Diet and Glycemic Index

Diet and Glycemic Index


The glycemic index ( GI ) is a parameter which, referring to foods , meals or diet in general, evaluates their ability to increase blood sugar (amount of glucose in the blood ).

The function of the glycemic index would be to predict the short-term metabolic impact of energy nutrients , or of one or more foods , allowing you to draw up a nutritional program that keeps glycemic fluctuations under control and the related consequences for the body.“In theory”, all this would allow to:

  • decrease ” hunger crises ” or the sense of weakness due to reflex hypoglycaemia ;
  • lower chronic hyperglycemia in sick subjects ( type 2 diabetes mellitus ) and its consequences (such as protein glycation );
  • facilitate weight loss (in overweight subjects , especially insulin-resistant).

However, as we will see, the glycemic index is a less relevant parameter than one might believe, even if the logic in support of a possible therapeutic utility would seem to be flawless (apparently).

Let’s go into detail.

Glycemic index

What is the glycemic index?

The glycemic index concept was introduced in 1981, after which it has been corrected and rearranged several times.

The glycemic index (GI) of a nutrient, food or meal identifies:

<< the trend with which, after oral intake , glycemia increases and then decreases *>>.

* Blood glucose concentration measurable in mg/dl or mmol/L .

This index is expressed as a percentage (%), which refers to a specific test food , to which a value of 100% is attributed .

The measurement is performed by feeding a portioned food containing the same amount of carbohydrates as the test – therefore, not the same portion as the test.

There are two distinct equations, both available in the literature, different from each other because they are based on different test foods.

The most used one uses a solution of water and 50 g of glucose as a parameter of comparison . The other instead, 50 g of white bread .

Since glucose in solution is 1.37 times faster than white bread, to obtain the conversion from one index to another it is sufficient to multiply or divide the value of the second or first scale by 1.37.

The glycemic trend, which would graphically draw a bell-shaped curve , is observed for a period of time equal to 120′ (2 hours).

For example, a food with a glycemic index of 10 (%) is capable of increasing blood sugar with a rate equal to 1/10 compared to the glucose solution.

According to the logic of the glycemic index, foods can be divided into 4 categories:

  • Very low GI up to 40;
  • Low GI 41 to 55;
  • Medium GI between 56 and 69 ;
  • High > 70 .

Critical issues

Some criticalities of the glycemic index

First of all, there are numerous factors that influence the glycemic index of foods; some are: individual responses and nutritional composition – also influenced by other characteristics which we will discuss below.

Subjective criticalities of the glycemic index

The individual response is the variable that most undermines the practical use of this criterion.

One could also reply that “the glycemic index is designed to give a trace on large numbers, by averaging a large sample”.

Certainly, but the dietary “work” is still applicable to the individual and not to the community (for which the famous “guidelines” are more suitable).

By observing different “apparently similar” people in terms of body size and habits, the glycemic index of a single food can change significantly. This is mainly due to ” digestive ” differences – exocrine secretions ( saliva , pancreas ), motility of the digestive tract , state of the mucosa , enzymatic density, absorption capacity, etc.

Even the acute metabolic state can give very flexible values. First of all, the same subject who is subjected to the test several times , even allowing glycemic normalization, offers different numbers before and after. This, of course, respecting identical procedural crimes. This is due to subliminal extra- and intracellular “sensitivity” mechanisms which need, conceivably, a longer time to normalize than glycemia.

Pertinently to the acute metabolic set, it must be said that the glycemic trend – we recall having an observation of 2 hours – of a subject ” emptied” of his muscle and liver glycogen reserves , with a high post -workout oxygen debt , which participates in opening a wide metabolic window, is completely different when observed in conditions of general homeostasis .

Discussing instead of metabolism in the chronic, the same subject with a different body composition (perhaps observed at different times in life) will give inconsistent results. When sedentary and overweight (probably insulin resistant), vice versa normal weight , muscular and active, the subject will have a totally different metabolic management of glucose.

Nutritional criticalities of the glycemic index

The glycemic index is influenced by the overall nutritional composition of the food or meal.

In fact, if a test like the water solution of glucose is independent of any other aspect of composition, for foods it is not so simple.

  • Fibres , fats and proteins reduce the glycemic index;
  • The ripening of the fruits increases the glycemic index;
  • Type of carbohydrate in question; glucose has the highest glycemic index, while fructose has the lowest. Raw starch ( polysaccharide ) is indigestible and all starches have a different structure , with different glycemic indexes. Resistant starch has a very low glycemic index;
  • State of hydration , since “dry” carbohydrates are not easily digested , hydrating them increases the glycemic index. However, too much can have the opposite effect ;
  • Cooking carbohydrates hydrolyzes them, making them more easily digestible and quickly absorbed, increasing the glycemic index However , if the composition is mixed, factors that reduce protein digestibility, glycation, etc. may occur;
  • Cooking the fibers makes them more soluble, therefore less indigestible but more capable of gelling the digestive content. The impact is controversial.

