Published May 6, 2016
Tryptophan metabolism, growth responses, and postprandial
insulin metabolism in weaned piglets according to the dietary
provision of niacin (vitamin B3) and tryptophan1
J. Jacques Matte,*2 Etienne Corrent,† Aude Simongiovanni,† and Nathalie Le Floc’h‡§
*Sherbrooke Research and Development Centre, Agriculture and Agri-Food Canada, 2000 College Street,
P.O. Box 90, Lennoxville STN, Sherbrooke, QC J1M 0C8, Canada; †Ajinomoto Eurolysine S.A.S.,
153 rue de Courcelles, F-75817 Paris Cedex 17, France; ‡INRA, UMR1348 Physiologie, Environnement, Génétique
pour l’Animal et les Systèmes d’Elevage, F-35590 Saint-Gilles, France; and §Agrocampus Ouest, UMR1348
Physiologie, Environnement, Génétique pour l’Animal et les Systèmes d’Elevage, F-35000 Rennes, France
ABSTRACT: The present experiment aimed to determine if Trp metabolism and growth responses to dietary
Trp are modulated by dietary niacin (B3) in weanling
piglets. Piglets weaned at 3 wk of age were distributed
1 wk later (7.6 kg of BW, SEM = 0.1) in 52 pens of 2
animals each. Pens were assigned to factorial dietary
treatments with 2 additions of B3, 15 mg/kg (LB3) vs.
45 mg/kg (HB3) and 2 additions of Trp, 0 mg/kg (-Trp)
vs. 1 mg/kg (+Trp) for Trp to Lys ratios of 0.16 vs.
0.24, respectively. Growth performance was recorded
every week from 4 to 10 wk of age. Fasting blood samples were taken at 4, 6, 8, and 10 wk of age. From 4 to
10 wk of age, ADFI tended to be greater (P = 0.10) in
HB3 than in LB3 (1,031 vs. 1,003 g, SEM = 7), and
this was reflected (P = 0.06) by ADG (642 vs. 623 g,
SEM = 7). No treatment effect was observed on plasma
Trp or kynurenine (Kyn), an intermediate metabolite of
Trp catabolism. The response of plasma nicotinamide
(Nam), a product of Trp catabolism and an indicator of
B3 status, to dietary B3 differed according to treatments
(interaction Trp × B3, P < 0.01) with values of 1.4, 3.3,
4.1, and 5.3 μM (SEM = 0.1) in LB3-Trp, HB3-Trp,
LB3+Trp, and HB3+Trp, respectively. At 11 wk of
age, postprandial blood samples were collected from
6 piglets per treatment for measurements of Trp and
insulin metabolism. Postprandial plasma Trp (96.4 vs.
72.2 μM, SEM = 3.4) and Kyn (1.7 vs. 1.3 μM, SEM =
0.1) were greater (P < 0.01) in +Trp vs. -Trp. Postprandial plasma Nam was greater (P < 0.01) in +Trp
vs. -Trp (3.4 vs. 1.9 µM, SEM = 0.3) and in HB3 vs.
LB3 piglets (3.4 vs. 1.9 µM, SEM = 0.3). Postprandial
peaks and areas under curves of C-peptide and glucose
were not affected by treatments. However, for insulin,
the postprandial peak was lower in +Trp vs. -Trp piglets in the LB3 group (interaction Trp × B3, P < 0.05);
values were 1.3, 1.0, 0.7, and 1.0 nM (SEM = 0.1) in
LB3-Trp, HB3-Trp, LB3+Trp, and HB3+Trp, respectively. The peak value of the molar ratio insulin:Cpeptide was lower (P < 0.02) in +Trp vs. -Trp piglets
(0.56 vs. 0.73, SEM = 0.05). The responses observed
on growth performance and plasma Nam suggest
that the LB3 level was suboptimal. According also to
plasma Nam, it appears that supplemental dietary B3
can attenuate Trp oxidation toward niacin metabolites.
Postprandial profiles of insulin and C-peptide indicate
that Trp action is exerted on insulin clearance rather
than on insulin secretion in piglets, without apparent
consequences on glucose utilization.
Key words: insulin, niacin, piglets, tryptophan
© 2016 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2016.94:1961–1971
doi:10.2527/jas2015-0221
1This study was made possible through the financial support
feed formulation; to the animal care team under supervision of
D. Morissette (AAFC); and to S. Méthot (AAFC) for expertise
in statistical analyses.
2Corresponding author: [email protected]
Received December 17, 2015.
Accepted February 23, 2016.
provided by Ajinomoto Eurolysine, France, and the Matching
Investment Initiative of Agriculture and Agri-Food Canada (AAFC).
The authors are grateful to M. Guillette and V. Lévesque (AAFC),
N. Mézière (INRA), and M. Eudaimon (Ajinomoto Eurolysine)
for technical assistance; to M. Vignola (Nutreco-Shur-Gain) for
1961
1962
Matte et al.
