Chromium is considered an essential mineral that is involved in the metabolism of carbohydrates, lipids, and proteins. A number of studies support the use of supplemental chromium as an adjunctive treatment for type 2 diabetes and impaired glucose tolerance (1). A detailed understanding of the molecular action of chromium is lacking; several lines of evidence point to enhancement of insulin action. Chromium increases insulin-stimulated glucose uptake in cultured muscle cells (14,20) and adipocytes (29). Chromium may increase insulin binding to cells, insulin receptor number, and insulin receptor tyrosine kinase activity (7). The enhancement of insulin action by chromium is associated with phosphorylation of insulin receptor substrate-1 (IRS-1) (14) and phosphatidylinositol 3-kinase (PI 3-kinase) (25) and is inhibited by wortmanin, an inhibitor of PI 3-kinase (1). Activation of these proteins in the insulin-signaling transduction pathway leads to translocation of glucose transporters from the cytosol to the plasma membrane. Indeed, chromium picolinate supplementation was recently shown to significantly enhance the membrane-associated Glut-4 content of skeletal muscle and rate of glucose disappearance in obese rats after insulin stimulation (5). In a follow-up study it was reported that improved glucose disposal rates in chromium-fed, obese, insulin-resistant animals were attributable to enhanced insulin-stimulated IRS-1 and PI 3-kinase activity in skeletal muscle (26). These cellular effects of chromium have not been established in humans supplemented with chromium.
If chromium acts to potentiate or autoamplify insulin signaling, thereby promoting greater glucose uptake in skeletal muscle, then one potential effect would be increased nonoxidative glucose metabolism (i.e., glycogen formation), a finding previously shown in cultured muscle cells (25). The magnitude of glycogen synthesis is regulated by glucose availability and glycogen synthase activity; both processes can be activated by exercise and insulin via independent upstream signaling pathways. Upstream elements in the insulin-mediated pathway include the insulin receptor IRS-1 and PI 3-kinase. We examined glycogen synthesis after a bout of intense exercise followed by a high-carbohydrate feeding regimen because this is a time period when glycogen synthesis can be enhanced by insulin-mediated mechanisms (21,30) and is therefore an opportune time for chromium to display an effect on insulin action and glycogen metabolism. The primary purpose of this study was to assess the potential of chromium picolinate supplementation in humans to enhance the rate of glycogen synthesis after intense exercise. We hypothesized that chromium would stimulate insulin signaling as demonstrated by an increase in IRS-1-associated PI 3-kinase activity and subsequent glycogen synthase.
MATERIALS AND METHODS
Subjects
Sixteen healthy, moderately overweight (BMI > 25 kg·m−2) male subjects aged 20-23 yr were recruited from the student population (mean ± SE: age 22 ± 1 yr, height 174.5 ± 0.9 cm, weight 96.1 ± 2.8 kg, BMI 31.1 ± 0.7 kg·m−2, body fat 28.8 ± 1.3%). Exclusion criteria included vitamin or mineral supplementation containing chromium, recent weight loss (>2 kg in the previous month), musculoskeletal problems, and/or significant history of diabetes or cardiovascular disease. Subjects were sedentary or recreationally active, but not highly trained. We studied moderately overweight non-highly trained subjects because we reasoned they would be more likely to experience benefits from chromium supplementation (1,4). Subjects signed a written informed consent document in accordance with the guidelines of the institutional review board at the University of Connecticut.
Experimental Design
Subjects were matched on the basis of BMI and percent body fat and then randomly assigned in a double-blind fashion to supplement with chromium picolinate (Cr) (N = 8) or an identical-looking placebo (N = 8) for 4 wk. Power analysis indicated that a sample size of 16 (eight subjects per group) would have a power of 80% to detect a 15% difference in the rate of glycogen synthesis during recovery. We obtained a fasting blood sample, muscle biopsy, and body composition scan before supplementation. During the last 2 d of supplementation, subjects consumed a very-low-carbohydrate diet and then performed an intense bout of cycling exercise. The combination of a low-carbohydrate diet and intense exercise ensured that participants would achieve significant glycogen depletion. After the exercise challenge, subjects were provided a high-carbohydrate diet for 24 h. Blood glucose, lactate, and insulin were determined at rest and during 2 h of recovery. Muscle biopsies were obtained immediately after exercise and 2 and 24 h after exercise to assess glycogen, PI 3-kinase activity, and glycogen synthase activity. A crossover design was not used because this would have resulted in a large number of muscle biopsies for each subject.
