Relative glycaemic potency and glycaemicglucose equivalents

Control of postprandial glycaemia - the blood glucose response to food intake -is an increasingly important health issue. Diabetes mellitus, marked by an inability to control blood glucose levels, is increasing rapidly in many developed countries, in which an over-supply of high energy and highly digestible carbohydrate foods is coupled with predisposing factors, including physical inactivity, obesity, and inheritance.28 Many consumers need to be able to manage postprandial glycaemia by selecting foods and food combinations according to glycaemic impact, but food labels at present give them little assistance.

7.6.1 End-point

Health consequences of hyperglycaemia are multiple and most evident in the diabetes mellitus syndrome.28,49,50 Persistently raised blood glucose causes protein glycation throughout the body, leading to cumulative, diffuse damage, emerging as pathology in a number of organ systems. Basal membrane damage is commonly an underlying factor in changes to micro-vessels involving the eyes, kidneys and nerves.51 Intense insulin production in response to diabetic hyper-glycaemia, or to repeated acute glucose loading from large intakes of highly digestible carbohydrate, is thought to contribute to the progression of glucose intolerance, through b-cell toxicity, leading to loss of the capacity of the pancreas to produce insulin.52 Hyperinsulinaemia as a response to elevated blood glucose favours elevated blood lipids, obesity and hypertension, all risk factors in heart disease.49,50,51

Post-prandial glycaemia may also lead to a number of acute and sometimes serious disorders, as the body attempts to counter the osmotic effects of high blood sugar levels. The excretion of sugar by the kidneys leads to water loss, excessive thirst, and in extreme cases, to fatal electrolyte imbalances.53

7.6.2 Markers

As blood glucose response is causal in glycation, insulin response, osmotic effects and other aspects of diabetic pathology, it is a highly relevant marker of the influence of foods and carbohydrates on progression towards disease end-points related to diabetes. Indeed, persistent elevation of blood glucose is clinically a defining feature of diabetes mellitus.

7.6.3 Current indices

Glycaemic carbohydrate components most commonly seen on food labels are 'carbohydrate', 'available carbohydrate', 'complex carbohydrate' (starch), and 'sugars'. One of the main reasons for distinguishing between sugars and complex carbohydrates is the once-held belief that sugars have a more acute impact on blood glucose levels than starch. However, some starches are so rapidly digested that they induce a blood glucose response similar to that of pure glucose. For instance, starch in rice bubbles has a glycaemic index (GI) of 97 (gives 97% of the response to an equicarbohydrate dose of glucose) and baked potato a GI of 85, whereas starch in noodles has a GI of 46.54 The relative amounts of sugar versus complex carbohydrate in a food is not, therefore, a reliable guide to its impact on blood glucose.

Sugar type is another reason that 'sugar' content does not indicate glycaemic effect. Sucrose ('cane sugar') for instance has a GI of 61 because it is a disac-charide containing a fructose (GI = 23) and a glucose (GI = 100) unit.55 While dietary sugars include sucrose, lactose, fructose, glucose, and other mono- and disaccharides, 'blood sugar' is blood glucose.

GI is now being used to classify and promote foods by glycaemic impact. The GI of a food is the incremental effect of carbohydrate in a food on blood glucose, as a percentage of the effect of an equal weight of glucose. It is usually based on the glycaemic effect of enough food to provide a 50 g dose of carbohydrate, compared with the effect of 50 g glucose, or its carbohydrate equivalent in white bread, as the reference.

Gi _ Blood glucose increment due to 50 g carbohydrate in a food ^ ^ Blood glucose increment due to 50g glucose

The glycaemic index was devised to take account of the relative differences in the impact of food carbohydrates on blood glucose resulting from the types of carbohydrate and their rates of digestion.43

Two intrinsic characteristics of GI make it difficult to use alone in accurate blood glucose management. Firstly, because GI is based on an equicarbohydrate comparison it should be used to compare foods only at equal carbohydrate doses whereas most foods differ enormously in available carbohydrate content. It is often incorrectly stated that GI ranks foods according to their impact on blood glucose,56 but it ranks carbohydrates in food, not foods, and ranks foods only if they contain equal amounts of carbohydrate. Secondly, as a percentage, GI does not change with food quantity, so cannot be used to predict relative glycaemic responses to servings or intakes of food. It is an example of an impractical index.

