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Central Research Institute, Mitsukan Group Company Limited, Handa 475-8585, Japan and * Research Center of Health, Physical Fitness and Sports, Nagoya University, Nagoya 464-8601, Japan
2To whom correspondence should be addressed. E-mail: tfushimi{at}mitsukan.co.jp.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: • acetic acid • glycogen repletion • liver • skeletal muscle • rats
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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. Liver glycogen is used to maintain
concentrations of blood glucose, for example, between meals. Glucose
supplied from the liver is metabolized in a number of tissues, in
particular, the brain. Glycogen is utilized as a fuel in skeletal
muscle; therefore, glycogen depletion in liver and skeletal muscle
results in central and peripheral fatigue. Thus, the rapid
replenishment of glycogen in these tissues is very important for
recovery from fatigue. Fasting as well as exercise results in glycogen
depletion. Food deprivation for >6 h leads to reduced glycogen
concentrations in liver and skeletal muscle of rats.
(2
,3)
. The consumption of carbohydrates aids glycogen
repletion. To our knowledge, however, there have been no other
nutritional studies showing the benefits of food in replenishing
glycogen.
Vinegar is a commonly used seasoning. Its main component is acetic acid
at a concentration of 3–9% for consumer use (4)
. Foods
such as sushi and marinated meats and vegetables that are prepared with
vinegar contain 0.2–1.5 g acetic acid/100 g (5
6
7
8)
.
Vinegar is also used traditionally as a folk medicine and is believed
to have several beneficial effects such as improving appetite,
enhancing mineral absorption and speeding recovery from fatigue.
Recently, it was shown that a diet containing vinegar at a dietary
concentration of 1.6 mL vinegar/100 g diet, for example, enhances the
intestinal absorption of calcium (9)
. However, no other
reports have proved experimentally other nutritional effects of vinegar
at concentrations consumed in a normal diet.
Acetic acid administered orally is immediately absorbed; uptake then
occurs in liver and peripheral tissues (10
,11)
. It is
metabolized via acetyl-CoA in the tricarboxylic acid cycle in liver
and skeletal muscle (12
13
14)
. In vitro, citrate inhibits
the activities of phosphofructokinase type 1
(PFK-1)3
and type 2 (PFK-2) in those tissues (15
16
17)
. Hence, we
hypothesized that acetic acid might stimulate glycogenesis by
increasing the influx of glucose 6-phosphate (G-6-P) into the glycogen
synthesis pathway through the inhibition of glycolysis due to an
increase in citrate concentration.
Here, to evaluate whether supplementing a meal with vinegar might aid
in recovery from fatigue, we examined whether a diet containing acetic
acid at concentrations corresponding to that in sushi would enhance
glycogen repletion in the liver and skeletal muscle using
food-deprived rats. In addition, we studied the dose-response
effect of dietary acetic acid. We used diets containing glucose as a
sugar source to prevent the effect of acetic acid on the stimulation of
amylase secretion (18)
and polyethylenglycol (PEG) to
examine the effect of acetic acid on gastric emptying
(19)
.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Male 5-wk-old rats (Sprague-Dawley, Japan SLC, Hamamatsu, Japan) weighing 135 ± 1 g were individually housed in a temperature-controlled room (24 ± 1°C) with a 12-h light:dark cycle. The light period began at 0700 h. The rats had free access to water and were allowed access to a powdered commercial nonpurified diet (MF, Oriental Yeast, Tokyo, Japan) twice a day (from 0900 to 1100 h and from 1700 to 1800 h) for 6 d, after which they were divided into five groups of five rats, each with the same mean body weight (161 ± 2 g). The rats were cared for in accordance with the Guidelines for Animal Experimentation outlined at the 34th Annual Meeting of the Japanese Association for Laboratory Animal Science (May 22, 1987).
The experimental diets, based on the AIN-76 diet (20)
,
contained glucose instead of sucrose, PEG as an indigestible marker and
various concentrations of acetic acid. Each 100 g of diet
contained 62.2 g glucose, 0.5 g PEG and 2.5 g/100 g solution
containing concentrations of 0, 4, 8 or 16 g acetic acid/100 g
diet [designated as control (C), A(0.1), A(0.2) and A(0.4),
respectively]. There was no difference in the energy intake per gram
body (195 ± 2 J/g) among the fed groups.