Supplementation Protocol
Each subject was provided with a bottle of capsules containing either chromium picolinate (600 μg of Cr+3 per capsule; Nutrition21, Purchase, NY) or an identical-looking placebo (dicalcium phosphate). Subjects ingested one capsule with breakfast per day for 28 d. The last day of supplementation coincided with the exercise protocol (described below). They recorded the time of day the supplement was ingested on a log sheet. At the end of the study, subjects returned the bottles and signed the log sheet to document compliance. Subjects were instructed to follow their normal exercise and dietary patterns. Subjects received thorough instructions for completing detailed weighed food records during weeks 1, 2, and 4 of the supplementation period (15 d total). Food diaries were analyzed for energy and nutrient content (Nutritionist Pro, Version 2.1, First Databank, The Hearst Corporation).
Experimental Protocol
Two days before the end of supplementation and before the intense exercise bout, subjects were familiarized with the cycle ergometer protocol. At that visit, subjects were provided with detailed instructions to consume a very-low-carbohydrate diet for the next 48 h until the morning of the exercise protocol. The rationale for this carbohydrate-restricted period was to partially deplete glycogen levels before the exercise bout. Specific menus were generated for each subject to provide 30 kcal·kg−1, distributed as 30% protein, 60% fat, and 10% carbohydrate.
The experimental protocol is shown in Figure 1. Subjects were required to fast overnight for >12 h and consume 16 oz of water on the morning of the exercise protocol to ensure euhydration. On arriving at the laboratory, a flexible catheter was inserted into an arm vein for blood sampling via a three-way stopcock. Subjects rested for 10 min in a semirecumbent position, and a preexercise blood sample was collected. A local anaesthetic was injected subcutaneously into the quadriceps, and an incision was made so that the immediate postexercise biopsy could be obtained quickly after completion of the exercise protocol. Subjects then performed a 5-min warm-up with no load on a Monark Ergomedia cycle ergometer, followed by stretching. After this warm-up, subjects cycled at 60 rpm for 150 s at a resistance of 10% body weight, immediately followed by a 30-s all-out sprint. The protocol finished with a 30-s recovery with no load on the ergometer. This protocol was chosen because a similar intense bout of cycling in conjunction with a high-carbohydrate diet has been shown to result in rapid glycogen synthesis and attainment of supranormal muscle glycogen levels in 24 h (8).
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FIGURE 1: Experimental protocol timeline.
Immediately after the exercise protocol, a muscle biopsy was performed and a blood sample was obtained. Subjects then consumed a carbohydrate-protein beverage, which was also ingested again at 30, 60, and 90 min after exercise. Thus, subjects consumed a total of four shakes at 30-min intervals. The beverage provided 0.35 g·kg−1·h−1 whey protein isolate (Precision Protein, EAS, Inc. Golden, CO) and 0.7 g of carbohydrate per kilogram per hour (50% dextrose/50% maltodextrin). Thus, each beverage consumed at 30-min intervals contained one half these amounts. The beverage was designed to maximally stimulate glucose, insulin, and glycogen synthesis on the basis of previous research (21-23). Additional blood samples were obtained at 30, 60, 90, and 120 min after exercise, and another muscle biopsy was obtained at 120 min. Subjects remained seated in the laboratory, and a meal was provided 4 h after exercise. Additional meals were prepared and given to subjects to take home and consume at 8, 12, and 22 h after exercise. During the 24-h postexercise period, the shakes and meals provided to subjects provided 40 kcal·kg−1 and 7 g of carbohydrate per kilogram. Protein and fat contributed equally to the remaining calories. Thus, the macronutrient distribution as a percent of energy was 70% carbohydrate, 15% protein, and 15% fat. Subjects were instructed to minimize physical activity after leaving the laboratory to avoid compromising glycogen synthesis (9). Subjects returned to the laboratory the next morning, and a final muscle biopsy was obtained 24 h after exercise.
