Vitamin B3: Absorption, Function, RDA


Side note #1: Pictured above, are the chemical structures of the various forms of niacin. Even though the names are similar to nicotine, the chemical found in tobacco products, they are not related. 

Vitamin B3, also known as niacin and nicotinic acid, is a water-soluble vitamin that participates alongside vitamin B1 (thiamin) and vitamin B2 (riboflavin) in oxidation and reduction reactions in the Krebs Cycle. 

Nicotinamide is the main form of niacin in foods and supplements and it is also the main form of niacin circulation in the bloodstream. This niacin derivative is used by the body to form the coenzymes nicatinamide adenine dinucleotide (NAD) and nicotinamide adanine dinucleotide phosphate (NADP), both of which are synthesize by all tissues. 

Niacin can also be biosynthesized in the liver from the amino acid tryptophan. About 60 mg of tryptophan is needed to produce 1 mg of niacin. 

Side note #2: Iron, riboflavin, and B6 are all required to synthesize niacin. Which means a deficiency in either of these nutrients can ‘mimic’ a niacin deficiency. 

Niacin is absorbed in the small intestine via facilitated absorption. However, if a large amount is available, niacin is absorbed through passive diffusion. Niacin is absorbed 100% for doses up to 3-4 grams. 

In plant foods, niacin may be bound covalently to complex carbohydrates (niacytin) or peptides (niacinogens) which reduces the bioavailability (especially in corn). HCL or treatment with lime increases the bioavailability but only  about 10% is freed and available for absorption

The RDA (Recommended Daily Allowance) for niacin is: 

  • 16 mg/day for men
  • 14 mg/day for women

Although niacin is not toxic in high doses, the UL (Upper Intake Level) has been set at 35 mg/day to avoid skin flushing. 

Vitamin B2: Fun Facts

  • Riboflavin is light sensitive, so milk is packaged in opaque plastic or cardboard containers as opposed to glass bottles. 
  • Antacids decrease the absorption of riboflavin. Achlorhydria in the elderly also decreases the absorption. 
  • Alcohol and divalent minerals (Zn, Fe, Cu) inhibit the absorption of riboflavin. 
  • Animal sources of riboflavin are more bioavailable than plant sources. 
  • Excess riboflavin is excreted in the urine and can turn the urine a bright yellow color. 
  • Riboflavin is also used as a food coloring, e.g., yellow fruit loops.

Ariboflavinosis is a result of a riboflavin deficiency that is characterized by:
mouth lesions
inflamed tongue
sore throat 
However, ariboflavinosis is curable with the consumption of large quantities of riboflavin. 

Ariboflavinosis is a result of a riboflavin deficiency that is characterized by:

  • weakness
  • fatigue
  • mouth lesions
  • inflamed tongue
  • sore throat 
  • cataracts 

However, ariboflavinosis is curable with the consumption of large quantities of riboflavin. 

Vitamin B2: Absorption, Function, RDA

The Vitamin B2, or riboflavin, molecule is composed of two parts: the isoalloxazine ring (bottom) and the ribitol side chain (top).

Riboflavin is non-covalently bound to proteins in foods and is freed by the HCl is stomach acid and intestinal enzymatic hydrolysis. Antacids and achlorhydria in the elderly might hinder the denaturing of the riboflavin.

The freed riboflavin is then absorbed into the small intestine via active transport and diffusion and passed on into the bloodstream. Once in the blood, the riboflavin is transported to the liver via the hepatic portal vein. The liver then phosphorylates the ribitol side chain via flavokinase which converts the vitamin into flavin mononucleotide, or FMN. Under hormone regulation, the liver can convert FMN into FAD.

Both FMN and FAD function as coenzymes for oxidative enzyme systems like energy production, vitamin B6, B12 and folate metabolism, and synthesizing niacin out of tryptophan. FMN and FAD are also coenzymes for glutathione reductase which produces glutathione, an antioxidant.

The RDA (Recommended Daily Allowance) for riboflavin is:

  • 1.1 mg/day for women 
  • 1.3mg/day for men.

There is no UL because it is water soluble and any excess will be excreted in the urine, but the small intestine is only able to absorb 25mg of riboflavin at a time.

Vitamin B1: Fun Facts

Antithiamin Factors:

  • Thiaminase found in raw fish destroys thiamin so it is not recommended to eat raw fish in conjunction with a thiamin-dense meal. However, cooking the fish destroys the enzymatic activity of the thiaminase.
  • Polyhydroxyphenols found in coffee, tea, blueberries and red cabbage, oxidize the thiazole ring. The damage is promoted by magnesium and calcium and is reduced by vitamin C.

Beriberi (Literally: “I can’t, I can’t” in Singhalese) is a result of Thiamin deficiency.

Dry Beriberi usually occurs in older adults and is characterized by muscle weakness and wasting and peripheral neuropathy but is not usually life-threatening.

Wet Beriberi affects the cardiovascular system and causes rapid heart rate and peripheral edema. Wet Beriberi is more likely to be fatal.

Acute Beriberi causes rapid lactic acidosis when glucose is administered intravenously at the hospital to a person with low Thiamin status.

Wernicke-Korsakoff Syndrome is also a result of Thiamin deficiency but this ailment is frequently observed in alcoholics. Symptoms of Wernicke-Korsakoff Syndrome include: confusion, loss of muscle coordination, leg tremors, double vision, eyelid drooping, inability to form new memories, loss of memory, and hallucinations.

Vitamin B1: Absorption, Function, RDA


Vitamin B1, also known as thiamin, is a water-soluble vitamin that’s main function is to remove carboxyl groups in the Krebs/TCA Cycle and produce Carbon Dioxide. Thiamin exists in the human body as free-forming thiamin and in 3 phosphorylated forms: thiamin monophosphate (TMP), thiamin triphosphate (TTP), and thiamin pyrophosphate (TPP), which is also known as thiamin diphosphate (TDP)

Thiamin in the form of TPP/TDP is obtained by eating animal products. TPP/TDP is absorbed through the lumen of the small intestine where it gets broken down into the free form of thiamin and then is released into the bloodstream. The thiamin found in plants is already in it’s free-forming state so nothing needs to be done to it chemically, it just gets absorbed by the small intestine and immediately released into the bloodstream.

Once in the blood, the majority of thiamin binds to albumin via the thiamin-binding protein (TBP) and is then distributed to various tissues.

The RDA (Recommended Daily Allowance) for Thiamin is:

  • 1.2mg/day for men
  • 1.1mg/day for women. 

There is no UL (Upper Intake Level) for Thiamin because it is not harmful in large doses. However, consuming more than 5mg/day decreases the absorption.

Vitamin C: Fun Facts

  • Plants and most animals can make their own Vitamin C. Humans, guinea pigs, fruit bats and primates are the exception.
  • Vitamin C was the third vitamin ever to be discovered, hence the “C.”
  • Vitamin C is a natural antihistamine and can decrease a person’s chances of getting upper respiratory infections like the common cold.
  • Vitamin C also regenerates Vitamin E and glutathione, other antioxidants.
  • Diabetics tend to have low Vitamin C levels because the GLUT 1 transporter that takes Vitamin C into the cells, is also responsible for taking glucose into cells. And since diabetics typically have a higher concentration of glucose in their blood, the glucose competes with Vitamin C (and usually wins) for transport into the cell by the GLUT 1 transporter. For this reason, it is wise for diabetics to take a supplement.