For example, this table says corn has a GI of 55 whereas corn flakes are rated at 92!

How can these ratings be so different when regular, unsweetened corn flakes are composed of 98%+ corn (according to the ingredients list on the box, for example Kellog's Original)?

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    It's missing 50% of the fibre content found in unprocessed corn. – Graham Chiu Dec 16 '17 at 18:31
  • @GrahamChiu I see. So this is kind of like Orange juice having higher GI than oranges? – t0x1n Dec 17 '17 at 1:49
  • Yes, something like that. – Graham Chiu Dec 17 '17 at 3:04
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    @GrahamChiu in that case, would you mind formulating this as an answer (perhaps with some references or more info) so I could accept it? – t0x1n Dec 17 '17 at 11:15
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    These flakes are practically pre-digested (cooked, crushed, rolled, heat-dried etc) and then have some actual sugar added in. – LаngLаngС Dec 17 '17 at 11:19

Cornflakes were specifically invented as a religiously inspired way to make the grains more easily digestible and to decrease masturbation desires. This "health food" tasted awful and made the eaters not very happy. Today the now very ancient memory of healthy food choice still reverberates through the marketing efforts and public memory. Despite these flakes now being a very highly processed food that many people might object to eat at all if they knew how these are made (analogous to the sausage making process).

A switch of emphasis occurred early in the 1900s from promoting corn flakes as a health food to a breakfast food that ‘tastes good’. This occurred especially after barley malt extract and sugar were added to enhance the flavor of the basic toasted corn flakes. Cereal producers in those early 1900s also turned to prominent artists to paint scenes of ‘the wholesome life situations’ that always included the product being advertised in a prominent position. […]

Market forces also drove the quest for more nutritious cereals. The emphasis was aimed at stressing the importance of breakfast as the day’s most important meal in a world whose pace was beginning to accelerate faster and faster. A ‘vitamin’s horsepower’ race soon evolved where manufacturers and marketers tried to outdo each other with vitamin additions and marketing strategies. […]

We will limit our discussion here on liquids blending to the four basic flavor materials; water, sugar, salt and malt. The workhorse mixing vessel for these has been, and still is, the steam jacketed kettle. Liquid sucrose, dry salt, and liquid malt extract are slurried in water in a kettle equipped with agitation, and a steam jacket sufficient in capacity to heat the mix to 125°F (52°C). This is a high enough temperature to make a good useable slurry of such viscosity for ease of handling. [from: Gavin Owens: "Cereals processing technology", Woodhead Publishing: Abington, 2001.]

Nobody needs to chew cornflakes, as they are sugary pre-digested carbohydrates that dissolve on their own upon contact with watery liquids. The pre-digestion artificial stomach machine looks like this: cornflake extruder assembly line

Looking at the finished product some orienting average numbers might be for typical cornflakes and unprocessed maize:

        starches sugars fibre salt
flakes  72g      8g     4g    2.75g
Maize   63g      1.29g  9.2   0.02g

Source: Naehrwertrechner For one popular product and others

Cornflakes are not 98% corn. Just using dry weight ratios of the finished product: The amount of salt, malt and sugar alone is above 10%. Another "official" recipe for typical corn flakes is:

Formulation: The basic raw material for the traditional corn flake is derived from the dry milling of regular field corn. Dry milling removes the germ and the bran from the kernel, and essentially what is left is chunks of endosperm. The size needed for corn flakes is one half to one third that of the whole kernel. […]

A typical formula for corn flakes is as follows: corn grits, 100 lb (45 kg); granulated sugar, 6lb (3.7 kg); malt syrup, 2lb (1 kg); salt, 2lb (1 kg); and water sufficient to yield cooked grits with a moisture content of not more than 32% after allowing for steam condensate. [From Elwood F. Caldwell & Robert B. Fast: "Breakfast cereals and how they are made", American Association of Cereal Chemists: St. Paul, 2000, p19.]

Compared to the corn going into these machines: Fibre content removed, heated several times, sugar and carbohydrates ratio increased. Everything pulverised and only afterwards baked together into something resembling solid food. Although on a much lower level this is of course also true for anything grain based, like good old bread, the level of processing is much higher in these flakes.