Anthropometrics and Body Composition
Body mass, waist circumference, and body composition were measured in the morning after a 12-h overnight fast by the same technician. Body mass was recorded to the nearest 100 g on a digital scale (OHAUS Corp., Florham Park, NJ) with subjects wearing only underwear. Waist circumference was measured to the nearest millimeter at the half distance between the bottom of the rib cage and the iliac crest, with subjects in a relaxed, standing position with weight distributed evenly on both legs. An average of three values was used. Whole-body and regional body composition were assessed using a fan-beam dual-energy x-ray absorptiometer (Prodigy, Lunar Corporation, Madison, WI). Regional analysis of the trunk was assessed according to anatomic landmarks by the same technician using computer algorithms (enCORE version 6.00.270).
Blood Processing and Analyses
Whole blood was collected into tubes with no preservative and allowed to clot before being centrifuged at 3000 rpm (1500 × g) for 15 min. A portion of serum was immediately analyzed in duplicate for glucose and lactate using a YSI glucose/lactate analyzer (YSI 2300 STAT, Yellow Springs, OH). Approximately 0.3 mL of serum was transferred into 1.5-mL storage tubes and frozen at −80°C for later determination of insulin. On fasting draws only before and after supplementation, the remaining serum was sent to a certified medical laboratory (Quest Diagnostics, Wallingford, CT) to perform a comprehensive metabolic screening profile that assessed serum glucose, albumin, total protein, minerals (sodium, potassium, chloride, calcium, phosphorus, magnesium, iron), renal function (blood urea nitrogen, uric acid, creatinine, bilirubin), and liver function (alkaline phosphatase, alanine aminotransferase, asparate aminotransferase, gamma glutamyl transferase, lactate dehydrogenase). Insulin was determined using an enzyme-linked immunosorbent assay (ELISA) with a sensitivity of 1.8 pM (Diagnostic Systems Laboratory, DSL, Webster, TX). All samples for insulin were determined in duplicate and were thawed only once. Intra- and interassay variances were 6.5 and 10.7%, respectively. The homeostasis model assessment (HOMA) was used to estimate insulin resistance using the formula: glucose (mM)·(insulin (mU·L−1)/22.5) (13).
Muscle Biopsy and Analyses
Muscle biopsies were performed at baseline, immediately after exercise (within 2 min of completion of the bout), 2 h after exercise, and 24 h after exercise. Muscle biopsies were obtained from the superficial portion of the vastus lateralis using the percutaneous needle technique with suction. The biopsy site was prepped and the skin was anesthetized by a local subcutaneous injection of 2% lidocaine HCL. A small incision (~1 cm) was made through the skin and muscle fascia, and a 5-mm-diameter sterile biopsy needle (Surgical Instruments Engineering Ltd, Midlothain, United Kingdom) was introduced into the muscle to a depth of approximately 2 cm. Because of possible variations in fiber-type distribution from superficial to deep and proximal to distal, postexercise biopsy sites were determined using the baseline biopsy scar and depth markings on the needle. To ensure adequate sample sizes, a double-chop method combined with suction was used. To avoid possible impairment of glycogen synthesis resulting from microtrauma in the area near the biopsy (6), we performed the four biopsies in the following manner: baseline = right leg; immediate postexercise = left leg; 2 h postexercise = right leg (3 cm below the first site); and 24 h postexercise = left leg (3 cm below the first site). The muscle sample was removed from the needle and divided into three pieces of roughly equal size. The specimens were cleaned of connective tissue and blood, frozen within 30 s in liquid nitrogen, and stored at −80°C for later determination of glycogen content, glycogen synthase activity, and PI 3-kinase activity.
