Sprint interval exercise elicits near maximal peak VO2 during repeated bouts with a rapid recovery within 2 minutes.

AIM: We investigated the cardiorespiratory response during acute sprint interval exercise (SIE; 4 x 30 sec maximal efforts, each separated by 4 min recovery) vs. continuous endurance exercise (CEE; 30 min) at 70% VO2max. METHODS: Oxygen consumption (VO2) and heart rate were measured in 8 males (age: 23±2.3 y, height: 181±6.4 cm, body mass: 78±8.6 kg, VO2max: 52±3.1 ml·kg-1·min-1, mean±SD). Pre-exercise diet was controlled. RESULTS AND CONCLUSION: Total VO2 was greater with CEE vs. SIE (87.6±13.1 vs. 35.1±4.4 L O2) with small differences (P=0.06) in average heart rates (CEE: 157±10 bpm vs. SIE: 149±6 bpm) and peak heart rates (CEE: 166±10 vs. SIE: 173±6; P=0.14). VO2 increased during the sprint bouts (53-72% of VO2max) and attained near maximal values (84-96%) in the immediate recovery period (within 20 sec). Thereafter a rapid decrease occurred so that at 2 min of recovery VO2 was ~1.5 L/min (~38% VO2max). During the remaining 2 min of recovery VO2 declined more slowly to ~1.3 L/min or ~33% of VO2max. Similar heart rate responses with CEE and SIE and a greater VO2 during SIE suggest increased muscle oxygen extraction with SIE, which might explain the greater peripheral adaptations, observed previously with sprint vs. continuous training. The potential value of shorter recovery durations to SIE needs to be examined.

Two minutes of sprint-interval exercise elicits 24-hr oxygen consumption similar to that of 30 min of continuous endurance exercise.

Six weeks (3 times/wk) of sprint-interval training (SIT) or continuous endurance training (CET) promote body-fat losses despite a substantially lower training volume with SIT. In an attempt to explain these findings, the authors quantified VO2 during and after (24 h) sprint-interval exercise (SIE; 2 min exercise) vs. continuous endurance exercise (CEE; 30 min exercise). VO2 was measured in male students (n = 8) 8 times over 24 hr under 3 treatments (SIE, CEE, and control [CTRL, no exercise]). Diet was controlled. VO2 was 150% greater (p < .01) during CEE vs. SIE (87.6 ± 13.1 vs. 35.1 ± 4.4 L O2; M ± SD). The observed small difference between average exercise heart rates with CEE (157 ± 10 beats/min) and SIE (149 ± 6 beats/min) approached significance (p = .06), as did the difference in peak heart rates during CEE (166 ± 10 beats/min) and SIE (173 ± 6 beats/min; p = .14). Total O2 consumed over 8 hr with CEE (263.3 ± 30.2 L) was greater (p < .01) than both SIE (224.2 ± 15.3 L; p < .001) and CTRL (163.5 ± 16.1 L; p < .001). Total O2 with SIE was also increased over CTRL (p < .001). At 24 hr, both exercise treatments were increased (p < .001) vs. CTRL (CEE = 500.2 ± 49.2; SIE = 498.0 ± 29.4; CTRL = 400.2 ± 44.6), but there was no difference between CEE and SIE (p = .99). Despite large differences in exercise VO2, the protracted effects of SIE result in a similar total VO2 over 24 hr vs. CEE, indicating that the significant body-fat losses observed previously with SIT are partially due to increases in metabolism postexercise.

Impaired superficial femoral artery vasodilation and leg blood flow in young obese women following an oral glucose tolerance test.

