4.4 Carbohydrate Uptake, Absorption, Transport, and Liver Uptake
Carbohydrate Uptake, Absorption, Transport & Liver Uptake
Monosaccharides (glucose, galactose, and fructose) are taken up into the enterocyte by two processes. Glucose and galactose are taken up by the sodium-glucose cotransporter 1 (SGLT1, active carrier transport). The cotransporter part of the name of this transporter means that it also transports sodium along with glucose or galactose. Fructose is taken up by facilitated diffusion through glucose transporter 5 (GLUT5). There are 12 glucose transporters that are named GLUT 1-12, and all use facilitated diffusion to transport various monosaccharides.
The different GLUTs have different functions and are expressed at high levels in different tissues. Thus, the intestine might be high in GLUT5, but not in GLUT12.
Once inside the enterocyte, all three monosaccharides are then transported out of the enterocyte and into capillaries or lacteals (absorption) through GLUT2 as shown in Figure 4.411.
Figure 4.41 Carbohydrate uptake and absorption
The capillaries and lacteals are located within each villus as shown below. Capillaries are the smallest blood vessels in the body, while lacteals are also small vessels but are part of the lymphatic system, as will be described further in a later subsection.
Figure 4.42 Anatomy of a villus2
The following video does a nice job of illustrating capillaries and lacteal and provides some basic detail on uptake into enterocytes and absorption into capillaries/lacteals.
Required Web LinkVideo: Absorption in the Small Intestine
The capillaries in the small intestine join with the portal vein (a.k.a. hepatic portal vein), which transports monosaccharides directly to the liver. The figure below shows the portal vein and all the smaller vessels from the stomach, small intestine, and large intestine that feed into it.
Figure 4.43 The portal vein transports monosaccharides and amino acids to the liver3
In the liver, galactose and fructose are completely taken up by the hepatocytes, while only 30- 40% of glucose is taken up (more on this shortly.) The monosaccharides are phosphorylated by their respective kinase enzymes forming galactose-1-phosphate, fructose-1-phosphate, and glucose-6-phosphate as shown in Figure 4.44.
Figure 4.44 Hepatic monosaccharide uptake
Galactose-1-phosphate, fructose-1-phosphate, and glucose-6-phosphate are important for energy (ATP) production by cells as they can all enter glycolysis directly, or after undergoing conversion to another molecule. This will be covered in greater detail in Chapter 6.
References & Links
- Stipanuk MH. (2006) Biochemical, Physiological, & Molecular Aspects of Human Nutrition. St. Louis, MO: Saunders Elsevier.
- http://en.wikipedia.org/wiki/File:Intestinal_villus_simplified.svg
- https://commons.wikimedia.org/wiki/File:Gray591.png
Video
Absorption in the Small Intestine – http://www.youtube.com/watch?v=P1sDOJM65Bc
Glycemic Response, Insulin, & Glucagon
If only 30-40% of glucose is being taken up by the liver, then what happens to the rest? How the body handles the rise in blood glucose after a meal is referred to as the glycemic response. The pancreas senses the blood glucose levels and responds appropriately. After a meal, the pancreatic beta-cells sense that glucose levels are high and secrete the hormone insulin, as shown in Figure 4.511.
Figure 4.51 Pancreatic beta-cells sense high blood glucose and secrete insulin
Thus, as can be seen in the following figure, blood insulin levels peak and drop with blood glucose levels over the course of a day.
Figure 4.52 Representative figure of blood glucose and insulin levels during a 24-hour period2
Blood glucose and insulin levels rise following carbohydrate consumption, and they drop after tissues have taken up the glucose from the blood (described below). Higher than normal blood sugar levels are referred to as hyperglycemia, while lower than normal blood sugar levels are known as hypoglycemia.
Insulin travels through the bloodstream to the muscle and adipose cells. There, insulin binds to the insulin receptor located within the cell membrane of the muscle and adipose cells. This causes GLUT4 transporters that are in vesicles inside the cell to move to the cell surface as shown below.
