Summary: brief exercise before eating shunts the resulting energy to your muscles, not fat
Action needed: 3-5 minutes of high-intensity, whole-body exercise soon before eating.

Why should we care?

Food is digested to glucose, a simple carbohydrate. The sugar either ends up in fat cells (bad) or muscle (potentially good). Because skeletal muscles account for 80% of glucose uptake [3], that is the obvious place to start.

Mechanism

Glucose uptake is rate-limited by GLUT-4 (GLUcose Transporter) [3], which carries it across cell membranes. There are two separate pools of GLUT-4 in cells [3]; transporters can only have an effect if translocated to the surface. One signaling pathway is via insulin (us laymen understand it regulates blood sugar – it is interesting to know exactly how this works) and “PKB” [10]. The other involves muscle contractions and “APMK” [10].

The good news is that exercise has an immediate effect on GLUT-4 gene expression [2]. Surface GLUT-4 roughly doubles [7, 11], with the exception of the calf muscle [8] and possibly others. These effects only last a few hours [1], but are similar to the maximum effective dose of insulin [7]. As a bonus, the action of the insulin pathway is also increased for many hours [6]. That is helpful because diabetes (an increasingly common problem due to our high sugar intake) is characterized by insulin resistance [1].

It was previously believed that GLUT-4 was maximized through several hours of low-intensity exercise. Happily, only a few minutes of fairly intense exercise – for example, 30x 3 second contractions of the quadriceps – are similarly effective [3].

This is the appealing picture painted by an excerpt from Tim Ferriss’ 4 Hour Body;
others have also taken up the message:
http://fitnessblackbook.com/diet-tips/gone-in-60-seconds-one-minute-of-activity-to-avoid-storing-calories-of-a-meal-as-body-fat/
http://jarmankinesiology.com/2011/04/09/exercise-might-be-able-to-make-it-ok-to-eat-a-bad-meal/

Reality Check

Unfortunately, it is an oversimplification. Serious exercise requires more energy than the normal glucose supply can provide. Oxidizing glycogen (a concentrated form of glucose similar to starch) is essential for endurance [4]. Unfortunately there are two interactions with GLUT-4 and eating.

First, muscle glycogen content, by a not yet fully understood mechanism, inhibits glucose uptake and reverses the increase in insulin sensitivity [9]. A full glycogen store inhibits GLUT-4 translocation [11] – both the PKB and AMPL signaling pathways are affected [10]. It is reasonable for the muscle to reject additional glucose when it is not required. The consequence is that anyone except maybe couch potatoes (with their untrained muscles) cannot rely on this trick to completely save them from over-indulgence.

Second, carbohydrate intake after exercise can lead to glycogen supercompensation [10] – a probably undesirable overshoot in glycogen synthesis. This has been reported for rat chow; one such formulation (though not necessarily the same) consists of only 4.7% sugar and a modest 58% of calories from carbohydrates [13]. It is probably good to avoid excessive carbs in general, and especially after light exercise – UNLESS glycogen reserves have been seriously depleted. A 90 minute soccer game uses up 70-90%, which are not fully re-generated even after 2 days [12] – hence the necessity of rest days when engaging in competitive sports.

Conclusions

Exercise is helpful for diverting calories from fat to muscle, and decreasing insulin resistance (a hallmark of diabetes). Reported increases of glucose uptake by factors of 2..3 assume fasting after exercise. Muscles with glycogen reserves will provide a more modest but still beneficial increase. It would probably be good to reduce carbs after mild exercise (and possibly in general, but that is a separate topic) to avoid glycogen supercompensation.

References

[1] http://diabetes.diabetesjournals.org/content/61/5/1090.full
[2] http://jap.physiology.org/content/88/2/794.full
[3] http://jap.physiology.org/content/90/6/2019.full
[4] http://jeb.biologists.org/content/212/2/238.full
[5] http://brage.bibsys.no/nih/bitstream/URN:NBN:no-bibsys_brage_14288/1/Dahl%20MedSciSportExcer%202009.pdf
[6] http://jap.physiology.org/content/94/4/1373.full
[7] http://www.jappl.org/content/85/4/1218.full
[8] http://ajpendo.physiology.org/content/272/5/E864.abstract
[9] http://jap.physiology.org/content/85/1/133.full
[10] http://ajpendo.physiology.org/content/279/6/E1311.full
[11] http://ajpendo.physiology.org/content/277/6/E1103.long
[12] https://www.uni-ulm.de/fileadmin/externe_websites/ext.dzsm/content/Archiv2013/Heft_1/45_%C3%9Cbersicht_Niess.pdf
[13] http://www.bio-services.nl/en/docs/03_catalogue_rat_mouse_new.pdf