This study was designed to test the hypothesis that glucose ingestion following an overnight fast increases leg vascular conductance (LVCd) and superficial femoral artery (SFA) vasodilation in lean but not obese young women. Obese (23.5 ± 4.0 years, 84.7 ± 14.7 kg, 37.2% ± 6.4% fat; mean ± SD, n = 8) and lean (23.8 ± 2.4 years, 60.6 ± 4.0 kg, 22.3% ± 2.8% fat; n = 8) women arrived in the laboratory at 0830 h after a 12-h overnight fast for body composition (densitometry) assessment. Then, capillary blood glucose (BGlu), plasma insulin, heart rate, cardiac output, mean arterial pressure, leg blood flow (Doppler ultrasound), and LVCd were measured (after 15 min in the supine position), and at 30-min intervals for 2 h following glucose ingestion (75 g glucose load, 12.5% solution). Fasting BGlu concentration was not different between groups (obese = 5.1 ± 0.47 vs. lean = 4.9 ± 0.37 mmol·L(-1), p = 0.71) but 60, 90, and 120 min post ingestion BGlu was elevated (p ≤ 0.03) in the obese women. Insulin differences were not significant. Fasting LVCd was not different between groups (lean = 0.72 ± 0.49 vs. obese = 0.70 ± 0.19 mL·min(-1)·mm Hg(-1); p = 0.48); however, LVCd, as well as Δ in SFA diameter were significantly elevated (p ≤ 0.04) in the lean compared with the obese group at 60, 90, and 120 min postglucose ingestion (LVCd, peak lean = 1.4 ± 0.5 vs. peak obese = 0.8 ± 0.1 mL·min(-1)·mm Hg(-1); Δ in SFA, peak lean = 0.51 ± 0.30 vs. peak obese = 0.09 ± 0.45 mm). The reduced LVCd following glucose ingestion could contribute to impaired glucose tolerance. Further, the lack of SFA dilation may be evidence of impaired vascular insulin responsiveness in these obese young women.

Beyond the zone: protein needs of active individuals.

There has been debate among athletes and nutritionists regarding dietary protein needs for centuries. Although contrary to traditional belief, recent scientific information collected on physically active individuals tends to indicate that regular exercise increases daily protein requirements; however, the precise details remain to be worked out. Based on laboratory measures, daily protein requirements are increased by perhaps as much as 100% vs. recommendations for sedentary individuals (1.6-1.8 vs. 0.8 g/kg). Yet even these intakes are much less than those reported by most athletes. This may mean that actual requirements are below what is needed to optimize athletic performance, and so the debate continues. Numerous interacting factors including energy intake, carbohydrate availability, exercise intensity, duration and type, dietary protein quality, training history, gender, age, timing of nutrient intake and the like make this topic extremely complex. Many questions remain to be resolved. At the present time, substantial data indicate that the current recommended protein intake should be adjusted upward for those who are physically active, especially in populations whose needs are elevated for other reasons, e.g., growing individuals, dieters, vegetarians, individuals with muscle disease-induced weakness and the elderly. For these latter groups, specific supplementation may be appropriate, but for most North Americans who consume a varied diet, including complete protein foods (meat, eggs, fish and dairy products), and sufficient energy the increased protein needs induced by a regular exercise program can be met in one's diet.

Effects of exercise on dietary protein requirements.

This paper reviews the factors (exercise intensity, carbohydrate availability, exercise type, energy balance, gender, exercise training, age, and timing of nutrient intake or subsequent exercise sessions) thought to influence protein need. Although there remains some debate, recent evidence suggests that dietary protein need increases with rigorous physical exercise. Those involved in strength training might need to consume as much as 1.6 to 1.7 g protein x kg(-1) x day(-1) (approximately twice the current RDA) while those undergoing endurance training might need about 1.2 to 1.6 g x kg(-1) x day(-1) (approximately 1.5 times the current RDA). Future longitudinal studies are needed to confirm these recommendations and asses whether these protein intakes can enhance exercise performance. Despite the frequently expressed concern about adverse effects of high protein intake, there is no evidence that protein intakes in the range suggested will have adverse effects in healthy individuals.

Moderate physical activity can increase dietary protein needs.

Six healthy men completed three 1-hr bouts of treadmill walk-jogging at low (L; 42 +/- 3.9% VO2max), moderate (M; 55 +/- 5.6%), and high (H; 67 +/- 4.5%) exercise intensity in order to determine whether moderate physical activity affects dietary protein needs. Both sweat rate and sweat urea N loss were greater (p < .10) with increasing exercise intensity. Seventy-two hour postexercise urine urea N excretion was elevated (p < .05) over nonexercise control (26.6 +/- 2.96 g) with both M (31.0 +/- 3.65) and H (33.6 +/- 4.39), but not L (26.3 +/- 1.86), intensities. Total 72-hr postexercise urea N excretion (urine + sweat) for the M and H exercise was greater than control by 4.6 and 7.2 g, respectively. This suggests that 1 hr of moderate exercise increases protein oxidation by about 29-45 g, representing approximately 16-25% of the current North American recommendations for daily protein intake. These data indicate that the type of exercise typically recommended for health/wellness can increase daily protein needs relative either to sedentary individuals or to those who exercise at lower intensities.