INTRODUCTION
Tryptophan is an essential AA recognized not only
for its structural role in protein accretion but also for its
functional properties in metabolism (Le Flocʹh and Sève,
2007). For instance, in the oxidative pathway of Trp, after
an initial step releasing kynurenine (Kyn) and alanine,
Kyn is directed toward acetyl-CoA for total oxidation
or to synthesis of quinolinic acid and to nicotinamide
(Nam), NAD, and NAD phosphate. These active metabolites of niacin or vitamin B3 (B3) are recognized for
their fundamental role in redox processes (Combs, 2012).
Besides this basic cellular function, B3 has been associated with prevention of onset of type I (insulin-dependent)
diabetes mellitus in high-risk people through effects on
activity and synthesis of insulin (Knip et al., 2000), a key
hormone not only for glucose balance but also for protein
synthesis and deposition (Davis et al., 1996).
In weaned piglets (2 to 9 wk of age), supplements
of 0.5 g/kg of Trp to basal dietary levels varying with
age between 1.9 and 2.5 g/kg had few impacts on plasma
profiles of Trp or Kyn but increased by over 45% plasma
Nam (Matte et al., 2011) in spite of a dietary supplement
of B3 2 times greater (30 mg/kg) than recommended by
NRC (2012). Such a phenomenon has also been observed
in humans (Fukuwatari and Shibata, 2007) with similar
ranges of dietary B3 supplies. However, in rats, it has
been shown that although the initial steps of Trp oxidation are independent of B3 supply, the last steps (quinolinic acid to Nam) can be perturbated by large amounts
of dietary preformed B3 (Fukuwatari et al., 2004). To our
knowledge, there is no information on the influence of B3
provision on the Trp oxidation pathway in pigs.
The present experiment aimed to determine if responses of Trp metabolism, postprandial insulinemia,
and growth performance to dietary supplement of Trp
are modulated by the dietary supply of B3.
MATERIALS AND METHODS
Animals, Treatments, and Samplings
The experimental procedures followed the guidelines of the Canadian Council on Animal Care (1993)
and were approved by the Institutional Animal Care
Committee of the Dairy and Swine Research and Development Centre of Sherbrooke (QC, Canada). All
animals were cared for according to the recommended
code of practice of Agriculture Canada (1993).
A total of 104 piglets (male Duroc × female [Landrace × Yorkshire]) were selected at weaning (21 d of
age). They were fed ad libitum 3 commercial-type diets:
one (phase 0, adaptation) for the week following weaning, a second one (phase 1) for up to 6 wk of age, and
a third one (phase 2) for up to 10 wk of age (Table 1).
After adaptation (phase 0), piglets were distributed in 52
pens of 2 individuals and assigned to blocks of 4 pens
according to BW. Within each block, piglets were allocated to 1 of the 4 factorial treatments for a total of 13
pens (experimental units) per treatment with 2 dietary
additions of synthetic niacin (nicotinic acid, 99.5%) at
15 mg/kg (LB3) vs. 45 mg/kg (HB3) and 2 dietary additions of synthetic Trp (l-Trp, 98%) at 0 mg/kg (-Trp)
vs. 1 mg/kg (+Trp) corresponding to Trp to Lys ratios
of 16.3% vs. 24.1%, respectively (weighed average for
both phases 1 and 2). Those treatments were given in
phase 1 and 2 diets. Growth performance measurements,
BW, and feed intake were recorded every week from 4
to 10 wk of age. Blood samples were collected in all
animals after an overnight fasting period (approximately
16 h) at 4 (before initiation of treatments), 6, 8, and 10
wk of age for determination of plasma Trp, Kyn, and
Nam. At 10 wk of age, 6 piglets representative of each
treatment in terms of BW (33.7 to 34.7 kg) were trained
to 1 morning meal of 1.0 kg during a period of 5 to 6
d. This allowance corresponded to approximately 70%
of ad libitum intake recorded during the last week of
experiment and was totally consumed in less than 6 h.
After this training period, pigs were fitted with a catheter
in the jugular vein (Matte, 1999). On the day of postprandial profiles, the feed allowance of the meal was
limited to 0.5 kg and was ingested within 30 min. Blood
samples were collected at 0, 30, 60, 90, 120, 180, and
240 min after the meal to follow profiles of plasma Trp,
Nam, glucose, C-peptide, and insulin. Piglets remained
on the same dietary treatment during the whole period.
Laboratory Analyses
Plasma Trp and Kyn were measured by HPLC according to a method adapted from Widner et al. (1997).
