Carbohydrates

The primary fuel of the body is glucose. Inside every cell in the body, glucose is converted into a substance known as adenosine triphosphate, or ATP, which provides the energy for that cells' function. Simple and complex carbohydrates in the diet provide glucose, or sugar units which are readily converted into glucose, for cellular energy production. Excess glucose is stored by the body in the liver and muscle tissue in a more complex form known as glycogen. When glycogen stores reach capacity, any additional glucose can be converted into fat. To satisfy energy needs, the body preferentially uses glucose in the blood, followed by stored glycogen. When these sources are depleted, as may be the case when following a carbohydrate-restricted diet for example, the body begins to burn protein tissue in order to generate glucose, an obviously unsustainable process!

In addition to energy production, dietary carbohydrates also provide nutrients for bacteria that aid digestion, and contribute to the absorption of certain vitamins and minerals. The sugar units contained in dietary fibre cannot be digested by the body, and so cannot contribute to energy production, but play a vital role in maintaining a healthy digestive tract and may also contribute to the regulation of blood cholesterol.

One gram of carbohydrate is equal to 3.75 kcals of energy. The Reference Intake (RI) for carbohydrate is 260 g per day. However, specific individual requirements may be dictated by physical activity levels relative to bodyweight – endurance athletes for example may need to consume 6 – 10 grams of carbohydrate per kilogram of body weight to fuel training and competition, while less than 4 grams of carbohydrate per kilogram may be adequate for the needs of sedentary individuals. EU recommendations for dietary fibre specify an RI of 25 g / day.

The most important sources of dietary carbohydrates and dietary fibre are plant-based foods – fruits, vegetables and grains. Aim to consume 6 – 11 portions of complex carbohydrate foods per day (breads, cereals, pasta, rice, and potatoes), and 5 portions of fruit and vegetables per day (the majority of which should comprise of vegetables). The following table details sources of dietary carbohydrate, their portion size, and approximate amount of carbohydrate and fibre per portion;

Intake of carbohydrate-containing foods causes an increase and subsequent decrease in the concentration of glucose in the blood, known as the glycaemic response. This process reflects the rate of digestion and absorption of glucose in addition to the ability of insulin to regulate blood glucose levels. The rate and duration of the glycaemic response is influenced by the type of food consumed (sugar and starch composition / cooking and processing methods / relative fat, protein and fibre content), and the person consuming the food (extent of oral digestion by chewing / rate of gastric emptying and small bowel transit time / metabolic rate / time of day).

The influence of different carbohydrate-containing foods on the glycaemic response of the body is classified relative to the effects of glucose, within 2 hours following consumption. This measurement is referred to as the glycaemic index (GI). A GI of 70 indicates that the carbohydrate-containing food or drink causes 70 % of the blood glucose response observed with the same amount of carbohydrate from pure glucose. High GI foods cause a greater blood glucose response than low GI foods, which are disgested and absorbed at a slower rate. Evidence suggest that a diet based on low GI foods is associated with a reduced incidence of metabolic disorders such as obesity and type 2 diabetes.

What does the evidence say about the effects of carbohydrates on health?

In June 2014, The Scientific Advisory Committee on Nutrition (SACN), a group of independent experts that provides advice to UK government agencies and departments to inform public health policy, published a detailed document entitled 'Draft Carbohydrates and Health report'. In this report (http://www.sacn.gov.uk/reports_position_statements/reports/scientific_consultation_draft_sacn_carbohydrates_and_health_report_-_june_2014.html), SACN members performed an evaluation of the existing scientific evidence on the relationship between dietary carbohydrates and health. Based on combined results from the research studies investigated, the conclusions of the committee can be summarised thus:

• In children and adolescents, consumption of sugar-sweetened beverages, as compared with non-calorie sweetened beverages, results in weight gain and an increase in body mass index
• In children and adolescents, higher consumption of sugars, sugar-containing foods, and sugar-containing beverages is associated with a greater risk of dental carries
• Sugar, or sugar-sweetened beverage intake is not associated with the incidence of colorectal cancer
• There is no relationship between the incidence of type 2 diabetes mellitus and total or individual sugar intake, but a greater risk is associated with higher intake of sugar-sweetened beverages
• Increasing intake of sugar and sugar-sweetened foods and beverages does not effect inflammatory markers or cardiovascular risk factors
• There is no relationship between total carbohydrate intake and the incidence of cardiovascular disease endpoints, type 2 diabetes mellitus, glycaemia, or colorectal cancer
• In children and adolescents, there is no relationship between total carbohydrate intake and body mass index or body fatness
• An energy-restricted higher carbohydrate, lower fat diet, as compared with a lower carbohydrate average fat diet, may be effective in reducing body mass index 
• A diet rich in dietary fibre is associated with a lower incidence of cardiovascular diseases, coronary events, type 2 diabetes mellitus and colo-rectal cancer
• Higher intakes of oat bran and isolated β-glucan (a polysaccharide contained in cellulose) have beneficial effects on fasting blood lipid concentrations and blood pressure

The results of this authorative report show that carbohydrates are not the anathema to good health that many would have us believe. Whilst the evidence clearly indicates that the intake of sugars and sugar-sweetened foods and drinks should be minimised to control weight gain and the risk of dental carries in the young, total carbohydrate intake as a whole is not associated with any adverse health effects investigated in the report. It also indicates that decreases to body mass index can be achieved without cause to dietary carbohydrate reduction per se. Furthermore, the report shows that intake of carbohydrate in the form of dietary fibre is associated with lower rates of cardiovascular disease, type 2 diabetes, and certain cancers.