After 15 h of food deprivation, the experiments commenced at 0900 h. One group of rats (designated Pre) was killed, and the remaining four groups [C, A(0.1), A(0.2) and A(0.4) according to the diet provided] were each given 2 g of the appropriate experimental diet. All rats consumed the diet within 90 min. The fed rats were decapitated at 1100 h, and immediately serum, liver, gastrocnemius muscles (comprising fast-twitch and slow-twitch fibers) and soleus muscles (comprising mainly slow-twitch fibers), stomach and small intestine were collected. Liver and muscles were freeze-clamped in liquid nitrogen within 2 and 7 min after decapitation, respectively, and stored at -80°C until assay.
Measurements of gastrointestinal PEG and glucose.
Contents of the stomach and small intestine were collected by washing
with 3 and 5 mL of saline, respectively. PEG content was measured by
turbidimetry (21)
. The glucose content of the small
intestine was determined by colorimetry (Glucose CII test Wako; Wako
Pure Chemicals, Osaka, Japan).
Determinations of liver and skeletal muscle metabolites.
Glycogen concentrations were measured by the method of Lo et al.
(22)
. G-6-P, fructose 6-phosphate (F-6-P) and fructose
1,6-bisphosphate (F-1,6-P2) were measured by the method of
Lowry and Passonneau (23)
. Xylulose 5-phosphate (X-5-P),
citrate and fructose 2,6-bisphosphate (F-2,6-P2) were
determined as previously described (24
25
26
, respectively).
cAMP was measured by an enzyme immunoassay kit (Direct Cyclic AMP,
Assay Designs, Ann Arbor, MI).
Measurements of serum metabolites and hormones.
Serum glucose and free fatty acids (FFA) were measured by colorimetry
with individual assay kits (Glucose C II test Wako and NEFA C-test
Wako, respectively; Wako Pure Chemicals). Serum insulin and glucagon
were assayed by RIA kits (Shionolia Insulin, Shionogi, Osaka, Japan,
and Glucagon kit Daiichi, Daiichi Radioisotope Laboratory, Tokyo,
Japan, respectively). Serum acetate was determined by capillary gas
chromatography (27)
.
Enzymatic analysis.
Glycogen synthetase (GS) activity was measured as the ratio of form I
GS, which is independent of G-6-P activation, to total enzyme activity.
Tissue powdered in liquid nitrogen was homogenized in 
10 volumes of
the extraction buffer (50 mmol/L Tris-HCl, pH 7.8, 10 mmol/L EDTA,
100 mmol/L sodium fluoride, 1 mmol/L dithiothreitol, 1 mmol/L
phenylmethylsulfonyl fluoride) and then centrifuged at 10,000 x g for 15 min. The supernatant was assayed for GS
activity as previously described (28)
.
Glucokinase (GK) in liver and hexokinase type II (HK II) in
gastrocnemius muscle were assayed by the technique of Newgard et al.
(29)
and the method of Burcelin et al. (30)
,
respectively.
Pyruvate dehydrogenase complex (PDC) activity was determined as the
ratio of the active enzyme to the total enzyme activity. Liver and
gastrocnemius muscle mitochondria were prepared by the technique of
Denyer et al. (31)
and the activity was measured
fluorometrically by coupling to arylamine acetyltransferase
(32)
.
Statistical analysis.
Data are expressed as means ± SEM. Homogeneity of
variance was analyzed by Levene’s test. Statistical evaluation of the
results with homogeneous variances was performed by one-way ANOVA.
Fisher’s least significant difference test (33)
was used
to determine whether mean values were significantly different at
P < 0.05. The result of the PDC activity in liver
had heterogeneous variance and was analyzed by the Kruskal-Wallis
nonparametric test.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Among the fed groups, there was no difference in either gastrointestinal PEG concentration [mean values were 4.0, 4.0, 3.6 and 4.4 mg (pooled SEM, 0.3) in gut and 4.8, 4.9, 5.1 and 4.5 mg (pooled SEM, 0.3) in small intestine for the C, A(0.1), A(0.2) and A(0.4) groups, respectively.] or the ratio of glucose/PEG in the small intestine [mean values are 5.8, 5.2, 6.2 and 5.1 (pooled SEM, 0.9) for the C, A(0.1), A(0.2) and A(0.4) groups, respectively]; 88% of PEG was recovered.