Further steps in processing: Mixing, Cooking, Dumping, Delumping, Drying, Cooling and Tempering, Flaking, Toasting, which then usually results in:

The moisture content of flakes is usually in the range of 1.5–3%.

Do not bet your farm on these numbers, products differ in one and the same assembly line and even much more so in different markets!

The amount of processing leads to some very minor enhancements in availability of the remaining nutrients, removal or destruction of many other nutrients:

Effects of Different Processing Methods on the Micronutrient and Phytochemical Contents of Maize: From A to Z: The effects of different processing methods on nutrient content in maize, from field to plate, indicate that, generally, the fresher and less processed the maize is, the more nutrients it retains. […] Losses to micronutrients during processing can be mitigated by changes in processing methods or reduction in processing, and also by encouraging consumption of whole-grain maize products over degermed, refined products. When losses cannot be mitigated and populations consuming the product are at risk of specific micronutrient deficiencies, these can be potentially reduced through fortification

This leads to the gycemic index and its differences even in seemingly similar foods:

International table of glycemic index and glycemic load values: 2002
[…] 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. […]


Many people have raised concerns about the variation in published GI values for apparently similar foods. This variation may reflect both methodologic factors and true differences in the physical and chemical characteristics of the foods. One possibility is that 2 similar foods may have different ingredients or may have been processed with a different method, resulting in significant differences in the rate of carbohydrate digestion and hence the GI value. Two different brands of the same type of food, such as a plain cookie, may look and taste almost the same, but differences in the type of flour used, in the moisture content, and in the cooking time can result in differences in the degree of starch gelatinization and consequently the GI values. In addition, it must be remembered that the GI values listed in the table for commercially available processed foods may change over time if food manufacturers make changes in the ingredients or processing methods used.

Another reason GI values for apparently similar foods vary is that different testing methods are used in different parts of the world. Differences in testing methods include the use of different types of blood samples (capillary or venous), different experimental time periods, and different portions of foods (50 g of total rather than of available carbohydrate). Recently, 7 experienced GI testing laboratories around the world participated in a study to determine the degree of variation in GI values when the same centrally distributed foods were tested according to the laboratories’ normal in-house testing procedures (31). The results showed that the 5 laboratories that used finger-prick capillary blood samples to measure changes in postprandial glycemia obtained similar GI values for the same foods and less intersubject variation. Although capillary and venous blood glucose values have been shown to be highly correlated, it appears that capillary blood samples may be preferable to venous blood samples for reliable GI testing. After the consumption of food, glucose concentrations change to a greater degree in capillary blood samples than in venous blood samples. Therefore, capillary blood may be a more relevant indicator of the physiologic consequences of high-GI foods.

Although it is clear that GI values are generally reproducible from place to place, there are some instances of wide variation for the same food. Rice, for example, shows a large range of GI values, but this variation is due to inherent botanical differences in rice from country to country rather than to methodologic differences. Differences in the amylose content could explain much of the variation in the GI values of rice (and other foods) because amylose is digested more slowly than is amylopectin starch (32). GI values for rice cannot be reliably predicted on the basis of the size of the grain (short or long grain) or the type of cooking method. Rice is obviously one type of food that needs to be tested brand by brand locally. Carrots are another example of a food with a wide variation in published GI values; the oldest study showed a GI of 92 ± 20 and the latest study a GI of 32 ± 5. However, the results of an examination of the SEs (20 compared with 5) and the number of subjects tested (5 compared with 8) suggest that the latest value for carrots is more reliable, although differences in nutrient content and preparation methods contributed somewhat to this variation.

An important reason GI values for similar foods sometimes vary between laboratories is because of the method used for determining the carbohydrate content of the test foods. GI testing requires that portions of both the reference foods and test foods contain the same amount of available carbohydrate, typically 50 or 25 g. The available or glycemic carbohydrate fraction in foods, which is available for absorption in the small intestine, is measured as the sum of starch and sugars and does not include resistant starch. Most researchers rely on food-composition tables or food manufacturers’ data, whereas others directly measure the starch and sugar contents of the foods.

