Presently carbohydrate nutrition is unofficially ruled by a concept that seems so reasonable we've become blind to its flaws, especially when applied to the current epidemic of obesity. It's the concept of glycemic index (GI), the basis for the claim that there are good carbs and bad carbs delineated by rate of sugar absorption. Put simply, glycemic index is a method of evaluating carbohydrates based solely on their immediate effect on blood sugar. This is how you establish glycemic of foods quoted directly from the website of the University of Sidney, Australia, recognized as the leading authorities on GI.
To determine a food's GI rating, measured portions of the food containing 10 - 50 grams of carbohydrate are fed to 10 healthy people after an overnight fast. Finger-prick blood samples are taken at 15-30 minute intervals over the next two hours. These blood samples are used to construct a blood sugar response curve for the two hour period. The area under the curve (AUC) is calculated to reflect the total rise in blood glucose levels after eating the test food. The GI rating (%) is calculated by dividing the AUC for the test food by the AUC for the reference food (same amount of glucose) and multiplying by 100 (see Figure 1). The use of a standard food is essential for reducing the confounding influence of differences in the physical characteristics of the subjects. The average of the GI ratings from all ten subjects is published as the GI of that food.
High glycemic foods are 70 or above, while low glycemic foods are below 55. High is good, low is bad.
We've accepted glycemic index as legitimate even though its flaws are evident. The most obvious is that blood sugar can be affected by many factors, some of which are consistent with health and some of which are not. For example, both soluble fiber and fat slow digestion and thus lower the glycemic index of a food. Beans have a low glycemic index because of their soluble fiber content, but so do high-fat foods like potato chips and ice cream. Here are some values taken off this same website:
These are by no means exceptions to the rule that glycemic index can separate good carbs from bad. Glycemic index only appears to have this power when you look at certain foods. But then the sun seems to circle the earth if we don't look too closely. Despite its obvious weakness as a determinant of carbohydrate value, glycemic index is treated as fact. The Harvard School of Public Health shapes its guidelines regarding carbohydrates on GI, which is insulated from criticism by virtue of both its plausibility and its vagueness. Consider the following again from the University of Sidney website under FAQs:
Question: "Do I need to eat only low GI foods at every meal to see a benefit?"
Answer: No you don't, because the effect of a low GI food carries over to the next meal, reducing its glycemic impact. This applies to breakfast eaten after a low GI dinner the previous evening or to a lunch eaten after a low GI breakfast. This unexpected beneficial effect is called the "second meal effect". But don't take this too far, however. We recommend that you aim for at least one low GI food per meal.
While you will benefit from eating low GI carbs at each meal, this doesn't have to be at the exclusion of all others. So enjoy baking your own bread or occasional treats. And if you combine high GI bakery products with protein foods and low GI carbs such as fruit or legumes, the overall GI value will be medium.
In other words, GI can't be wrong because the application is so general. And nobody really wants to test it by doing something radical like creating two groups, each with the same glycemic index but very different foods. For example, group 1 would have an otherwise balanced diet with fruits and vegetables and include 30% of calories from boiled brown rice, beans, and potatoes. Group 2 would consume the same diet but with 30% of calories coming from selected French fries, cake, and ice cream. Follow both groups over time, months to years. This should easily demonstrate the fallacy of this form of evaluation.
Paying homage to GI distorts our understanding of carbohydrates and their contribution to the current epidemic of obesity because it equates all digestible carbohydrates regardless of structural differences (sugar vs. starch) on the basis of one number.
Terms such as complex carbohydrates and sugars, which commonly appear on food labels, are now recognised as having little nutritional or physiological significance. The WHO/FAO recommend that these terms be removed and replaced with the total carbohydrate content of the food and its GI value. However, the GI rating of a food must be tested physiologically and only a few centres around the world currently provide a legitimate testing service.
In other words, GI tells us a story we want to hear. It can't be wrong because we'll never try to falsify it. Rather than do the scientific thing, like test whether GI separates good carbohydrates from bad, we'll simply define good and bad in terms of GI, insulating it from criticism.
Glycemic index reinforces the notion that carbohydrates are inherently suspect when it comes to obesity. It does this by perpetuating a flawed paradigm, that there are three energy-yielding macronutrients, protein, fat, and carbohydrate. This paradigm essentially wipes out the distinction between sugar and starch, between simple and complex carbohydrates because it treats all carbohydrates as sugar. Though on the surface, GI is used as a method of separating carbohydrates according to their nutritional value; value only means restricted access to sugar. And since GI is really only a snapshot of the combined effect of sugar entering the blood and sugar exiting the blood (fate undetermined), the value is arbitrary. The underlying assumption is that structure and function have no meaning when it comes to digestible carbohydrates. They are either fast sugar or slow sugar, period.
Consider how different the picture is from the perspective of the four macronutrient paradigm there are four energy-yielding macronutrients, protein, fat, sugar, and starch. Starch, which is structurally unique carbohydrate, the digestible polymer of glucose, functions to increase responsiveness of cells to insulin, particularly muscles cells that then store the starch in a similar polymer called glycogen. In this paradigm, structure and function matter, a concept that is consistent throughout biology. The role of the polymer, starch, is different from that of the monomer, sugar. Sugar supplies glucose to the blood, while starch has the further role of reducing blood sugar by enhancing the cells responsiveness to insulin. The response to a diet deficient in starch is insulin resistance, the precursor to both obesity and type-2 diabetes, which is why both are so closely associated with each other in the recent epidemic. In fact, they are so linked as to be virtually inseparable. About a third of the nation is obese, and it is estimated that a fourth of the nation displays insulin resistance, suggesting that once insulin resistance is present obesity is inevitable.
The four macronutrient paradigm (FMP) settles many questions that the three macronutrient paradigm (TMP) leaves a mystery. These include the following, each of which will be the subject of further discussion:
1. The relationship between low muscle glycogen and type-2 diabetes
2. The transition from insulin resistance to obesity
3. Why we crave sugar and blame it for obesity
4. The reason for insulin resistance
5. The obvious flaw in the thrifty gene hypothesis
6. Why high fat diets promote insulin resistance
7. Why insulin resistance is so difficult to overcome even with weight loss
8. Why starch-eating populations don't suffer obesity and type 2 diabetes
9. Why athletes need starch
10. Why we've never reconciled the glucose fatty acid cycle with diet and obesity
11. Why type-2 diabetes mimics starvation
12. Why complex carbohydrates matter
13. How insulin resistance is linked to inflammatory conditions
14. The role of the immune system in insulin resistance
15. Why the liver carries out gluconeogenesis even with sugar in the diet
16. Why glycemic index appears to make sense only part of the time
17. And more
Abolishing the distinction between sugar and starch has been a part of the disastrous legacy that the fraudulent concept of Glycemic Index has left behind.