Consumption of fructose- but not glucose-sweetened beverages for 10 weeks increases circulating concentrations of uric acid, retinol binding protein-4, and gamma-glutamyl transferase activity in overweight/obese humans
Nutrition & Metabolism
Consumption of fructose- but not glucose- sweetened beverages for 10 weeks increases circulating concentrations of uric acid, retinol binding protein-4, and gamma-glutamyl transferase activity in overweight/obese humans
Chad L Cox 0
Kimber L Stanhope 0 1
Jean Marc Schwarz
James L Graham 0 1
Bonnie Hatcher 0
Steven C Griffen
Andrew A Bremer
John P McGahan
Nancy L Keim 0
Peter J Havel 0 1
0 Department of Nutrition, University of California, Davis , One Shields Ave, Davis, CA 95616 , USA
1 Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis , One Shields Avenue, Davis, CA 95616 , USA
Background: Prospective studies in humans examining the effects of fructose consumption on biological markers associated with the development of metabolic syndrome are lacking. Therefore we investigated the relative effects of 10 wks of fructose or glucose consumption on plasma uric acid and RBP-4 concentrations, as well as liver enzyme (AST, ALT, and GGT) activities in men and women. Methods: As part of a parallel arm study, older (age 40-72), overweight and obese male and female subjects (BMI 25-35 kg/m2) consumed glucose- or fructose-sweetened beverages providing 25% of energy requirements for 10 wks. Fasting and 24-h blood collections were performed at baseline and following 10 wks of intervention and plasma concentrations of uric acid, RBP-4 and liver enzyme activities were measured. Results: Consumption of fructose, but not glucose, led to significant increases of 24-h uric acid profiles (P < 0.0001) and RBP-4 concentrations (P = 0.012), as well as plasma GGT activity (P = 0.04). Fasting plasma uric acid concentrations increased in both groups; however, the response was significantly greater in subjects consuming fructose (P = 0.002 for effect of sugar). Within the fructose group male subjects exhibited larger increases of RBP-4 levels than women (P = 0.024). Conclusions: These findings suggest that consumption of fructose at 25% of energy requirements for 10 wks, compared with isocaloric consumption of glucose, may contribute to the development of components of the metabolic syndrome by increasing circulating uric acid, GGT activity, suggesting alteration of hepatic function, and the production of RBP-4.
Between 1970 and 2005 there has been a 19% increase
in the consumption of added sugars and sweeteners in
the U.S. . Current estimations suggest that sweetener
consumption in the U.S. has increased to an average of
477 kcal/person, or approximately 24% of a typical
2000 kcal/day diet . The monosaccharide fructose is
a primary component of both of the two most
common sweeteners in the U.S., sucrose and high fructose
corn syrup. An increase in the consumption of
sweeteners containing fructose has occurred in parallel with
the increasing prevalence of overweight and obesity
over the past three decades , suggesting that
increased consumption of fructose may contribute to
the current epidemic of obesity-related metabolic
disorders [2,3] including increased incidence of the
metabolic syndrome [4,5].
Recently we reported that consumption of
fructosesweetened beverages for 10 wks, at 25% of energy
requirements, increases several risk factors for the
metabolic syndrome in older (age 4072), overweight/obese
humans (BMI 2535 kg/m2) as compared to isocaloric
glucose consumption . Consumption of fructose, but
not glucose led to increased hepatic de novo lipogenesis
(DNL), and promoted accumulation of intra-abdominal
fat, the development of a more atherogenic lipid profile,
and reduced insulin sensitivity in overweight/obese
adults . We have hypothesized that the increased rate
of DNL following consumption of fructose leads to an
accumulation of hepatic lipid, which promotes
dyslipidemia and decreased insulin sensitivity .
