would actually decrease with time, perhaps relating to the anecdotal observation of"hitting the wall" on weight loss diets. Very low carbohydrate diets, in their early phases, also must supply substantial glucose to the brain from gluconeogenesis. For example, the early phase of the popular Atkins or Protein Power diet restricts dieters to about 20-30 grams of carbohydrate per day, leaving 60-65 grams to be made up from protein-originated gluconeogenesis. One hundred grams of an"average" protein can supply about 57 grams of glucose so 110 grams protein would be needed to provide 60-65 grams glucose. Increased gluconeogenesis has been directly confirmed using tracer studies on day 11 of a very low carbohydrate diet(approx 8 grams/day) [29]. If indeed, 110 grams of endogenous protein is broken down for gluconeogenesis and re- synthesized, the energy cost, at 4-5 kcal/gram could amount to as much as 400-600 kcal/day. This is a sizable metabolic advantage. Of course, the source of proteln for gluconeogenesis may be dietary rather than endogenous. Whereas endogenous protein breakdown is likely to evoke energetically costly re-synthesis in an organism in homeostasis, dietary protein may conserve energy. The source of protein for the observed gluconeogenesis[29] remains an open question, but there is no a priori reason to exclude endogenous rather than dietary sources. This is therefore a hypothesis that would need to be tested. The extent to which the protein for gluconeogenesis is supplied by endogenous protein would explain very high- energy costs. It should be noted, however, that even if limited to breakdown of dietary protein sources, there would be some energy cost associated with gluconeogenesis.