Table 7.4 Foods within food groupings ranked by glycaemic index (GI), with corresponding values for glycaemic glucose equivalents (GGE) per common standard measure (CSM), and GGE per 100g (RGP).44 Rankings by GI did not similarly rank either RGP or GGE/CSM, showing that GI does not rank foods by glycaemic impact

Food grouping

CSM wt(g) RGP GGE/CSM

Bakery products

Doughnut ring doughnut

Bagels, plain bagel

Bread, roll, white, soft roll

Bread, white, sliced med slice

Bread, wholemeal med slice

Croissants small

Crispbread, rye biscuit

Biscuit, digestive biscuit

Cake, sponge slice

Bread, multigrain 'heavy' med slice

Breakfast cereals

Puffed rice cup

Corn flakes, Kelloggs serving

Wheat, puffed cup

Wheat biscuit, 'Weet-Bix' biscuit

Muesli, non toasted cup

Muesli, toasted, sweet cup

42

44

76

33

14

74

47

72

33.8

25

51

49

70

34.3

17.5

26

43

70

30.1

7.8

28

37

69

25.5

7.1

57

39

67

26.1

14.9

6

64

65

41.6

2.5

14

63

58

37

5

89

60

46

28

25

28

37

52

19.2

5.4

14

78

89

69

10

30

85

84

71.4

21.4

14

64

74

47.4

6.6

15

62

70

43.4

6.5

260

10.5

61

6.4

16.7

107

57

56

32

31.9

110

53

43

22.8

25.1

7.6.4 Relevant indices

To overcome the limitations of GI, a food-based 'GI', termed Relative Glycaemic Potency (RGP), was calculated, and defined as the theoretical response to 50g of a food as a percentage of the response to 50g glucose:57,58

Blood glucose increment due to 50 g food RGP = X 100

Blood glucose increment due 50g glucose

RGP is simply GI adjusted for the carbohydrate content of a food (%CHO), and it allows a comparison of foods, rather than food carbohydrates, on an equal weight basis. Adjustment for the carbohydrate content of a food results in a completely different ranking of foods than is obtained with GI, as shown in Table 7.4.

7.6.5 Practical units

Because they are based on whole foods, RGP values can easily be expressed as practical units that are related to food intakes. RGP is a percentage of the effect of glucose, so the RGP of a food can be regarded as the amount of glucose that would be equivalent to 100g of the food in its glycaemic impact. In other words, an RGP 30 can be expressed as 30 glycaemic glucose equivalents (GGE) per 100g food:

The number of GGEs donated by a food item or meal may be termed its relative glycemic impact (RGI):

RGI = GGE intake at a time [7.5]

  • Food weight • GGE/g of food [7.6]
  • no. CSMs food • CSM weight • (%CHO • GI//10000)

Glycaemic impact (RGI), as GGE intake, then becomes a function, not of GI alone, but of GI, carbohydrate content, and serving size or food intake, so it takes into account all of the factors determining the RGI of a food, and gives freedom from the equicarbohydrate limitation of GI. The term 'glycaemic loading' is similar to RGI, but has been used in epidemiology to express cumulative or chronic exposure to glycaemia in response to foods over periods of months,59 whereas RGI refers to acute impact. The inaccuracy of food classifications for glycaemic control based on GI alone, when foods differ in carbohydrate content and serving size, is shown in Fig. 7.1.

Expressing glycaemic potency as a content of GGEs enables it to be treated like a nutrient, so that RGI and nutrient intake can be presented simultaneously, allowing complete nutritional management, and use in computer-based diet management systems.44 GGEs should also enable the combined effect of foods in meals to be gauged, because the GGE contents of different foods may, in theory, be added to give the glycaemic impact of the meal.

Table 7.5 The glycaemic index (GI) of foods does not indicate relative glycaemic impact (glycaemic glucose equivalents per common standard measure; GGE/CSM). GI is not a useful guide to food choice for blood glucose control unless carbohydrate intakes are equal

Common standard

measure of food

Table 7.5 The glycaemic index (GI) of foods does not indicate relative glycaemic impact (glycaemic glucose equivalents per common standard measure; GGE/CSM). GI is not a useful guide to food choice for blood glucose control unless carbohydrate intakes are equal

