Possible Mechanisms
It has been observed that the human liver cannot safely metabolise much more than 285-365 g of protein per day (for an 80 kg person), and human kidneys are similarly limited in their capability to remove urea (a byproduct of protein catabolism) from the bloodstream. Exceeding that amount results in excess levels of amino acids, ammonia (hyperammonemia), and/or urea in the bloodstream, with potentially fatal consequences, especially if the person switches to a high-protein diet without giving time for the levels of his or her hepatic enzymes to upregulate. Since protein only contains 4 kcal/gram, and a typical adult human requires in excess of 1900 kcal to maintain the energy balance, it is possible to exceed the safe intake of protein if one is subjected to a high-protein diet with little or no fat or carbohydrates. However, given the lack of scientific data on the effects of high-protein diets, and the observed ability of the liver to compensate over a few days for a shift in protein intake, the US Food and Nutrition Board does not set a Tolerable Upper Limit nor upper Acceptable Macronutrient Distribution Range for protein. Furthermore, medical sources such as UpToDate do not include listings on this topic.
Read more about this topic: Rabbit Starvation