Chromium (Cr) is a trace element that functions in carbohydrate, fat, and protein metabolism by serving as a cofactor that potentiates the action of insulin (18), although the precise mechanism of Cr action remains unknown (7,19). Insufficient dietary intakes of Cr have been documented in humans eating their habitual diet (3) and linked to various metabolic abnormalities associated with non-insulin-dependent diabetes and cardiovascular disease(9,20). Furthermore, Cr supplementation has been shown to be of benefit for Cr deficient humans and experimental animals resulting in increased glycogen synthesis (24), improved glucose tolerance (7), and improved lipid-lipoprotein profiles (2,6,23,24).
Studies involving the effects of acute endurance exercise on Cr status have shown that 2 h post exercise, urinary Cr concentration is increased≈five-fold and that 24-h urinary excretion of Cr is ≈two-fold higher on an exercise versus a nonexercise day (4,5). This suggests that exercise may deplete Cr stores; however, subjects who exercise regularly have lower basal Cr losses and may compensate for the increased losses associated with acute exercise (4). Even so, there have not been any longterm studies documenting that individuals who perform strenuous exercise training show signs of Cr deficiency.
Recently, Cr picolinate supplements have been vigorously promoted for individuals undergoing strength training and for body builders as a safe alternative to anabolic steroids and growth hormone as a means to increase muscle mass and reduce body fat(10,15,16). Theoretically, Cr supplementation could enhance the uptake of amino acids into muscle cells by potentiating the action of insulin (12).
Two preliminary experiments in a study by Evans (11) suggested that increases in lean body mass and decreases in percent body fat occurred with Cr supplementation over the span of 6 wk in young males participating in a resistive exercise training program. In a more recent study by Hasten et al. (14), female but not male college students demonstrated an increase in body weight when supplemented with Cr compared with unsupplemented controls as a result of 12 wk of resistive exercise training. These initial findings should be interpreted with caution since percent body fat and lean body mass were estimated via anthropometry. In an attempt to improve on the design of previous studies, our purpose in the present investigation was to assess the effects of Cr picolinate supplementation (200 μg · d-1) and a progressive resistive exercise training program on muscle strength, body composition, and Cr excretion in young, untrained male subjects.
METHODS
Subjects
Sixteen untrained healthy male subjects between the ages of 18-35 yr (mean age 24 ± 4 yr) participated in the study, which was approved by the Institutional Review Board at the University of Maryland, after providing written informed consent. The subjects had a mean body weight of 82 ± 3 kg, body fat of 20 ± 8%, were untrained at entry into the study; and while two subjects had performed resistive training previously, they had stopped training at least 6 months before the investigation. All subjects were asked to maintain their habitual level of physical activity in conjunction with the resistive training program as outlined below. The subjects were free of medical conditions which would preclude safe participation in regular, progressive resistive exercise training. Determination of health status was made from a medical history questionnaire that was completed by each subject before entry into the study.
The 16 subjects were pair-matched on initial strength levels recorded from the 1 repetition maximum strength tests (1 RM) and then randomly assigned to the Cr or placebo groups. Matching of the subjects was carried out in order to insure that strength levels in the two groups were approximately the same before the subjects began the 12 wk of progressive resistive exercise training.
Cr Supplementation
The subjects were randomly assigned, in a doubleblind fashion, to receive either the Cr supplement or a placebo containing lactose in capsule form. The Cr supplement (Nutrition 21, San Diego, CA) contained 200 μg of Cr3+ as chromium picolinate. Chromium content by analysis was 188 ± 15 μg(mean ± SE) for the Cr capsules and 0.9 ± 0.03 μg for the placebo. Specifically, the contents of the capsules were weighed in 16 × 100 mm, acid-washed, glass tubes. One ml of 6 M HCL was added and the tubes were placed in a heating bath at 85°C. Two ml of concentrated nitric acid and 2 ml of distilled water were added to the samples and heated to 80°C for 1 h to ensure complete solubilization of the material. Samples were cooled and diluted in 0.1 N HCL and run in a graphite furnace as described previously(3).
The supplement or placebo capsules were ingested daily without food beginning on the first day of resistive training, 1 wk after the baseline 24-h urine collection. On exercise days, the subjects ingested the capsules approximately 3 h before their resistive exercise training session. To enhance compliance, the subjects completed a daily checklist after ingestion of the capsules. The subjects were given a second supply of the supplements after 6 wk of training and were instructed to return the unused capsules at the end of the study.