    That of Meals

    What does the glycemic index of meals depend on?

    The glycemic index of the diet depends above all on the chemical composition of the individual foods. It increases if the meal is composed only of high glycemic index foods , in particular of glucose or short polymers of the same – such as maltodextrins .

    On the other hand, it decreases if the meal also contains a lot of fat, protein, fiber, too much or too little water. Among the sugars , fructose and galactose have a lower glycemic index – the same is true for their digestible polymers – since they must first be converted by the liver into glucose.

    Paradoxically , a very large meal can have a lower glycemic index than a medium-small one.

    But how could this be an advantage? It is not. This makes us understand the senselessness of using the glycemic index as a reference when what really matters is the amount of calories in the diet – given by the glycemic load of all meals, but also by the amount of protein and fat. The metabolic impact, however, referring above all to the release of insulin (insulin index and load), deserves a separate discussion and will not be dealt with in this short article.

    The glycemic index increases with deep cooking , even if parallel factors such as water absorption, protein glycation, etc. can take over. On the other hand, it is lower , in the case of starch, if it remains raw or if it undergoes inversion ( resistant starch ).

    Attention! The factors that increase or decrease the glycemic index must be suitably contextualised, in the sense that although they have an impact, they may not do so in a significant or decisive way.

    High GI carbohydrates

    What are high glycemic index carbohydrates?

    Carbohydrates with a high glycemic index (JIA) are those capable of raising the concentration of glucose in the blood suddenly, an event which – associated with a high glycemic load – hyperactivates the secretion of insulin which, in turn, creates various effects on metabolism; In summary:

    • Promotes glycogen synthesis (accumulation of glycogen, a storage sugar ) in the liver and muscles ;
    • Depresses the consumption of fats ( lipolysis ) and neoglucogenetic amino acids ( neoglucogenesis );
    • It promotes liposynthesis (synthesis of fatty acids and triglycerides ) starting from excess carbohydrates and food proteins , and the storage of fats in adipose tissue ;
    • In conditions of muscle fatigue, it promotes the regeneration of damaged fibers;
    • Inhibits the action of glucagon ;
    • Stimulates the secretion of leptin by adipose tissue – which promotes the feeling of satiety.

    Regardless of the glycemic index, what drastically increases blood sugar and insulin levels is the excess of carbohydrates, therefore the glycemic load.

    Furthermore, AIG carbohydrates can find a very useful use in sport . For example, after very intense and prolonged activity, which favors the depletion of glycogen stocks , the body needs a certain amount of carbohydrates to restore (replenish) muscle and liver reserves. High glycemic index carbohydrates are especially helpful in replenishing these stores as efficiently and quickly as possible.

    Foods with high glycemic index and high caloric density Foods with Medium and High Glycemic Index Foods with low celiac index
    Sugar White bread All fruits and vegetables (excluding exceptions)
    Some refined grains ( puffed rice , bagged potato chips, wafers , cornflakes) Cookies Milk and yoghurt
    Sweets and cakes Potatoes Fructose
    Sugary drinks ( fizzy drinks , “summer” drinks, iced tea , etc.) Croissant Whole grains (especially oats and barley )
    Foods containing “sugar” or ” glucose syrup ” in the ingredients Raisins Pasta cooked al dente
    Some types of fruit and vegetables (carrot, melon , pumpkin , etc.)

    What is it for

    What is the use of understanding the glycemic index of the diet?

    Since hyperglycemia is one of the most widespread and harmful metabolic derangements, also due to the resulting hyperinsulinemia , the evaluation of the GI has the purpose of understanding and assessing which foods can be considered “recommendable” and which ones are not.

    On the other hand, it is now known that blood sugar “surges” more, rather than in response to the intake of high GI foods, after consuming a high glycemic load (CG). This other parameter corresponds to the quantity of glucose that a food, in a standard portion, is capable of releasing into the plasma – an aspect which, not surprisingly, is also correlated to the caloric density of the product.

    It must also be said that the glycemic load should be evaluated as a whole ; furthermore, what matters is, on balance, total calories .

    While considering the daily division of energy into several meals (for various reasons) to be suitable , absurdly, if our daily requirement were 2000 kcal and we assumed only 1800 kcal with a single meal, we would lose weight more than by eating 5, the sum of which reaches a total of 2000 kcal.

    Also on the glycemic load of the single meal then, a separate chapter should be opened. Is it really incorrect to drastically increase or decrease that of a single meal? It depends, above all on what you did before and what you will do after this meal.

