Nutrition: Macronutrients and Carbohydrates Quiz

Macronutrients and Micronutrients

• Macronutrients are needed in larger quantities than micronutrients
• Macronutrients include carbohydrates, proteins, and fats
• Micronutrients include vitamins and minerals

Carbohydrate Sources

• Sucrose (cane sugar) is a disaccharide
• Lactose is a disaccharide found in milk
• Starches are large polysaccharides
• Cellulose is a large amount of polysaccharide
• Amylose, glycogen, alcohol, lactic acid, pyruvic acid, pectins, dextrins, and minor carbohydrate derivatives are also found in meats

Simple Carbohydrates

• Simple carbohydrates are found in fruits, milk, and vegetables
• Cake, candy, and refined sugars are simple carbohydrates
• Simple carbohydrates provide energy but lack vitamins, minerals, and fiber

Dietary Carbohydrates and Blood Glucose

• Some carbohydrate-containing foods cause a rapid rise and fall in blood glucose
• Other foods result in a gradual rise and fall in blood glucose levels
• The glycemic index (GI) measures these differences in the speed of post-prandial glucose concentration

Glycemic Index (GI)

• The GI measures the area under the blood glucose curve after consuming a carbohydrate-rich meal
• It's compared with the area under a blood glucose curve after consuming the same amount of glucose
• A low GI (<55) results in a slower increase in blood glucose compared to a high GI (270)
• A low GI diet can improve glycemic control in diabetic individuals
• The glycemic load (GL) measures how much a typical serving size of a food raises blood glucose

Carbohydrate Digestion

• Digestion occurs in the mouth and the intestinal lumen
• Specific glycosidases hydrolyze polysaccharides and oligosaccharides
• Disaccharidases hydrolyze tri- and disaccharides into reducing sugars
• The final digestion products are glucose, galactose, and fructose

Hydrolysis of Carbohydrates

• Carbohydrates in our diet are large polysaccharides or disaccharides
• Digestive enzymes hydrolyze these molecules by returning hydrogen and hydroxyl ions from water to separate the monosaccharides from each other

Digestion of Carbohydrates

• In the mouth, the digestive enzyme ptyalin (α-amylase) hydrolyzes some starches into the disaccharide maltose
• In the mouth, not more than 5% of the starches are hydrolyzed via ptyalin
• In the stomach, between 30 and 40% of the starches are hydrolyzed
• The digestion in the stomach halts when the acidic contents reach the small intestine
• Pancreatic amylase (several times powerful than salivary amylase) works on the carbohydrates that are entering the small intestine (duodenum)
• Carbohydrates are almost completely transformed to maltose and/or other smaller glucose polymers entering the upper jejunum by the pancreatic amylase

Hydrolysis of Disaccharides

• Enterocytes lining the small intestine contain four enzymes (lactase, sucrase, maltase, and α-dextrinase) that hydrolyze lactose, sucrose, and maltose into monosaccharides
• Lactose produces glucose and galactose molecules
• Sucrose splits into fructose and glucose molecules
• Maltose and other small glucose polymers are split into molecules of glucose

Absorption of Carbohydrates

• Carbohydrates are absorbed as monosaccharides
• Glucose accounts for more than 80% of carbohydrate calories absorbed
• Remaining 20% are mostly galactose and fructose

Absorption of Glucose

• Glucose is absorbed via a sodium co-transport mechanism
• Sodium ions are actively transported across the basolateral membranes of the intestinal cells, reducing intracellular sodium
• The decreased intracellular sodium concentration causes sodium ions from the intestinal lumen to move through the brush border of the cells to the blood
• Glucose molecules are transported with the sodium ions to the interior of the cell
• Other transport proteins and enzymes facilitate glucose diffusion into the interstitial fluid and then into the blood

Absorption of Other Monosaccharides

• Fructose is absorbed via facilitated diffusion and does not rely on sodium transport
• Fructose enters the cell and is phosphorylated into a glucose form
• Galactose absorption follows the same mechanism as glucose

Absorption of Carbohydrates (duodenum and jejunum)

• Absorption of carbohydrates occurs in the duodenum and upper jejunum
• Different sugars have different absorption mechanisms
• Galactose and glucose are absorbed via sodium-dependent glucose cotransporter (SGLT-1)
• Fructose requires a sodium-independent monosaccharide transporter (GLUT-5)

Clinical Aspect - Defects

• Defects in disaccharidase activity can impair carbohydrate digestion, leading to undigested carbohydrates entering the large intestine
• Various intestinal diseases, malnutrition, and certain drugs can also disrupt disaccharide digestion

Lactose Intolerance

• Lactose intolerance is prevalent in many adults, commonly due to a deficiency of lactase, the enzyme needed to digest lactose
• Lactose intolerance can cause symptoms like bloating, pain, gas, and diarrhea from undigested lactose entering the large intestine (fermentation)
• People dealing with this condition can reduce consumption of lactose-containing foods, substitute with alternative sources to maintain energy, use lactase products, or eat yogurt with live and active cultures
• Lactose intolerance is sometimes age-dependent and related to the DNA sequence on chromosome 2 that influences lactase expression

Protein Digestion

• Dietary proteins are hydrolyzed by proteolytic enzymes to amino acids for absorption
• These enzymes include pepsin in the stomach and various pancreatic proteases and peptidases in the small intestine
• Protein digestion in the stomach starts with pepsin, which denatures proteins
• Enzymes in the small intestine further hydrolyze polypeptides to release various amino acids

Digestion in small intestine - steps

• Pepsin begins protein digestion in the stomach
• Pancreatic enzymes act on proteins in the small intestine: Trypsin, chymotrypsin and others hydrolyze polypeptides further
• Peptidases in the small intestine break down peptides into amino acids.

Protein Absorption

• Proteins are absorbed in their di-, tri-peptide forms or as free amino acids
• Sodium co-transport mechanism facilitates di- and tri-peptide absorption
• Amino acids are absorbed via facilitated diffusion

Diseases Associated with Protein Digestion

• Pancreatic insufficiency (acute/chronic pancreatitis, cystic fibrosis, surgical removal of the pancreas) can lead to protein maldigestion
• Celiac disease damages the intestinal villi leading to malabsorption of dietary components such as proteins

Other Notes:

• Carbohydrates, proteins, and fats are the three major food types digested and absorbed in the body in their constituent parts. Their amounts and percentages might be different
• The images show diagrams and figures showing the process
• The listed enzymes, some with their inactive form, are produced in one part of the body and their actions occur in another part of the body.