Glycogen concentrations in liver and gastrocnemius and soleus muscles.
Feeding increased glycogen concentrations in the liver (Fig. 1A
) and skeletal muscles (Fig. 1B
, C
). Further, in
the liver, the glycogen concentration in group A(0.2) was significantly
higher than in group C. In gastrocnemius muscle, glycogen
concentrations were significantly greater in the acetic acid–fed
groups compared with the Pre group. Moreover, the glycogen
concentration in group A(0.2) was significantly higher than in group C.
In soleus muscle, the glycogen concentrations in the acetic acid–fed
groups were 60% greater than the Pre group but not different from the
C group.
|
Feeding significantly increased serum glucose and decreased serum FFA
(Table 1
), but there were no differences among the fed groups. Serum insulin
concentrations in the fed groups, except in group A(0.2), were
significantly higher than in the Pre group, but there were no
differences among the fed groups nor were there differences in glucagon
or acetate concentrations (data not shown).
|
The concentrations of F-2,6-P2 in groups C and
A(0.1) were significantly higher than in the Pre group (Table 2
), with a generally lower level in group A(0.2) than in group C
(P < 0.08). Feeding increased the concentration of
X-5-P, but this value was lower in the acetic acid–fed groups than in
group C [C vs. A(0.1), P < 0.06; C vs. A(0.2) and
A(0.4), P < 0.01]. Concentrations of G-6-P, F-6-P,
F-1,6-P2, citrate and cAMP were not different
among the groups, although the citrate and F-6-P concentrations in the
acetic acid–fed groups were >1.3-fold (P > 0.1) and
<0.9-fold (P > 0.1), respectively, of those values in
group C. There were no significant differences in GK, GS or PDC
activities among any of the groups (data not shown).
|
The concentration of G-6-P in group A(0.2) was significantly higher
than in the Pre, C and A(0.1) groups (Table 3
). There were no significant differences among any of the groups in the
concentrations of citrate, F-2,6-P 2, F-6-P or
F-1,6-P2, but the citrate concentrations in
groups A(0.2) and A(0.4) were 1.4-fold that in group C (P
> 0.1). The F-1,6-P2/F-6-P ratio in group C
generally was higher than in the other groups [Table 3
; C vs. Pre and
A(0.2), P < 0.02; C vs. A(0.1) and A(0.4),
P < 0.09]. Feeding significantly increased GS
activity (Pre vs. all fed groups: P < 0.05), but there
were no differences among the fed groups. There were no differences
among any of the groups in HK II or PDC activities (data not shown).
|
There were no significant differences among any of the groups in the concentrations of G-6-P, F-6-P, F-1,6-P2 or citrate and the F-1,6-P2/F-6-P ratio (data not shown). The means of F-1,6-P2/F-6-P ratio in acetic-acid-fed groups were <0.8-fold that in group C (P > 0.1).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, B
, C
). The effect in liver and gastrocnemius
muscle appeared to be linear up to 0.2 g acetic acid/100 g diet.
Neither the gastrointestinal PEG content nor the glucose/PEG ratio
differed among the fed groups, indicating that dietary acetic acid did
not influence gastric emptying, the rate of food intake or glucose
absorption. The serum acetate concentrations in the acetic acid–fed
groups were > 1.4-fold that in group C. Acetate administered
orally to humans has been shown to be absorbed and metabolized within
1.5 h (11)
. Thus, it seems likely that the absorbed
acetate had been metabolized in the acetic acid–fed groups, which
might explain why the citrate concentrations in liver and skeletal
muscles tended to be greater than that in group C.
In liver, the concentration of F-2,6-P2 was
lower in group A(0.2) than in group C (Table 2)
.