Niacin status was evaluated from plasma Nam measured
also by HPLC but according to the method described by
Santschi et al. (2005). Plasma glucose was measured
by an enzymatic colorimetric assay (GLU GOD-PAP;
Roche Diagnostics, Indianapolis, IN), whereas insulin
(Porcine Insulin RIA Kit PI-12K; Linco Research Inc.,
St. Charles, MI) and C-peptide (Porcine C-peptide RIA
Kit PCP-22K; Linco Research Inc.) were assayed by
commercial RIA. Inter- and intra-assay CV for insulin
and C-peptide were lower than 4.0%.
Statistical Analyses
Data were analyzed using the MIXED procedure
of SAS (SAS Inst. Inc., Cary, NC; Littell et al., 1996) as
a 2-way ANOVA according to a randomized complete
block design (combinations of sex and initial weight,
Tryptophan and niacin in postweaned piglets
Table 1. Ingredients and analytical composition of the
basal experimental diets (as-fed basis)
Basal diet
Ingredient, g/kg
Maize
Soybean meal, 480 g CP
Oatmeal
Lactoserum powder, 700 g CP
Plasma protein
Wheat
Micronized soybeans, 380 g CP
Meat meal
Hulled barley
Fat
Canola meal
Vegetable fat
Porcine peptide protein
Yeast
Limestone
Monocalcium phosphate
Phytase, 5,000 phytase units/kg
Phase 0,
3 to 4 wk
of age
319
124
125
156
41.7
50.0
40
25
20
—
17
10.4
10.4
7
6
—
Lincomycin hydrochloride,
1
spectinomycin sulfate premix
(L-S 20)
Salt
—
l-Lys, 780 g Lys
1.0
dl-Met, 990 g Met
—
l-Thr, 980 g Thr
—
Choline chloride, 750 g choline
0.5
Premix1
46
Analytical nutrient composition (except for ME)
ME, MJ/kg
14.8
CP, g/kg
199
Fat, g/kg
51
ADF, g/kg
29
Lys, g/kg
14.1
Met + Cys, g/kg
7.8
Thr, g/kg
8.8
Trp,2 g/kg
2.6
Trp/Lys, %2
18.6
Val, g/kg
10.0
Pyridoxine (B6),3 mg/kg
6.0
Niacin (LB3)2, mg/kg
43
43
Niacin (HB3)2, mg/kg
Phase 1,
4 to 6 wk
of age
602
170
—
100
—
Phase 2,
6 to 10 wk
of age
677
217
—
—
—
—
—
—
23
—
—
—
35.0
—
19
18
—
—
—
1
—
—
8.0
10.2
1
—
—
1.8
0.3
0.9
—
100.0
3.8
4.5
1.3
1.8
0.5
3.9
14.2
181
56
32
14.0
6.7
8.9
2.1
15.0
8.5
6.0
25
79
13.7
185
51
37
13.0
6.2
8.2
2.2
16.9
8.6
5.6
32
87
1Provided per kilogram of feed for phases 0, 1, and 2, respectively: Zn,
2.9, 2.0, 0.3 g; Cu, 120, 120, 115 mg; vitamin E, 110, 110, 60 IU; biotin,
200, 200, 78 µg; choline, 298, 298, 303 mg. For all phases (3 to 10 wk of
age), provided per kilogram of feed: 130, 40, 2.1, 0.3, 2.7, 2.7, 8.8, 15, 0.7,
and 21 mg of Fe, Mn, I, Se, menadione, thiamin, riboflavin, niacin, folic
acid, and pantothenic acid, respectively; 14,600 IU, 1,500 IU, and 25 µg of
vitamins A, D3, and B12, respectively.
2Factorial design of dietary treatments from 4 to 10 wk of age: two levels of supplemented niacin (B3; nicotinic acid, 99.5%) at 15 (LB3) and 45
mg/kg (HB3) and two levels of supplemented Trp (synthetic L-Trp, 98%)
at 0 (-Trp) and 1.0 g/kg (+Trp). For Trp, the analytical values corresponded
to dietary concentration in –Trp diets for Phases 1 and 2. The analytical
value of synthetic Trp (L-Trp, 98%) added to -Trp diets corresponded to
1.04 g/kg. Trp/Lys ratios of 22.4 and 24.9% in +Trp diets during Phases 1
and 2, respectively.
3According to Matte et al. (2011).
1963
heavy and light) with a factorial arrangement of treatments (2 dietary additions of niacin [LB3 vs. HB3]
and 2 dietary additions of Trp [-Trp vs. +Trp]) given
during the postweaning period. For repeated measurements, the variables were analyzed as described above
with the added effect of age of piglets (4 to 10 wk of
age) or postprandial time of sampling along with their
appropriate interactions with the factorial contrasts for
treatments. For variables related to the age of piglets,
a pen of 2 pigs was the experimental unit, whereas for
those related to postprandial time it corresponded to
the individual piglet selected within the pen. Effects of
the model were considered significant at P ≤ 0.05, and
tendencies were considered at 0.05 < P ≤ 0.10.
