Chapter 43 Nutrition

What type of bonds do amylases hydrolyze in starch?

→ α(1 → 4) glycosidic bonds

What are the products of starch digestion by amylases?

→ Maltose, maltotriose, and dextrins

What enzyme breaks down maltose at the brush border?

→ Maltase

What two enzymes are combined in sucrase-isomaltase?

→ Sucrase and isomaltase

What enzyme deficiency causes lactose intolerance?

→ Lactase deficiency

How are glucose and galactose absorbed in the intestine?

→ Sodium-dependent transport by SGLT1

How is fructose absorbed in the intestine?

→ Carrier-mediated diffusion (GLUT)

What happens when large amounts of fructose or sugar alcohols remain unabsorbed in the intestine?

→ Osmotic diarrhea

What happens to lactase activity in most humans after weaning?

→ It declines

Which populations retain lactase activity into adulthood?

→ Northern Europeans and nomadic African/Arab tribes

What are the major lipids in the diet?

→ Triacylglycerols (main), phospholipids (minor)

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.

What are the end products of pancreatic lipase digestion?

→ 2-monoacylglycerols and free fatty acids

How do bile salts aid lipid absorption?

→ They emulsify fats into micelles for transport through the watery intestinal environment.

Where are bile salts reabsorbed?

→ Ileum (enterohepatic circulation)

What enzyme requires colipase for activation?

→ Pancreatic lipase

How do plant sterols lower cholesterol levels?

→ They compete with cholesterol for esterification, leading to increased excretion of unesterified cholesterol.

Where are long-chain fatty acids absorbed?

→ Repackaged into chylomicrons → absorbed into lymphatics

What inhibits pancreatic lipase and is used to treat obesity?

→ Orlistat

What happens to short- and medium-chain fatty acids after absorption?

→ Absorbed directly into the hepatic portal vein

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).

How is pepsinogen activated into pepsin?

→ By gastric acid and active pepsin in the stomach.

What enzyme activates trypsinogen into trypsin?

→ Enteropeptidase (secreted by duodenal epithelial cells).

What is the role of chymotrypsin in protein digestion?

→ Hydrolyzes peptide bonds next to aromatic amino acids.

How are free amino acids absorbed in the intestine?

→ By sodium-dependent active transport.

What enzyme hydrolyzes small peptides at the amino-terminal end?

→ Aminopeptidase.

What type of amino acids does elastase target?

→ Small neutral aliphatic amino acids (e.g., alanine, serine).

What is the role of dipeptidases and tripeptidases?

→ Hydrolyze di- and tripeptides into free amino acids.

What are zymogens, and why are they important?

→ Inactive precursors of proteases; they prevent self-digestion.

What condition results from defective amino acid transporters?

→ Cystinuria — leads to increased excretion of amino acids in urine.

What are the two types of vitamins based on their absorption mechanism?

→ Fat-soluble (via micelles) and water-soluble (via active transport or diffusion)

How is vitamin B12 absorbed in the intestine?

→ Requires intrinsic factor secreted by gastric parietal cells

What role does vitamin D play in calcium absorption?

→ Increases calbindin synthesis and recruits calcium transporters

What compound enhances iron absorption by reducing Fe³⁺ to Fe²⁺?

→ Vitamin C

What protein regulates iron absorption by blocking ferroportin?

→ Hepcidin

Why can high fat intake impair calcium absorption?

→ Forms insoluble calcium soaps with fatty acids

What dietary factors reduce calcium absorption?

→ Phytic acid and oxalates

What happens to iron that is not transported into the bloodstream?

→ Stored in mucosal cells bound to ferritin and lost when cells are shed

Why is iron absorption from meat better than from plants?

→ Heme iron is absorbed separately and more efficiently

How does hypoxia affect iron absorption?

→ Reduces hepcidin production, increasing iron absorption

What is the primary source of energy for the body?

→ Carbohydrates, fats, and proteins

What is the formula for BMI?

→ BMI = weight (kg) ÷ height² (m²)

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).

What happens to BMR with age?

→ Decreases due to loss of muscle and increase in fat.

What are the key differences between marasmus and kwashiorkor?

→ Marasmus = calorie deficiency, muscle + fat loss; Kwashiorkor = protein deficiency, edema + fatty liver.

What is diet-induced thermogenesis?

→ Energy used for digestion, absorption, and nutrient storage (~10% of meal energy).

What causes increased energy expenditure in cachexia?

→ Increased protein catabolism, anaerobic glycolysis, and futile lipid cycling.

What is nitrogen balance and how is it calculated?

→ Nitrogen balance = dietary nitrogen intake – nitrogen excretion (mainly in urea).

What are the three states of nitrogen balance?

→ Positive nitrogen balance (intake > output), negative nitrogen balance (intake < output), nitrogen equilibrium (intake = output).

What conditions lead to positive nitrogen balance?

→ Growth, pregnancy, recovery from illness.

What conditions lead to negative nitrogen balance?

→ Trauma, infection, starvation, inadequate protein intake.

How is protein intake calculated from nitrogen intake?

→ mg protein = mg nitrogen × 6.25 (since nitrogen is 16% of protein).

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).

Why do athletes not need very high protein intake?

→ Increased energy needs are more important than increased protein intake.

How does high protein intake affect protein turnover?

→ Increases both protein synthesis and protein catabolism, leading to increased diet-induced thermogenesis.

Why do children have higher protein requirements than adults?

→ Due to increased muscle and tissue growth during development.

What is the main form of nitrogen excretion?

→ Urea (via urine).

What are the main hormones involved in protein catabolism after trauma?

→ Cytokines (e.g., IL-1, TNF) and glucocorticoids (e.g., cortisol)

Why does protein breakdown increase after trauma or infection?

→ Increased demand for threonine and cysteine for acute-phase protein synthesis and hormonal response

How much total body protein can be lost in 10 days after severe trauma?

→ 6–7%

What type of nitrogen balance occurs during recovery from trauma?

→ Positive nitrogen balance

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

Which two amino acids are synthesized from essential amino acids?

→ Cysteine (from methionine) and tyrosine (from phenylalanine)

What are the three truly dispensable amino acids?

→ Alanine, Aspartate, Glutamate

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

What happens to protein turnover with a high-protein diet?

→ Both protein synthesis and catabolism increase, maintaining nitrogen balance

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