Proteins

Question 1

Essential Amino Acids

Answer 1

HV MILK FTW

1. Histidine (H)
2. Valine (V)
3. Methionine (M)
4. Isoleucine (I)
5. Leucine (L)
6. Lysine (K)
7. Phenylalanine (F)
8. Threonine (T)
9. Tryptophan (W)

Arginine (R) becomes essential under certain circumstances such as illness or injury.

Question 2

Protein Requirements

Answer 2

Normal protein requirements:

• Infancy ⇒ 2 g/kg
• Ages 1-2 y/o ⇒ 1 g/kg
• Adulthood ⇒ 0.8 g/kg for both males and females
• Pregnancy & lactation ⇒ extra 10-15 g/day

Disease states may affect protein needs:

• Increased need
  
  • wasting such as disuse atrophy
  • physiological stress (fractures, burns, traumas, illness)
• Decreased need
  
  • acute liver failure (to avoid hepatic trauma)
  • kidney disease

Question 3

Protein Quality

Answer 3

A “good” protein supplies all needed AA in appropriate ratios.

Many dietary proteins, esp. plant proteins, do not supply proper mix.

Must eat complementary proteins.

Protein abundance of amino acids score (PDCAAS):

Chemical score accounts for amount of AA based on content of most limiting essential AAs.

Corrects for digestibility.

A poor quality protein would generate a high BUN due to excess AAs which are metabolized.

Question 4

Digestion and Absorption

Answer 4

1. Proteins hydrolyzed by peptidases into free AAs and oligopeptides
2. Oligopeptides hydrolyzed by peptidases on brush border to form FAA, di- and tri-peptides
3. Free AAs, di- and tri-peptides absorbed along entire small intestine, most proximally
   
   • AA via symport with Na⁺
   • Di- and tri-peptides via H⁺-linked transport system
4. Di- and tri-peptides hydrolyzed by proteases in enterocyte cytoplasm
   
   • Enterocytes use glutamine as 1° energy source & generate alanine
5. Mostly free AAs enter circulation
   
   • Escaped oligopeptides hydrolyzed by peptidases in plasma or at cell membranes of organ tissues
6. AA uptake primarily by the liver via portal circulation
   
   • 57% catabolized
   • 20% converted into proteins
   • 23% released into general circulation
     
     • 70% BCAA ⇒ taken up by muscle, heart, and kidneys for energy

Question 5

Celiac’s Disease

Answer 5

Gluten Enteropathy

• Peptide fragment from gluten indigestible
• Causes autoimmune destruction of enterocytes
• Must avoid wheat, barley, and oats (rice ok)

Question 6

Protein Utilization

Answer 6

• Fates of circulatory AAs
  
  • catabolism
  • protein synthesis
  • production of biogenic amines
• Homeostatic balance occurs in healthy persons
  
  • Ingested protein = “excreted” or catabolized protein
    
    • Neither proteins or AAs are actually excreted intact

Question 7

Amino Acid Catabolism

Answer 7

When amino acids are catabolized:

Carbons exhaled as CO₂

Nitrogens excreted mostly as urea

• Found in blood as BUN (blood urea nitrogen)
• Excreted primarily as UUN (urinary urea nitrogen)
  
  • measured as total amount of urea nitrogen found in a 24 hr urine collection

Question 8

Nitrogen Balance

Answer 8

Nitrogen constitutes 16% of protein mass.

1 gram of nitrogen ⇔ 6.25 grams of protein

Nitrogen balance = Intake - Output

N-balance = protein N intake (g/24h) - (UUN + 4g)

4 g accounts for N in other forms & losses in feces and skin

⊕ N-balance (intake > loss) during growth or pregnancy

⊖ N-balance (intake < loss) during starvation, sickness, etc.

Adults are normally in nitrogen balance.

Question 9

Protein-Energy Undernutrition (PEU)

aka

Protein-Energy Malnutrition (PEM)

Answer 9

Deficiencies of macronutrients.

• Spectrum of states of malnutrition
• Differ in acuity and need for intervention
• Two extreme states:
  
  • Kwashiorkor
  • Marasmus
  • Marasmic Kwashiorkor may occcur (combined form of PEU)

Question 10

Kwashiorkor

Answer 10

Non-adapted Starvation

Protein deficiency with adequate caloric nutrition.

• Seen in weaned children 1.5-3 y/o or adults with certain chronic diseases
• Mildest anthropomorphic changes
  
  • little wasting in adults
  • some wasting in children
• Greatest depression of serum proteins
• Symptoms:
  
  • edema characteristic
    
    • intracellular > extracellular water increases
  • hair and skin changes
    
    • diffuse “flaky paint” dermatitis with desquamation and depigmentation
    • lightened hair at roots
  • hepatomegaly or fatty liver
    
    • due to inc. FA synthesis from carbs
    • lack AA prevents apolipoprotein synthesis required for lipid transport
  • hypoalbuminemia
  • hyperinsulinemia (in response to carbs)
    
    • decreased tissue lipolysis
    • decreased muscle protein breakdown
  • lethargy, apathy, or irritability
  • stunted growth and some muscle wasting
  • anorexia, vomiting, diarrhea
  • significant immune dysfunction
    
    • marked ↓ T-cells
    • atrophy of thyrmus
    • B-cells & splenic/LN B-cells regions fairly normal but defects in Ab production

Question 11

Marasmus

Answer 11

Adapted Starvation

All macronutrients are inadequate and calories are inadequate.

• Manifests with greatest anthropomorphic changes
  
  • significant generalized wasting
  • growth stunting
  • absence of subQ fat
  • non-edematous
  • abdominal distention may be present
• Body has adjusted to reduced caloric intake
  
  • hypoglycemia with hypothemia can occur
• Some immune depression

Question 12

Marasmic Kwashiorkor

Answer 12

Combined form of PEU

Biochemical features of both may be seen.

Protein deficiency usually predominates.

• Muscle wasting and decreased subQ fat
• Also exhibits edema

Question 13

Cachexia

Answer 13

Malnutrition resulting in low body weight and bodily decline associated with chronic diseases.

Ex. cancer, tuberculosis, malaria.

Question 14

PEU

Treatment

Answer 14

1. Kwashiorkor
   
   • rapid deterioration over weeks
   • life-threatening due to immune compromise
   • requires immediate and aggressive refeeding
2. Marasmus
   
   • deterioration over years
   • slower, progressive re-introduction of calories especially carbs
   • aggressive refeeding can be harmful
     
     • rapid intro of carbs ⇒ G-6-P formation but poor utilization
     • depletion of inorganic phosphate pool leads to inability to reform ATP
     • can result in cardiac failure and death

Question 15

PEU

Assessment

Answer 15

• Anthropometric measurements mainly used for severity determination.
  
  • weight-for-height
  • weight-for-age
  • height-for-age
  • upper arm circumference
  • BMI
• Biochemical parameters often unchanged in early stages or are not specific.
  
  • Parameters may help distinguish mild from severe PEU.
  • *Need to know reference albumin values only.