Evidence suggests that elevated levels of uric acid and
increased activity of the liver enzymes gamma-glutamyl
transferase (GGT) and alanine aminotransferase (ALT)
are also associated with development of the metabolic
syndrome [8,9]. Fructose has been demonstrated to
increase circulating uric acid concentrations in both animals
and humans, and elevated concentrations of uric acid are
correlated with increased fructose intake [10,11]. It has
been suggested that fructose-induced hyperuricemia may
mediate some of the abnormalities associated with
metabolic syndrome including hypertriglyceridemia, insulin
resistance and hypertension . Elevated serum uric acid
levels have also been found to be associated with the
development of cirrhosis and increased plasma activity of
GGT and ALT . GGT and ALT activities (and to a
lesser extent aspartate aminotransferase (AST)) have been
shown to be strong predictors of metabolic syndrome and
are considered to be markers of non-alcoholic fatty liver
disease (NAFLD) [13,14]. Uric acid levels have also been
found to be strongly associated with concentrations of the
recently described adipokine retinol binding protein-4
(RBP-4) . Increased circulating concentrations of
RBP4 have been linked with increased visceral adiposity 
and have been shown to directly contribute to hepatic
insulin resistance via induction of hepatic glucose
production  and impairment of insulin signaling in muscle
[15,18]. To determine if fructose-induced changes of
DNL, intra-abdominal fat deposition, circulating lipids
and insulin sensitivity  were accompanied by increases
of these biological markers associated with the
development of metabolic syndrome, 24-h plasma uric acid levels
as well as fasting concentrations of RBP-4 and activities of
the liver enzymes GGT, AST and ALT were measured in
overweight/obese men and women consuming
fructoseor glucose-sweetened beverages for 10 wks.
period; and 3) a 2-wk inpatient intervention period.
During baseline, subjects resided in the University of
California-Davis Clinical and Translational Science
Centers Clinical Research Center (CCRC) for 2 wks.
Subjects then began the 8-wk outpatient intervention
and consumed either fructose- (n = 17) or
glucosesweetened (n = 15) beverages at 25% of energy
requirements with self-selected ad libitum diets. Subjects
returned to the CCRC for the final 2 wks of
intervention during which the glucose- or fructose-sweetened
beverages were consumed as part of an
Participants were recruited through newspaper
advertisements and underwent a telephone and an in-person
interview with medical history, a complete blood
count, and a serum biochemistry panel to assess
eligibility. Inclusion criteria included age 4072 years and
BMI 2535 kg/m2 with a self-report of stable body
weight during the prior six months. Women were
post-menopausal based on a self-report of no
menstruation for at least one year. Exclusion criteria included:
evidence of diabetes, renal or hepatic disease, fasting
serum triglyceride (TG) concentrations >400 mg/dl,
hypertension (>140/90 mm Hg), and surgery for
weight loss. Individuals who smoked, reported exercise
of more than 3.5 hours/wk at a level more vigorous
than walking, or having used thyroid, lipid-lowering,
glucose-lowering, anti-hypertensive, anti-depressant, or
weight loss medications were also excluded.
Dietrelated exclusion criteria included habitual ingestion of
more than one sugar-sweetened beverage/day (12 ounces
or ~350 mL) or more than two alcoholic beverages/day
(one alcoholic beverage is defined as 5 ounces of wine,
12 ounces of beer, 8 ounces of malt liquor, or 1.5 ounces
of spirits (80 proof ) which is equivalent to 14 grams or
0.6 ounces of ethanol). The UCD Institutional Review
Board approved the experimental protocol, and subjects
provided informed consent to participate in the study.
Thirty-nine subjects enrolled in the study and
experimental groups were matched for gender, BMI, and
fasting TG and insulin concentrations. As reported by
Stanhope et al., baseline anthropometric and metabolic
characteristics did not differ between the two
experimental groups (Table 1) . Seven subjects (3 in the
glucose group, 4 in the fructose group) did not complete
the study because of inability/unwillingness to comply
with the protocol or due to personal or work-related
conflicts. We were unable to measure uric acid, liver
enzymes or RBP-4 in one female subject in the fructose
group due to a lack of plasma samples. We were also
unable to collect a complete set of 24-h blood samples
from one subject in the glucose group. Therefore, 24-h
Table 1 Baseline anthropometric & metabolic parameters
GLM 2-factor ANOVA (type of sugar & gender). There were no significant differences among groups. Data represent mean SEM.