RESULTS AND DISCUSSION
Growth Performance
Animals were in good health, and there was no
loss of piglets during the experimental period. For either phase 1, phase 2, or the whole experimental period,
there was no main effect of Trp (P > 0.46) or interaction Trp × B3 (P > 0.17) on ADFI, ADG, and final
BW, and there was no effect of any treatments on G:F
(P > 0.65; Table 2). At the time when the present experimental diets were designed, the Trp to Lys ratios
were chosen to represent a suboptimal (-Trp groups) or
a “more than adequate” (+Trp groups) level according
to NRC (1998) to create 2 distinct metabolic levels of
Trp oxidation. This model remains relevant according
to NRC (2012) and to a recent meta-analysis (Simongiovanni et al., 2012) showing that the Trp to Lys ratio
(standardized ileal digestible basis) required to maximize ADFI and ADG of postweaning piglets could
be as high as 22%. The present lack of growth performance responses to dietary Trp supplementation could
be related to the dietary profile of other essential AA
that were also balanced according to NRC (1998). For
sulfur AA, threonine, and valine, the present diets were
possibly limiting for growth performance compared to
the recent recommendation by Gloaguen et al. (2013).
According to the ideal protein concept, if 1 AA is suboptimal, the others are not utilized at their full potential
for protein deposition and then are catabolized (Chung
and Baker, 1992). Therefore, in the present experiment,
the formulation of basal diets and the resulting ratios
among essential AA in both phases 1 and 2 might have
further amplified the difference in terms of Trp oxidation between Trp treatments and allowed this model
to better assess how dietary B3 supplementation may
spare Trp by reducing its conversion into Nam.
For the overall experimental period (4 to 10 wk
of age), ADFI and ADG tended to be greater by 2.9%
1964
Matte et al.
Table 2. Growth performance of piglets from 4 to 10 wk of age according to supplements of niacin (B3) and Trp
and to age of piglets
Treatment1
Variable
LB3-Trp
Initial BW, kg
7.6
Phase 1 (4 to 6 wk of age)
ADFI, g·d−1·piglet−1
670.1
ADG, g·d−1·piglet−1
486.5
G:F
0.73
Phase 2 (6 to 10 wk of age)
ADFI, g·d−1·piglet−1
1,179.7
ADG, g·d−1·piglet−1
692.2
G:F
0.59
Overall period (4 to 10 wk of age)
ADFI, g·d−1·piglet−1
1,009.9
ADG, g·d−1·piglet−1
623.6
G:F
0.62
Final BW, kg
33.8
HB3-Trp
B3 effect
Trp effect
7.6
7.6
0.1
0.99
0.74
676.7
489.3
0.72
633.5
456.9
0.72
693.1
502.4
0.73
13.2
16.9
0.02
0.10
0.14
0.74
0.61
0.60
0.76
1,175.9
705.2
0.60
1,211.9
717.6
0.59
16.3
10.5
0.01
0.14
0.09
0.85
0.74
0.46
0.65
995.1
622.5
0.63
33.7
1,039.0
645.8
0.62
34.7
7.3
6.5
0.01
0.3
0.10
0.06
0.84
0.07
0.97
0.81
0.82
0.78
1,023.0
639.9
0.63
34.4
HB3+Trp
Pooled
SEM3
7.6
1,196.2
715.3
0.6
LB3+Trp
P-value2
1Factorial design of dietary treatments from 4 to 10 wk of age: 2 levels of supplemented niacin (B ; nicotinic acid, 99.5%) at 15 mg/kg (LB3) and 45
3
mg/kg (HB3) and 2 levels of supplemented Trp (synthetic l-Trp, 98%) at 0 g/kg (-Trp) and 1.0 g/kg (+Trp).
2The interaction B × Trp was not significant (P < 0.18) whatever the variables.
3
3Standard error of the mean across treatments. Values are means of 13 pens (26 piglets) per treatment.
(P = 0.097) and 3.2% (P = 0.056), respectively, in HB3
compared to LB3 piglets (Table 2). Those effects between 4 and 10 wk of age were related to separate phase
responses: HB3 tended to be greater than LB3 for ADFI
in phase 1 and for ADG in phase 2 (P = 0.095 and 0.091,
respectively). In fact, the final BW of HB3 piglets tended to be greater by more than 0.8 kg (P = 0.069) as
compared to LB3 animals. Inconsistent responses to
supplements greater than LB3 levels were reported in
the literature. In accordance with the present results, increased feed intake was observed in early weaned piglets (4.3 kg of BW) receiving 50 mg/kg of supplemental
niacin during the first 2 wk postweaning (Blodgett et al.,
2002). However, no niacin effect was reported in piglets weaned at 4 wk of age (7.6 kg of BW) and receiving 0 to 81 mg/kg of dietary niacin (Ivers et al., 1993).