Daily exercise reduces fat, protein and body mass in male but not female rats.

This study was designed to compare the estimated energy balance, linear growth (body and bone lengths) and body composition (all components including body mass, total body water, fat, protein and ash) response to daily spontaneous running (DSR) in young male and female rats. We tested the hypothesis that due to gender differences in energy efficiency, DSR would reduce linear growth and body composition more in male rats. Fourteen male and sixteen female weanling Sprague-Dawley rats were randomly assigned to either a sedentary (SED) control (male 7, female 8) or DSR (male 7, female 8) group. The DSR rats were allowed to run spontaneously in running wheels while SED rats remained in standard rat cages for 9 weeks. Body mass, running distance and food intake were measured over the nine week period. Subsequently, chemical analysis was performed to measure carcass content of water, protein, fat and ash. Linear growth was assessed by measures of body and bone lengths. The estimated energy balance of the DSR rats was computed and compared between genders. Estimated energy balance was significantly more negative in females than males due to significantly greater DSR distance. Body and bone lengths were similar among the SED and DSR female and SED and DSR male rats. However, whole body mass, fat mass and protein mass were significantly lower only in DSR males. These results demonstrate that DSR reduced body mass, body fat and protein mass in male rats but not in female rats despite a more negative estimated energy balance in female rats. These findings suggest that females are better protected from an energy deficit due to DSR. Possible mechanisms include gender-specific hormonal responses.

Preexercise sugar feeding does not alter prolonged exercise muscle glycogen or protein catabolism.

The purpose of this study was to determine the effects of type of preexercise sugar feedings (glucose [GLU] or fructose [FRU]) on muscle glycogen and protein catabolism during prolonged exercise in fed men. Seven men cycled to exhaustion on three different occasions at 70% VO2max, 45 min after ingestion (700 ml) of either a 0.476 mol.L.1 carbohydrate (CHO) solution or a sweetened placebo (PLA). With GLU, serum insulin was significantly increased prior to exercise. As a result, serum glucose was significantly lower at 15 and 30 min of exercise with GLU, but was similar to the other treatments thereafter. Time to fatigue was absolutely longer with the GLU feeding, and exercise muscle glycogen catabolism was absolutely lower during the FRU trial, but the observed differences did not attain statistical significance due to intersubject variability. Protein catabolism was similar for all treatments. These data indicate that a 60-g preexercise glucose or fructose feeding following CHO loading procedures has minimal effects on muscle glycogen or protein catabolism during prolonged exercise.

Effects of exogenous growth hormone on skeletal muscle of young female rats.

We examined the effects of exogenous growth hormone (GH) treatment on the soleus and rectus femoris muscles of young female rats. Rat GH (1.8 IU/mg) was administered for 3 weeks by subcutaneous injection, twice a day, at doses of 0.5, 0.6, and 0.8 mg/day during the 1st, 2nd and 3rd week, respectively. Final body weight, as well as wet and dry weight, of the soleus and rectus femoris muscles were significantly greater in the GH-treated group, compared to controls. Muscle weight to body weight ratios did not differ between the two groups. The fiber type composition of the soleus muscle was determined by histochemical staining for myosin ATPase activity. No statistically significant difference was found between the GH-treated and the control groups in the percentages of fiber types. However, GH treatment significantly increased the cross-sectional area of type II fibers of the soleus muscle. These results suggest that, in young female rats, acceleration of body weight gain by homologous GH administration is accompanied by a proportional hypertrophy of skeletal muscle mass. Increased muscle mass is due to hypertrophy of muscle fibers. Type II muscle fibers appear to be more sensitive to GH stimulation.

Diabetes reduces growth and body composition more in male than in female rats.