Data previously published: Stanhope KL, et al. (2009). J Clin Invest 119: 13221334.
uric acid exposure was calculated with n = 30 (fructose:
n = 16, glucose: n = 14), while all other measurements
were calculated with n = 31 (fructose: n = 16, glucose:
n = 15).
During the inpatient metabolic phases, subjects
consumed diets designed to maintain energy balance
providing 15% of energy as protein, 30% as fat, and 55%
as carbohydrate. Subjects were required to consume all
of the food and were limited to only the food provided.
Daily energy intake was based on an estimate of energy
needed to maintain body weight, calculated at baseline
using the Mifflin equation to estimate resting energy
expenditure  and adjusted for activity using a
multiplication factor of 1.3 on the day of 26-h stable
isotope infusions and 24-h serial blood collections, and
a factor of 1.5 for other inpatient days. Energy intake
was distributed across the day: 25% at breakfast
(09:00 h), 35% at lunch (13:00 h), and 40% at dinner
(18:00 h). During baseline, the carbohydrate content
consisted primarily of complex carbohydrates. For the
final 2-wk inpatient intervention period, subjects
consumed diets at the baseline energy level and
macronutrient composition except that 30% of energy was from
complex carbohydrates and 25% was provided by
fructose- or glucose-sweetened beverages. The
macronutrient and micronutrient content of the diets
consumed by the two experimental groups during the
inpatient baseline and inpatient intervention phases of
the study was identical, with the exception of the
sugars provided in the beverages. Additional details
about the dietary intake for inpatient and outpatient
phases have been described previously .
Meals consumed during and prior to 24-h blood
Intervention meals (10 wk) were matched as closely as
possible to the baseline meals (0 wk), except for the
substitution of 25% of energy from sugars for the
complex carbohydrate. The baseline (0 wk) and final (10
wk) intervention 24-h blood collections were performed
after subjects had consumed energy-balanced,
weightmaintaining diets in the CCRC for 10 days.
24-hour fasting and postprandial blood profiles
24-h blood collections were conducted during baseline
(0 wk) and after 10 wks of intervention (10 wk). At
07:30 h, an i.v. catheter was inserted into an arm vein by
a Registered Nurse and kept patent with slow saline
infusion. Three fasting blood samples were collected in
EDTA at 08:00, 08:30, and 09:00 h. Thirty-three
postprandial blood samples were collected at 3060 minute
intervals from 09:30 until 08:00 h the next morning
[20,21]. Meals were served at 09:00, 13:00 and 18:00 h.
An additional 36 ml of blood was collected at each of
the following time-points: 08:00, 08:30, 09:00 and 22:00,
23:00, 23:30 h. The plasma from the 3 fasting samples
(08:00, 08:30, 09:00 h) was pooled, as was the plasma
from the 3 postprandial blood samples (22:00, 23:00,
23:30 h); multiple aliquots of each pooled sample were
stored at 80 C.
Uric Acid, RBP-4, GGT, AST and ALT
Uric Acid was measured using a colorimetric assay from
Wako (Richmond, VA). RBP-4 was quantified using an
ELISA from R&D Systems (Minneapolis, MN) and GGT,
AST and ALT activities were determined using a
Polychem Chemistry Analyzer (PolyMedCo, Inc., Cortlandt
Average 24-h uric acid and TG exposure were
determined by averaging values for the 33 postprandial time
periods. Peak TG exposure was defined as the highest
TG measurement over the 33 postprandial time periods,
and postprandial TG peak was calculated by averaging
TG measurements taken at 22:00, 23:00, and 23:30 h.
Statistical tests were performed with SAS 9.2. The
percent change for each outcome was calculated and
analyzed in a 3-factor (type of sugar, gender, and presence
of metabolic syndrome) mixed procedures analysis using
PROC MIXED. Outcomes with least squares means (LS
means) of the change (10wk versus 0wk) significantly
different than zero were identified. Risk factors for
metabolic syndrome (MSRFs) were identified in all subjects
and the presence of metabolic syndrome was defined as
having at least 3 MSRFs (subjects with metabolic
syndrome: Fructose n = 5, Glucose n = 4; subjects without
metabolic syndrome: Fructose n = 11, Glucose n = 11).
MSRFs were those defined by the American Heart
Association/National Heart Lung and Blood Institute [22,23].