Nonetheless, the present results on growth performance
support the fact that the minimum niacin requirement
of postweaned piglets could be higher than the dietary
niacin levels in LB3 treatments especially during phase
2.This LB3 level was higher than those suggested by Ivers and Veum (2012) or recommended by NRC (2012)
at 14 and 30 mg/kg, respectively. In fact, the analytical dietary concentration of niacin in LB3 diets was
slightly lower in phase 1 and slightly higher in phase
2 than NRC (2012). At the time when these treatments
were designed, the level of total niacin in LB3 diets
were much higher than NRC (1998) recommendations
(10 to 15 mg/kg) but were close to typical industry levels according to a survey performed by BASF (2001).
The present LB3 levels were therefore representative of
practical dietary levels of this vitamin. Further studies
are needed to better assess the minimum niacin requirement of postweaned piglets.
Tryptophan-Related Metabolites According to Age
Plasma Trp increased (P < 0.01) with age from
34.3 ± 1.4 to 45.3 ± 1.9 µM between 4 and 10 wk of
age (Fig. 1). There was no main Trp or B3 effect (P >
0.16) on plasma Trp, and in spite of an interaction of
Trp × age (P < 0.03), no steady Trp treatment trend
could be observed. These responses for plasma Trp
are in accordance with Le Floc’h et al. (2010) within
similar dietary concentrations of Trp fed ad libitum to
piglets of similar ages. These last authors hypothesized
that tryptophan dioxygenase activity would limit and
stabilize Trp accumulation in plasma when the partition of this AA toward growth is maximized. In such a
situation, one could expect, in the present experiment,
a greater appearance of intermediary metabolites such
as Kyn in plasma according to age and/or to supplementation of Trp. However, the result was the opposite
as plasma Kyn values declined by approximately 17%
(P < 0.01; Fig. 2) between 4 and 10 wk of age and was
not influenced by treatments. It appears that a biweekly
profile of plasma Kyn, as an intermediary (and transient) metabolite, might not be a reliable indicator of
Trp oxidation compared to plasma Nam, which was influenced by treatments (Trp, B3, and Trp × B3 effects, P
< 0.01; Fig. 3). In LB3-Trp piglets, plasma Nam values
decreased by approximately 47% between 4 and 10 wk
Tryptophan and niacin in postweaned piglets
1965
Figure 1. Plasma tryptophan according to supplements of niacin (B3) and Trp and to the age of piglets (interaction Trp × age, P < 0.03). Factorial
design of dietary treatments from 4 to 10 wk of age with 2 levels of supplemented B3 (nicotinic acid, 99.5%) at 15 mg/kg (LB3) and 45 mg/kg (HB3) and
2 levels of supplemented Trp (synthetic L-Trp, 98%) at 0 g/kg (-Trp) and 1.0 g/kg (+Trp). Values are means ± SEM of 13 couples of piglets per treatment.
of age, whereas an increase was observed in all other
treatments during the same period (Trp × B3 × age, P <
0.03; Fig. 3). This decline of plasma Nam concentrations in LB3-Trp piglets can be interpreted as a negative niacin metabolic balance in these animals as the
postweaning period progressed. In line with the tendencies for a B3 effect on performance responses, this result further supports the earlier suggestion that the basal
dietary supplement of niacin at 15 mg/kg in LB3 diets
for a total provision of 25.6 and 32.0 mg/kg in phases 1
and 2, respectively (Table 1), did not correspond to the
minimum niacin requirement of postweaned piglets.
The impact of Trp supplements on plasma Nam
from 6 to 10 wk of age was considerable (increase of
127% in +Trp vs. -Trp piglets), as observed previously
by Matte et al. (2011), and was greater than the impact
of B3 supplements (74% of increase in HB3 vs. LB3
piglets). The link between Trp and niacin is recognized for several species (Le Grusse and Watier, 1993),
including pigs (Markant et al., 1993). However, the
present interaction Trp × B3 suggests that the partition
of Trp toward oxidation and synthesis of niacin was
not totally independent of dietary niacin provisions as
the numerical increase of Nam due to Trp supplements
in LB3 piglets (+3.79 µM) was greater than the cor-
responding value in HB3 piglets (+2.67 µM). Such estimations contrast with the lack of influence of dietary
niacin on Trp:Nam conversion reported in humans
(Fukuwatari and Shibata, 2007), but they are in agreement with observations in rats, where large amounts of
dietary preformed niacin interfere with the last steps
of the Trp:Nam pathway (quinolinic acid to NAD; Fukuwatari et al., 2004). Although the mechanism cannot be confirmed here for piglets (as quinolinic acid
was not measured), these results suggest that there
are situations where Trp oxidation can be attenuated
by high dietary niacin supplies in postweaned piglets.