Food restriction and/or starvation has a consistently greater and more permanent effect on physical growth in males than in females. Because diabetes may be viewed as being analogous to starvation, we tested the hypothesis that diabetes would reduce growth more in male than in female rats. Diabetes was induced with streptozotocin (65-125 mg/kg IP) at 3 weeks of age in 7 female and 10 male Lewis rats. Body weight (BW) and blood glucose (bGlc) were measured over the following 8 weeks. Subsequently, animals were assessed for body (ano-nasal; ANL) and bone length (tibia; TBL) and chemically analyzed for body composition. Results were compared to age-matched controls (male = 11; female = 9). A 2-way factorial analysis of covariance (ANCOVA), with body weight as the covariate, was used to test for statistical significance for the effects of gender and diabetes on body composition (fat and protein mass) and linear growth because control males and females had significantly different body weights. There were no significant differences in bGlc between genders. However, males had a greater decrease from controls in BW (-45% vs. -13%), protein (-48% vs. -11%), fat (-89% vs. -65%), TBL (-13% vs. 0%), and ANL (-17% vs. -5%) compared to females. In addition, males had a greater absolute decrease from controls in protein (-40 g vs. -5 g) and fat (-39 g vs. -23 g) mass. These results suggest that male rats are more susceptible than females to the deleterious effects of diabetes on linear growth and body composition.

Is increased dietary protein necessary or beneficial for individuals with a physically active lifestyle?

For most of the 20th century, scientists have believed that protein needs are not altered by physical exercise. In contrast, athletes are typically convinced that additional dietary protein can significantly enhance exercise performance. Until recently, the opinion of the athletes has been largely unsubstantiated in the scientific literature. However, since the 1970s, an increasing number of studies have appeared that indicate dietary protein needs are elevated in individuals who are regularly physically active. Together, these data suggest that the RDA for those who engage in regular endurance exercise should be about 1.2-1.4 g protein/kg body mass/d (150-175% of the current RDA) and 1.7-1.8 g protein/kg body mass/d (212-225% of the current RDA) for strength exercisers. Fortunately, the typical North American diet contains protein near these quantities, so most individuals who decide to begin an exercise program will obtain sufficient protein as long as their diet is mixed and they are careful to consume adequate energy. Populations at greatest risk for consuming insufficient protein include any group that restricts energy intake (those on diets) or high quality protein sources (vegetarians) as well as any group that has a requirement higher than normal due to another existing condition (growing individuals). Future studies should focus on these groups. Moreover, few exercise performance measures have been made, so any negative effect of insufficient dietary protein on athletic success needs to be determined. Supplementation of several individual amino acids may be beneficial for physically active individuals, but considerable potential risk is also present. Intake of large quantities of individual amino acids is not recommended until much more information is available.

Oxidative capacity of human muscle fiber types: effects of age and training status.

Morphometry and oxidative capacity of slow-twitch (type I) and fast-twitch (type IIa and IIb) muscle fibers obtained from vastus lateralis needle biopsies were compared between younger (21-30 yr) and older (51-62 yr) normal fit (maximal O2 uptake = 47.0 vs. 32.3 ml.kg-1.min-1) and endurance-trained (66.3 vs. 52.7 ml.kg-1.min-1) men (n = 6/group). The older groups had smaller type IIa (31%) and IIb (40%) fiber areas and fewer capillaries surrounding these fibers than did younger groups. The reduced type II fiber areas and capillary contacts associated with aging were also observed in the older trained men. However, the capillary supply per unit type II fiber area was not affected by age but was enhanced by training. Additionally, on the basis of quantitative histochemical analysis, succinate dehydrogenase activities of type IIa fibers in the older trained men [4.07 +/- 0.68 (SD) mmol.min-1.l-1] were similar to those observed in younger trained men (4.00 +/- 0.48 mmol.min-1.l-1) and twofold higher than in older normal fit men (2.01 +/- 0.65 mmol.min-1.l-1; age x fitness interaction, P < 0.05). Type I muscle fibers were unaffected by age but were larger and had more capillary contacts and higher succinate dehydrogenase activities in the trained groups. The findings of this study suggest that aging results in a decrease in type II fiber size and oxidative capacity in healthy men and that this latter effect can be prevented by endurance training. Conclusions regarding the effects of age and training status on muscle capillarization depend largely on how these data are expressed.

Do athletes need more dietary protein and amino acids?

The current recommended daily allowance (RDA) for protein is based primarily on data derived from subjects whose lifestyles were essentially sedentary. More recent well-designed studies that have employed either the classic nitrogen balance approach or the more technically difficult metabolic tracer technique indicate that overall protein needs (as well as needs for some specific individual amino acids) are probably increased for those who exercise regularly. Although the roles of the additionally required dietary protein and amino acids are likely to be quite different for those who engage in endurance exercise (protein required as an auxiliary fuel source) as opposed to strength exercise (amino acids required as building blocks for muscle development), it appears that both groups likely will benefit from diets containing more protein than the current RDA of 0.8 g.kg-1.day-1. Strength athletes probably need about 1.4-1.8 g.kg-1.day-1 and endurance athletes about 1.2-1.4 g.kg-1.day-1.