The percent change in response variables between 0 and
10 wks was determined, and Pearsons correlation
coefficients describing the relationship between response
variables were calculated using PROC CORR. Statistical
tests with P values <0.05 were considered significant.
Data are presented as mean SEM.
Fasting uric acid and 24-h circulating uric acid profiles
At 10 wks of intervention fasting plasma uric acid
concentrations were significantly increased from
baseline in subjects consuming both fructose- (absolute
= +0.82 0.08 mg/dL; P < 0.0001) and
glucosesweetened (absolute = +0.23 0.09 mg/dL; P = 0.02)
beverages, but the effect was significantly greater in those
consuming fructose-sweetened beverages (P = 0.002 for
effect of sugar) (Table 2). 24-h serum uric acid profiles
were increased significantly from baseline in subjects
consuming fructose (absolute = +0.42 0.05 mg/dL;
P < 0.0001) but not in those consuming glucose (absolute
= +0.07 0.03 mg/dL; P = 0.26) (P < 0.0001 for effect of
sugar) (Table 2) (Figure 1).
After 10 wks fasting plasma RBP-4 concentrations
increased significantly from baseline in subjects
consuming fructose-sweetened beverages (absolute = +2.2
1.0 ng/mL; P = 0.012) and decreased significantly
from baseline in those consuming glucose-sweetened
beverages (absolute = 3.7 0.9 ng/mL; P = 0.0005)
(P < 0.0001 for effect of sugar) (Table 2). Although
RBP-4 concentrations decreased comparably in both
men and women in the glucose group, the increase of
RBP-4 from baseline levels in subjects consuming
fructose was significantly greater in men (P = 0.007)
compared to women (P = 0.908) (P = 0.024 for effect of
sugar gender) (Figure 2).
Plasma GGT, AST, and ALT Activities
Plasma GGT activity was significantly elevated compared
with baseline values following 10 wks of fructose
consumption (absolute = +3.7 1.1 U/L; P = 0.04) but
decreased significantly in subjects consuming glucose
(absolute = 6.4 2.6 U/L; P < 0.0001)(P < 0.0001 for
effect of sugar) (Table 2). The increases of GGT at 10
wks in subjects consuming fructose were greater in
those with less than 3 MSRFs (Baseline: 23.0 5.1 U/L,
absolute = +4.1 1.4 U/L) as compared to those with
metabolic syndrome (3 MSRFs) (Baseline: 23.1 7.3
U/L, absolute = +2.7 1.9 U/L) (P = 0.002 for effect of
MSRF and P = 0.002 for effect of sugar MSRF). Fasting
activities of AST and ALT decreased slightly following
10 wks of fructose consumption; however, these changes
Table 2 Baseline values and % change at 10 wks of intervention for fasting uric acid levels, mean 24-h uric acid
exposure, fasting plasma RBP-4 concentrations, and fasting GGT, ALT and AST activities1
1Values are means SEM, n = 31 (fructose n = 16; glucose n = 15) for all measurements except 24-h uric acid exposure (n = 30, fructose n = 16, glucose n = 14).
PROC MIXED 3-factor (sugar, gender, and presence of metabolic syndrome) ANOVA; 2sugar gender: P =0.037; 3effect of metabolic syndrome: P = 0.002,
sugar metabolic syndrome: P = 0.002.
*P < 0.05, **P < 0.01, ***P < 0.001 for changes significantly different from zero.
08:00 12:00 16:00 20:00 24:00 08:00 12:00 16:00 20:00 24:00
Time (hr) Time (hr)
Figure 1 Circulating uric acid concentrations over 24-h in subjects before and after 10 wks of consuming (A) fructose- or (B)
glucose- sweetened beverages. ***P < 0.0001 compared with baseline and P < 0.0001 for effect of sugar using PROC mixed 3-factor (sugar,
gender, and presence of metabolic syndrome) ANOVA; glucose, n = 14; fructose, n = 16. Data represent mean SEM.
were not statistically significant (Table 2). In subjects
consuming glucose both AST (absolute = 4.5 2.2
U/L, P = 0.002; effect of sugar: P = 0.08) and ALT
(absolute = 6.9 3.0 U/L, P = 0.002; effect of sugar:
P = 0.04) activities were decreased after 10 wks (Table 2).
