STUDY OBJECTIVES: Although increasing aerobic fitness by exercise training is advocated as part of a healthy lifestyle, studies examining the different effects of intensity and amount on peak consumption (VO2) remain sparse. DESIGN: This randomized controlled trial compared the effects of three different exercise regimens differing in amount and intensity on fitness improvements. PARTICIPANTS: Overweight men and women with mild-to-moderate dyslipidemia were recruited. INTERVENTIONS: The exercise groups were as follows: (1) low amount/moderate intensity (LAMI, n = 25), the caloric equivalent of walking 19 kilometers (km)/wk at 40 to 55% of peak VO2; (2) low amount/high intensity (LAHI, n = 36), the equivalent of jogging 19 km/wk at 65 to 80% of peak VO2; (3) high amount/high intensity (HAHI, n = 35), the equivalent of jogging 32 km/wk at 65 to 80% of peak VO2; and (4) a control group (n = 37). MEASUREMENTS AND RESULTS: Peak VO2 and time to exhaustion (TTE) were tested before and after 7 to 9 months of training. All exercise groups increased peak VO2 and TTE compared to baseline (p <= 0.001). Improvements in peak VO2 were greater in the LAHI and HAHI groups compared to the control group (p < 0.02); HAHI group improvements were greater than the LAMI group (p < 0.02) and the LAHI group (p < 0.02). Increased TTE for all exercise groups was higher compared to the control group (p < 0.001) CONCLUSIONS: Exercising at a level of 19 km/wk at 40 to 55% of peak VO2 is sufficient to increase aerobic fitness levels, and increasing either exercise intensity or the amount beyond these parameters will yield additional separate and combined effects on markers of aerobic fitness. Therefore, it is appropriate to recommend mild exercise to improve fitness and reduce cardiovascular risk yet encourage higher intensities and amounts for additional benefit.

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