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Chapter 43 Nutrition
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1
What type of bonds do amylases hydrolyze in starch?
α(1 → 4) glycosidic bonds
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2
What are the products of starch digestion by amylases?
Maltose, maltotriose, and dextrins
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3
What enzyme breaks down maltose at the brush border?
Maltase
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4
What enzyme deficiency causes lactose intolerance?
Lactase deficiency
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5
How are glucose and galactose absorbed in the intestine?
Sodium-dependent transport by SGLT1
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6
How is fructose absorbed in the intestine?
Carrier-mediated diffusion (GLUT)
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7
What happens when large amounts of fructose or sugar alcohols remain unabsorbed in the intestine?
Osmotic diarrhea
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8
What happens to lactase activity in most humans after weaning?
It declines
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9
Which populations retain lactase activity into adulthood?
Northern Europeans and nomadic African/Arab tribes
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10
What are the major lipids in the diet?
Triacylglycerols (main), phospholipids (minor)
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11
Why must lipids be emulsified before absorption?
Lipids are hydrophobic and need to be broken into small droplets (micelles) to be absorbed in a watery environment.
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12
What are the end products of pancreatic lipase digestion?
2-monoacylglycerols and free fatty acids
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13
How do bile salts aid lipid absorption?
They emulsify fats into micelles for transport through the watery intestinal environment.
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14
Where are bile salts reabsorbed?
Ileum (enterohepatic circulation)
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15
What enzyme requires colipase for activation?
Pancreatic lipase
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16
How do plant sterols lower cholesterol levels?
They compete with cholesterol for esterification, leading to increased excretion of unesterified cholesterol.
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17
Where are long-chain fatty acids absorbed?
Repackaged into chylomicrons → absorbed into lymphatics
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18
What inhibits pancreatic lipase and is used to treat obesity?
Orlistat
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19
What happens to short- and medium-chain fatty acids after absorption?
Absorbed directly into the hepatic portal vein
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20
What are the two main classes of proteolytic enzymes?
Endopeptidases (cut within the protein chain) and Exopeptidases (cut at the ends of the protein chain).
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21
How is pepsinogen activated into pepsin?
By gastric acid and active pepsin in the stomach.
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22
What enzyme activates trypsinogen into trypsin?
Enteropeptidase (secreted by duodenal epithelial cells).
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23
What is the role of chymotrypsin in protein digestion?
Hydrolyzes peptide bonds next to aromatic amino acids.
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24
How are free amino acids absorbed in the intestine?
By sodium-dependent active transport.
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25
What enzyme hydrolyzes small peptides at the amino-terminal end?
Aminopeptidase.
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26
What type of amino acids does elastase target?
Small neutral aliphatic amino acids (e.g., alanine, serine).
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27
What is the role of dipeptidases and tripeptidases?
Hydrolyze di- and tripeptides into free amino acids.
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28
What are zymogens, and why are they important?
Inactive precursors of proteases; they prevent self-digestion.
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29
What condition results from defective amino acid transporters?
Cystinuria — leads to increased excretion of amino acids in urine.
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30
What are the two types of vitamins based on their absorption mechanism?
Fat-soluble (via micelles) and water-soluble (via active transport or diffusion)
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31
How is vitamin B12 absorbed in the intestine?
Requires intrinsic factor secreted by gastric parietal cells
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32
What role does vitamin D play in calcium absorption?
Increases calbindin synthesis and recruits calcium transporters
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33
What compound enhances iron absorption by reducing Fe³⁺ to Fe²⁺?
Vitamin C
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34
What protein regulates iron absorption by blocking ferroportin?
Hepcidin
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35
Why can high fat intake impair calcium absorption?
Forms insoluble calcium soaps with fatty acids
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36
What dietary factors reduce calcium absorption?
Phytic acid and oxalates
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37
What happens to iron that is not transported into the bloodstream?
Stored in mucosal cells bound to ferritin and lost when cells are shed
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38
Why is iron absorption from meat better than from plants?
Heme iron is absorbed separately and more efficiently
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39
How does hypoxia affect iron absorption?
Reduces hepcidin production, increasing iron absorption
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40
What is the primary source of energy for the body?
Carbohydrates, fats, and proteins
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41
What is the formula for BMI?
BMI = weight (kg) ÷ height² (m²)
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42
What is the significance of the respiratory quotient (RQ)?
RQ indicates the type of metabolic fuel being used (1.0 = carbs, 0.7 = fats, 0.8 = proteins).
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43
What happens to BMR with age?
Decreases due to loss of muscle and increase in fat.
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44
What are the key differences between marasmus and kwashiorkor?
Marasmus = calorie deficiency, muscle + fat loss; Kwashiorkor = protein deficiency, edema + fatty liver.
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45
What is diet-induced thermogenesis?
Energy used for digestion, absorption, and nutrient storage (~10% of meal energy).
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46
What causes increased energy expenditure in cachexia?
Increased protein catabolism, anaerobic glycolysis, and futile lipid cycling.
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47
What is nitrogen balance and how is it calculated?
Nitrogen balance = dietary nitrogen intake – nitrogen excretion (mainly in urea).
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48
What are the three states of nitrogen balance?
Positive nitrogen balance (intake > output), negative nitrogen balance (intake < output), nitrogen equilibrium (intake = output).
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49
What conditions lead to positive nitrogen balance?
Growth, pregnancy, recovery from illness.
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50
What conditions lead to negative nitrogen balance?
Trauma, infection, starvation, inadequate protein intake.
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51
How is protein intake calculated from nitrogen intake?
mg protein = mg nitrogen × 6.25 (since nitrogen is 16% of protein).
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52
What is the average daily protein requirement for adults?
0.66 g/kg body weight; recommended = 0.825 g/kg body weight (~55 g/day).
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53
Why do athletes not need very high protein intake?
Increased energy needs are more important than increased protein intake.
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54
How does high protein intake affect protein turnover?
Increases both protein synthesis and protein catabolism, leading to increased diet-induced thermogenesis.
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55
Why do children have higher protein requirements than adults?
Due to increased muscle and tissue growth during development.
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56
What is the main form of nitrogen excretion?
Urea (via urine).
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57
What are the main hormones involved in protein catabolism after trauma?
Cytokines (e.g., IL-1, TNF) and glucocorticoids (e.g., cortisol)
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58
Why does protein breakdown increase after trauma or infection?
Increased demand for threonine and cysteine for acute-phase protein synthesis and hormonal response
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59
How much total body protein can be lost in 10 days after severe trauma?
6–7%
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60
What type of nitrogen balance occurs during recovery from trauma?
Positive nitrogen balance
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61
Why can't the body maintain protein synthesis if even one essential amino acid is missing?
Protein synthesis requires all essential amino acids in the correct proportion
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62
Which two amino acids are synthesized from essential amino acids?
Cysteine (from methionine) and tyrosine (from phenylalanine)
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63
What are the three truly dispensable amino acids?
Alanine, Aspartate, Glutamate
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64
Why do athletes and bodybuilders not need a high-protein diet?
A normal diet providing 14% of energy from protein is enough for muscle synthesis.
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65
What happens to protein turnover with a high-protein diet?
Both protein synthesis and catabolism increase, maintaining nitrogen balance.
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66
Why might some non-essential amino acids become essential during stress or trauma?
The body’s ability to synthesize them may be insufficient under stress.
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