Relationships between response variables
In subjects consuming fructose the percent increases of
both fasting concentrations of RBP-4 and GGT activity,
but not uric acid concentrations, were positively
associated with previously reported increases of TG exposure,
peak TG exposure, and postprandial TG peak (Table 3)
(Figure 3) . These relationships were not observed in
subjects consuming glucose (Table 3). There were no
significant relationships observed between elevations of uric
acid and RBP-4 levels or GGT activity, nor between
elevations of RBP-4 levels and GGT activity.
Figure 2 Percent changes of fasting RBP-4 concentrations after
10 wks of consuming fructose- or glucose-sweetened
beverages in male and female subjects. Values are means SEM,
n = 31 (fructose group, n = 16; glucose group, n = 15) PROC MIXED
3-factor (sugar, gender, and presence of metabolic syndrome)
ANOVA; *P < 0.05 and **P < 0.01 for changes significantly different
To our knowledge, this is the first study to report that
prolonged fructose consumption increases 24-h
circulating uric acid concentrations, GGT activity and RBP-4
levels in humans. Importantly, these findings also
complement our previously reported results (in these same
subjects) which demonstrated that hepatic fractional de novo
lipogenesis (DNL) and plasma lipids and lipoprotein
concentrations increased in subjects consuming
fructosesweetened beverages, but remained unchanged in subjects
consuming glucose-sweetened beverages for 10 wks .
Despite comparable weight gain (~1-2% of initial body
weight), subjects consuming fructose primarily exhibited
increases of visceral adipose tissue (VAT), whereas only
GGT vs. peak TG exposure
RBP-4 vs. 24-h TG exposure
GGT vs. 24-h TG exposure
RBP-4 vs. postprandial TG peak
GGT vs. postprandial TG peak
Pearsons correlation coefficients (r) describing the relationship between
response variables (% change) were calculated using PROC CORR. Average 24-h
TG exposure was determined by averaging values for the 33 postprandial time
periods. Peak TG exposure was defined as the highest TG measurement over
the 33 postprandial time periods, and postprandial TG peak was calculated by
averaging TG measurements taken at 22:00, 23:00, and 23:30 h. *P < 0.05 **
P < 0.01.
-10 0 10 20 30 40 50 60 70
24-h TG exposure (% change)
-10 0 10 20 30 40 50 60 70
24-h TG exposure (% change)
Figure 3 Correlations between the percent changes of 24-h TG exposure and percent changes of (A) RBP-4 levels and (B) GGT activity
in subjects consuming fructose. Pearsons correlation coefficients (r) were calculated using PROC CORR. Average 24-h TG exposure was
determined as the mean of TG concentrations in the 33 postprandial samples. n = 31 (fructose group n = 16).
subcutaneous adipose tissue (SAT) was increased in
subjects consuming glucose . In addition, indices of insulin
sensitivity and glucose tolerance were decreased during 10
wks of fructose consumption, but were unaffected by
isocaloric consumption of glucose .
Prolonged fructose consumption significantly increased
both fasting uric acid concentrations and 24-h uric acid
exposure. The increase of fasting uric acid
concentrations exhibited after 10 wks of fructose consumption is
consistent with previous reports of the effects of up to 5
wks of fructose consumption at 20-30% of total calories
in humans [24-26]. Reiser et al. demonstrated that
consumption of 20% of energy from fructose for 5 wks as
part of a diet designed to maintain body weight
increased fasting uric acid levels in men  and our
results support these findings.
The mechanism by which fructose consumption
leads to increased uric acid concentrations is thought
to be initiated by the depletion of adenosine
triphosphate (ATP) and inorganic phosphate (Pi) resulting
from unregulated production of fructose-1-phosphate
and glyceraldehyde-3-phosphate from fructose, which
bypasses the rate-limiting step of glycolysis,
phosphofructokinase (PFK) [3,27]. Fructose-induced depletion
of ATP and Pi leads to a concomitant increase of
purine nucleotide degradation and, subsequently, uric acid
production . It has also been reported that, in
addition to increased nucleotide degradation, prolonged
fructose consumption promotes increases in the
incorporation of glycine into urate suggesting that
upregulation of de novo purine nucleotide synthesis may
also contribute to fructose-induced increases of uric
acid . While uric acid is a potent and
physiologically relevant antioxidant, recent evidence suggests that
elevated levels may be associated with increased
oxidative stress, and there is still considerable debate as to
which of these roles is more important in the context
of metabolic disease .
