The Glycemic Index, Glycemic Load, and How They Affect Digestion of Carbohydrates in The Human Body

Written by Eric Morris Jr., CPT, Pn1

 
The Glycemic Index

The Glycemic Index (GI) is a scale that rates carbohydrate-based foods from 0 to 100 based on how fast they spike blood sugar levels after eating. Blood sugar begins to rise about 10-15 minutes after a meal and the degree of rise and subsequent fall is usually measured over a period of two hours. Low glycemic foods are ranked 55 or less, high glycemic foods are ranked 70 or higher, anything in between 55 and 70 is moderate. It effectively rates the quality or kind of carbohydrate (simple or complex). This rating is in comparison to reference glucose with a GI of 100. The higher the rating on the GI scale the faster the carbohydrate food raises blood sugar levels. If blood glucose (blood sugar) is raised too fast after a meal this leads to high insulin levels then a crash shortly after resulting in hypoglycemia (low blood sugar). The raise in insulin and resulting hypoglycemia in turn leads to feeling hungry again shortly after meals. This can lead to overconsumption of calories (especially when eating calorie dense or high GI foods once again) and to weight gain. Eating low-GI foods results in slower and smaller fluctuations in blood sugar and therefore lower insulin levels. This keeps you fuller longer and prevents overeating. In short, high-GI foods spike glucose and insulin making you feel hungry again soon after eating. Low-GI foods do not spike glucose or insulin as fast and keep you satisfied for longer, preventing overeating. Fats and proteins are not rated on the GI.



The Glycemic Load


There’s also a scale called the Glycemic Load (GL). The difference is while the GI which rates the quality of the carbohydrate, the GL rates the quality and quantity of the carbohydrate. The GL is determined by not just using a linear scale but by taking the GI of the food, multiplying the amount of carbohydrates in the portion size in grams and dividing that number by 100 (GI x amount in grams/100 = GL). Taking into account portion size as well as type of carbohydrate is another method used in determining how a carbohydrate will react in the body. The reasoning behind the GL is that a small amount of a high GI food can have the same effect on blood glucose as a large amount of a low GI food.


Both the GI and GL are useful tools in helping to decide which food choices to make and how the carbohydrate will affect blood sugar and insulin levels at any given meal. Bear in mind that although a food might have a high GI or GL on their own, it doesn’t necessarily mean that it will have a glucose or insulin spiking effect when eaten in combination with other foods. For instance, mashed potatoes have a GI of about 73 (high). However, eaten with a meal containing protein, fat, fiber, and veggies it no longer has as great of an effect on glucose or
insulin. So, it’s good to consider GI, GL and food combinations and use all these as a tool to effectively manage your blood sugar.


While all carbohydrates are broken down or converted into glucose (sugar) there are different types of carbohydrates.


Simple carbohydrates

There are two types of simple carbohydrates. Monosaccharides and
Disaccharides:
Monosaccharides: Are the simplest form of sugar that make up more complex carbohydrates. They consist of a single sugar unit (hence the prefix ‘mono’). Examples of monosaccharides would be glucose, fructose, galactose, and mannose.
Disaccharides: Are carbohydrates that consist of two sugar units (hence the prefix ‘di’). Examples include sucrose, which is made of one molecule of fructose and one molecule of glucose; and lactose (the sugar in milk and dairy), which is made up of one molecule of glucose and one molecule of galactose.


*Fun Fact: When dairy is said to be ‘lactose free’ that means the disaccharide lactose has already been broken down into its two separate constituent molecules glucose and galactose. For those who are lactose intolerant (they lack the enzyme to break down lactose on their own) this makes it easier to digest and can help prevent or alleviate stomach upset, gas, and bloating. Other options that can also help mitigate lactose intolerance is taking a lactase enzyme with food containing lactose and/or opting for fermented or cultured dairy such as yogurt or kefir (drinkable yogurt).


Complex Carbohydrates
There are two main types of complex carbohydrates. Oligosaccharides and polysaccharides:
Oligosaccharides: Are complex carbohydrates that have 3 to 10 sugar sub-units. Some examples would be stachyose and raffinose.
Polysaccharides: Are complex carbohydrates that have more than 10
monosaccharide sugars linked together. Starch, which is made of amylose and amylopectin (multiple molecules of glucose) would be an example of a polysaccharide.