Common standard

measure of food

1 apricot

54

57

3

1 banana

128

58

18

2 scoops ice-cream

50

61

6.5

1 blueberry muffin

80

59

24

1 can Fanta™

375

68

35

2 slices pineapple

125

66

6.6

1 glass orange juice

256

46

9.9

1 slice banana cake

80

47

22

1 slice dark rye bread

70

86

19

1 apple muffin

80

44

19

1 cup broad beans

160

79

14.2

1 cup spaghetti

180

41

23

Breads

Breakfast cereals

Fruit

Vegetables

Breads

Breakfast cereals

Fruit

Vegetables

Food grouping

Fig. 7.1 Glycaemic impact as glycaemic glucose equivalents (GGE) per serving of foods classified as low (<55; A), medium (55-70; •) and high (>70; ■) glycaemic index (GI), in each of the food groupings breads, breakfast cereals, fruit and vegetables. GI has not usefully classified foods in each food grouping by glycaemic impact, as GGE contents in the different GI classes overlap.

GGE has potentially great practical advantages over GI in dietary management of glycaemia and in food labelling. Responsiveness of GGE intake to food intake could greatly improve the precision with which glycaemia could be managed by diet, insulin and medication. GGE values on food labels would allow the relative glycaemic impact of any food item or quantity to be directly specified, as the examples in Table 7.5 show.

Because atherosclerosis is a common long-term complication of diabetes mel-litus, food products for diabetes should also contain a low proportion of saturated fat. Less than 7% of energy from saturated fat is the recommended guideline of the American Heart Association.60

Yam Biscuits Porridge Noodles Yam Biscuits Porridge Noodles DM DM DM DM

Fig. 7.2 Glycaemic responses (IAUC; mmol-min/L) per glycaemic glucose equivalent (GGE) for foods of differing glycaemic index (GI), carbohydrate (CHO) content, and intake, consumed at a single and a double GGE dose, by subjects with (DM) and without Type 2 diabetes. Yams, (GI, 35; CHO, 8.84%; intakes: 10GGE, 323.2g; 20GGE, 646.4g), biscuits (GI, 49.5; CHO, 72.9%; intakes: 10GGE, 27.7g; 20GGE, 55.4g), porridge (GI, 67.5; CHO, 13.1%; intakes: 10GGE, 132.4g; 20GGE, 246.8g), noodles (GI, 48; CHO, 16%; intakes: 24GGE, 323.2g; 48GGE, 646.4g). Results show that GGE content is a robust measure of glycaemic impact.

7.6.6 Validity of GGE intake in predicting glycaemic response

GGE is a new concept, and only one study to date has directly tested the predictive validity of GGE intake per se.61 But GGEs are a combination of GI and carbohydrate dose so studies of joint effects of GI and carbohydrate dose on blood glucose are a test of GGE, and have shown that both amount of carbohydrate and its glycaemic potency (GI) in a food determine glycaemic response.62,63 We fed subjects several foods, each at two GGE doses, one double the other. After adjusting for individual glycaemic responsiveness, glycaemic responses per unit GGE intake were similar across food types and doses to over 100 g carbohydrate intake. Figure 7.2 shows the increment in blood glucose response per GGE for several foods of differing GI, carbohydrate content and intake; it confirms that GGEs are a robust predictor of glycaemic impact.

7.6.7 Product screening for GGE content

GGE content is calculated from GI which is a clinical measurement of relative blood glucose responses that may not be feasible to use in a food processing context. However, several tests of in vitro digestibility that correlate well with glycaemic responses in humans are now available and are suitable for screening starchy foods to gauge their relative glycaemic potencies.64,65,66 Glucose release after 20min of starch digestion appears to predict glycaemic response well.66 More slowly digested starch has less impact on blood glucose, because the slower rate of glucose loading into the blood does not exceed the rate of cellular glucose uptake enough to produce an acute glycaemic response.

The most accurate measurement of GGE content requires measurement of blood glucose responses to foods in humans, but, because of costs and time, in vitro digestion is a more practical option for the product development stages.

The Most Important Guide On Dieting And Nutrition For 21st Century

The Most Important Guide On Dieting And Nutrition For 21st Century

A Hard Hitting, Powerhouse E-book That Is Guaranteed To Change The Way You Look At Your Health And Wellness... Forever. Everything You Know About Health And Wellness Is Going To Change, Discover How You Can Enjoy Great Health Without Going Through Extreme Workouts Or Horrendous Diets.

Get My Free Ebook


Post a comment