Bottom line: you burn more fat doing cardio after weight training.
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Med Sci Sports Exerc. 2007 Feb;39(2):308-15.

Effects of Resistance Exercise on Lipolysis during Subsequent Submaximal Exercise.Goto K, Ishii N, Sugihara S, Yoshioka T, Takamatsu K.
1Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Komaba, Tokyo, JAPAN; and 2Institute of Health and Sport Sciences, University of Tsukuba, Tsukuba, Ibaraki, JAPAN.

PURPOSE:: This study examined effects of prior resistance exercise on fat metabolism during subsequent submaximal exercise with different recovery periods between exercise bouts. METHODS:: Ten male subjects performed three types of exercise regimens: 1) submaximal endurance exercise only (E), 2) submaximal endurance exercise with prior resistance exercise and 20 min of rest (RE20), and 3) submaximal endurance exercise with prior resistance exercise and 120 min of rest (RE120). Resistance exercise consisted of six exercises, each with three to four sets at 10-repetition maximum. Subjects performed cycle ergometer exercise at 50% of the maximal oxygen uptake for 60 min. RESULTS:: Prior resistance exercise caused increases in blood lactate, plasma norepinephrine, serum growth hormone (GH), insulin, and glycerol concentrations (P < 0.01). Before the submaximal exercise, serum free fatty acid (FFA) concentration was higher in the RE120 than in the RE20 and E trials (P < 0.01), although concentrations of plasma norepinephrine, serum GH, insulin, and glycerol were higher in the RE20 than in the RE120 and E trials (P < 0.05). Concentrations of FFA and glycerol during the 60-min submaximal exercise were higher in the RE120 and RE20 trials than in the E trial (P < 0.05). No significant difference was observed in the acetoacetate and 3-hydroxybutyrate responses. In the RE20 trial, fat oxidation throughout the 60-min submaximal exercise (mean value) was greater than in the E trial (P < 0.05), but no significant difference was found between the RE120 and E trials. CONCLUSION:: Fat availability during the submaximal exercise was enhanced by prior resistance exercise. However, augmentation of fat oxidation was observed only in the trial with shorter rest between resistance exercise and submaximal exercise bouts (RE20 trial).

PMID: 17277595 [PubMed - in process]