Although this was apparently not sufficient to impact
G:F, it cannot be ruled out that this interference of dietary B3 on the Trp:Nam pathway is related to other
aspects of growth performance (ADFI and ADG) that
were marginally affected by the present niacin supplementation. In such a case, however, the mechanism of
action remains to be elucidated.
The similarity of plasma Nam responses between
LB3+Trp and HB3-Trp piglets at 8 and 10 wk of age
allows us to make an equivalence between effects of
B3 and Trp supplies. In fact, using the Nam response in
blood plasma pool as an indicator, the total dietary Trp
supply (from feedstuffs and basal vitamin supplements)
1966
Matte et al.
Figure 2. Plasma kynurenine according to supplements of niacin (B3) and Trp and to the age of piglets (age effect, P < 0.01). Factorial design of
dietary treatments from 4 to 10 wk of age with 2 levels of supplemented B3 (nicotinic acid, 99.5%) at 15 mg/kg (LB3) and 45 mg/kg (HB3) and 2 levels of
supplemented Trp (synthetic l-Trp, 98%) at 0 g/kg (-Trp) and 1.0 g/kg (+Trp). Values are means ± SEM of 13 couples of piglets per treatment.
Figure 3. Plasma nicotinamide according to supplements of niacin (B3) and Trp and to the age of piglets (Trp and B3 effects and interaction Trp × B3, P <
0.01). Factorial design of dietary treatments from 4 to 10 wk of age with 2 levels of supplemented B3 (nicotinic acid, 99.5%) at 15 mg/kg (LB3) and 45 mg/kg
(HB3) and 2 levels of supplemented Trp (synthetic l-Trp, 98%) at 0 g/kg (-Trp) and 1.0 g/kg (+Trp). Values are means ± SEM of 13 couples of piglets per treatment.
Tryptophan and niacin in postweaned piglets
1967
Figure 4. Plasma tryptophan response after a single meal of 500 g given to piglets at 11 wk of age and supplemented with Trp and niacin (B3; Trp effect
and interaction Trp × postprandial time, P < 0.01). Factorial design of dietary treatments from 4 to 10 wk of age with 2 levels of supplemented B3 (nicotinic
acid, 99.5%) at 15 mg/kg (LB3) and 45 mg/kg (HB3) and 2 levels of supplemented Trp (synthetic l-Trp, 98%) at 0 g/kg (-Trp) and 1.0 g/kg (+Trp). Values are
means ± SEM of 6, 5, 6, and 6 piglets for treatments LB3-Trp, HB3-Trp, LB3+Trp, and HB3+Trp, respectively. See Material and Methods for details.
of 3.2 g/kg of diet in LB3+Trp piglets was equivalent
to the B3 supply of 87 mg/kg in HB3-Trp piglets for an
estimated Trp:Nam conversion ratio of 37:1. This value
is lower than the sole estimation of 50:1 previously reported for young piglets (1 to 8 kg BW) by Firth and
Johnson (1956). Such variations in Trp:Nam conversion ratios might be related to differences in methodology of estimation, but it cannot be excluded that they
are also linked to differences in age and to tremendous
changes brought to housing conditions, feeding regimen, and animal genotype during the last 60 yr.
It is noteworthy to mention that, globally, the present Kyn and Nam values were 2 to 3 times greater than
those reported by Le Floc’h et al. (2010), although those
measurements were performed in the same laboratory.
This discrepancy of Trp oxidation indicators between
experiments might be related to diet formulation and/
or husbandry conditions but also might be because the
genotypes of pigs were different (male Duroc × female
[Landrace × Yorkshire] in Canada vs. male Piétrain ×
female [Landrace × Large White] in France). The importance of this last factor remains to be better understood and is currently being studied in our laboratories.
Postprandial Tryptophan-Related Metabolites
In contrast to the lack of treatment effects on the
long-term response of plasma Trp and Kyn, the postprandial response was clearly greater for these 2 metabolites in +Trp compared to -Trp piglets (Fig. 4 and
5). For the plasma Trp profile (Fig. 4), there were a
main effect of Trp and an interaction of Trp × postprandial time (P < 0.01). The statistical model applied for
each postprandial time (slice effect) indicated that the
Trp effect appeared rapidly, from 30 min after the meal,
and persisted during the whole postprandial period. For
the plasma Kyn profile (Fig. 5), there was no main effect of Trp (P > 0.13), but an interaction of Trp × postprandial time was detected (P < 0.01). Compared to
plasma Trp, the slice effect indicated that the Trp effect
was delayed, at 120 min after the meal, and persisted
during the whole postprandial period. For the plasma
Nam profile (Fig. 6), there was a global interaction of
Trp × B3 × postprandial time (P < 0.03) due to variable
treatment responses identified by the slice effect during
the first postprandial 60 min. However, thereafter, the
postprandial response was similar to the long-term one.
1968
Matte et al.