Glycogen was determined from muscle samples that were homogenized on 0.3 M PCA and glycogen digested by the amyloglucosidase method. The resulting glucose moieties were quantified spectrophotometrically in the presence of hexokinase and glucose-6-phosphate dehydrogenase (18). To assess glycogen synthase activity, approximately 4 mg of freeze-dried muscle tissue was cut on ice and homogenized (1:50) in a buffer solution (50% glycerol, 20 mM K2HPO4, 5 mM 2-mercaptoethanol, 0.5 mM EDTA, 0.02 % bovine serum albumin) using a glass grinder (Duall 21, Kontes, Vineland, NJ). After 5 min of centrifugation, supernatant was incubated either in the presence (D form) or absence (L form) of 10 mM glucose 6-phosphate. A series of enzymatic reactions then led to a fluorescent product, which was measured by a fluorometer (Quantech, Barnstead International, Dubuque, IA). Each form was analyzed in triplicate. Glycogen synthase activity was determined as the active fraction of the total enzyme (L/(L + D)).
IRS-1-associated PI 3-kinase activity was determined from frozen muscle samples that were homogenized in a buffer solution (50 mM hepes, 137 mM NaCl, 1 mM MgCl2, 1 mM CaCl2, 10 mM Na pyrophosphate, 10 mM NaF, 2 mM EDTA, 2 mM Na VO4, 1% NP-40, 10% glycerol, 2 μg·mL−1 aprotinin, 5 μg·mL−1 leupeptin, 1.5 μg·mL−1 bentamidine, 0.2 M AEBSF, 10 μg·mL−1 antipain, 0.5 μg·mL−1 pepstatin) and analyzed as previously described (12). Protein concentration in the tissue homogenates was determined by the Bio-Rad protein assay following the manufacturer's instructions (Bio-Rad Laboratories). A sample of the total muscle protein was immunoprecipitated with 4 μg of the IRS-1 polyclonal antibody (Upstate Biotechnology Inc., Lake Placid, NY), rocking overnight at 4°C. A 40-μL sample of slurry protein-A sepharose was added to the immunoprecipitate for 2 h, and an immunocomplex was obtained by brief centrifugation at 9000 rpm and washed three times in PBS-1% NP-40, twice in 500 mM LiCl/100 mM Tris pH 7.6, and once in 10 mM Tris/HCl pH 7.4, 100 mM NaCl, 1 mM EDTA. The pellet was centrifuged one more time and washed in PI 3-kinase adenosine assay buffer (20 mM Tris pH 7.4, 100 mM NaCl, 10 mM MgCl2, 0.5 mM EGTA, 120 μM adenosine). The final pellet was resuspended in 40 μL of PI 3-kinase adenosine assay buffer. A 50-μL sample of phosphatidylinositol and phosphatidylserine was dried down in a nitrogen stream and sonicated in 100 μL of 20 mM hepes/1 mM EDTA, pH 7.4. The lipid mixture was kept on ice, and 5 μL of this mixture (2 μg·mL−1 of phosphatidylinositol) was added to each sample. The solution was mixed by sonication and incubated for 10 min at 30°C on a heat block. A mixture consisting of 170 μCi of γ32P and 280 μM unlabeled ATP was prepared, and the reaction was started by adding 5 μL of this mixture to each sample. After 10 min at 30°C, the reaction was stopped by the addition of 200 μL of 1N HCl to each sample. The phosphatidylinositol 3-phosphate (PI3-phosphate) was extracted with 160 μL of chloroform:methanol (1:1). The phases were separated by centrifugation, and the lower organic phase was removed and separated by thin-layer chromatography. The radioactivity incorporated into PI3-phosphate was determined by PhosphoImaging (Molecular Dynamics Inc. Sunnyvale, CA).
