Chapter 4_Macronutrient Uptake, Absorption, Transport

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From The Kansas State University Human Nutrition (HN 400) Flexbook by Brian Lindshield is in the public domain in the United States. 4 Macronutrient Uptake, Absorption & Transport The term absorption can have a number of different meanings. Not everything that is taken up into the enterocyte from the lumen will be absorbed, so the term uptake refers to compounds being transported into the enterocyte. Absorption means that a compound is transported from the enterocyte into circulation. Under most circumstances, compounds that are taken up will then be absorbed. After this chapter, hopefully this distinction between these terms will be clear. After later micronutrient chapters, hopefully you will understand the reason for emphasizing this distinction. Sections: 4.3 Types of Cell Uptake/Transport 4.4 Carbohydrate Uptake, Absorption, Transport & Liver Uptake 4.5 Glycemic Response, Insulin & Glucagon 4.6 Protein Uptake, Absorption, Transport & Liver Uptake 4.7 Lipid Uptake, Absorption & Transport

4.3 Types of Cell Uptake/Transport There are a number of different forms of uptake/transport utilized by your body. These can be classified as passive or active. The difference between the two is whether energy is required and whether they move with or against a concentration gradient. Passive transport does not require energy and moves with a concentration gradient. Active transport requires energy to move against the concentration gradient. The energy for active uptake/transport is provided by adenosine triphosphate (ATP), which is the energy currency in the body. Tri- means three, thus ATP is adenosine with three phosphate groups bonded to it. Phosphorylation is the formation of a phosphate bond. Dephosphorylation is removal of a phosphate bond. Overall phosphorylation is a process that require energy. The net effect of dephosphorylation is the release of energy. Thus, energy is required to add phosphates to ATP, energy is released through removing phosphates from ATP. Subsections: 4.31 Passive Uptake/Transport 4.32 Active Uptake/Transport

4.31 Passive Uptake/Transport There are three forms of passive uptake/transport: 1. Simple Diffusion 2. Osmosis 3. Facilitated Diffusion 1. Simple Diffusion Simple diffusion is the movement of solutes from an area of higher concentration (with the concentration gradient) to an area of lower concentration without the help of a protein, as shown below. Figure 4.311 Simple diffusion 2. Osmosis Osmosis is similar to simple diffusion, but water moves instead of solutes. In osmosis water molecules move from an area of lower concentration to an area of higher concentration of solute as shown below. The effect of this movement is to dilute the area of higher concentration. Figure 4.312 Osmosis

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The following videos do a nice job of illustrating osmosis. Web Links: Video: Osmosis (0:47) Video: Osmosis in the Kitchen (0:58) Another example illustrating osmosis is the red blood cells in different solutions shown below. Figure 4.313 Effect of salt solution concentration on red blood cells 1 We will consider the simple example of salt as the solute. If the solution is hypertonic, that means that there is a greater concentration of salt outside (extracellular) the red blood cells than within them (intracellular). Water will then move out of the red blood cells to the area of higher salt concentration, resulting in the shriveled red blood cells depicted. Isotonic means that there is no difference between concentrations. There is an equal exchange of water between intracellular and extracellular fluids. Thus, the cells are normal, functioning red blood cells. A hypotonic solution contains a lower extracellular concentration of salt than the red blood cell intracellular fluid. As a result, water enters the red blood cells, possibly causing them to burst. 3. Facilitated Diffusion The last form of passive absorption is similar to diffusion in that it follows the concentration gradient (higher concentration to lower concentration). However, it requires a carrier protein to transport the solute across the membrane. The following figure and video do a nice job of illustrating facilitated diffusion. Figure 4.314 Facilitated diffusion examples 2

Web Link: Video: Facilitated Diffusion (0:27) References 1. http://en.wikipedia.org/wiki/File:Osmotic_pressure_on_blood_cells_diagram.svg 2.https://en.wikipedia.org/wiki/Facilitated_diffusion#/media/File:Scheme_facilitated_diffusion_in_cell_membrane -en.svg Videos 1. Osmosis - http://www.youtube.com/watch?v=sdiJtDRJQEc 2. Osmosis in the Kitchen - http://www.youtube.com/watch?v=H6N1IiJTmnc&NR=1&feature=fvwp 3. Facilitated Diffusion - http://www.youtube.com/watch?v=s0p1ztrbXPY

