Vitamin B6

[Guest guest]

Tom,

I read up on the B6 issue and here is a summary and a few thoughts -- I

hope you don't mind, but I am also passing on this information to other

colleagues who might be interested.

As I mentioned, there are three primary forms of B6 which are absorbed

mostly in the jejunum in a passive process. As to the bioavailability that

we discussed, the general rule of thumb is that ~75% of most B6 in foods is

bioavailable. Dietary intake should average >1.25-1.5 mg/day over the

course of a week.

One historical reference noted that adults who were fed a diet low in

Vitamin B6 and high in protein for 3 or 4 weeks had abnormal EEG's -- a

lesser period of time (< 21 days) did not result in abnormalities. It

would be possible for a patient on a self-selected protein diet that was a

protein supplement versus a meal replacement and who was not taking their

supplements routinely to have low B6 intake. Throw in a period of

starvation and it gets even more interesting.

We also discussed B6 requirements and I mentioned that one of the factors

that affects B6 is protein intake. Protein intake is inversely correlated

with plasma PLP concentration (pyridoxal 5'phosphate) -- a direct measure

of status (adequate is >30 umol/L). I would reason that on a high protein

diet, PLP concentrations would be decreased without adequate B6 intake or

supplementation. The vitamin B6 :protein ratio (mg/g) proposed by Leklem

(J Nutr 1990;120:1503-7) is >0.016, though the RDA is between .032 - .035

with males and older females at the lower end. Your 18 yr old would

require .030 mg/g protein. Compliance could be an issue and should be

addressed.

In order to determine B6 status, both direct and indirect methods are

employed. The direct methods measure one or more of the metabolites of the

B6 vitamers. The most common is the plasma PLP concentration which is

supported by both human and animal models, yet its use as a status model

remains controversial. The problem is that this level is also influenced

by several factors -- it would be increased by 1) increased dietary B6

intake 2) aerobic exercise (acutely). Factors that would decrease PLP

include 1) increased protein intake 2) hyperglycemia (acute) 3) increased

age 4) pregnancy 5) increased alk phos activity and 6) chronic smoking.

Plasma B6 (total) and plasma PL are also direct measures that may be

useful. The PL is the form that enters the cell so it may be the most

relevant. One interesting note is that the PLP and PL are carried or

transported by albumin and hemoglobin...so it is also possible that if the

patient had issues with the carriers it could impact the ability to

transport adequate levels to stores.

Indirect measures include products of metabolic pathways or specific

enzymes that require PLP -- so some tissue levels might not be reflected.

The most common are a) the metabolites of the tryptophan pathway or the

methionine pathway and erythrocyte transaminase activity and

stimulation. As you know, the methionine pathway includes not only B6, but

also B12 and folate which have been previously found to be disrupted after

surgery.

Thinking about your patient with the asthma or CF-like syndrome, one

thought is that the PLP concentration could be affected by the asthma or

her diabetes (decreasing it). The asthma could also disrupt tryptophan

metabolism. I did not find anything that would note a drug interaction

from the list you gave me and the B6 being given.

As far as clinical presentation -- in addition to an abnormal EEG and

possibly convulsions in a severe case, you would find stomatitis,

cheilosis, glossitis, irritability and depression and confusion.

I would again, go back to the report to determine what the lab actually

measured to determine if it was sensitive. The results should be viewed in

conjunction with clinical presentation. In most cases, the administration

of pyridoxine corrects altered B6 metabolism. The neurological toxicity

seen with massive doses of 500-1000 mg/d are rarely seen with doses of 2 to

250 mg/day -- and are usually seen with chronic over supplementation.

Normal Lab Values:

Direct measures:

Plasma PLP (pyridoxal 5'phosphate) >30 nmol/L

Plasma total vitamin B6 >40 nmol/L

Urinary 4-pyridoxic acid >3.0 umol/day

Urinary total vitamin B6 >0.5 umol/day

Indirect measures:

Erythrocyte alanine transaminase index <1.25

Erythrocyte aspartic transaminase index <1.80

Thanks for keeping on my toes...

JB

Jeanne Blankenship, MS RD CLE

Senior Clinical and Research Dietitian

Department of Surgery

UC Medical Center

Sacramento, CA 95817

916-734-7260