    In fact, if we were talking about unhealthy people, therefore insulin resistant and obese, we could say that it is always advisable to moderate the glycemic and insulin index (when possible of course), however giving priority to the glycemic load and total calories.

    Conversely, considering an endurance sportsman , we could also find it profitable to create a maxi-glycemic load to be administered immediately after the performance, or even add up the entire glucid quota by spreading it over the 2 hours before, intra- and post-workout.

    It’s not over. Thanks to the process of neoglucogenesis , foods mainly containing proteins ( neoglucogenic amino acids ) and triglycerides ( glycerol ) are also able to increase blood sugar .

    Furthermore, amino acids and fatty acids also have insulin stimulating properties . This means that not only foods high in carbohydrates, but others as well, can raise insulin, both in a blood sugar – dependent and independent manner .


    In 2003, on the “International table of glycemic index and glycemic load values” ( Foster -Powell K, Holt SH, Brand-Miller JC. HumanNutrition Unit, School of Molecular and Microbial Biosciences, University of Sydney, NSW, Australia – Am J Clin Nutr. 2003 Apr;77(4):994 ) A new and updated glycemic index table was published.

    The novelty of this publication was the introduction of the concept of variability. In fact, it was discovered that the glycemic index of a food can vary according to:

    • Varieties (e.g. different varieties of a fruit have different glycemic indexes)
    • Degree of ripeness (an unripe fruit has a different glycemic index from a very ripe fruit)
    • Geographical area of ​​production (for example an apple grown in Denmark or Italy)
    • Mode of production (e.g. various “industrial” products)
    • Fat and protein content (e.g. ice cream )
    • Fiber content (e.g. real corn flakes , high in fiber , vs. higher calorie corn flakes that are more cookie-like)
    • Storage and drying
    • Cooking method (e.g. boiling or baking varies glycemic index)
    • Duration of cooking (for example pasta al dente or slightly overcooked)
    • Other ingredients of the recipe (pasta with pesto will have a different glycemic index than pasta with tomato sauce ).
      Click on the image to view the GLYCEMIC INDEX TABLE.


      How to consider the glycemic index of the diet?

      Setting an entire eating pattern to the glycemic index is conceptually wrong .

      Comparing the GI between foods only makes sense if the difference is clear-cut and, even in this case, subordinates to other criteria.

      For the purposes of a healthy and balanced diet , the GI takes on a marginal role after all. Instead, it is essential to carefully evaluate the amount of calories and, possibly in sick or obese subjects , the insulin index (II) and the insulin load (CI) – which, as we have said, also concern foods without carbohydrates – to establish the most suitable portions of foods rich in carbohydrates such as pasta, bread and sweet fruit.

      Preferring pasta to rice or potatoes unnecessarily restricts our possibilities for variety, pushing us towards orthorexia . Orthorexia is a real obsession with healthy eating , comparable with anorexia and bulimia with the difference that these two pathologies are related to the quantity, orthorexia to the quality of food.

      Those who already follow a balanced diet , integrating the glycemic index concept, would not add anything useful to their eating behavior . In fact, in a healthy diet , the breakdown of macronutrients is calculated as a percentage of the subject’s caloric needs . A normocaloric diet composed of approximately 55-60% carbohydrates, 25-30% fat and the remainder in proteins, appropriately distributed throughout the day, does not require anything.

      The concepts of glycemic index and load, related to the resulting insulin load and index, are particularly important in pathologies such as type 2 diabetes mellitus and related complications, the consequent hypertriglyceridemia and obesity.

      In fact, chronic hyperglycemia, by compromising the functionality of LDL ( cholesterol transport lipoproteins ) increases cholesterolemia and favors the atherosclerotic process – responsible for serious cardiovascular events . It can also damage nerve tissue and eye tissue. If that were not enough, by decompensating insulin production and function, it promotes overweight, determines reduced glucose tolerance due to insulin resistance , etc.

      Common sense and good food education remain the most useful precautions. Eating everything but in moderation is in fact an optimal system for keeping the glycemic index, the glycemic load, the insulin index and the insulin load of the diet under control.

      Overweight and metabolic pathologies are not caused by occasional transgression, but by bad daily habits. A healthy person can indulge in a small dessert at the end of each meal as part of a healthy diet , provided that a neutral calorie balance is respected.

      However, it would be an excellent measure to reduce the use of sugar to sweeten drinks, limit the consumption of sugary drinks (caca cola , fruit juices , etc.) instead drinking more water. To this it is advisable to associate an appropriate protocol of motor physical activity , which allows you to better control your weight and above all to optimize the metabolism of dietary carbohydrates .



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