates into available and unavailable carbohydrates. Available carbohydrates are those that are hydrolysed by enzymes of the human gastrointestinal system to monosaccharides that are absorbed in the small intestine and enter the pathways of carbohydrate metabolism, while unavailable carbohydrates are not hydrolysed by endogenous human enzymes, but may be fermented in the large intestines to varying extents.
Carbohydrate may be define as “polyhydroxy aldehydes, ketones, alcohols, acids, their simple derivatives and polymers having polymeric linkages of the acetal type”. 2 Carbohydrates are further classified according to the degree of polymerization as sugars (mono- and disaccharides); oligosaccharides (three to nine monosaccharide units); and polysaccharides (ten or more monosaccharide units). 3 Each of these three groups may be subdivided on the basis of the monosaccharide compositions. Sugars comprise monosaccharide, disaccharides and polyols (sugar alcohols). Oligosaccharides include malto-oligosaccharides, principally those occurring from the hydrolysis of starch, and other oligosaccharides, e.g. -galactosides (raffinose, stachyose etc.) and fructo-oligosaccharides. The polysaccharides may be divided into starch (-glucans) and non-starch polysaccharides of which the major components are the polysaccharides of the plant cell wall such as cellulose, hemicelluloses and pectin.
A number of carbohydrates are only partly or not at all digested in the small intestine and are fermented in the large bowel to short chain fatty acids. These include the non-digestible oligosaccharides, resistant starch and non-starch polysaccharides. The process of fermentation is metabolically less efficient than absorption in the small intestine and these carbohydrates provide the body with less energy.
Metabolism Of Carbohydrate Meals
The digestion of dietary carbohydrates starts in the mouth, where salivary -amylase initiates starch degradation. The starch fragments thus formed include maltose, some glucose and dextrins containing the 1,6- -glycosidic branching points of amylopectin. The -amylase degradation of starch is completed by the pancreatic amylase active in the small intestine. Dietary disaccharides, as well as degradation products of starch, need to be broken down to monosaccharides in order to be absorbed. This final hydrolysis is accomplished by hydrolases attached to the intestinal brush-border membrane, referred to as “disaccharidases”. Glucose and galactose are transported actively against a concentration gradient into the intestinal mucosal cells by a sodium dependent transporter (SGLT 1). Glucose is pumped out of the enterocyte into the extracellular space by the glucose transporter 2 (GLUT 2). Fructose undergoes facilitated transport by another mechanism (GLUT 5).
When delivered to the circulation, the absorbed carbohydrates cause an elevation of the blood glucose concentration. Fructose and galactose have to be converted to glucose mainly in the liver and therefore produce less pronounced blood glucose elevation. The extent and duration of the blood glucose rise after a meal is dependent upon the rate of absorption, which in turn depends upon factors such as gastric emptying, as well as the rate of hydrolysis and diffusion of products of hydrolysis in the small intestine.
Insulin is secreted as a response to blood glucose elevation but is modified by many neural and endocrine stimuli. Insulin secretion is also influenced by food related factors, especially by the amount and the amino acid composition of dietary proteins, particularly arginine, lysine, leucine and phenylalanine. Insulin has important regulatory functions in both carbohydrate and lipid metabolism and is necessary for glucose uptake by most body cells.
Fermentation is the colonic phase of the digestive process and describes the breakdown in the large intestine of carbohydrates not digested and absorbed in the upper gut. This process involves gut microflora and is unique to the colon of humans because it occurs without the availability of oxygen. It thus results in the formation of gases e.g. hydrogen, methane and carbon dioxide; as well as short chain fatty acids (SCFA) (acetate, propionate and butyrate) which are rapidly absorbed and metabolized by the body. The gases are either absorbed and excreted in breath, or passed out
Dietary Carbohydrates And Food Processing
Glycaemic Index Of Food
Epidemiological studies 9 have reported that as nations become more affluent, the nature of the people’s carbohydrate consumption changes such that the ratio of complex (starches) to simple carbohydrates decreases. It has been suggested that this change in dietary pattern is responsible for the occurrence of various diseases, such as atherosclerosis, diabetes and hyperlipidaemia. One proposed physiologic basis underlying such suggestions is a traditionally held tenet that simple carbohydrates are more readily available for immediate absorption by the gut than are more complex carbohydrates and that they therefore produce a greater and faster rise in postprandial plasma glucose and insulin responses than do the supposedly more gradually digested and absorbed complex carbohydrate. Consequently, diets restricted in simple carbohydrates have been recommended in disease states in which control of plasma glucose and/or insulin is felt to be important. Dahiqvist and Borghstrom 10 as well as Fogel and Gray 11 have challenged this concept of carbohydrate digestion and absorption. These workers demonstrated that after test meals, more than enough intraluminal amylase is present to rapidly hydrolyze ingested starch. They concluded that absorption, not intraluminal digestion, was the rate-limiting step in over-all starch assimilation.
