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An Alternative Glycemic Ranking

posted Dec 17, 2011, 4:53 AM by jp bangkatan   [ updated May 28, 2015, 6:56 PM ]

The current Glycemic Index undoubtedly provides some insights towards the understanding of the impact of carbohydrates on our health. It does however contain many counter-intuitive ratings that are of significant concern. These include the notions that bread is more glycemic than sugar, some potatoes are more glycemic than pure glucose and that such a close cousin of sugar as fruit juice is low GI.

The concept of a glycemic ranking is such a nice idea it would be great if these problems could be overcome and maybe they can as a recent study - A review of the Glycemic Effect and Glycemic Rankings - suggests. This study may be found on the links given on our home page. In the event you wish to preview the content of the study before wading through it or simply or are just interested in the basics the following outline is provided.

Firstly a quick recap. When we eat carbohydrates these are digested and produce glucose which enters our blood stream. After a given meal this glucose level will rise to a certain level and then decrease back to a normal level. To obtain the Glycemic Index or GI the increase in blood glucose at various times after eating some food is plotted against time. The area under this curve is then measured and compared to the area for a similar test after consuming glucose.  

The study makes the contention that the most fundamental measure of glycemic impact is the maximum rate at which blood glucose is accumulated in the blood stream or MGR and that this can be reasonably estimated using blood glucose versus time curves supported by correlations with the GI for various food groups.

The MGR represents the maximum difference in the rate at which glucose enters the blood stream and the rate at which the body is able to remove it making it a direct indicator of the strain on the body. The study also asserts there are theoretical relationships between the MGR and the area as well as the peak height for the blood glucose curves for homogeneous solids and liquid carbohydrates.  As the MGR occurs early in the process this means the curves that follow are also dependent on the MGR.

The relationship between the MGR and GI obtained from the study is shown below.

The study establishes that there is a strong correlation between the MGR and GI for homogeneous carbohydrate solids i.e. foods that essentially consist of similar types of carbohydrate source such as potatoes, rice, pasta and plain bread. There is also a strong correlation between the MGR and GI for carbohydrate based liquids and another distinct trend for dairy products. The study also interestingly shows that there is strong correlation between average values of MGR and GI for mixed carbohydrates although this correlation would not extend to individual foods.  The significant finding from the study is all of these correlations are quite different.  

The following table from the study (where N is the number of tests) rates various food groups in terms of the MGR. It should be noted that values in the table are average values for food groups and the glycemic response for various foods within the broad groups given will vary.

Food Group

Category

N

GI

MGR-N

MGR-T

Homogeneous Solids

 

 

 

 

 

White Breads

Low GI

21

49

51

49

White Breads

Medium GI

30

63

61

63

White Breads

High GI

27

76

69

76

Brown Breads

High GI

8

74

68

74

Rice

Low GI

39

50

52

50

Rice

Medium GI

46

62

60

62

Rice

High GI

22

79

71

79

Potatoes & Pasta

Pasta &Noodles

13

43

47

43

Potatoes & Pasta

Potato

9

90

77

90

Fruit

Fruit

17

 

46

42

Legumes &Vegetables

Legumes

8

38

43

38

 

 

 

 

 

 

Liquids

 

 

 

 

 

Soft Drinks

Low GI

7

39

64

69

Soft Drinks

Medium GI

7

61

82

100

Fruit

Juice

29

45

71

79

Glucose

Glucose

 

100

100

140

 

 

 

 

 

 

Dairy

 

 

 

 

 

Dairy

Ice Cream

21

38

57

57

Dairy

Yogurt

22

 

49

46

Dairy

Flavoured Milk

30

36

55

54

Dairy

Milk

5

25

25

17

 

 

 

 

 

 

Mixed Carbohydrates

 

 

 

 

 

Wholegrain Breads

Low GI

55

50

57

57

Wholegrain Breads

Medium GI

28

62

68

74

Breakfast cereals

Low GI

24

46

53

52

Breakfast cereals

Medium GI

22

63

68

75

Breakfast cereals

High GI

22

81

83

104

Cereal Bars & protein

Protein

29

28

36

29

Cereal Bars & protein

Low GI

43

43

51

48

Cereal Bars & protein

Medium GI

23

62

68

74

Cereal Bars & protein

High GI

14

77

80

98

 

The MGR-N scale is directly proportional to the MGR (measured in (mmol/L)/hr) and has been scaled so that glucose has a value of 100.  The MGR-N scale is not directly comparable to the existing GI scale however it may be transformed using the correlation and the theoretical relationship between MGR and GI for homogeneous solids to provide an index directly comparable to the Glycemic Index. This results in the MGR-T index.