Statistical Analyses
All statistical analyses were done with Statistica software, version 5.5 (StatSoft Inc, Tulsa, OK). A two-way analysis of variance (ANOVA) was used to evaluate the main effect of exercise response (before exercise and 0, 30, 60, 90, and 120 after exercise for circulating markers; before exercise and 0, 2, and 24 h after exercise for muscle markers), the main effect of supplement condition (chromium and placebo), and the interaction effect (exercise response × supplement condition). Significant main or interaction effects were further analyzed using a Fisher's LSD post hoc test. Independent t-tests were used to compare the total area under the curve, calculated using the trapezoidal method for the exercise-induced glucose, lactate, and insulin responses. Delta changes between supplement conditions were also compared using independent t-tests. Relationships among selected variables were examined using Pearson's product-moment correlation coefficient. The alpha level for significance was set at 0.05. All data are presented as means ± SE.
RESULTS
Compliance to the supplementation protocol was 100% according to log sheets and returned bottles. No adverse responses were reported to the supplementation protocol as determined by serum hematological responses, and dietary nutrient intake during the supplementation period did not change during the 4 wk and was not significantly different between Cr and Pl (data not shown). There were no significant differences between groups in physical characteristics and biochemical parameters except for a greater height (P = 0.02) and lean body mass (P = 0.03) in the Pl group (Table 1).
TABLE 1: Physical characteristics and fasting biochemical parameters before the exercise protocol.
The high-intensity exercise and nutritional supplementation protocol led to significant increases in lactate, glucose, and insulin (Table 2). Lactate peaked immediately after exercise and remained significantly above preexercise levels for the entire 120-min postexercise period. There was a significant interaction (group × time), as reflected by a 23% higher area under the lactate curve in the Cr group. There was a significant main time effect for exercise-induced glucose responses; values were significantly higher immediately after exercise, peaked at 60 min after exercise, and remained elevated above rest for the 120-min recovery period. The exercise-induced increase in insulin mirrored the glucose response; values at 60 min were more than 15-fold higher than at rest (> 1000 pM).
TABLE 2: Plasma lactate, glucose, and insulin at rest and during recovery from exercise.
There was a significant main time effect for muscle glycogen (Fig. 2). Compared with baseline, the combination of a low-carbohydrate diet and intense exercise resulted in a significant depletion of glycogen immediately after exercise, followed by a rapid increase at 2 h and a further increase at 24 h. Compared with resting preexercise values, glycogen was significantly higher at 24 h, indicating glycogen supercompensation. The rate of glycogen synthesis during the 2-h period after exercise was not different between groups (Cr: 25.8 ± 8.0 and Pl: 17.1 ± 4.7 mmol·kg−1·h−1, respectively) (Fig. 3). The rate of glycogen synthesis during the next 22 h was much lower and was not different between groups (Cr: 3.4 ± 0.8 and Pl: 3.0 ± 0.9 mmol·kg−1·h−1). The amount of glycogen synthesized during the initial 2 h represented 41 and 34% of the total amount synthesized during the 24-h recovery period for Cr and Pl, respectively (P > 0.05).
FIGURE 2: Muscle glycogen in response to high-intensity exercise and carbohydrate feedings. IP, immediately after exercise; 2 h, 2 h after exercise; 24 h, 24 h after exercise. Values are means ± SE. There was a significant main effect of time. Time points with a different letter are significantly different (P ≤ 0.05).
FIGURE 3: Rate of muscle glycogen synthesis during the initial 2 h of recovery from exercise and from 2 to 24 h after exercise. Values are means ± SE. IP, immediately after exercise.
A main time effect for glycogen synthase activity was observed. There was a significant increase immediately after exercise (20%) that returned to resting levels at 2 and 24 h of recovery (Fig. 4). IRS-1-associated PI 3-kinase activity was expressed as multiples of increase in activity above baseline for each person (Fig. 5). There was a main time effect for muscle PI 3-kinase. There was a significant decrease immediately after exercise, followed by a significant increase at 2 h after exercise. There was a trend for a group effect (P = 0.08), with lower PI 3-kinase observed in the Cr group. Effect sizes for PI 3-kinase at immediately postexercise, 2-, and 24-h recovery time points were 0.47, 0.69, and 0.87, respectively. Glycogen synthase activity at 2-h recovery was negatively related to the amount of glycogen synthesized during the 2-h period after exercise (r = −0.73) and positively related to the amount of glycogen synthesized from 2 to 24 h (r = 0.56).