It has also been shown 12 that complex carbohydrates resulted in lower glucose and insulin responses than equivalent amounts of glucose or sucrose, indicating that despite adequate amylase, the process of starch digestion proceeds more slowly. It was also found that glucose and insulin responses to cooked potato were significantly higher than the glucose and insulin responses to rice. These latter findings suggest that not all starches are treated identically by gastrointestinal digestive and absorptive processes.
Evidence favouring the active reduction of blood lipids continues to accumulate, and several major diabetes associations now recommend that diabetic patients should reduce fat intake and increase carbohydrate intake to approximately 50% of total calories. Since one aim of diabetic therapy is to prevent large fluctuations in blood glucose throughout the day diabetics are advised to select carbohydrate foods that minimize the postprandial blood glucose excursions. In the absence of adequate information specific advice on food selection is not given, although high-fibre foods have been advocated 13 . Before detailed advice can be given, comparative data on the physiological effects of carbohydrate foods are required.
where the valuable carbohydrate content of the test and reference foods is the same. This method allows results of food testing in individuals with different glucose tolerance to be pooled and comparison made between findings of different investigators. In general, results of studies in young adult, normal weight, non-diabetic volunteers agree well with those in middle-aged and elderly, overweight, diabetic patients. 14
Definition Of Glycaemic Index
The blood glucose responses of carbohydrate foods can be classified by the glycaemic index (GI). The GI is considered to be a valid index of the biological value of dietary carbohydrates. It is defined as the glycaemic response elicited by a 50g carbohydrate portion of a food expressed as a per cent of that elicited by a 50g carbohydrate portion of a standard food. 17 The glycaemic response is defined as the incremental area under the blood glucose response curve, ignoring the area beneath the fasting concentration (i.e. the area beneath the curve). 91819 The standard food has been glucose or white bread. If glucose is the standard, (i.e. GI of glucose = 100) the GI values of foods are lower than if white bread is the standard by a factor of 1.38 because the glycaemic response of glucose is 1.38 times that of white bread. GI values for several hundreds foods have been published. 2021
Factors Influencing Glycaemic Response
Clinical Significance Of Glycaemic Index
More than twenty years ago the first index of the relative glycaemic effects of carbohydrate exchanges from 51 foods was published by Jenkins
In 1997 the Food and Agriculture Organization (FAO) of the United Nations and the World Health Organization (WHO) reviewed available research evidence regarding the importance of carbohydrates in human nutrition and health, 8 and endorsed the use of the GI method for classifying carbohydrate-rich foods and recommended that the GI values of foods be used in conjunction with information about food composition to guide food choices.
Reliable tables of GI compiled from the scientific literature are instrumental in improving the quality of research examining the relation between the dietary glycaemic effect and health. Several studies from Harvard University indicate that the long-term consumption of a diet with a high glycaemic load (GL) (GL= GI x dietary carbohydrate content) is a significant independent predictor of the risk of developing type 2 diabetes and cardiovascular disease. More recently, evidence has been accumulating that a low-GI diet might also protect against the development of obesity, colon cancer, and breast cancer. 30
The EURODIAB (Europe and Diabetes) study, involving >3000 subjects with type 1 diabetes in 31 clinics throughout Europe, showed that the GI rating of self-selected diets was independently related to blood concentrations of glycated haemoglobin in men and women 31 and to waist circumference in men. 32 In addition, higher blood HDL-cholesterol concentrations were observed in patients consuming low-GI diets from the northern, eastern, and western European centers participating in the study. 32 In parallel with these advances there have been studies documenting the importance of postprandial glycaemia
In addition, evidence from both short-term and long-term studies in animals and humans indicate that low-GI foods may be useful for weight control. Laboratory studies examining the short-term satiating effects of foods have shown that low-GI foods are relatively more satiating than are their high-GI counterparts. 35 Compared with low-GI meals, high-GI meals induce a greater rise and fall in blood glucose and a greater rise in blood insulin, leading to lower concentrations of the body's 2 main fuels (blood glucose and fatty acids) in the immediate postabsorptive period. The reduced availability of metabolic fuels may act as a signal to stimulate eating.