Figure 5. Plasma kynurenine response after a single meal of 500 g given to piglets at 11 wk of age and supplemented with Trp and niacin (B3; interaction Trp × postprandial time, P < 0.01). Factorial design of dietary treatments from 4 to 10 wk of age with 2 levels of supplemented B3 (nicotinic acid,
99.5%) at 15 mg/kg (LB3) and 45 mg/kg (HB3) and 2 levels of supplemented Trp (synthetic l-Trp, 98%) at 0 g/kg (-Trp) and 1.0 g/kg (+Trp). Values are
means ± SEM of 6, 5, 6, and 6 piglets for treatments LB3-Trp, HB3-Trp, LB3+Trp, and HB3+Trp, respectively. See Material and Methods for details.
Those treatment effects on postprandial profiles of
Kyn contrasted with the lack of responses for this metabolite for the long-term effect of age but are in accordance with the fact that this is an intermediary (and
transient) metabolite. It is also noteworthy to mention
that plasma Kyn concentrations observed at 10 wk of
age (1.29 ± 0.14 µM) were substantially greater (more
than 60%) than values of Kyn recorded 1 wk later, before postprandial measurements (0.80 ± 0.08 µM; Fig.
5). A similar observation can be made for plasma Nam
with corresponding values of 3.86 ± 0.28 µM at 10 wk
of age (Fig. 3) vs. 2.05 ± 0.27 µM a week later before
the postprandial measurements (Fig. 6), a difference of
more than 85%. In contrast, the corresponding plasma
Trp values were much closer, with a difference of less
than 8% between 10 wk of age (45.2 ± 1.9 µM) and 11
wk of age (41.9 ± 3.4 µM). For both ages, those concentrations were obtained after a fasting period of more
than 16 h, but at 11 wk of age, a feed restriction at 1.0
kg/d for training the animals to 1 morning meal was
imposed during a week before the postprandial profiles.
This restriction of feed intake could have changed the
partition of Trp toward less oxidation and a more efficient metabolic utilization, as has been suggested for
the overall metabolism after a feed restriction (Prince et
al., 1983). The stability of the fasting plasma Trp status
whatever the feed regimen (restriction or ad libitum)
may indicate that a sparing effect of this AA occurs at
the expense of Trp oxidation (likely toward protein deposition) when the dietary provision of Trp is rationed.
Postprandial Insulin-Related Responses
There was no treatment effect on either the peak
or the area under the curve (AUC; 0 to 90 or 0 to 360
min) of C-peptide after the meal (P > 0.14; Table 3),
suggesting that the postprandial insulin secretion was
not influenced by treatments. Indeed, peripheral Cpeptide is considered a better indicator of insulin secretion than peripheral insulin because of its greater
stability to hepatic metabolism (Polonsky and Rubenstein, 1984). Insulin concentrations represent, in fact,
a balance between secretion, on the one hand, and
hepatic extraction and tissular internalization, on the
other hand (Morgan, 1992). For instance, there were
no treatment effects on AUC of insulin (0 to 90 or
0 to 360 min; P > 0.18; Table 3), but for the whole
postprandial profile of insulin concentrations (Fig. 7),
1969
Tryptophan and niacin in postweaned piglets
Figure 6. Plasma nicotinamide response after a single meal of 500 g given to piglets at 11 wk of age and supplemented with Trp and niacin (B3; Trp and
B3 effects and interaction Trp × B3 × postprandial time, P < 0.01). Factorial design of dietary treatments from 4 to 10 wk of age with 2 levels of supplemented
B3 (nicotinic acid, 99.5%) at 15 mg/kg (LB3) and 45 mg/kg (HB3) and 2 levels of supplemented Trp (synthetic l-Trp, 98%) at 0 g/kg (-Trp) and 1.0 g/kg (+Trp).
Values are means ± SEM of 6, 5, 6, and 6 piglets for treatments LB3-Trp, HB3-Trp, LB3+Trp, and HB3+Trp, respectively. See Material and Methods for details.
