19 Nutrition of Carbohydrates, Proteins, and Lipids (1) Flashcards
1
Q
Carbohydrates (p.2)
- Carbohydrates (CHO)
- Carbohydrates serve a number of functions:
- Carbohydrates and their digestive enzymes
- monosaccharides,
A
-
Carbohydrates (CHO)
- hydrates of carbon ( CxH2xOx).
- No single carbohydrate is essential in the diet, and most are digested and used very efficiently, with the notable exception of those individuals with deficiencies in CHO digestive enzymes.
-
Carbohydrates serve a number of functions:
- fuel storage (liver, muscle) and energy production
- mediators of cellular signaling
- components of cellular structures
-
Carbohydrates and their digestive enzymes
- are water soluble
- easily mix in the lumen of the gastrointestinal tract.
-
monosaccharides,
- the final products of digestion,
- require specific carriers to cross the hydrophobic barrier of the plasma membrane.
2
Q
Dietary Carbohydrates (p.3-6)
- Average CHO Consumption:
- Adult male
- Adult female
- CHO provide/
- A typical diet is composed of/
- The relative proportions of these vary among different populations.
- Developing countries
- infants
- the “Western” diet
A
-
Average CHO Consumption:
- Adult male: 300g/d,
- Adult female: 200 g/d
- CHO provide about 45-50% of the average American’s daily caloric intake, at 4kcal/gram.
- A typical diet is composed of starch (50%), sucrose (30%), lactose (5%); maltose (2%); other monosaccharides, sugar alcohols such as sorbitol and mannitol, and polysaccharides such as cellulose and pectins (13%).
-
The relative proportions of these vary among different populations.
- Developing countries take in most of their CHO as starch,
- infants’ diet focuses on lactose,
- the “Western” diet has gradually become enriched in sucrose and fructose.
3
Q
Dietary Carbohydrates: Simple CHO (p.7-8)
- Monosaccharides
- Glucose
- Fructose
- Galactose
- Disaccharides
- Sucrose
- Lactose
- Maltose
A
-
Monosaccharides
- Glucose: Blood sugar
- Fructose: Fruit sugar, honey, high fructose syrups
- Galactose: Usually as lactose component
-
Disaccharides
- Sucrose = glucose + fructose, Table sugar
- Lactose = glucose + galactose, Milk sugar
- Maltose = glucose + glucose, Various foods, digestion product of starches
4
Q
Dietary Carbohydrates: Complex CHO (p.9-10)
- Starch
- The two forms of starch are characterized by different branching patterns that require separate digestive enzymes.
- Amylose
- Amylopectin
A
-
Starch
- a complex mixture of dietary polymers of glucose.
- Most starch is derived from plant sources such as cereals, rice and potatoes.
-
The two forms of starch are characterized by different branching patterns that require separate digestive enzymes.
- Amylose: α-1, 4 links of glucose, straight chain. Average chain length ~ 600 glucose units
- Amylopectin: α-1,4 linked straight chains with α-1,6 linked branch points. ~6000 glucose units.
5
Q
Dietary Carbohydrates: Dietary fiber (p.11-14)
- Dietary fiber consists of/
- Passing into the colon, fiber can be digested by/
- Short-chain fatty acids stimulate/
- Butyrate
- Soluble
- Insoluble
A
- Dietary fiber consists of non-starch polysaccharides that cannot be digested by GI enzymes.
- Passing into the colon, fiber can be digested by resident bacterial flora that partially break down dietary fiber to short-chain fatty acids (acetate, propionate, butyrate), hydrogen, carbon dioxide and methane gas.
- Short-chain fatty acids stimulate colonic absorption of salt and water and regulate colonocyte proliferation and differentiation.
- Butyrate is a primary energy source for the large intestine.