It is also important to emphasize that many low-GI foods are relatively less refined than are their high-GI counterparts and are more difficult to consume. The lower energy density and palatability of these foods are important determinants of their greater satiating capacity. In obese children, the
In a meta-analysis it was concluded that choosing low-GI foods in place of conventional or high-GI foods has a small but clinically useful effect on medium-term glycemic control in patients with diabetes. 40
Nutritional Management Of Diabetes Mellitus
The prevalence of diabetes mellitus (DM) is increasing around the world and at a rate that appears as dramatic as to have been characterized as an epidemic. Many factors have been postulated to contribute to the DM epidemic. Environmental factors have drawn particular attention because of the rapidity of the increase in type 2 DM. Because DM is a disease directly related to carbohydrate, lipid, and protein metabolism, nutrition has always had an integral role in its management. The contemporary term used to describe the dietary prescriptions is medical nutrition therapy
The goals of MNT for diabetes include:
Achieve and maintain near normal blood glucose goals
Achieve and/ or maintain optimal blood lipid levels
Achieve and/ or maintain normal blood pressure
Prevent, delay or treat nutrition related complications
Provide adequate kcalories for achievement of reasonable body weight
Provide optimal nutrition for maximizing health and for growth, development, pregnancy, and lactation
Diabetes Mellitus And Sugars
The ADA recommends that the classifications sugars, starch, and fiber
Sucrose and other sugars can be consumed by those with DM and need to be considered primarily from the perspective of energy consumed and thus substituted for other sources of carbohydrate. 41 This perception of the sensitivity of metabolic control in DM to energy balance underlies the recommendations that emphasise carbohydrate content as a key point for patient education. The 3 points stressed are:
Knowledge concerning which foods contains carbohydrates,
Recognition of the portion size for carbohydrate within a meal (with 15 g being the basis for estimating 1 carbohydrate serving), and
Knowledge of how many carbohydrate servings is appropriate within a meal or snack.
Negative energy balance can promptly induce reductions in hyperglycaemia and hypertriglyceridaemia, even before the achievement of substantial weight loss, whereas consumption of excess energy has the opposite effect. Increased energy consumption regardless of source, but certainly including carbohydrate, directly induces insulin resistance. 43
Metabolic studies have been used to compare the glycaemic response to sugars consumption in persons with DM with isocaloric consumption of other sources of carbohydrates. Bantle
Diabetes Mellitus And Dietary Fibre
There has been specific interest in the role that dietary fiber may have in the nutritional management of DM. Benefits of fiber were found with regard to glycaemic control, HDL and LDL cholesterol, and triacylglycerols. 45 However, a 3-month study by Jenkins
In consideration of the available data as a whole, the ADA expert committee did not perceive that there was value in recommending an increase in fiber intake above general recommendations for persons with DM. 48
Diabetes Mellitus And The Dietary Genome
The progression from a healthy state to a chronic disease state such as diabetes mellitus must occur by changes in gene expression or by differences in activities of proteins and enzymes. Since dietary chemicals are regularly ingested and participate indirectly and directly in regulating gene expression, it follows that a subset of genes regulated by diet must be involved in disease initiation, progression, and severity (79, 113). The clearest example of genotype-diet interactions in chronic disease is type 2 diabetes; once diagnosed with type 2 diabetes, some individuals can control symptoms by increasing physical activities and by reducing caloric intake, thus the expression of genomic information is changed by changing environmental (dietary) variables.
The link between high GI diets and diabetes may relate to glucose peaks and increased insulin demand. High GI foods lead to rapid rises in blood glucose and insulin levels. Hyper-insulinemia, in turn, may down-regulate insulin receptors and therefore reduce insulin efficiency, resulting in insulin resistance a known risk factor for type 2 diabetes. Low-GI foods tend to delay glucose absorption thereby resulting in reduced peak insulin concentrations and overall insulin demand. Several studies have found improvements in glycemic control with low-GI diets.
A recent study by Uyeda et al showed that feeding on a high carbohydrate diet induces transcription of more than 15 genes involved in the metabolic conversion of glucose to fat. Carbohydrate responsive element-binding protein (ChREBP), the newly discovered transcription protein is activated in response to high glucose and up-regulates these genes. Cyclic AMP and a high fat diet inhibit ChREBP and slow down glucose utilization. ChREBP is able to control transcription of lipogenic enzyme genes in response to nutritional and hormonal inputs, and may play an important role in disease states such as diabetes, obesity, and hypertension.
Carbohydrate provides the majority of energy in the diets of most people. In addition to providing easily available energy for oxidative metabolism, carbohydrate-containing foods are vehicles for important micronutrients and phytochemicals. Dietary carbohydrate is important to maintain glycemic homeostasis and for gastrointestinal integrity and function. Unlike fat and protein, high levels of dietary carbohydrate, provided it is obtained from a variety of sources, is not associated with adverse health effects. An optimum diet should consist of at least 55% of total energy coming from carbohydrate obtained from a variety of food sources. Diabetes mellitus remains a universal health problem, and its management requires a multidisciplinary healthcare approach which includes a combination of diet, drug or insulin therapy, exercise, and behavioural modifications to ensure long-term compliance. However whether the disease is managed by insulin or oral drugs, dietary measures are necessary and inevitable to uphold the control of blood glucose, improve metabolic control and prevent complications. Despites its limitation glycemic index still has a major role to play in diabetic nutritional management. Further studies on the role of diet on gene modification in causation and management of chronic diseases will shed more light on the role of carbohydrate on diabetic nutrition.