Link to the study:
...The major finding in this study is that barefoot running overground and on the treadmill was significantly more economical than running with shoes. In contrast, no significant differences in VO2 (volume of oxygen used by the runner) were found between the treadmill and the overground track regardless of the footwear condition (barefoot/shod). The magnitude of the lower VO2 while running barefoot yielded a PRE of 0.41, meaning that 41% of the variance in VO2 was explained by the barefoot vs. shod difference. The associated PRE values due to the barefoot vs. shod difference for HR (heart rate) and RPE (relative perceived effort) were 0.50 and 0.69, respectively.
Note, only 6-10 runners sampled in this study so it shows a trend, not definitive data. The source material in this study is very valuable, plenty of reading material. I plan to summarize more such studies in the near future