1. Glycemic Response
Glycemic response refers to the effect that foods and drinks have on blood glucose after consumption. After eating a meal, carbohydrates (excluding fibre) are absorbed from the intestine into the bloodstream, causing a temporary increase in blood glucose concentration – this is called a glucose excursion. In response, the hormone insulin is released and blood glucose concentration returns to fasting levels, or may fall slightly below.
There is clear scientific evidence that persistent elevation of blood glucose is linked with conditions such as type 2 diabetes, cardiovascular disease and obesity. A systematic review and meta-analysis confirmed that diets with a low glycemic impact (i.e. resulting in only moderate elevations of blood glucose) have a role in reducing the risk of such chronic conditions.
There are several markers for assessing glycemic status, all of which are used to diagnose diabetes.
Normal and abnormal values are shown in Table 1.
- Fasting blood glucose levels
- Oral Glucose Tolerance Test (OGGT) which measures how quickly a 75g dose of glucose is cleared from the blood in a fasting individual
- Concentrations of glycated proteins e.g. HbA1c, which reflect the impact of long term elevations in blood glucose.
The glycemic effect of foods also depends on the sensitivity of tissues to insulin, a hormone secreted by the pancreas. When released, this stimulates insulin-sensitive tissues, such as muscle and adipose tissue, to take up glucose from the blood. As blood glucose returns to normal, insulin release is slowed to normal resting levels.
In cases of ‘insulin resistance’, muscle and adipose tissue do not respond adequately to insulin production. This results in a reduced rate of glucose uptake from blood with the consequence that blood glucose levels remain elevated. More insulin is then released leading to chronically high plasma insulin levels. In the long-term, there is a risk that the pancreas may become exhausted resulting in falling insulin levels and a need for insulin medication.
The homeostatic model assessment (HOMA) is often used to assess the risk of insulin resistance. This marker shows the dynamic between baseline (fasting) blood sugar and the responsive hormone insulin; the healthy range is 0.5–1.4.
2. Glycemic index (GI)
Carbohydrates provide a significant energy source in our diets. The glycemic response depends on the type of carbohydrate and its molecular composition. In addition, GI is influenced by the food matrix in which the carbohydrates are present. The matrix, in turn, is influenced by food processing and the presence of dietary fibre, protein and fat. Carbohydrate-containing foods that result in a slow increase in blood glucose may be called low glycemic foods.
Pure glucose is rapidly taken up and is given a standard glycemic value of 100. By comparing carbohydrate foods to this standard of 100, it is possible to give these a relative index score. In this respect, GI is defined as the area under the glucose response curve after consumption of 50g carbohydrate from a test food divided by the area under the curve after consumption of 50g glucose (sometimes white bread is taken as standard, instead of glucose).
Generally, there are three categories of foods based on their GI values:
- High GI foods (≥ 70)
- Intermediate GI foods (56 – 69)
- Low GI foods (≤ 55) .
Consuming low GI foods, instead of high GI foods, has a positive effect on keeping post-meal glucose excursions and related insulin requirements relatively low (Fig. 1). Fully and readily digestible carbohydrates, such as glucose, maltodextrin, white bread and cooked potato starch, produce a rapid increase in blood glucose and insulin, followed by an equally rapid fall. If high GI foods are eaten in significant amounts, this has implications for the overall glycemic impact of the meal. Consuming high glycemic meals frequently in conditions of overweight and inactivity, will drive the development of insulin resistance and type 2 diabetes.
Figure 1: Blood sugar response in healthy adults
Legend: After a high glycemic meal there is a rapid increase in blood glucose. This triggers a significant insulin release which stimulates tissues to take up glucose from blood. As a result, the blood glucose level will fall rapidly, sometimes even below the pre-meal level (hypoglycemic effect). In contrast, consumption of a low glycemic meal will produce a moderate increase in blood glucose and insulin, and a slow return to the pre-meal blood glucose level.
Consumption of fructose, for example found in fruits, table sugar, honey and fruit juices, only minimally increases blood glucose, explaining why fructose-containing foods tend to have a relatively low glycemic response. This is also the reason why 100% fruit juices have a low GI, contrary to media and lay person claims that they produce a significant rise in blood glucose (Fig 2.).
Figure 2: Post-meal glycemia, and the shape of the curve depends on GI
3. Glycemic load (GL)
The glycemic response to a food depends not just on GI but also on the total carbohydrate ingested. On this basis GL was created; defined by how much each portion of carbohydrate raises blood glucose levels. GL is classified as: low (< 10), intermediate (11–19) and high (> 20). Fruits and 100% fruit juices have a low GI and intermediate GL (Table 2). Table 2: GI/GL of typical products
A systematic review and meta-analysis1 of data from 45 studies reported that diets comprised of low glycemic foods reduced fasting blood glucose and HbA1c, particularly in people with poor fasting blood glucose control. Adding non-digestible carbohydrates enhanced these effects. Lower GL was more important than GI for lowering triglyceride levels.
4. 100% fruit juice, glycemic control and type 2 diabetes
In one trial, 36 overweight subjects with raised plasma cholesterol were enrolled in a randomised, single-blinded, placebo-controlled clinical study to investigate the metabolic effects of daily 100% orange juice consumption. During the 12-week intervention, participants received either 250 ml of orange juice or a control orange drink matched for energy and sugars. The results revealed that 100% orange juice had no adverse effects on insulin sensitivity (HOMA-IR), blood lipid profile or body weight. The authors concluded that: “Daily consumption of 250 ml of orange juice for 3 months did not result in an increase in dietary sugars intake in a cohort of overweight men with elevated total cholesterol concentration, and despite media concern, an increase in body weight or decreased insulin sensitivity did not occur over the intervention”.
A meta-analysis of 12 randomized controlled trials involving over 400 participants who were obese or had risk factors for diabetes or cardiovascular disease, examined the effect of sugar-containing beverages on fasting glucose and insulin levels. In half of these studies, the intake of 100% fruit juice was 400g per day or more. The overall results showed that consumption of 100% fruit juices had no significant effect on fasting blood glucose or insulin levels.
Another meta-analysis of four cohorts of adults found that consumption of fruit drinks with added sugars was significantly associated with an increased risk of type 2 diabetes (RR = 1.28: p = 0.02) while consumption of 100% fruit juices (i.e. no added sugars) had no effect (RR = 1.03, p = 0.62).
The most recent systematic review and meta-analysis, based on 18 randomised and controlled studies, examined the effects of 100% fruit juice on glucose-insulin homeostasis. Compared with controls, 100% fruit juices had no significant effect on fasting blood glucose (mean difference: -0·13 mmol/L; 95 % CI -0·28, 0·01; p = 0·07), fasting blood insulin (-0·24 mmol/L; 95 % CI -3·54, 3·05; p = 0·89), HOMA-IR (-0·22; 95 % CI -0·50, 0·06; p = 0·13) or HbA1c (-0·001%; 95 % CI -0·38, 0·38; p =0·28). The authors concluded that their meta-analysis suggested: “a neutral effect of 100% fruit juice on glycaemic control” and, consequently, “consumption of 100% fruit juice is not associated with increased risk of diabetes”.