-
Soluble: (pectin, guar gum, oat bran)
- Absorbs water
- Delay absorption of sugar
- Binds bile salts, cholesterol
-
Insoluble: (cellulose, lignin)
- Speeds GI transit time
6
Q
Carbohydrate Digestion:
Salivary alpha-Amylase (p.15)
- CHO digestion begins/
- salivary amylase breaks down/
- The stomach/
- Digestion continues through/
A
- CHO digestion begins in the mouth, with salivary alpha-amylase, which is more prevalent in infants and children than adults (oral phase).
- salivary amylase breaks down internal alpha1-4 links that comprise amylase, the straight chain version of starch.
- The stomach has no essential role in CHO digestion.
- Digestion continues through the pancreatic phase, and finally the brush border phase, both in the small intestine.
7
Q
Carbohydrate Digestion:
Pancreatic alpha-amylase (p.15)
- most important in CHO digestion.
- As the partially digested food (chyme) moves into the duodenum/
- pancreatic alpha-amylase only breaks/
- The main products generated by alpha-amylase in the small intestine
- While digestion with alpha-amylase is rapid, none of its products are/
- Amylase concentration is usually/
A
- The small intestine, particularly the jejunum, is most important in CHO digestion.
- As the partially digested food (chyme) moves into the duodenum, CCK is released to stimulate the exocrine pancreas to secrete alpha-amylase.
- pancreatic alpha-amylase only breaks internal alpha1-4 links, not terminal alpha1-4 links, nor the beta1-4 (of lactose) or the alpha1-6 (of amylopectin).
- The main products generated by alpha-amylase in the small intestine are maltose and maltotriose plus limit dextrins that consist of 4-5 glucose units and includes all of the alpha1-6 bonds.
- While digestion with alpha-amylase is rapid, none of its products are monomers that can bind to CHO transporters.
- Amylase concentration is usually in excess; severe pancreatic insufficiency at levels <10% of normal will eventually limit starch digestion.
8
Q
Carbohydrate Digestion:
Brush border disaccharidases/oligosaccharidases (p.16-17)
- Digestion of small fragments and disaccharides takes place/
- These enzymes are expressed/
- CHO can only be absorbed as/
- At the completion of CHO digestion, there is essentially/
- there exists a virtual unsaturable capacity/
- Glucoamylase
- Lactase
- Isomaltase/Sucrase (double-headed enzyme) has two activities located in different domains.
A
-
Digestion of small fragments and disaccharides takes place at the brush border membrane of the intestinal villi where the disaccharidases extend into the gut lumen as integral membrane proteins of the glycocalyx,
- These enzymes are expressed in two gradients with concentrations increasing from crypt to villus and decreasing from duodenum to ileum.
- CHO can only be absorbed as monomers.
- At the completion of CHO digestion, there is essentially no loss of sugar units, with the exception of those in the non- or limited-digestible CHOs.
- Between the extensive folding and length of the small intestine and an excess of pancreatic enzymes there exists a virtual unsaturable capacity for absorbing glucose molecules.
- Glucoamylase: alpha-glucosidase, hydrolyzes terminal alpha1-4 links , including maltose to 2 molecules of glucose.
-
Lactase: β-galactosidase that hydrolyzes the β1-4 link of lactose to galactose and glucose. This enzyme decreases after childhood.
- Lactose intolerance is due to inadequate levels of the enzyme lactase in adults.
-
Isomaltase/Sucrase (double-headed enzyme) has two activities located in different domains.
- One domain hydrolyzes sucrose (with alpha1-2 links) to fructose and glucose,
- the other domain hydrolyzes isomaltose (with alpha1,6 links) to 2 molecules of glucose.
9
Q
Carbohydrate Absorption (p.18)
- The final digestion of CHO to monosaccharides places them/
- Hexoses:
- none of these three sugar transporters are/
- Sodium-Dependent Glucose Transporter (SGLT1)
- located/
- high affinity for/
- also carries/
- powered by/
- can concentrate/
- Fructose Transporter (GLUT5)
- located/
- occurs in/
- Sodium-Independent Glucose Transporter (GLUT2)
- located/
- transports/
- high capacity and low affinity for/
A
- The final digestion of CHO to monosaccharides places them in the direct vicinity of the villus CHO transporters.
