what is rabbit starvation?
"...This ceiling, Cordain thinks, could be imposed by the way we process protein for energy. The simplest, fastest way to make energy is to convert carbohydrates into glucose, our body’s primary fuel. But if the body is out of carbs, it can burn fat, or if necessary, break down protein. The name given to the convoluted business of making glucose from protein is gluconeogenesis. It takes place in the liver, uses a dizzying slew of enzymes, and creates nitrogen waste that has to be converted into urea and disposed of through the kidneys. On a truly traditional diet, says Draper, recalling his studies in the 1970s, Arctic people had plenty of protein but little carbohydrate, so they often relied on gluconeogenesis. Not only did they have bigger livers to handle the additional work but their urine volumes were also typically larger to get rid of the extra urea. Nonetheless, there appears to be a limit on how much protein the human liver can safely cope with: Too much overwhelms the liver’s waste-disposal system, leading to protein poisoning—nausea, diarrhea, wasting, and death" Discovery magazine-
The Inuit Paradox
How can people who gorge on fat and rarely see a vegetable be healthier than we are?
posted by vanwash at 11:16 AM
3 Comments:
Possible mechanisms[edit]
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,[1] 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.[2] Furthermore, medical sources such as UpToDate[3] do not include listings on this topic.
nt J Sport Nutr Exerc Metab. 2006 Apr;16(2):129-52.
A review of issues of dietary protein intake in humans.
Bilsborough S1, Mann N.
Author information
Abstract
Considerable debate has taken place over the safety and validity of increased protein intakes for both weight control and muscle synthesis. The advice to consume diets high in protein by some health professionals, media and popular diet books is given despite a lack of scientific data on the safety of increasing protein consumption. The key issues are the rate at which the gastrointestinal tract can absorb amino acids from dietary proteins (1.3 to 10 g/h) and the liver's capacity to deaminate proteins and produce urea for excretion of excess nitrogen. The accepted level of protein requirement of 0.8g x kg(-1) x d(-1) is based on structural requirements and ignores the use of protein for energy metabolism. High protein diets on the other hand advocate excessive levels of protein intake on the order of 200 to 400 g/d, which can equate to levels of approximately 5 g x kg(-1) x d(-1), which may exceed the liver's capacity to convert excess nitrogen to urea. Dangers of excessive protein, defined as when protein constitutes > 35% of total energy intake, include hyperaminoacidemia, hyperammonemia, hyperinsulinemia nausea, diarrhea, and even death (the "rabbit starvation syndrome"). The three different measures of defining protein intake, which should be viewed together are: absolute intake (g/d), intake related to body weight (g x kg(-1) x d(-1)) and intake as a fraction of total energy (percent energy). A suggested maximum protein intake based on bodily needs, weight control evidence, and avoiding protein toxicity would be approximately of 25% of energy requirements at approximately 2 to 2.5 g x kg(-1) x d(-1), corresponding to 176 g protein per day for an 80 kg individual on a 12,000kJ/d diet. This is well below the theoretical maximum safe intake range for an 80 kg person (285 to 365 g/d).
What limits the liver's capacity to convert amino acids to
glucose?
Conversion of amino acids to glucose involves several metabolic processes;
deamination or transamination, conversion of the released NH4 + to urea and finally
synthesis of glucose from amino acid residues. The key to understanding the
physiological limitation of glucose formation from amino acids lies in the large
amount of energy required to fuel these processes.
Energy in the sense used here
means the hydrolysis of adenosinetriphosphate (ATP) to either AMP + PPi or ADP +
Pi. Four ATP molecules are used to convert two NH4+ to urea and six more are
required to convert the carbon skeletons of these amino acids to glucose. One ATP is
also required to add a glucosyl group to a glycogen molecule so, you see, a lot of
energy is used in this process. All cells and tissues are built up such that ATP levels
are relatively stable. This is a basic prerequisite for life. Under gluconeogenesis the
liver must rely upon aerobic metabolism to replace the ATP that is consumed. By
definition this is an oxygen-dependent process. The "catch" is that the liver obtains
most of its oxygen from the portal vein where the partial pressure of oxygen is rather
low. This limits uptake of oxygen, limits ATP production and, therefore, the synthesis
of glucose from amino acids.
We have data about the total amount of oxygen supplied to the human liver.
Calculations based on this (and assuming the all of this oxygen goes to support
conversion of amino acids to glucose) suggest that the maximum capacity of hepatic
glucose synthesis from amino acids lies around 400 grams/day. This is the
equivalent of approximately 1600 kcal, close to the metabolic rate of a bed-ridden
person and hardly enough to support an active life.
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