Carb Binges Against a Low-carb Background

In low carb diets, the protein->glucose pathway is mostly utilized to fill the liver with glycogen (a long-lasting store of glucose, to feed the brain, and if any is left over, to refill muscle glycogen or store in fat). So it’s hard to do anything other than lose fat (although with enough dietary fat you may be able to gain).

For athletes, eating carbs on workout day, or to near-bursting once or twice a week (following a muscle glycogen depleting workout, possibly), is thought to help maintain or build muscle while losing fat (just cutting calories while maintaining enough protein in diet, you’ll still lose plenty of muscle mass along with fat mass). I first heard of this from Lyle McDonald (look for “depletion workout” and “refeed”). I’m not sure how strong the evidence is, but there are several plausible mechanisms beyond the claim below that alternating carb-loading and carb-avoiding will increase glycogen storage capacity, which would make for a better low-carb diet experience (more energy available, especially compared to the Atkins-type 50g/day or less of carbs and the accompanying ketosis-only malaise - although I guess burned proteins contribute less to cancer than burned glucose or fat).
Excess amino acids are oxidized for energy. This may be why many people feel a slight surge of energy after a high-protein meal. (A related effect is associated with alcohol consumption, which is often masked by the relaxing effect also associated with alcohol consumption.) Amino acid oxidation is not associated with cancer. Neither is fat oxidation. But glucose oxidation is; this is known as the Warburg effect.

A high-protein LC approach will not work very well for athletes who deplete major amounts of muscle glycogen as part of their daily training regimens. These folks will invariably need more carbohydrates to keep their performance levels up. Ultimately this is a numbers game. The protein-to-glucose conversion rate is about 2-to-1. If an athlete depletes 300 g of muscle glycogen per day, he or she will need about 600 g of protein to replenish that based only on protein. This is too high an intake of protein by any standard.

A recreational exerciser who depletes 60 g of glycogen 3 times per week can easily replenish that muscle glycogen with dietary protein. Someone who exercises with weights for 40 minutes 3 times per week will deplete about that much glycogen each time. Contrary to popular belief, muscle glycogen is only minimally replenished postprandially (i.e., after meals) based on dietary sources. Liver glycogen replenishment is prioritized postprandially. Muscle glycogen is replenished over several days, primarily based on liver glycogen. It is one fast-filling tank replenishing another slow-filling one.

Recreational exercisers who are normoglycemic and who do LC intermittently tend to increase the size of their liver glycogen tank over time, viacompensatory adaptation, and also use more fat (and ketones, which are byproducts of fat metabolism) as sources of energy. Somewhat paradoxically, these folks benefit from regular high carbohydrate intake days (e.g., once a week, or on exercise days), since their liver glycogen tanks will typically store more glycogen. If they keep their liver and muscle glycogen tanks half empty all the time, compensatory adaptation suggests that both their liver and muscle glycogen tanks will over time become smaller, and that their muscles will store more fat.