Elevations of fasting uric acid levels have been shown to
be associated with hypertriglyceridemia and insulin
resistance [30,31]. Our results are not consistent with these
findings as we did not detect any relationships between
changes in fasting uric acid levels or 24-h uric acid
exposure and the previously reported marked increases of TG
exposure or reductions of insulin sensitivity in subjects
consuming fructose . Also in contrast to the findings of
previous investigations [12,15], increases of fasting uric
acid concentrations and 24-h uric acid exposure were not
associated with elevations of RBP-4 levels or GGT activity.
Although we did not detect a correlation between
elevations of uric acid and changes of TG exposure, insulin
sensitivity, RBP-4 levels or GGT activity, it is important to
point out that our sample size was fairly modest and thus
these findings do not rule a contribution of uric acid to
the reported changes in these metabolic parameters.
Additional intervention studies are needed to investigate the
possibility that these changes are related. It is interesting
that fasting uric acid concentrations (but not 24-h uric
acid exposure) also increased significantly in subjects
consuming glucose-sweetened beverages for 10 wks despite
the fact that these subjects did not exhibit any of the
adverse changes measured in subjects consuming fructose
. This finding suggests that changes of 24-h uric acid
exposure may be a more sensitive indicator of increased
metabolic dysfunction than changes of fasting levels.
No previous studies have investigated the effects of
glucose or fructose consumption on circulating levels of
RBP-4. There is increasing evidence suggesting that
RBP-4 may be an important link between increases of
visceral adiposity and insulin resistance . In animal
studies RBP-4 has been clearly shown to reduce glucose
uptake and impair insulin signaling in muscle, as well as
to increase hepatic glucose production via induction of
the gluconeogenic enzyme phosphoenolpyruvate
carboxykinase (PEPCK) . In humans, increases of
circulating RBP-4 are strongly associated with insulin resistance
in adipose tissue , and elevations of circulating
RBP4 are predictive of a diagnosis of metabolic syndrome
. While hepatocytes are the primary source of
RBP-4, it has been demonstrated that adipocytes can
also contribute significantly to circulating
concentrations of RBP-4 . Reduced expression of the
insulinstimulated glucose transporter, GLUT4, in adipocytes
has been shown to lead directly to increased adipocyte
secretion of RBP-4 . Moreover, expression of
RBP4 in humans is significantly greater in visceral adipose
tissues (VAT) compared with subcutaneous adipose
tissue (SAT) and is associated with an increase of
adipocyte size .
Circulating RBP-4 concentrations are significantly
elevated following 10 wks of fructose consumption (Table 2)
and these changes are correlated with increases of
postprandial TG (Table 3, Figure 3). We have hypothesized
that fructose consumption can promote reductions in
insulin sensitivity by providing substrate for hepatic DNL
leading to hepatic triglyceride accumulation, PKC
activation, and increased hepatic insulin resistance  and have
suggested that this mechanism is responsible for the
reductions in insulin sensitivity previously reported in
these same subjects . Although the reported increases
of RBP-4 were relatively modest, our results suggest the
possibility that fructose-induced increases of RBP-4 levels
may also have contributed to the previously reported
reductions of insulin sensitivity in these subjects .