Figure 4.53 Response of muscle and adipose cells to insulin; 1) binding of insulin to its receptor,
- movement of GLUT4 vesicles to the cell surface.
The movement of the GLUT4 to the cell surface allows glucose to enter muscle cells and adipocytes (fat cells). The glucose is then phosphorylated to glucose-6-phosphate by hexokinase (different enzyme but same function as glucokinase in liver) to maintain gradient.
Figure 4.54 Response of muscle and adipose cells to insulin part 2; hexokinase phosphorylates glucose to glucose-6-phosphate
Glucagon is a hormone that has the opposite action of insulin. Glucagon is secreted from the
alpha-cells of the pancreas when they sense that blood glucose levels are low, as shown below.
Figure 4.55 Glucagon secretion from pancreatic alpha-cells in response to low blood glucose levels.
Glucagon binds to the glucagon receptors located in the cell membrane of hepatocytes, which causes the breakdown of the glycogen stored in the hepatocytes to glucose (glycogenolysis) as illustrated below.
Figure 4.56 Glucagon binding to its receptor leads to the breakdown of glycogen to glucose.
This glucose is then released into circulation which causes blood glucose levels to rise as shown below.
Figure 4.57 Glucagon leads to the release of glucose from the liver. Subsections:
- 4.51 Diabetes
- 4.52 Glycemic Index
- 4.53 Glycemic Load
References & Links
- Webb, Akbar, Zhao, Steiner . (2001) Expression profiling of pancreatic beta-cells: Glucose regulation of secretory and metabolic pathway genes. Diabetes 50 Suppl 1: S135.
- http://en.wikipedia.org/wiki/File:Suckale08_fig3_glucose_insulin_day.jpg
Diabetes
Diabetes is a condition of chronically high blood sugar levels. The prevalence of diabetes in the US has been rapidly increasing; the link below provides some statistics about prevalence.
Required Web LinkDiabetes Statistics
There are 2 forms of diabetes: Type 1 and Type 2.
In Type 1 diabetes, not enough insulin is produced, as shown in the figure below.
Figure 4.512 Type 1 diabetes
Without insulin, GLUT4 does not move to the surface of muscle and adipose cells, meaning glucose will not be taken up into these cells. This results in an increase in the amount of glucose remaining in circulation (i.e. increased blood sugar.)
Type 1 diabetes was previously known as juvenile-onset, or insulin-dependent diabetes and is estimated to account for 5-10% of diabetes cases1. Type 1 diabetics receive insulin through injections or pumps to manage their blood sugar.
In Type 2 diabetes, the body produces enough insulin, but the person’s body is resistant to it. In Type 2 diabetics the binding of insulin to its receptor does not cause GLUT4 to move to the surface of the muscle and adipose cells as it normally should, thus no glucose will be taken up by these cells.
Figure 4.513 Type 2 diabetes
Type 2 diabetes accounts for 90-95% of diabetes cases, and was once known as non-insulin- dependent diabetes or adult-onset diabetes1. However, with the increasing rates of obesity, many younger people are being diagnosed with Type 2, making the adult-onset distinction no longer appropriate. Some people with Type 2 diabetes can control their condition with a diet and exercise regimen. This regimen improves their insulin sensitivity, or their response to the body’s own insulin. Others with Type 2 diabetes must receive insulin. These individuals are producing enough insulin, but are so resistant to it that more is needed for glucose to be taken up by their muscle and adipose cells.
The video below illustrates Type 2 diabetes.
Required Web LinkVideo: Understanding Type 2 Diabetes (3:45)
References & Links
1. http://diabetes.niddk.nih.gov/dm/pubs/statistics/#what
Link
Diabetes Statistics – http://www.diabetes.org/diabetes-basics/statistics/
Video
Understanding Type 2 Diabetes – https://www.youtube.com/watch?v=JAjZv41iUJU
Glycemic Index
Research has indicated that hyperglycemia is associated with chronic diseases and obesity. As a result, measures of the glycemic response to food consumption have been developed so that people can choose foods with a smaller glycemic response. The first measure developed for this purpose was the glycemic index. The glycemic index is the relative change in blood glucose after consumption of 50 g of carbohydrate in a test food compared to 50 g of carbohydrates of a reference food (white bread or glucose). Thus, high glycemic index foods will produce a greater rise in blood glucose concentrations compared to low glycemic index foods, as shown in Figure 4.521.