Protein requirements of soccer.

Although the physical demands of soccer have been studied frequently over the years, there has been little attention to the dietary protein needs of soccer players. Recent data from both moderate-intensity, prolonged (endurance) and heavy-resistance (strength) exercise studies indicate that the current recommendations (0.8 g per kg body mass per day) for protein intake are probably suboptimal for individuals who are chronically active. Endurance athletes need more dietary protein than sedentary individuals to maintain an auxiliary fuel source which appears to become increasingly important as exercise is prolonged. Strength athletes can also benefit from a greater protein intake than is currently recommended because it appears that, in combination with heavy-resistance training, it can provide an enhanced stimulus for muscle development. Soccer is a high-intensity, intermittent activity which requires aspects of both strength and endurance over a period of 90 min. As a result, soccer players would be likely to benefit from protein intakes above current recommendations not only because of their potential to enhance strength, but also to provide a supply of amino acids for any increased amino acid oxidation that may occur during training and in competition. Based on the related exercise studies completed to date, it appears that a protein intake of 1.4-1.7 g kg-1 day-1 should be adequate for soccer players. Assuming free access to a wide variety of foods, this protein intake can be easily obtained by most soccer participants. Individuals at greatest risk of falling short of this intake include those who are growing (especially children in developing countries where suboptimal protein intake may be common) or those who consume proteins of lower quality. Although diets high in protein are frequently condemned because of possible kidney problems, it appears these concerns have been over-emphasized. There is no evidence that protein intakes in the range recommended will cause healthy individuals any concerns.

Protein requirements and muscle mass/strength changes during intensive training in novice bodybuilders.

This randomized double-blind cross-over study assessed protein (PRO) requirements during the early stages of intensive bodybuilding training and determined whether supplemental PRO intake (PROIN) enhanced muscle mass/strength gains. Twelve men [22.4 +/- 2.4 (SD) yr] received an isoenergetic PRO (total PROIN 2.62 g.kg-1.day-1) or carbohydrate (CHO; total PROIN 1.35 g.kg-1.day-1) supplement for 1 mo each during intensive (1.5 h/day, 6 days/wk) weight training. On the basis of 3-day nitrogen balance (NBAL) measurements after 3.5 wk on each treatment (8.9 +/- 4.2 and -3.4 +/- 1.9 g N/day, respectively), the PROIN necessary for zero NBAL (requirement) was 1.4-1.5 g.kg-1.day-1. The recommended intake (requirement + 2 SD) was 1.6-1.7 g.kg-1.day-1. However, strength (voluntary and electrically evoked) and muscle mass [density, creatinine excretion, muscle area (computer axial tomography scan), and biceps N content] gains were not different between diet treatments. These data indicate that, during the early stages of intensive bodybuilding training, PRO needs are approximately 100% greater than current recommendations but that PROIN increases from 1.35 to 2.62 g.kg-1.day-1 do not enhance muscle mass/strength gains, at least during the 1st mo of training. Whether differential gains would occur with longer training remains to be determined.

Protein intake and athletic performance.

For most of the current century, exercise/nutritional scientists have generally accepted the belief that exercise has little effect on protein/amino acid requirements. However, during the same time period many athletes (especially strength athletes) have routinely consumed diets high in protein. In recent years, the results of a number of investigations involving both strength and endurance athletes indicate that, in fact, exercise does increase protein/amino acid need. For endurance athletes, regular exercise may increase protein need by 50 to 100%. For strength athletes, the data are less clear; however, protein intakes in excess of sedentary needs may enhance muscle development. Despite these observations increased protein intake may not improve athletic performance because many athletes routinely consume 150 to 200% of sedentary protein requirements. Assuming total energy intake is sufficient to cover the high expenditures caused by daily training, a diet containing 12 to 15% of its energy from protein should be adequate for both types of athletes.

Protein and amino acid needs of the strength athlete.

