- The status of vitamin B-6 (B6) nutriture of nine
persons (4F;5M) with insulin dependent diabetes mellitus
(IDDM), nine persons (5F;4M) with non-insulin dependent
diabetes mellitus (NIDDM), and 18 control individuals
(9F;9M) was evaluated, using biochemical and dietary
indicators of B6 status. The biochemical indices employed
were plasma concentration of pyridoxal 5'-phosphate (PLP),
urinary 4-pyridoxic acid (4PA) excretion, and urinary
kynurenic acid (KA) and xanthurenic, acid (XA) excretion
following a tryptophan load test (2 g L-tryptophan oral
load). Dietary B6 intake and the ratio of B6 (mg) to
dietary protein (g) (B6:protein) were determined.
Fasting blood, two consecutive 24 h urine collections
and three consecutive daily weighed diet records were
obtained on each of two occasions, separated by 30-70 d.
Diet records were analyzed for vitamin B-6 and protein
intake using nutrient data bases. Samples of 70 foods, for
which the data bases lacked B6 values, were obtained and
analyzed for total B6 content by a microbiological method.
The plasma concentration of PLP was determined by an
enzymatic method, and plasma alkaline phosphatase activity
by a colorimetric method. Urinary 4PA was separated by
HPLC, urinary KA and XA by ion exchange, and each
metabolite was determined fluorometrically.
The mean daily vitamin B-6 intake of each group
exceeded the recommended dietary allowance (RDA). The mean
B6:protein ratios ± standard deviations (SD) for the groups
of females were 0.0200±0.0027, 0.0304±0.0101, and
0.0254±0.0099 for IDDM, NIDDM and control (C),
respectively. The respective B6:protein ratios for the
males were 0.0280±0.0040, 0.0242±0.0038 and 0.0241±0.0078.
The mean±SD plasma PLP concentrations for females were
22.4±6.8, 21.8±9.6 and 37.4126.8 nmol/L for IDDM, NIDDM and
C, respectively. The mean plasma PLP concentrations of the
two groups of females with diabetes were at the low end of
a range (22.4-25.3 nmol/L) suggested to indicate marginal
status, and 56% of the females with diabetes had PLP
concentrations below the lower boundary of the marginal
range. For the three groups of males the PLP
concentrations were in the same rank order as dietary B6
intake; 53.9±18.2, 43.6±7.2 and 37.5±17.7 nmol/L for IDDM,
NIDDM and C, respectively. Plasma PLP concentration was
strongly and significantly correlated with B6 intake in
both diabetes (n=18, r=.744, p<.001) and C (n=18, r=.695,
p<.001) groups, but was also negatively associated with
plasma AP activity only for the diabetes group (n=18, r=-
.454, a=.058). The mean plasma AP activity of females with
NIDDM was significantly higher than that of the female C
group (p<.01). Greater than normal AP hydrolysis of PLP is
thought to have contributed to the low plasma PLP
concentrations observed in the females with NIDDM.
Levels of urinary 4PA excretion by females were
8.76±2.10, 7.61±12.57 and 8.15±14.43 μmol/d for IDDM, NIDDM
and C, respectively, or 87, 63 and 72% of B6 intake. For
males the urinary 4PA levels were 12.76±14.53, 10.32±11.77
and 9.81+3.34 μmol/d, respectively, or 76, 68 and 78% of B6
intake. All subjects excreted 4-PA in amounts indicative
of adequate B6 status.
All means for tryptophan metabolites were within
ranges seen for normal subjects, both pre and post-tryptophan load. None of the subjects with diabetes and
only one female C subject excreted more than 65 μmol XA in
24 h after the tryptophan load (upper boundary of normal
response to 2 g tryptophan load). Mean post-load excretion
of XA and KA of diabetes groups was numerically lower than
that of same sex controls in all comparisons, although in
only one instance was the difference significant (NIDDM
females post-load KA, p<.05). The results of the
tryptophan load test suggest adequate B6 function in the
kynurenine pathway those with diabetes and controls.
Individuals with diabetes were found to consume
adequate or above amounts of B6 by the standard of the RDA.
Low plasma PLP levels were observed in females with IDDM
who had the lowest B6 intake, and in females with NIDDM who
had the highest plasma AP activity. The present research
indicates that low PLP may be present in diabetes, as
observed by other investigators, despite seemingly adequate
B6 nutriture. However, normal to above normal amounts of
urinary 4-PA excretion indicated adequate body stores of
B6, and normal response to the tryptophan load test
suggested adequate function of B6 in the liver of persons
with diabetes. Plasma PLP concentration alone may not be
an adequate B6 status indicator in persons with diabetes.
Based upon the levels of multiple indicators, the vitamin
B-6 status of those persons with diabetes studied was
judged to be adequate.