Figure 4.521 Blood glucose response to a high glycemic index (GI) food compared to a low glycemic index food1
As a general guideline, a glycemic index that is 70 or greater is high, 56-69 is medium, and 55 and below is low. A stop light graphical presentation has been designed to emphasize the consumption of the low glycemic index foods while cautioning against the consumption of too many high glycemic index foods2.
Figure 4.522 Food glycemic index classifications2
The main problem with the glycemic index is that it does not take into account serving sizes. Let’s take popcorn (glycemic index 89-127) as an example. A “serving size” of popcorn is 20 g, 11 g of which is carbohydrate3. This is equal to approximately 2.5 cups of popcorn4. Thus, a person would have to consume over 11 cups of popcorn to consume 50 g of carbohydrate needed for the glycemic index measurement. Another example is watermelon, which has a
glycemic index of 103, with a 120 g serving containing only 6 g of carbohydrates3. To consume the 50 g needed for glycemic index measurement, a person would need to consume over 1000 g (1 kg or 2.2 lbs.) of watermelon. Assuming this is all watermelon flesh (no rind), this would be over 6.5 cups of watermelon4.
The website glycemicindex.com (link provided below) contains a database you can search to see the glycemic index and glycemic load (covered in the next section) of various foods. The database also contains detail on how the measurement was done, and more information on the product itself. The top link below will take you to this website. The second link is to another database that contains the same information that might be easier for some people to use.
However, please note that in the second link the glycemic loads are calculated using 100 g serving sizes for all foods. This might not be the actual serving size for all foods, which is what is typically used, so it is important to keep this in mind.
Required Web LinksGlycemicindex.comGlycemic Index & Glycemic Load of Foods
References & Links
- http://upload.wikimedia.org/wikipedia/commons/e/ec/Glycemic.png
- www.glycemicindex.com
- Foster-Powell K, Holt SHA, Brand-Miller J. (2002) International table of glycemic index and glycemic load values: 2002. Am J Clin Nutr 76(1): 5.
- USDA National Nutrient Database – http://www.nal.usda.gov/fnic/foodcomp/search/
Links
Glycemicindex.com – http://www.glycemicindex.com/
Glycemic Index & Glycemic Load of Foods – http://dietgrail.com/gid/
Glycemic Load
To incorporate serving size into the calculation, another measure known as the glycemic load
has been developed. It is calculated as shown below:
Thus, for most people, the glycemic load is a more meaningful measure of the glycemic impact of different foods. Considering the two previous examples from the glycemic index section, their glycemic loads would be:
Popcorn:
Glycemic load = (89-127 X 11 g)/100 = 9.79-13.97
Watermelon:
Glycemic load = (103 X 6 g)/100 = 6.18
As a general guideline for glycemic loads of foods: 20 or above is high, 11-19 is medium, and 10 or below is low1,2.
Figure 4.531 Food glycemic load classifications1,2
Putting it all together, popcorn and watermelon have high glycemic indexes, but medium and low glycemic loads, respectively.
You can also use the top two links below to find the glycemic loads of foods. However, please note that in the second link the glycemic loads are calculated using 100g serving sizes for all foods. This might not be the actual serving size for all foods, which is what is typically used, so it is important to keep this in mind. The third link is to the NutritionData estimated glycemic load tool that is pretty good at estimating the glycemic loads of foods, even if actual glycemic indexes have not been measured.
Required Web LinksGlycemicindex.comGlycemic Index & Glycemic Load of Foods Estimated Glycemic Load
References & Links
Links
Glycemicindex.com – http://www.glycemicindex.com/
Glycemic Index & Glycemic Load of Foods – http://dietgrail.com/gid/
Estimated Glycemic Load – http://www.nutritiondata.com/help/estimated-glycemic-load