We have suggested that the differential effects of
fructose and glucose on regional adipose deposition may be
explained in part by the increased sensitivity of SAT
relative to VAT to insulin-stimulated lipoprotein lipase
(LPL) activation , and by our reported observations
that insulin responses were decreased in subjects
consuming fructose and increased in subjects consuming
glucose . The differential changes in RBP-4 levels in
subjects consuming fructose or glucose are consistent
with this mechanism considering that reductions of
post-meal insulin exposure in subjects consuming
fructose would lead to decreased expression of GLUT4 in
adipocytes, which would be expected to be associated
with an increase in the production and secretion of
RBP-4 from adipose tissue (VAT in particular). Thus, it
is possible that reduced insulin exposure in subjects
consuming fructose led to increased plasma RBP-4 levels
directly by decreasing expression of GLUT4 in adipose
tissue, and indirectly by increasing deposition of TG into
VAT, leading to increased visceral adipocyte size, and
increased secretion of RBP-4. The significant reduction
of circulating RBP-4 concentrations observed in subjects
consuming glucose-sweetened beverages is consistent
with the observations that glucose consumption did not
result in increased DNL, postprandial
hypertriglyceridemia, accumulation of VAT or reduced postprandial
insulin exposure .
The finding that increases of RBP-4 during fructose
consumption were larger in men than in women was
not unexpected considering that TG responses and
increases of VAT deposition were also considerably
greater in men  (Figure 2). We have also reported that
women exhibited greater decreases in insulin sensitivity
than men in response to fructose consumption, and have
hypothesized that this may be due to decreased rates of
VLDL production and secretion, leading to larger
increases of hepatic lipid . Our findings suggest that
the increased rate of VLDL production/secretion
following fructose consumption in men is accompanied by an
increase of RBP-4 levels. These elevations of RBP-4
levels are likely the result of increased accumulation of
VAT and decreased insulin-stimulated GLUT4
expression in adipocytes. These data suggest that in men
fructose consumption likely contributes to increases of
hepatic insulin resistance both directly by providing
substrate for hepatic DNL leading to increased hepatic TG
accumulation and decreased hepatic insulin sensitivity
, and indirectly by increasing visceral adiposity ,
while in women it is primarily a direct effect mediated
by increased hepatic DNL and lipid content.
There have been very few prospective investigations of
the effects of fructose consumption on the activity of the
liver enzymes GGT, AST and ALT in humans. However,
the results from one recent study suggest that
shortterm fructose overfeeding does not alter AST or ALT
activities , and our results following 10 wks of fructose
consumption are in agreement with these findings.
Reports on the effects of fructose consumption on GGT
activity in humans are not available, and the results of
animal studies are not likely to be physiologically
relevant since these investigations have primarily been
conducted rats, which have been reported to have hepatic
and plasma GGT activities over 20-fold lower than
humans . The results of the present study
demonstrate that prolonged fructose consumption leads to
marked increases of GGT activity in older, overweight/
obese adults (Table 2).
We also report that fructose-induced increases of
plasma GGT activity are positively associated with
increases of 24-h TG exposure, peak TG exposure and
the postprandial TG peak reported previously in these
same subjects  (Table 3, Figure 3). Martin et al. first
demonstrated that GGT activity is strongly associated
with postprandial plasma TG levels , which has since
been confirmed by other investigators . The authors
hypothesized that the upregulation of hepatic
microsomal enzymes that results in increased TG synthesis
and DNL is accompanied by a concomitant increase in
GGT activity and speculated that this process could be
initiated by excessive carbohydrate intake . The
increases of GGT activity in subjects consuming fructose
support the mechanism proposed by Martin et al.
However, since GGT activity significantly decreased during
10 wks of glucose consumption, and these changes were
not related to measures of TG exposure, we suggest that,
under energy balanced conditions, it is not carbohydrate
in general (as speculated by Martin et al.) but rather
intake of fructose specifically that mediates increases of
hepatic DNL, TG synthesis, and GGT activity.
