The Glycemic Index (GI), is a number associated with a food product indicating its effect on the person’s blood sugar level. In 1981, David Jhenkins and Thomas Wolever ranked different carbohydrates along a scale of 100 in which pure glucose has the highest value.The glycemic index can be technically defined as, “The incremental area under the blood glucose response curve of a specific portion of test food expressed as a percent of the response to the same amount of carbohydrate from a standard food taken by the same subject.” Based on these rankings, this GI concept is used worldwide by people (mainly diabetics) to select the foods which will keep their blood sugar level in control and prevent insulin resistance. But there are so many factors which have to be taken into account before assessing a reliable glycemic value to a food product.
It is a well known fact that fats have zero glycemic index value. So, it can be concluded that fats when added to carbs will lower their glycemic response. While Fats alone have a negligible impact on major fat storing hormone, that is insulin, but when fats are combined with carbs, things don’t work the same way. While carbs induce insulin, fats induce acylation stimulating protein (ASP), both of which are fat storing hormones. Both insulin and ASP stimulate the secretion of each other. So when we combine carbs with fats, the secretion of both ASP and insulin gets doubled, leading to excessive fat storage from individualistic actions of both insulin and ASP. Then there is also another fat storing hormone that is, glucosedependent insulinotropic peptide (GIP). GIP promotes fat storing action of LPL and induces more insulin in the body. GIP is primarily induced by carbs and fats. Protein and fibers also induce GIP but to a much lesser extent. Hence, fats along with carbs will result in excessive insulin released due to the combined effect of :
- Insulin response to carbs.
- Insulin induced by ASP in response to carbs.
- Insulin induced by ASP in response to fats.
- Insulin induced by GIP in response to carbs.
- Insulin induced by GIP in response to fats.
Similar to fats, proteins also have zero GI. So proteins, when added to carbs, are believed to lower their glycemic response. But some studies have shown that protein lowers the blood glucose level when it it is orally ingested with glucose. For this reason, many people consume a protein/carbohydrate drink post workout.
A study carried out on people with type II diabetes also showed that protein, when given with glucose, will increase insulin secretion and reduce the plasma glucose rise in at least some type II diabetic subjects.This can be mainly attributed to the fact that protein slows down the gastric emptying of the food and also due to the insulinotropic effect of the proteins. Hence, this puts a big question on the relationship between the rise in blood glucose level and the insulin spike.
The amount of fiber in a carbohydrate is also an important factor which has to be considered while looking at it’s GI. Some types of fibers lower the GI of a food, but some do not. Insoluble fiber, the type found in wheat, has minute effect on GI, so whole wheat bread has a GI similar to that of white bread. Soluble fiber, on the other hand, lowers the GI of food. Foods high in soluble fiber such as oats, barley, and legumes (dried beans and lentils) have low GI values. The results of a study suggest a beneficial effect of dietary fiber in the metabolic control of NIDDM, as the high fiber diet induced lower fasting blood glucose levels and decreased the ratio of low density lipoproteins to high density lipoproteins.
Also, it’s not necessary that the foods which are lower in GI are a better choice than high GI foods. For example, fructose has a fairly low GI of approximately 25. However, there are many harmful effects of fructose which need to be considered before selecting it as a source of low GI carbohydrate. Unlike other sugars, fructose can only be metabolised in our liver and cannot be used to replenish our muscle glycogen. The average liver glycogen content of a person is about 40-45gms and any type of carbohydrate will first be used to replenish the liver glycogen. Since any amount of carbs above the glycogen level is directly converted to the fats, that means if we are having fruits when our liver glycogen store is already full, it will be stored as fat. Hence, we can say that fruits can also make you fat. Along with that, several studies have indicated that excessive fructose consumption can result in high blood triglyceride, insulin insensitivity and high blood pressure.
So what does GI exactly reveal? It is just a measure of how fast a particular carbohydrate could raise blood glucose level relative to pure glucose. However, it can be concluded that GI alone is not a sufficient index to judge a food choice. We must also consider the composition of a carbohydrate, the effect of other macronutrients on the various fat storing hormones such as insulin, ASP and GIP, when combined with a carbohydrate and how they are metabolised in our body. Hence, its safe to say that GI hides the complete truth.
Author Credits – Akshita Arora [Faculty,INFS]
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