Diet Records
Diet records were completed by the subjects on three consecutive weekdays and reviewed with the investigators before training in order to roughly estimate the Cr content of the diet and to ensure adequate protein intake. The initial diet records were returned to the subjects so that they could replicate the diet as closely as possible for 3 d before the 6- and 12-wk urine collections. Repeat 3-d diet records were completed at 6 and 12 wk of the study and analyzed. Previously, Anderson and Kozlovsky(3) have shown using the duplicate diet technique that clinical and 7-d self-selected diets contain ≈15 μg of Cr · 1000 kcal-1. This figure along with the energy intake averaged from 3-d food records was used to roughly estimate the Cr content of the diet since it is not possible to estimate Cr intake based upon urinary Cr excretion. The subjects were instructed to maintain habitual dietary patterns throughout the training period. Diet records were analyzed via the Nutritionist III database(N2 computing, Silverton, OR, 1991) for total kJ, and the% of total kJ from protein, fat, and carbohydrate.
Strength Testing
The strength testing was conducted 1 wk before the start of the 12-wk resistive training program on Keiser K-300 pneumatic variable-resistance machines after the subjects had attended at least three training sessions to become familiar with the equipment. The one-repetition maximum test (1-RM) was defined as the maximal resistance that could be moved a single time through the full range of motion. The 1-RM test was administered after sufficient warm-up to determine baseline upper and lower body strength using the following exercises: leg press, leg extension, chest press, lat pulldown, seated rows, and overhead press. The initial resistance was set at a level estimated to be slightly above the subject's 1 RM and subsequently reduced until a resistance was achieved that the subject could only lift a single time. Each subject was allowed up to five trials with adequate rest periods between trials to achieve a peak 1-RM; this protocol was the same before and after training. A second strength test was performed within 1 wk of the last training session to assess increases in muscle strength.
Resistive Training Protocol
The subjects trained 3 times per week on nonconsecutive days for 12 wk using the Keiser pneumatic exercise equipment in the Wellness Research Laboratory in the College of Health and Human Performance at the University of Maryland. All training sessions were directly supervised by an exercise physiologist to monitor compliance and provide motivation. Compliance with the training program was excellent (100%) as a minimum of 36 exercise sessions were completed before the post-training strength measurements. The training program consisted of a 5-min warm-up on an air-dyne cycle ergometer, followed by a carefully supervised nonballistic stretching routine. After the stretching routine, the subjects were instructed in proper lifting technique and seat adjustment for each Keiser machine and free weight exercise. The resistive training program consisted of two sets of 8-10 repetitions using the following nine exercises: leg press, leg extension, leg curl, chest press, latissimus pulldown, overhead press, seated rows, tricep extensions(dumbbells), and bicep curls (dumbbells). The rest interval between sets was limited to 60 s. The subjects were encouraged to “lift until failure,” i.e. the subject was unable to perform one more repetition during a set. This was accomplished on the Keiser machines by varying the resistance during each set with the foot pedal/hand button controls so that each repetition was performed with maximal resistance. Ninety percent of the 1 RM determined during the strength test was used as the subject's starting resistance for the training program. Training logs were reviewed weekly and the subjects were instructed to progressively increase the resistance in 5-lb. increments when more than 10 repetitions could be completed.
Body Composition
Body composition was assessed before and after 12 wk of resistive training using hydrodensitometry. The subjects were weighed in the morning after an overnight fast, or in the afternoon after having fasted for a minimum of 4 h with pre- and post-training determinations conducted at the same time of the day for each subject. Body density was calculated as the average of the three highest underwater weights after several practice trials. Density was corrected for residual lung volume (RV) as determined via two to three trials on land using the closed-circuit oxygen-dilution technique(27) as measured on an Airspec 2000 mass spectrometer. Percent body fat was estimated from body density values using the formula of Brozek et al. (8). Skinfold and circumference measurements were assessed before and after 12 wk of resistive training to determine regional changes in subcutaneous fat and limb girth, respectively. The following anatomical sites were measured in duplicate: chest, scapula, triceps, midaxillary, suprailiac, and abdominal. Circumference measurements were performed in duplicate using a cloth tape at the chest, biceps, abdomen, hips, and thigh (17). All skinfold and circumference measurements were performed by the same investigator before and after training.