*Fun Fact: Since glycogen is the storage form of glucose it’s often mistaken for a simple carbohydrate as well. Although the mix-up is understandable, glycogen is actually a polysaccharide (complex carbohydrate) made up of thousands of glucose units. The molecular formula for glucose is C6H12O6 while glycogen’s formula is C24H42O21.


Breaking Down Carbohydrates for Energy
When it comes to digesting and breakdown carbohydrates for energy the first thing that needs to happen is mastication (chewing). There is a salivary enzyme called amylase that is released when we chew our food that starts the process of breaking down carbohydrates. That’s why it’s highly important to chew your food thoroughly or the entire process of digestion is hindered. Amylase breaks down disaccharides, oligosaccharides and starches. Salivary amylase only breaks down about 5 percent of carbohydrates to prevent too much glucose from being present in the mouth which can cause cavities. Once a bolus (mass of chewed food) is formed and swallowed it goes down the esophagus into the stomach where the process of digestion continues.


The stomach mixes the carbohydrates together into a substance called chyme and then passes into the small intestine. Once in the small intestine the pancreas secretes a digestive juice that also contains amylase while the villi (little projections on the inner surface of the intestines) release specific enzymes, such as sucrase, lactase and maltase to breakdown certain carbohydrate molecules, such as sucrose, lactose and maltose respectively.


After the carbohydrate chains have been sufficiently broken down into their simplest units of sugar (monosaccharides) they’re transported into the cells of the intestines called enterocytes. From there the monosaccharides enter the bloodstream with help of special transport proteins called GLUT transporters. When insulin is released into the bloodstream in response to the presence of glucose, insulin binds to the insulin receptor embedded in the cell membrane.
The insulin receptor is polarized (positively charged one side and negatively charged on the other) due to the phosphorylation (addition of a phosphate molecule) of ATP (energy molecule). This activates the GLUT4 vesicle which stimulates the GLUT4 transport protein in the cell membrane which enables glucose transport. There are 12 transport proteins in all named GLUT1 through GLUT12. All of them facilitate the diffusion of sugars into the bloodstream. Monosaccharides are shuttled to the liver by the portal vein that connects the intestines to the liver. The liver is the first stop for monosaccharides where they are converted to glucose and either released or stored as glycogen for a future energy source.


Glucose levels are regulated by the pancreas just like a thermostat system in your house. Once a certain temperature is reached in your home, the thermostat shuts off. The pancreas works in the same fashion. When blood sugar levels rise, cells in the pancreas release insulin into the bloodstream and cells take up the glucose and use it for energy through the process of glycolysis (breaking down of glucose into pyruvate). Once glucose levels have decreased because your cells have taken up the glucose, the pancreas secretes the hormone glucagon which tells the cells to stop using glucose and to release unused glucose back into the blood (from the cells that did not use the glucose) and then back to liver to be stored as glycogen.


The Role of Fiber
Fiber (roughage) is a polysaccharide, a type of complex carbohydrate. Fiber is a component of plant cell walls and plays an important role in digestion and nutrition because it helps with gut motility, efficient bowel movements and helps you absorb sugars and nutrients into your bloodstream. Examples of fiber include cellulose, pectin, gums, and mucilage just to name a few. There are two types of fiber, soluble and insoluble:


Soluble fiber: Soluble fiber dissolves in liquid and forms a gel-like substance when it dissolves. This type of fiber is broken down by gut bacteria in the colon (large intestine) and can help lower LDL (the ‘bad’ cholesterol) by blocking the absorption of dietary fat and cholesterol. It also slows digestion and prevents rapid spikes in blood sugar.
Insoluble fiber: Insoluble fiber doesn’t dissolve in water and passes through our digestive tract largely undigested and doesn’t provide any calories. This type of fiber is what bulks up your stool and helps speed up the removal of waste from your digestive system. This is also the type of fiber that’s listed on the nutrition label of food products.


*Quick tip: If you want to calculate your net carbohydrates, which are the carbs that are actually digested and provide calories, simply subtract the dietary fiber amount listed on the nutrition label from the total carbohydrate amount and you’ll get your net carbs (Total carbohydrate - Dietary fiber = Net carbohydrates). This can be useful when tracking calorie and/or carbohydrate intake.
​

You want to make sure you’re getting both types of fiber in your diet as they both have beneficial properties, help the proliferation of good bacteria and are needed.