The debate regarding optimal protein/amino acid needs of strength athletes is an old one. Recent evidence indicates that actual requirements are higher than those of more sedentary individuals, although this is not widely recognized. Some data even suggest that high protein/amino acid diets can enhance the development of muscle mass and strength when combined with heavy resistance exercise training. Novices may have higher needs than experienced strength athletes, and substantial interindividual variability exists. Perhaps the most important single factor determining absolute protein/amino acid need is the adequacy of energy intake. Present data indicate that strength athletes should consume approximately 12-15% of their daily total energy intake as protein, or about 1.5-2.0 g protein/kg.d-1 (approximately 188-250% of the U.S. recommended dietary allowance). Although routinely consumed by many strength athletes, higher protein intakes have not been shown to be consistently effective and may even be associated with some health risks.

Whole body leucine metabolism during and after resistance exercise in fed humans.

The effects of resistance exercise upon leucine oxidation and whole body protein synthesis were studied using stable isotope methodology. L-[1-13C]leucine was used as a tracer to calculate leucine oxidation and whole body protein synthesis in six healthy, fed, male athletes in response to a 1 h bout of circuit-set resistance exercise. The measurements were performed prior to, during, and for 2 h after exercise, and corrections were made for background 13CO2/12CO2 breath enrichment and bicarbonate retention factor changes. Results demonstrated significant (P less than 0.01) increases in the background 13CO2/12CO2 breath enrichment at 1 and 2h after exercise and in the bicarbonate retention factor (P less than 0.01) during exercise. At 15 min after exercise, the bicarbonate retention factor was significantly (P less than 0.05) lower than at rest. There were no significant effects of exercise on leucine oxidation or flux, whole body protein synthesis, or the rate of appearance of endogenous leucine. We concluded that circuit-set resistance exercise did not affect the measured variables of leucine metabolism. In addition, large errors in calculating leucine oxidation and whole body protein synthesis during resistance exercise can occur if background 13CO2/12CO2 breath enrichment and bicarbonate retention factor changes are not accounted for.

Effect of exercise on protein requirements.

The effect(s) of exercise on dietary protein requirements has (have) been a controversial topic for many years. Although most expert committees on nutrition have not provided an additional allowance of protein for active individuals, a considerable amount of experimental evidence has accumulated during the past 15 years which indicates that regular exercise does in fact increase protein needs. Part of the confusion is due to methodological difficulties and inadequate control of several interacting factors including: diet composition, total energy intake, exercise intensity, duration and training, ambient temperature, gender, and perhaps even age. Although definitive dietary recommendations for various athletic groups must await future study, the weight of current evidence suggests that strength or speed athletes should consume about 1.2-1.7 g protein/kg body weight.d-1 (approximately 100-212% of current recommendations) and endurance athletes about 1.2-1.4 g/kg.d-1 (approximately 100-175% of current recommendations). These quantities of protein can be obtained from a diet which consists of 12-15% energy from protein, unless total energy intake is insufficient. There is no evidence that protein intakes in this range will cause any adverse effects. Future studies with large sample sizes, adequate controls, and performance as well as physiological/biochemical measures are necessary to fine tune these recommendations.

Effect of exercise and obesity on skeletal muscle amino acid uptake.

The genetically obese Zucker rat has a reduced capacity to deposit dietary protein in skeletal muscle. To determine whether amino acid uptake by muscle of obese Zucker rats is impaired, soleus strip (SOL) and epitrochlearis (EPI) muscles from 10-wk-old lean and obese Zucker rats were studied in vitro by use of [14C]alpha-aminoisobutyric acid (AIB). Muscles from fasted rats were incubated under basal conditions at rest or after a 1-h treadmill run at 8% grade. To equate total work completed, lean and obese rats ran at 27 and 20 m/min, respectively. Muscles were pinned at resting length, preincubated for 30 min at 37 degrees C in Krebs-Ringer bicarbonate buffer containing 5 mM glucose under 95% O2-5% CO2, and then incubated up to 3 h in Krebs-Ringer bicarbonate with 0.5 mM AIB, [14C]AIB, and [3H]inulin as a marker of extracellular fluid. Basal AIB uptake in EPI and SOL from obese rats was significantly reduced by 40 and 30% (P less than 0.01), respectively, compared with lean rats. For both lean and obese rats, exercise increased (P less than 0.05) basal AIB uptake in EPI and SOL, but the relative increases were greater in the obese rats (EPI 54% and SOL 71% vs. EPI 32% and SOL 37%). These results demonstrate that genetically obese Zucker rats have reduced basal skeletal muscle amino acid uptake and suggest that physical inactivity may partially contribute to this defect.