This difference in the accuracy of measurements of the carbohydrate content might explain some of the variation in reported GI values for fruit and potatoes and other vegetables. Food labels may or may not include the dietary fiber content of the food in the total carbohydrate value, leading to confusion that can markedly affect GI values, especially those for high-fiber foods. Consequently, researchers should obtain accurate laboratory measurements of the available carbohydrate content of foods as an essential preliminary step in GI testing. The available carbohydrate portion of test and reference foods should not include resistant starch, but, in practice, this can be difficult to ensure because resistant starch is difficult to measure. There is also difficulty in determining the degree of availability of novel carbohydrates, such as sugar alcohols, which are incompletely absorbed at relatively high doses.

Measuring the rate at which carbohydrates in foods are digested in vitro has been suggested as a cheaper and less time-consuming method for predicting the GI values of foods (33). However, only a few foods have been subjected to both in vitro and in vivo testing, and it is not yet known whether the in vitro method is a reliable indication of the in vivo postprandial glycemic effects of all types of foods. It is possible that some factors that significantly affect glycemia in vivo, such as the rate of gastric emptying, will not change the rate of carbohydrate digestion in vitro. For example, high osmolality and high acidity or soluble fiber slow down the gastric emptying rate and reduce glycemia in vivo, but they may not alter the rate of carbohydrate digestion in vitro. It is difficult to mimic all of the human digestive processes in a test tube. In fact, research results from our laboratory have shown that GI values measured in vivo can be significantly different for the same foods measured in vitro. Until we know more about the validity of in vitro methods, it is not recommended that they be used in clinical or epidemiologic research applications or for food labeling purposes because of the potential for large over- or underestimates of true GI values.

As any grain from grasses maize needs some form of processing to be really palatable and nutritious for humans. Corn flakes may be actually one of the better choices among those horrid cereals, being relatively low in added sugar compared to other cereals and mainly raising concerns for their high amount of salt. But the destructive processing removes the taste along with the vitamins:

The managing director of Kellogg's Europe Tony Palmer confessed that 'if we'd known you could take out 25 per cent of the salt and make cornflakes taste even better, we would have done it earlier. But it's also about the interaction with the sugar – as you take the salt out, you've got to reduce the sugar because it starts to taste sweeter.' But isn't the target to reduce sugar consumption too? Why not just cut down on salt and sugar, we wondered. Well, sugar helps keep the crispness and is part of the bulk, so that would be difficult, we were told. Mr Palmer's eyebrows started working furiously as he answered: 'And the risk is, if you take the salt out you might be better off eating the cardboard carton for taste,' he said.

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    Wow, this is fantastic! One question though regarding your statement "Cornflakes are not 98% corn". I am looking at the ingredient list of Kellog's Corn Flakes ("The Original", as sold in Israel) and it clearly says Corn (98%). So what's going on here? – t0x1n Dec 17 '17 at 14:19
  • Not sure, food labeling laws differ. But look at the salt and sugars added, previously just not found in the corn. Maybe they make the sugary part from HFCS and that then counts as 'made from corn'? Dry weight vs volume as calculation base, Israel's CFs made different? Do you have a link to these ingredients specifically? – LаngLаngС Dec 17 '17 at 14:25
  • Well if they say 98% that presumably only leaves 2% for anything else (including sugar and salt). I doubt they'd get away with chalking HFCS as corn, it is usually mentioned explicitly. Not sure about weight/volume or how that would change things. Israel CF made different quite likely, we need it to be Kosher anyway. Nutritional facts uploaded here (translation in description): imgur.com/a/imHmf – t0x1n Dec 17 '17 at 14:54
  • just fyi: kosher cornflakes thejc.com/lifestyle/features/… – LаngLаngС Dec 17 '17 at 14:59
  • Yeah you're right, I looked and it's manufactured in the UK and just imported here. Any insights regarding the nutritional values I posted? – t0x1n Dec 17 '17 at 15:00

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