4.32 Active Uptake/Transport There are two forms of active uptake/transport: 1. Active Carrier Transport 2. Endocytosis 1. Active Carrier Transport Active carrier transport is similar to facilitated diffusion in that it utilizes a protein (carrier). However, energy is also used to move compounds against their concentration gradient. The following figure and video do a nice job of illustrating active carrier transport. Figure 4.321 Sodium-potassium ATPase (aka sodium-potassium pump) an example of active carrier transport 1 2. Endocytosis Endocytosis is the engulfing of particles, or fluids, to be taken up into the cell. If a particle is endocytosed, this process is referred to as phagocytosis. If a fluid is endocytosed, this process is referred to as pinocytosis as shown below. Figure 4.322 Different types of endocytosis 2 References 1. https://en.wikipedia.org/wiki/File:Scheme_sodium-potassium_pump-en.svg 2. http://commons.wikimedia.org/wiki/File:Endocytosis_types.svg

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4.4 Carbohydrate Uptake, Absorption, Transport & Liver Uptake The following video does a nice job of illustrating capillaries and lacteal and provides some basic detail on uptake and absorption. Web Link: Video: Absorption in the Small Intestine The capillaries in the small intestine join to the portal vein, which transports monosaccharides directly to the liver. No References Video 1. Absorption in the Small Intestine - http://www.youtube.com/watch?v=P1sDOJM65Bc

4.5 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 below 1 . Figure 4.51 Pancreatic beta-cells sense high blood glucose and secrete insulin Thus, blood insulin levels peak and drop with blood glucose levels over the course of a day 2 . 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. 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 receptor in the liver, which causes the breakdown of glycogen to glucose 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 to raise blood glucose levels 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 1. Webb , Akbar , Zhao , Steiner . (2001) Expression profiling of pancreatic beta-cells: Glucose regulation of secretory and metabolic pathway genes. Diabetes 50 Suppl 1: S135. 2. http://en.wikipedia.org/wiki/File:Suckale08_fig3_glucose_insulin_day.jpg

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4.51 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. Web Link: Diabetes Statistics There are 2 forms of diabetes, type 1 and type 2. In type 1 diabetes, not enough insulin is produced. Type 1 diabetes was previously known as juvenile-onset, or insulin-dependent diabetes and is estimated to account for 5-10% of diabetes cases 1 . 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, thus no glucose is taken up. Type 2 diabetes accounts for 90-95% of diabetes cases and was once known as non-insulin-dependent diabetes or adult-onset diabetes 1 . However, with the increasing rates of obesity , many younger people are being diagnosed with type 2, making the latter definition 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 dia betes 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. References 1. http://diabetes.niddk.nih.gov/dm/pubs/statistics/#what Link 1. Diabetes Statistics - http://www.diabetes.org/diabetes-basics/statistics/

4.53 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: Glycemic Load = (Glycemic Index X (g) Carbs/serving)/100 Thus, for most people, the glycemic load is a more meaningful measure of the glycemic impact of different foods. Considering the two examples from the glycemic index section, their glycemic loads would be: Popcorn: (89-127 X 11 g Carbs/ Serving)/100= 10-14 Watermelon: (103 X 6 g Carbs/Serving)/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 low 1,2 . Figure 4.531 Food glycemic load classifications 1,2 Putting it all together, popcorn and watermelon have high glycemic indexes, but medium and low glycemic loads, respectively. The following link is to the NutritionData estimated glycemic load tool that is good at estimating the glycemic loads of foods, even if actual glycemic indexes have not been measured. Web Link: Estimated Glycemic Load References 1. http://www.mendosa.com/gilists.htm 2. http://www.nutritiondata.com/help/estimated-glycemic-load Links 1. Estimated Glycemic Load - http://www.nutritiondata.com/help/estimated-glycemic-load

4.6 Protein Uptake, Absorption, Transport & Liver Uptake There are a number of similarities between carbohydrate and protein uptake, absorption, transport, and uptake by the liver. Like monosaccharides, amino acids are transported directly to the liver through the portal vein 1 . Amino acids are taken up into the hepatocyte through a variety of amino acid transporters. The amino acids can then be used to either make proteins or broken down to produce glucose. Figure 4.64 Hepatic amino acid uptake References 1. http://www.freebase.com/view/wikipedia/images/commons_id/560004

