Vitamin C - Ascorbic Acid, Ascorbate

Question 1

List dietary sources of vitamin C

Answer 1

Fruits: kiwi, papaya, strawberry, citrus
Vegetables: broccoli, red peppers & green leafy vegetables

Question 2

Describe the digestion of vitamin C

Answer 2

Digestion: none required

Question 3

Describe the absorption of vitamin C

Answer 3

Intestinal lumen of enterocyte
Three mechanisms

1. Sodium-dependent co-transporters
   Transports ascorbate only
   SVCT1 & SCVT2 (sodium vitamin cotransport)
2. Simple diffusion
   Transports ascorbate only
   Stomach and SI with ingestion of high amounts of vitamin C
3. Facilitated diffusion
   Transports dehydroascorbate only using sodium-independent carriers
   - ascorbate may be oxidized to dehydroascorbate in the intestinal lumen
   - dehydroascorbate also competes with glucose for uptake by GLUT1 and GLUT3
   - dehydroascorbate rapidly reduced back to Ascorbic acid in intestinal cells

Once in enterocyte vitamin C diffuses into ECF and enters the plasma

Percentage of vitamin C absorbed decreases with increasing intake, therefore IV administration is required to achieve high levels in the blood

Capacity to absorb oral vitamin C may decrease during acute illness

Unabsorbed vitamin C may be metabolized by intestinal flora

Question 4

Describe the transport and storage of vitamin C

Answer 4

Transported primarily in free form
Uptake into cells requires sodium-dependent carriers –> SVCT1 & SVCT2
Highest concentration of vitamin C found in pituitary and adrenal glands

Question 5

Describe the function and mechanism of action of vitamin C

Answer 5

Collagen synthesis (also need B6)
Carnitine synthesis
Tyrosine synthesis

Neurotransmitter synthesis: norepinephrine and serotonin

Activation of hormones: gastrin, CCK, calcitonin, oxytocin, corticotropin-releasing factor, growth hormone-releasing factor, vasopressin etc

Microsomal metabolism: enzymes required for synthesis of bile acids from cholesterol, also aldosterone and cortisol synthesis. Detoxification through mixed-function-oxidase (cytochrome P450) and flavin-monooxygenase
Antioxidant

Question 6

List interactions with other nutrients and vitamin C

Answer 6

Iron

• enhances the absorption of non-heme iron
• facilitates transfer of iron from plasma into ferritin of liver storage and stabilize ferritin molecule
• excessive iron causes oxidative catabolism of Ascorbic acid

Copper

• high doses of vitamin C (1.5g/d over >2 months) can decrease serum copper ceruloplasmin levels
• • vit C may decrease absorption of copper
• • vit C decrease copper binding to ceruloplasmin
• • therefore patients on high dose vit C for extended periods may need a copper supplement (~2mg/d)

Flavonoids
- Rustin and quercitin have a sparing effect on vitamin C

Vitamin E

• vitamins E and C function together in the antioxidant system
• large amounts of either will increase requirements of the other

Question 7

Describe the metabolism and excretion of vitamin C

Answer 7

Smoking accelerates the depletion of the body’s Ascorbic acid pool
Vitamin C is excreted primarily in the urine as Ascorbic acid or as dehydroascorbate
Dehydroascorbate can also be further broken down to oxalic acid (increase risk for kidney stones)
Excretion increases with increasing intake

Question 8

Describe consequences and symptoms of deficiency of vitamin C

Answer 8

Scurvy
Body pool, 1.5g, sufficient to protect from deficiency for 90 days
- declines 3-4% every day until 300mg left, first signs of deficiency appear

Symptoms

• bleeding abnormalities
• corkscrew hair
• impaired wound healing
• joint muscle and bone pains
• loose and decaying teeth
• anemia
• heart disease and edema/ascites
• emotional changes i.e. depression

Question 9

Who is at highest risk of vitamin C deficiency

Answer 9

Children

Question 10

Describe consequences and symptoms of toxicity of vitamin C

Answer 10

UL = 2g/d

Most common side effect are GI related due to:

• unabsorbed vitamin C digested by gut bacteria
• • abdominal pain
• • gas and bloating
• osmotic diarrhea

Question 11

Describe the assessment of nutrient status of vitamin C

Answer 11

Plasma and serum levels can be checked but leukocyte content better reflects tissue stores and is preferable

Urine concentration

However none of blood or urine tests are very reliable except in severely deficient patients

Question 12

Discuss some of the therapeutic uses vitamin C

Answer 12

Burns, wound healing
- collagen production

Cardiovascular disease

• collagen production
• antioxidant
• carnitine synthesis

Infectious disease
- decreases duration and severity of colds

Stress
- adrenal support

Cancer
- IV therapy