GGT has been well established as a reliable marker of
increased hepatic lipid content and hepatic insulin
resistance . In addition, elevations of plasma GGT activity
are positively associated with increases of visceral
adiposity, and this relationship is independent of hepatic fat
content [43,44]. It has been suggested that fructose
consumption may contribute to increases of hepatic insulin
resistance both directly, by providing substrate for
hepatic DNL leading to increased triglyceride accumulation
and novel PKC activation  and indirectly by increasing
visceral adiposity . Based on the established
associations between GGT activity, increased hepatic lipid
content and visceral adiposity it is possible that either or
both of these mechanisms may contribute to increases
of GGT activity during fructose consumption. However,
the association of fructose-induced increases of GGT
activity with measures of TG exposure, but not with
increases of visceral adiposity, suggests that an increase
of intrahepatic lipid may be a primary mechanism. It
should also be noted that while increased GGT activity
is considered a sensitive marker of increased intrahepatic
lipid, it lacks the specificity of other liver enzymes such
as ALT. Elevations of GGT activity have also been shown
to be associated with increased oxidative stress, all-cause
mortality, and mortality from cancer and diabetes as
opposed to liver disease alone . It is interesting but
not clear why the liver enzymes GGT, ALT and AST
decreased significantly from values measured during the
baseline complex carbohydrate diet in subjects
consuming glucose-sweetened beverages.
Elevations of plasma GGT activity are predictive of the
development of metabolic syndrome  and it has been
suggested that elevated GGT activity should be included
as an additional diagnostic risk factor for metabolic
syndrome . Our data suggest that under energy balanced
conditions, consumption of fructose at 25% of energy
requirements for 10 wks leads to greater increases of
plasma GGT activity in subjects with less than 3 MSRFs
compared to those with metabolic syndrome (3 MSRFs),
despite comparable baseline values in both groups (see
Results). The reasons for this differential response are
We report new data demonstrating that consumption of
fructose at 25% of energy requirements for 10 wks, when
compared with isocaloric consumption of glucose, leads
to significant increases of fasting uric acid and 24-h uric
acid exposure, as well as circulating concentrations of
RBP-4 and GGT activity in overweight/obese adults. The
increases of GGT activity and RBP-4 levels in subjects
consuming fructose were associated with previously
reported increases of TG exposure , while increases of
fasting uric acid and 24-h uric acid exposure were not.
The effect of fructose to increase levels of RBP-4 is much
more robust in men than in women and this may be
related to the association of these changes with increases
of measures of TG exposure, which were also larger in
men . Together, the results presented here indicate that
prolonged fructose consumption may contribute to the
development of the metabolic syndrome by increasing
circulating concentrations of uric acid, GGT activity (altered
hepatic function), and the production of RBP-4.
RBP-4: Retinol binding protein-4; AST: Aspartate aminotransferase;
ALT: Alanine aminotransferase; GGT: Gamma-glutamyl transferase; DNL: De
novo lipogenesis; TG: Triglyceride; MSRF: Metabolic syndrome risk factor;
VAT: Visceral adipose tissue; SAT: Subcutaneous adipose tissue; PKC: Protein
kinase C; GLUT4: Glucose transporter-4; VLDL: Very low density lipoprotein.
None of the authors have any personal or financial conflicts of interest.
The authors thank Marinelle Nuez, Brandi Bair, Rebecca Stewart, Sara
Wuehler, Barbara Gale, Artem Dyachenko and Patrick Lam for their excellent
technical support and Nicole Mullen and the nursing staff at the CCRC for
their dedicated nursing support. We also thank Janet Peerson for expert
advice on the statistical analysis of the data. This research was supported
with funding from NIH grant RO1 HL-075675. The project also received
support from Grant Number UL1 RR024146 from the National Center for
Research Resources (NCRR), a component of the National Institutes of Health
(NIH), and NIH Roadmap for Medical Research. Dr. Havels laboratory also
receives support from NIH grants HL-091333, AT-003545, and DK-097307. Dr.
Keims research is supported by intramural USDA-ARS CRIS 5306-51530-016
CLC: responsible for organization and analysis of data, primary preparation of
the manuscript, and assisted with execution of experimental procedures; KLS:
assisted with obtaining funding and design of the study, was responsible for
study implementation and supervision, formulation of experimental diets
and assisted with manuscript preparation; JLG: assisted with analysis of data
and manuscript preparation; JMS: assisted with study design and was
responsible for measurement of hepatic DNL; SCG: served as study physician
and assisted with design of the study; AAB: served as study physician; LB:
assisted with design of the study; NLK: assisted with design and
implementation of the study; and PJH: responsible for the conception and
design of the study, obtaining funding, and assisted with preparation of the
manuscript. All authors read and approved the submitted manuscript.
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