FIGURE 4: Muscle glycogen synthase activity in response to high-intensity exercise and carbohydrate feedings. IP, immediately after exercise; 2 h, 2 h after exercise; 24 h, 24 h after exercise. Values are means ± SE. There was a significant main effect of time. Time points with a different letter are significantly different from each other (P ≤ 0.05).
FIGURE 5: IRS-1-associated PI 3-kinase activity in skeletal muscle in response to high-intensity exercise and carbohydrate feedings. The top image is a representative immunoblot from a subject supplemented with chromium. IP, immediately after exercise; 2 h, 2 h after exercise; 24 h, 24 h after exercise. Values are means ± SE. There was a significant main effect of time. Time points with a different letter are significantly different (P ≤ 0.05). There was a trend for a significant group effect (P = 0.08).
DISCUSSION
Prior work has shown that chromium can alter insulin signaling and potentiate insulin action in culture (14,20,29) and in skeletal muscle of obese, insulin-resistant rats (26). The present study is the first to investigate the effects of chromium picolinate supplementation on insulin signaling in human skeletal muscle in vivo. Specifically, this study examined whether chromium supplementation had any effect on muscle glycogen synthesis, glycogen synthase activity, and PI 3-kinase activity at rest and during recovery from high-intensity exercise with carbohydrate feedings. The results show that chromium supplementation did not affect glucose and insulin levels, glycogen synthase, or the rate of glycogen synthesis above that achieved with a high-glycemic carbohydrate feeding protocol. Despite similar glycogen synthase activity and glycogen synthesis, there was a trend for a lower activation of PI 3-kinase in the chromium group. This was contrary to our a priori hypothesis that chromium supplementation would augment PI 3-kinase activity, and although speculative, this suggests that chromium may engage an alternative signaling pathway that is independent of PI 3-kinase to stimulate glycogen synthase and glycogen synthesis during recovery from a supramaximal exercise bout in humans.
The molecular mode of action of chromium in human skeletal muscle remains speculative. A unique chromium-binding oligopeptide (low-molecular weight chromium-binding substance or chromodulin) that binds four chromic ions in response to insulin-mediated chromic ion flux has been shown to bind to and activate insulin receptor tyrosine kinase activity (24). Chromium-mediated autoamplification of insulin signaling has been confirmed in studies showing that chromium augments insulin receptor kinase activity, IRS-1 phosphorylation, membrane-associated Glut-4 content, and glucose uptake into cells (5,14,28). The normal pathway of insulin-mediated glucose uptake involves autophosphorylation of the insulin receptor, phosphorylation of IRS-1, with docking of PI 3-kinase and subsequent downstream propagation of insulin signaling through Akt. Akt is an important downstream kinase involved in activation of Glut-4 (and subsequent glucose transport into the cell) and also glycogen synthase. Glycogen synthase is regulated by its inhibitory enzyme glycogen synthase kinase 3. Phosphorylation of Akt leads to inactivation of glycogen synthase kinase 3, activating glycogen synthase and glycogen formation.
There was a trend for chromium supplementation to be associated with lower activation of PI 3-kinase that was strongest at 24 h after exercise, yet this did not lead to a greater increase in glycogen synthesis. The protocol was unique because in terms of signal transduction, it involved an insulin-stimulated (due to feeding) and a contraction-mediated (due to high-intensity exercise) element, the latter being independent of PI 3-kinase (27). Prior studies have shown that increased glucose transport after exercise is associated with lower PI 3-kinase activation (16). The higher carbohydrate-induced insulin response during recovery in this study probably offset this normal inhibition of PI 3-kinase, as previously shown by O'Gorman et al. (16).
