I've done maybe 50 marathons and two-dozen ultras. This fat-burning idea really sounded great so I tried Atkins. After a few years of running on lowcarb, this is what I've come up with: We're constantly burning a mixture of carb, fat and protein. When running, the ratio shifts to more carb-burning the faster you go. And shifts back to more fat-burning the slower you go. But without enough carb on board, you will start catabolizing muscle tissue for energy.

You can lose weight on lowcarb but training will suffer. However even walking is training. If you schedule walking breaks during long runs it may give your body time to restock the muscles with glycogen. For 50 mile runs I found the cycle of walking for 2 minutes and running for eight minutes got me to the finish line in decent shape.

To get through the daily run, I suggest carbs that are absorbed fast taken shortly before the workout. Hammergel works for me. After the workout, a protein meal. In time you should find you need less carb to run but you'll never get to the point of not needing carbs.

Some on this site will claim to do tough cardiac workouts on Atkins-style diets. My guess is they're eating huge amounts of protein. Excess protein turns into carb, restocking the glycogen levels of their muscles and liver. That approach never worked for me.