F-2,6-P2 is a potent regulator of gluconeogenesis
and glycolysis through the inhibition of fructose-1,6-bisphosphatase
(F16BPase) and the activation of PFK-1, respectively [reviewed in
(34
35
36)
]. F-2,6-P2 activates
glycolysis at concentrations above 5 nmol/g (37)
. The
concentrations of F-2,6-P2 in the acetic
acid–fed groups were lower than the activating concentration of 5
nmol/g. Also, the citrate concentrations in the acetic acid–fed groups
tended to be greater than in group C. Together with the lack of
difference in PDC activity, it is possible that acetic acid feeding
might lead not only to enhanced gluconeogenesis by the activation of
F16BPase but also to the suppression of glycolysis by the inhibition of
PFK-1 activity. F-2,6-P2 synthesis is controlled
by the bifunctional enzyme,
phosphofructokinase-2/fructose-2,6-bisphosphatase
(PFK-2/F26BPase), which is regulated by both phosphorylation, an
active form of F-2,6-P2 degradation, and
dephosphorylation, an active form of F-2,6-P2
synthesis. Phosphorylation and dephosphorylation of PFK-2/F26BPase are
catalyzed by cAMP-dependent protein kinase and X-5-P-activated
protein phosphatase 2A (PP2A), respectively (38
,39)
. Among
the fed groups, there were no differences in cAMP. However, X-5-P
levels in groups A(0.2) and A(0.4) were significantly lower than in the
C group, which is consistent with results from studies that perfused
acetic acid into liver (40)
. In this way, the lower
concentrations of X-5-P in the acetic acid–fed groups increased the
phosphorylation state of the bifunctional enzyme, leading to lower PP2A
activity and finally resulting in the lower concentrations of
F-2,6-P2 in those rats. X-5-P is generated from
G-6-P and from F-6-P and glyceraldehyde 3-phosphate (Gly-3-P) catalyzed
by transketolase in the pentose pathway. A low
[NADP+]/[NADPH] ratio inhibits the activity
of glucose 6-phosphate dehydrogenase. Acetate administration has been
reported to decrease this ratio in liver (41)
. Because the
activity of transketolase is in equilibrium under both starved and fed
conditions (42)
, a decrease of either F-6-P or Gly-3-P
results in a low X-5-P concentration. The F-6-P concentrations in the
acetic acid–fed groups generally were below that in group C,
suggesting that the low concentrations of X-5-P in the acetic acid–fed
groups might be due to reduction of influx of hexose 6-phosphates
toward the pentose phosphate pathway.
The pathway of glycogen synthesis in liver is mainly via
gluconeogenesis (indirect pathway) or a glucose phosphorylation step
(direct pathway) (43)
. The direct pathway has two stages,
i.e., glucose phosphorylation, which was not affected by acetic acid
feeding because there were no differences in GK activity among any of
the groups, and the conversion of G-6-P into glycogen. Because G-6-P is
used as a precursor for not only glycogen synthesis but also glycolysis
and the pentose phosphate cycle, inhibition of these steps supports the
preferential utilization of G-6-P for glycogenesis. This suggests that
the enhancement of glycogen repletion by acetic acid may result from
the activation of both the indirect pathway by inhibition of
F-2,6-P2 synthesis caused by a decrease of X-5-P
accumulation and the direct pathway by activating the preferential
utilization of G-6-P for glycogenesis due to the suppression of
glycolysis and the pentose phosphate pathway.
In gastrocnemius muscle, we found a higher concentration of G-6-P in
group A(0.2) compared with group C (Table 3)
. G-6-P plays an important
role in the regulation of glycogen synthesis in skeletal muscle
(44)
. When 30–50% of the GS present in muscle is in the
G-6-P-independent form, physiologic concentrations of G-6-P can
accelerate glycogen synthesis in the presence of inhibitors of GS such
as AMP, ADP and ATP (45)
. Thus, an enhancement in
glycogenesis in gastrocnemius muscle by acetic acid feeding might
result from the higher concentration of G-6-P, although we found no
differences in the ratio of the G-6-P–independent form of GS to total
enzyme among the fed groups. Accumulation of G-6-P is caused by
activation of glucose transport/phosphorylation steps or by inhibition
of glycolysis. HK II activity has been reported to correlate with the
concentration of the insulin-sensitive glucose transporter GLUT4
(46)
and we found no differences among the fed groups in
HK II activity or in serum insulin concentration (Table 1)
; these
findings indicate that acetic acid feeding did not affect the glucose
transport/phosphorylation steps. That the
F-1,6-P2/F-6-P ratio, which reflects PFK-1
activity (47)
, was lower in the acetic acid–fed groups
than in group C (Table 3)
and that there were no differences in PDC
activity among the fed groups suggest that acetic acid may suppress
glycolysis through inhibition of PFK-1 activity. However, there were no
differences among the fed groups in the concentration of
F-2,6-P2, which is the activator of muscle PFK-1.