Table 3. Plasma C-peptide, insulin, and glucose response after a single meal of 500 g given to piglets at 11 wk of
age and supplemented with Trp and niacin (B3)1
Peak
Variable
Treatments
LB3-Trp
HB3-Trp
LB3+Trp
HB3+Trp
Pooled SEM2
P-value
Trp effect
B3 effect
Trp × B3
interaction
C-peptide,
pM
Insulin,
pM
Area under the curve
Insulin:C- Glucose,
peptide ratio
mM
C-peptide,
nM/min
Insulin,
nM/min
Glucose,
mM/min
1,849.4
1,685.6
1,350.6
1,633.2
177.4
1,270.1
988.2
699.6
1,008.9
141.9
0.71
0.74
0.51
0.61
0.07
7.80
7.31
7.24
7.28
0.34
0–90 min
98.2
101.3
83.7
95.0
15.1
0–360 min
194.9
221.8
207.4
219.9
27.1
0–90 min
61.5
45.7
40.5
53.9
10.8
0–360 min
97.0
91.6
84.9
99.5
16.3
0–90 min
511.5
573.3
580.0
575.9
63.4
0–360 min
1,842.5
2,189.9
2,240.1
2,235.0
223.2
0.14
0.74
0.22
0.05
0.92
0.04
0.02
0.32
0.58
0.40
0.51
0.44
0.49
0.63
0.78
0.84
0.46
0.78
0.54
0.91
0.18
0.90
0.77
0.53
0.64
0.57
0.60
0.43
0.32
0.43
1Factorial design of dietary treatments from 4 to 10 wk of age with 2 levels of supplemented B (nicotinic acid, 99.5%) at 15 mg/kg (LB3) and 45 mg/
3
kg (HB3) and 2 levels of supplemented Trp (synthetic l-Trp, 98%) at 0 g/kg (-Trp) and 1.0 g/kg (+Trp).
2Standard error of the mean across treatments. Values are means of 6, 5, 6, and 5 piglets for treatments LB3-Trp, HB3-Trp, LB3+Trp, and HB3+Trp,
respectively. See Material and Methods for details.
1970
Matte et al.
Figure 7. Plasma insulin response after a single meal of 500 g given to piglets at 11 wk of age and supplemented with Trp and niacin (B3; Trp effect,
P = 0.071, and interaction Trp × B3, P = 0.100). Factorial design of dietary treatments from 4 to 10 wk of age with 2 levels of supplemented B3 (nicotinic
acid, 99.5%) at 15 mg/kg (LB3) and 45 mg/kg (HB3) and 2 levels of supplemented Trp (synthetic l-Trp, 98%) at 0 g/kg (-Trp) and 1.0 g/kg (+Trp). Values
are means ± SEM of 6, 5, 6, and 5 piglets for treatments LB3-Trp, HB3-Trp, LB3+Trp, and HB3+Trp, respectively. See Material and Methods for details.
there were tendencies for a Trp effect (P = 0.071) and
for the interaction of Trp × B3 (P = 0.100). In fact,
the peak of insulin was approximately 24% lower in
+Trp than in -Trp (P < 0.05), and this was concentrated
within LB3 piglets (interaction of Trp × B3, P < 0.04;
Table 3). A main effect of Trp was also observed on
the peak of the molar ratio insulin:C-peptide (P < 0.02;
Table 3). Marginal or no Trp effects were previously
observed on the insulin response of younger piglets
(less than 5 wk of age) under different experimental
conditions in which Trp intake was controlled through
duodenal infusions (Matte et al., 1997) or through
acute vs. chronic (approximately 3 wk) variations in
gastric Trp intake (Ponter et al., 1994). The present results for C-peptide, insulin, and molar ratio insulin:Cpeptide suggest that the action of Trp is exerted on insulin clearance from the blood plasma pool and not on
insulin secretion. Although the present experimental
design did not allow us to characterize directly this
plasma insulin clearance between hepatic metabolism
and tissular internalization for glycemic utilization,
the former appears more likely because glycemia was
similar among treatments, as shown by the peak or
AUC (0 to 90 or 0 to 360 min) of plasma glucose after
the meal (P > 0.32; Table 3). In this way, because the
interaction of Trp × B3 observed for the insulin peak
alone is no longer apparent for the ratio insulin:C-peptide peak, it appears that the dietary supplement of B3
can attenuate the postprandial plasma clearance of insulin induced by Trp supplementation. In line with the
present results, it was recently shown that large dietary
doses of niacin (1 g/d) increased blood serum concentrations of both insulin and C-peptide in postmenopausal women, but the effect was more marked with
insulin (62%) than with C-peptide (46%; Koh et al.,
2014). To the best of our knowledge, such responses
of Trp and B3 (the present experiment and Koh et al.,
2014) on insulin clearance have never been reported
before. The mechanism of action remains to be further
investigated for reliable interpretations of the role of
these nutrients in insulin metabolism.
Conclusion
The tendencies for B3 treatment effects on some
aspects of growth performance (ADFI and ADG) and
Tryptophan and niacin in postweaned piglets
on plasma Nam suggest that the present LB3 dietary
level, corresponding approximately to the NRC (2012)
recommendation, was suboptimal. The responses of
plasma Nam to both dietary Trp and B3 suggest that
preformed dietary niacin may attenuate Trp oxidation
toward niacin metabolites. Plasma Nam concentrations allowed also a novel estimation of the conversion ratio of Trp to niacin in pigs (8 to 10 wk of age) of
37:1. Moreover, the postprandial insulinemia suggests
a Trp action on insulin clearance rather than on insulin
secretion (assessed by C-peptide), without apparent
metabolic consequences on glucose utilization.
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