Urinary Cr Analysis
A 24-h urine sample was collected for each subject pretraining (baseline) and at 6 and 12 wk. Prior to training the samples were collected 1 wk before the supplements/placebos were given on a day with no exercise, while at the 6- and 12-wk time point urine samples were collected on an exercise day. The subjects were provided with preweighed, disposable 3.5-1; Cr-free containers(Fisher/Scientific Products, McGaw Park, IL) and were instructed to collect a 24-h urine sample after the first void of the following morning. The urine samples were refrigerated and later analyzed for Cr content by atomic absorption spectrometry in the Vitamin and Mineral Nutrition Laboratory at the Beltsville Human Nutrition Research Center. Urinary Cr determinations were performed using a microcomputer-controlled atomic absorption spectrometer(Perkin-Elmer, Model HGA 500) according to the procedure of Veillon and coworkers (26). An in-house control urine sample, whose Cr concentration had been verified by two independent methods, was assayed at least 2 times daily as an internal check on the accuracy of the results. A value of 0.20 ± 0.20 μg · 1-1 was obtained by graphite furnace atomic absorption and 0.21 ± 0.04 by gas chromatography-mass spectroscopy (GC-MS).
Data Analysis
Statistical analyses were performed using repeated-measures analysis of variance to determine the effects of time and Cr supplementation on the dependent variables (28). The level of significance was set at P < 0.05 for all statistical analyses, and values were reported as means ± SE. Post-hoc analyses were conducted using selected treatment comparisons with the Bonferroni method only if a significant main effect was identified.
RESULTS
There were no significant group differences in age, LBM, percent fat, or strength at the beginning of the study. Dietary analysis of the 3-d food records indicated that total energy intake and the percentage of total calories derived from carbohydrate, fat, and protein were not significantly different between groups and were unaltered from pre- to post-training(Table 1). The chromium content of the diet before training was estimated from energy intake (3) and found to be ≈36 ± 4 μg · d-1 in both groups.
As shown in Table 2, there were no significant differences in muscle strength between the groups before training due to the fact that the subjects were pair-matched. In response to 12 wk of resistive training there were significant increases in upper, lower, and total body muscle strength in both groups (range 23-35%) although the magnitude of the strength gains were not significantly different between the Cr and placebo groups (376 vs 494 kg, respectively).
Table 3 shows the effect of resistive exercise training and Cr supplementation on body composition variables. There were no statistically significant changes in body weight, waist/hip ratio, percent body fat, or lean body mass in either the Cr or placebo group in response to 12 wk of progressive resistive exercise training. Resistive exercise training did not alter skinfold or circumference measurements in the Cr or placebo groups.
The Cr and placebo groups showed similar rates of Cr excretion at baseline(0.15 ± 0.08 vs 0.21 ± 0.07 μg · d-1;Figure 1). Twenty-four hour Cr excretion in the Cr group increased ≈nine-fold over baseline to 1.52 ± 1.26 μg · d-1 (P < 0.01) after 6 wk of supplementation. At 12 wk, the 24-h excretion of Cr was unchanged from that at 6 wk. There were no significant changes in Cr excretion in the placebo group at 6 or 12 wk of resistive training.
DISCUSSION
To our knowledge, this is the first study to investigate the effects of Cr supplementation and resistive exercise training on muscle strength, body composition and urinary Cr excretion while estimating the Cr content of the diet with 3-d food records. The analysis of dietary records taken before, at 6 and 12 wk of the resistive training protocol also enabled us to monitor the maintenance of the subject's habitual dietary patterns and ensure adequate protein intake.