In gastrocnemius muscle, the effect of acetic acid on PFK-1 was not the
same as in liver because skeletal muscle PFK-2/F26BPase lacks a
phosphorylation site (48)
. Torheim (49)
reported that in vitro citrate at a physiologic concentration, 250
µmol/L, inhibits the activation of PFK-1 by
F-1,6-P2 (at 0–90 µmol/L) and
F-2,6-P2 (at 0–5 µmol/L). Here we
found that the concentrations of citrate,
F-1,6-P2 and F-2,6-P2 were
110–170, 30–60 and 1.5–2.3 nmol/g, respectively. These
concentrations are in agreement with Torheim’s report
(49)
. Further, the citrate concentrations in groups A(0.2)
and A(0.4) were >1.3-fold that of group C, but there were no
differences in F-1,6-P2 and
F-2,6-P2 concentrations among any of the groups.
Hence, in gastrocnemius muscle, the enhancement of glycogen repletion
by acetic acid feeding might result from the accumulation of G-6-P
secondary to inhibition of PFK-1 activity.
In soleus muscle, glycogen concentrations in all of the acetic
acid–fed groups were 1.1-fold that in group C. Because glycogen
repletion of soleus muscle takes priority over that in gastrocnemius
muscle and liver (50)
, this indicated that the soleus
muscle in the acetic acid–fed groups might have approached a state of
glycogen repletion 2 h after feeding. The
F-1,6-P2/F-6-P ratios in the acetic acid–fed
groups were all <0.8-fold that in group C. This suggests that the
mechanism of the enhancement of glycogen repletion in this tissue by
acetic acid feeding might be the same as that in gastrocnemius muscle.
Our results show that dietary acetic acid can enhance glycogen repletion in both liver and skeletal muscle. The mechanism of this effect is different in liver and skeletal muscle. In liver, acetic acid feeding enhances glycogen repletion by activation of gluconeogenesis and the preferential utilization of G-6-P for glycogenesis. In skeletal muscle, the enhancement of glycogen repletion by acetic acid feeding results from the accumulation of G-6-P due to suppression of glycolysis. We used acetic acid at concentrations comparable to those found in a normal diet. Therefore, we conclude that supplementing meals with vinegar may be beneficial in the recovery of liver and skeletal muscle glycogen, for example, upon fatigue, after skipping meals, postexercise or as part of an athlete’s breakfast on the day of competition.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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3 Abbreviations used: A(0.1), A(0.2) and A(0.4), rats 2 h after being fed a diet containing 0.1, 0.2, 0.4 g acetic acid/100 g diet, respectively; C, rats 2 h after being fed a diet without acetic acid; FFA, free fatty acids; F-6-P, fructose 6-phosphate; F-1,6-P2, fructose 1,6-bisphosphate; F-2,6-P2, fructose 2,6-bisphosphate; F16Bpase, fructose-1,6-bisphosphatase; GK, glucokinase; Gly-3-P, glyceraldehyde 3-phosphate; G-6-P, glucose 6-phosphate; GS, glycogen synthetase; HK II, hexokinase type II; PDC, pyruvate dehydrogenase complex; PEG, polyethylenglycol; PFK, phosphofructokinase; PFK-2/F26Bpase, the bifunctional enzyme, phosphofructokinase-2 and fructose-2,6-bisphosphatase; PP2A, protein phosphatase 2A; Pre, rats killed before feeding; X-5-P, xylulose 5-phosphate.
Manuscript received November 2, 2000. Initial review completed December 29, 2000. Revision accepted April 17, 2001.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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