The most striking feature of the glycemic ratings based on MGR is that all carbohydrate Liquids and mixed carbohydrates are substantially more Glycemic than suggested by the GI.

The reason that liquids are more glycemic than solids with a similar GI is because all carbohydrates are immediately available to be produce glucose. For a solid the availability of some portion of the carbohydrates and entry of some glucose into the blood stream will be delayed. This in turn will cause the blood glucose curve to be extended in time and have a greater area under it.

Why then does wholesome wholegrain low GI bread rate so poorly against white low GI bread? The reason is quite simple. Sugars will always be higher in whole meal than in white bread because sugar levels are higher in the bran and endosperm boundary which is removed in refined flour. This and the fact that overall digestible  carbohydrate is lower in low GI wholemeal bread means sugar  comprises 8% to 10% of carbohydrate while in Low GI white breads typically sugars comprise 2.5 % of carbohydrate. The wholegrain bread is therefore significantly influenced by the sugar content 

In order to appreciate why the area under the curve is a poor indicator for mixures of sugar and a lower MGR carbohydrate consider the 25g and 50g glucose reference curves glucose shown below. From these curves it is seen that the maximum rate of glucose accumulation in the blood stream or MGR for the 25g glucose curve is around 95% of the rate for the 50g curve but the area under the curve is only around 64%.

As the GI is proportional to the area under the curve and set at set at 100 for 50g of glucose an enterprising food manufacturer could mix 25g of glucose with 25g of a carbohydrate that had a GI of around zero to produce a compound with a GI of around 64. The MGR-N however would only reduce to 95. By adding different carbohydrates this manufacturer could tailor a compound that had a GI anywhere between 64 and 100. The MGR-N however would remain between 95 and 100. It therefore follows the GI is not a satisfactory indicator for mixtures of sugars and low GI carbohydrates .

One noticeable omission from the table is sucrose or table sugar. However considering that this so readily dissolves in water it is appropriate to treat this as a carbohydrate based liquid along with the closely related soft drinks. This would give it a MGR-N of 85 and MGR-T rating of over 100 based on a GI of 65 and the appropriate transformation. Sucrose therefore whilst hiding under a medium GI cloak is able to impart a high glycemic effect to any food to which it is added in significant quantity.

Perhaps some of the most intriguing aspects of the study are the equations proposed for mixtures of various carbohydrates that replace the concept of GL. These would enable the prediction of the Maximum Glycemic Rate for sugar based carbohydrates such as fruit and fruit juice from a consideration of their basic composition. Amongst other possibilities they would also enable the prediction of the effects on the MGR caused by adding various amounts of sugar.

So what is good and bad food per the MGR rankings? Firstly this may well depend on an individual’s ability to process glycemic foods. The only food group in the table however, that humans have evolved to eat over a very long time is raw unprocessed fruit. We must therefore consider that natural selection has weeded out any inability to cope with this food and that the average glycemic reaction associated with eating fruit is a normal healthy event for all individuals. Allowing for different fruits in different states of ripeness having a different glycemic effect it would be appropriate to locate any acceptable bar somewhat higher than the average value for fruit. Similarly the bar for what is bad should be placed somewhat lower than the values of food known to cause problems such as fruit juice and high GI bread.

As noted above, fruit is the only food group on the list that humans have fully evolved with. The theory that the species is part of the way through the process of evolving to cope with other high glycemic foods and that the fatties are on the wrong side of natural selection may go some way towards explaining why some people are thin and others fat. This however is another story.

The glycemic rankings based on the MGR are seen to overcome the counter-intuitive rankings found in the Glycemic Index. When compared to the MGR the current Glycemic index significantly underestimates the glycemic impact of all carbohydrate based liquids (e.g. fruit juice and soft drinks), all mixed carbohydrate foods such as multi-grains and most dairy products (e.g. ice-cream). As few if any of these foods can really be considered as being ‘Low Glycemic” the impact on what we eat and our health may be significant.

 

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