-
Hexoses:
- none of these three sugar transporters are insulin dependent
-
Sodium-Dependent Glucose Transporter (SGLT1)
- Located in brush border membrane (intestine and kidney); functions in intestinal absorption and renal reabsorption
- High affinity for glucose; co-transports 2Na+ and D-glucose (or D-galactose)
- Also carries 210-260 molecules of H2O as water of hydration for sodium, and hydration of the glucose binding site
- Powered by the Na+ electrochemical gradient (NaK ATPase)
- Can concentrate glucose 10,000 fold
-
Fructose Transporter (GLUT5)
- Located in the brush border membrane as well as in the basolateral membrane
- Occurs in limited numbers in some individuals resulting in an osmotic effect if transporters are saturated.
-
Sodium-Independent Glucose Transporter (GLUT2)
- Located in the basolateral membrane
- Transports glucose (also galactose and fructose) out of the enterocytes and into the portal circulation
- High capacity and low affinity for glucose; not saturated at physiological concentrations of glucose.
10
Q
Carbohydrate Distribution (p.19-20)
- As monosaccharides exit the enterocytes/
- The liver therefore has the first opportunity to/
- liver and glucose
- Glycogen
- Stores of liver glycogen
- increased by/
- mobilized by/
- the liver is the only organ that can/
A
- As monosaccharides exit the enterocytes, they diffuse into fenestrated capillaries of the villus lamina propria and eventually enter the liver via the portal vein.
- The liver therefore has the first opportunity to assimilate dietary carbohydrates.
- Liver can store glucose as highly branched glycogen, or convert excess into triglycerides.
-
Glycogen
- insoluble in water
- forms cytosolic precipitates that have no effect on osmotic pressure inside the hepatocyte.
-
Stores of liver glycogen
- increased by insulin (increased glycogen synthase)
- mobilized by glucagon (increased glycogen phosphorylase and glucose-6-phosphatase).
- the liver is the only organ that can release glucose stores back into the circulation.
11
Q
Carbohydrate Distribution (p.21-24)
- Dietary components have different rates at which glucose is liberated and absorbed depending on/
- This alters/
- glycemic index
- Foods with a high glycemic index/
- Foods with a lower glycemic index/
- These differences are the basis for the
A
-
Dietary components have different rates at which glucose is liberated and absorbed depending on other associating molecules present in the food.
- This alters the concentration of blood glucose and affects how quickly insulin is released from the islets of Langerhans in the pancreas.
- This variation is called “glycemic index.”
- Foods with a high glycemic index tend to rapidly increase blood glucose, producing spikes of insulin and resulting in more stores of glycogen and triglycerides.
- Foods with a lower glycemic index require longer time periods for digestion and absorption, therefore lowering rates of insulin secretion.
- These differences are the basis for the South Beach Diet.
12
Q
Fructose (p.26-27)
- Once fructose does pass through the enterocytes it is/
- Fructose is almost entirely cleared from the/
- hepatic metabolism of fructose tends to favor/
- Fructose has no impact on/
- High- fructose diets have been correlated with/
A
- Once fructose does pass through the enterocytes it is metabolized differently than glucose.
- Fructose is almost entirely cleared from the portal circulation in the liver where it bypasses the rate-limiting phosphofructokinase step of the glycolysis pathway and can overflow that system.
- hepatic metabolism of fructose tends to favor lipogenesis and uric acid synthesis.
- Fructose has no impact on the release of insulin from the pancreas, resulting in lower plasma insulin than had the sugar source been glucose.
- High- fructose diets have been correlated with a decrease of leptin, and a lack of suppression of ghrelin, both of which would lead to an increased appetite.