Chromium supplementation, as a potentiator of insulin action, would be expected to increase muscle mass by increasing amino acid uptake into cells for incorporation into muscle contractile protein and thereby elicit muscle hypertrophy (21). The hypothesized increase in muscle mass would subsequently influence body composition as reflected by a decrease in percent body fat as well as result in increased muscle strength. However, the results of the present study in young, relativerly lean males show no significant changes in body composition or muscle strength with Cr supplementation, and are not in accord with the results of previous studies(11,14). The Cr supplemented group in the present study evidenced a non-significant 1.0-kg increase in LBM and a 1% decrease in body fat. In contrast, Evans (11) reported a significant increase in LBM of 1.6 kg in 10 college males after 6 wk of resistive exercise training in conjunction with 200 μg · d-1 of Cr picolinate supplementation. In a second experiment in the same study, 16 trained football players showed a large increase in LBM (2.6 kg) and a significant decrease in percent body fat (3.6%) 6 wk of resistive training and Cr supplementation. The methodological limitations of these experiments (11) may have accounted for the positive results since anthropometric measurements rather than hydrodensitometry were used to estimate changes in body composition. In addition, LBM and percent body fat data were estimated from nonvalidated skinfold prediction equations. Furthermore, it would be unlikely that a 6-wk period of resistive exercise training could elicit such dramatic increases in LBM in subjects who were well-trained to begin with. The present study applied a progressive, heavy resistive training stimulus of sufficient resistance and number of repetitions to elicit significant increases in muscle strength but the magnitude of the strength gains were the same in the Cr and placebo groups (≈25-35%). In this context, it has been shown that in young, sedentary males who perform 10-20 wk of progressive resistive exercise training, the magnitude of strength gains is typically between 30-50%(13); similar to the average strength gains in the present study.
The results of the current investigation agreed in part with the results of a recent study by Hasten and coworkers (14) that studied the effects of Cr supplementation and resistive exercise training on muscle strength and body weight (body composition was not measured) in both males and females. Male subjects in the Cr group showed no significant change in body weight after 12 wk of resistive training but female subjects who took Cr had a significant increase in body weight compared with controls (2.5 ± 2.0 vs 0.6 kg ± 1.8 kg). In contrast to the present study, Hasten and coworkers (14) reported a significant increase in the sum of three circumferences for both the males and females in the Cr supplementation groups. In the present study, the absence of significant regional changes in limb girth as estimated from the sum of five circumferences in both the Cr supplementation and placebo groups is consistent with the lack of significant changes in body weight, LBM, and percent body fat in either group. As in the present study, Hasten and coworkers(14) reported no significant differences between the Cr and placebo groups in terms of strength gains as measured by a 1 RM strength test.
The 3-d food diaries that the subjects recorded in the present study provided an estimate of the total energy intake plus a rough estimate of the dietary Cr intake (3). Total energy intakes of≈9,000-10,000 kJ per day may be somewhat of an underestimate for young exercising subjects but this is probably a function of the fact that subjects routinely underreport the quantity of food consumed in the diet when they are asked to keep food records. At entry into the study, both groups were consuming an estimated 36 μg of Cr per day in their habitual diet, below the suggested safe and adequate intake for Cr of 50-200 μg · d-1(22), although as with the energy intake, this may be an underestimate. It would have been ideal to more directly estimate the dietary Cr content via a duplicate diet technique, but this was not feasible due to time and monetary constraints. This is a limitation of the present study.
The baseline Cr excretion values of the Cr supplementation and placebo groups (0.15 ± 0.08 and 0.21 ± 0.07 μg · d-1, respectively) were normal when compared to previous studies that measured daily urinary Cr excretion rates in healthy subjects (1). The significant increase in Cr excretion over baseline in the Cr group due to supplementation at 6 wk of resistive exercise training is consistent with previous results showing an increase in urinary Cr losses at daily Cr intakes greater than 40 μg · d-1(6). Previous studies (4,5) have investigated the chronic effects of aerobic exercise on Cr excretion and reported significantly lower baseline Cr excretion rates in trained versus untrained runners (0.09 vs 0.21 μg· d-1). It was suggested that this decrease may indicate a training adaptation whereby athletes have increased their ability to conserve Cr, possibly through increased tissue storage (25). In contrast, the results of the present study did not show an adaptive effect on Cr excretion secondary to resistive exercise training, as there were no changes in Cr excretion in the placebo group, and no significant change in Cr excretion from 6 to 12 wk of training in the Cr group. However, the present study was not designed to separate out the acute effects of a single exercise bout from the effects of resistive training on Cr excretion or to determine whether Cr excretion was affected on nonexercise days.
In conclusion, 12 wk of progressive resistive exercise training resulted in equivalent improvements in upper and lower body muscle strength in both the Cr supplementation and placebo groups. Cr picolinate supplementation of 200 μg· d-1 in conjunction with a resistive training program did not result in significant alterations in body composition. Cr picolinate supplementation resulted in a significant increase in Cr excretion, which was not altered by chronic resistive exercise training. Additional research is warranted to confirm the present findings and to examine the ergogenic benefits of the amount and form of supplemental Cr for previously untrained individuals who are engaged in resistive exercise training programs.
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