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 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.