13
Q
Overview of Nitrogen Metabolism:
Amino Acid Pool and Protein Turnover (p.28)
- The majority of amino acids made available to cells for the synthesis of new proteins actually come/
- All body proteins/
- The rates vary from/
- Approximately 300 gm of protein turnover each day;
- 75% of these/
- The remainder is used for/
- during the degradation of proteins, the amino group/
- Free ammonia
A
-
The majority of amino acids made available to cells for the synthesis of new proteins actually come from turnover of existing body proteins.
- All body proteins turnover.
- The rates vary from a half-life of years for some of the immunoglobulins, to hours for some of the cytokine and regulatory proteins.
-
Approximately 300 gm of protein turnover each day;
- 75% of these are reutilized.
- The remainder is used for gluconeogenesis, ketogenesis, the synthesis of specialized products or are completely oxidized to CO2 and H20.
- during the degradation of proteins, the amino group is discarded and must be processed into urea or bound into glutamine.
- Free ammonia is toxic, especially to the brain.
14
Q
Overview of Nitrogen Metabolism:
Amino Acid Pool and Protein Turnover (p.29-30)
- storage reserve for excess proteins
- the amino acids that are not reutilized must be/
- If amino acids are oversupplied in the diet/
- After removing the amino groups there are three paths for the remaining carbon skeletons:
- Excess dietary protein will be utilized for/
- Due to the caustic and digestive environment in the stomach and small intestine/
- the highest rate of turnover within an organ occurs in/
A
- Unlike carbohydrates and lipids, there is no storage reserve for excess proteins in the body.
- Therefore, the amino acids that are not reutilized must be replaced by dietary intake or by the synthesis of the nonessential amino acids.
- Approximately 100gm of amino acids are consumed as dietary protein each day in the typical Western diet.
-
If amino acids are oversupplied in the diet, the body cannot store them.
-
After removing the amino groups there are three paths for the remaining carbon skeletons:
- oxidation for energy,
- synthesis of glucose for storage as glycogen,
- synthesis of fatty acids for storage in adipose tissue.
- Excess dietary protein will be utilized for fuel before the mobilization of fat reserves stored in adipose tissue.
-
After removing the amino groups there are three paths for the remaining carbon skeletons:
-
Due to the caustic and digestive environment in the stomach and small intestine, 40% of the proteins of these organs turnover each day.
- While the actual weight in grams of proteins synthesized/day in the skeletal muscle is the greatest in the body, the highest rate of turnover within an organ occurs in the gastrointestinal tract.
15
Q
Overview of Nitrogen Metabolism:
Nitrogen Balance and Dietary Protein (p.31)
- During normal health, the intake and loss of nitrogen in the body is at a steady state.
- The majority of the eliminated nitrogen is incorporated into/
- A small amount/
- A variety of normal and pathological conditions push the body into a positive or negative nitrogen balance.
- In a positive nitrogen balance/
- In a negative nitrogen balance/
- Conditions that produce these shifts include:
- Positive Nitrogen Balance
- Negative Nitrogen Balance
- The dietary requirement for protein varies with/
- preferred for healthy adults
- in childhood and during pregnancy
A
-
During normal health, the intake and loss of nitrogen in the body is at a steady state.
- The majority of the eliminated nitrogen is incorporated into urea (via the liver’s urea cycle) and excreted in the urine.
- A small amount is excreted in the feces due to mucosal turnover of the cells lining the gastrointestinal tract.
-
A variety of normal and pathological conditions push the body into a positive or negative nitrogen balance.
- In a positive nitrogen balance, the deposition of amino acids into new proteins exceeds degradation of protein.
- In a negative nitrogen balance the reverse is true.
-
Conditions that produce these shifts include:
-
Positive Nitrogen Balance
- Growth
- Pregnancy
- Lactation
- Recovery from metabolic stress
-
Negative Nitrogen Balance
- Uncontrolled diabetes
- Injury, trauma
- Surgery
- Infection, sepsis
-
Positive Nitrogen Balance
-
The dietary requirement for protein varies with age.
- While neutral nitrogen balance is preferred for healthy adults,
- in childhood and during pregnancy the emphasis is on adequate proteins for optimal growth.