Skeletal muscle glucose uptake and glycogen synthase activity are the two major regulators of glycogen synthesis and increase both after high-intensity exercise and insulin stimulation to promote glycogen repletion (15), as was observed in this study. Although unclear, the signals that regulate glucose transport and glycogen synthase by exercise are different from those used by insulin and do not involve the insulin receptor IRS-1 or PI 3-kinase (27). Both insulin and acute exercise seem to activate Akt and glycogen synthase kinase 3 (15). Chromium has been shown to enhance insulin-stimulated glucose uptake via a mechanism that involves insulin-stimulated phosphorylation of Akt but not tyrosine phosphorylation of the insulin receptor (28).
To study the in vivo effects of chromium in skeletal muscle, we designed a model that would result in a rapid and large amount of glycogen synthesis in a short time period. The calculated rate of glycogen synthesis during the 2-h period after exercise was more than 20 mmol·kg−1, representing one of the highest values recorded in the literature. Values in the range of 1.5-2.0 mmol·kg−1·h−1 are typical during recovery from prolonged endurance exercise, even with optimal provision of carbohydrate (17). Because the rapid phase of glycogen synthesis lasts from 30 to 60 min after exercise (11), the fact that our rates were calculated for a 2-h period may actually underestimate the peak rate of muscle glycogen synthesis. The effectiveness of the feeding protocol is noteworthy, with peak glucose levels reaching nearly 8 mM and insulin levels > 1000 pM during the 2-h recovery from exercise; however, chromium did not alter this response. It is possible that the high rate of muscle glycogen synthesis achieved with the exercise and nutrition protocol compromised or masked the ability of Cr to exert an effect on muscle signaling and insulin action; this should be acknowledged as a limitation of the current study.
Our cycling protocol was modeled after that used by Fairchild et al. (8), who showed that a similar exercise bout combined with a high-carbohydrate diet led to supercompensation of muscle glycogen levels in 24 h. Glycogen synthesis rates after short-term high-intensity exercise are much higher than after prolonged endurance exercise (17). It has been suggested that the rapid rates of glycogen resynthesis after high-intensity exercise, even without carbohydrate ingestion, are attributable largely to glycogen synthesis from lactate, with little contribution from glucose uptake (10,17). Interestingly, there was a significantly greater increase in circulating lactate in the chromium group. The greater plasma lactate in recovery in the chromium group could indicate greater lactate efflux from muscle, resulting from less lactate conversion to glycogen. Because glycogen synthesis was similar between groups, this could indicate greater contribution of glucose uptake to glycogen formation with chromium supplementation. Alternatively, the increased lactate in the chromium group could indicate a redistribution of intracellular glucose toward glycolysis rather than glycogen formation. Again, because glycogen synthesis was the same between groups, this would have to be achieved by a greater glucose uptake. However, glucose levels were not lower with chromium, which casts doubt on these speculations.
As expected, compared with the initial 2 h after exercise, the rate of glycogen synthesis was much lower during the subsequent 22-h period (slow phase of glycogen synthesis). Glycogen levels after 24 h of recovery were significantly higher than baseline (15%), indicating supercompensation (2). Classical methods involve 3 d of high-carbohydrate intake to supercompensate muscle glycogen (3,19), but when appropriate exercise-selection and nutritional protocols are used, 1 d is sufficient (8); our data confirm such a phenomenon.
In summary, the rapid phase of glycogen synthesis during 2 h of recovery from high-intensity exercise was not significantly enhanced by chromium supplementation. There was a trend for less activation of PI 3-kinase during recovery. Circulating lactate responses to exercise were significantly higher with chromium supplementation. The findings are specific to the time period after brief high-intensity exercise and under conditions of a large quantity of rapidly absorbed carbohydrates and high glucose availability. Chromium may have different effects after aerobic exercise and/or when carbohydrates are less abundant.
This work was sponsored by Nutrition 21, Inc.
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