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4.7 Lipid Uptake, Absorption & Transport L ipid uptake is not completely understood. Re-esterified lipids are packaged into chylomicrons , which are lipoproteins. These chylomicrons are too large to fit through the pores in the capillaries, but they can fit through the larger fenestrations (openings) in the lacteal. Lacteals (shown below) are small vessels that feed into the lymphatic system. Thus, the chylomicrons enter the lacteals and enter into lymphatic circulation. Figure 4.73 Anatomy of a villus, with the lacteal shown in blue 1 The lymphatic system is a system similar to the circulatory system in that it contains vessels that transport fluid. However, instead of blood, the lymphatic system contains a clear fluid known as lymph. There are a number of lymph nodes (small glands) within the lymphatic system that play a key role in the body's immune system. The figure below shows the lymphatic system. Figure 4.74 The lymphatic system 2

The following videos describe and illustrate how the lymphatic system and lymph functions. Web Links: Video: Lymphatic System (0:49) The animation below is an overview of lipid digestion, uptake, and initial transport. Web Links: Animation: Lipid Digestion, Uptake, and Transport Subsection: 4.71 Lipoproteins References 1. http://en.wikipedia.org/wiki/File:Intestinal_villus_simplified.svg 2. http://en.wikipedia.org/wiki/File:Illu_lymphatic_system.jpg 3. http://en.wikipedia.org/wiki/File:Gray505.png Link 1. Lipid Digestion, Uptake, and Transport - http://www.wiley.com/college/grosvenor/0470197587/animations/Animation_Lipid_Digestion_and_Absorption/E nergy/media/content/dig/anima/dig5a/frameset.htm Videos 1. Lymphatic system - http://www.youtube.com/watch?v=qTXTDqvPnRk

4.71 Lipoproteins Lipoproteins, as the name suggests, are complexes of lipids and protein. The following video does a nice job of illustrating the different lipoprotein components. Web Link: Video: Lipoproteins (0:28) There are a number of lipoproteins in the body. Lipoprotein Chylomicrons VLDL (very low-density lipoproteins) IDL (intermediate-density lipoproteins) LDL (low-density lipoproteins) HDL (high-density lipoproteins) Table 4.711 Different lipoproteins 1 Many of the lipoproteins are named based on their densities (i.e. very low-density lipoproteins). The lipoproteins released from the small intestine are chylomicrons . The video below does a nice job of showing, describing, and illustrating how chylomicrons are constructed and function. Web Link: Video: Chylomicrons (0:55) Now in the form of a chylomicron remnant, the digested lipid components originally packaged into the chylomicron are directed to the liver where the chylomicron remnant is endocytosed. This process of clearing chylomicrons from the blood takes 2-10 hours after a meal 1 . This is why people must fast 12 hours before having their blood lipids (triglycerides, HDL, LDL etc.) measured. This fast allows all the chylomicrons and chylomicron remnants to be cleared before blood is taken. However, whether patients should be asked been questioned as described in the link below. Web Link: Should you fast before a cholesterol test? You are probably familiar with HDL and LDL being referred to as "good cholesterol" and "bad cholesterol," respectively. This is an oversimplification to help the public interpret their blood lipid values, because cholesterol is cholesterol; it's not good or bad. LDL and HDL are lipoproteins, and as a result you can't consume good or bad cholesterol, you consume cholesterol. A more appropriate descriptor for these lipoproteins would be HDL "good cholesterol transporter" and LDL "bad cholesterol transporter." What's so bad about LDL? LDL enters the endothelium where it is oxidized. This oxidized LDL is

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engulfed by white blood cells (macrophages), leading to the formation of what are known as foam cells. The foam cells eventually accumulate so much LDL that they die and accumulate, forming a fatty streak. From there the fatty streak, which is the beginning stages of a lesion, can continue to grow until it blocks the artery. This can result in a myocardial infarction (heart attack) or a stroke. HDL is good in that it scavenges cholesterol from other lipoproteins or cells and returns it to the liver. References 1. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw's perspectives in nutrition. New York, NY: McGraw-Hill. Links 1. Ask Well: Should you fast before a cholesterol test - http://well.blogs.nytimes.com/2016/05/24/ask-well-should- you-fast-before-a-cholesterol-test/ Videos 1. Lipoproteins - https://www.youtube.com/watch?v=x-4ZQaiZry8 2. Chylomicrons - http://www.youtube.com/watch?v=hRx_i9npTDU

Chapter 4_Macronutrient Uptake, Absorption, Transport

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