In article <96123183wnr at concern1.demon.co.uk>, steve collins <steve at concern1.demon.co.uk> writes:
> Is there any evidence for metabolic adaptations to spare protein during
> starvation / prolonged fasting? Much of what I read seems to be saying
> that after the initial loss of labile protein the
> utilistaion of protein / kg body weight (as reflected by urinary N
> losses) is constant (i.e. no metabolic adaptations to spare protein). I
> find this conclusion slightly at odds with the observations that obese
> people survive very much longer during prolonged fasting than
> leaner people. If anybody could shed any light on
> where my thinking is going wrong I'd be most greatful -- steve collins
Gluconeogenesis (the process of converting noncarbohydrate material to
glucose) starts to increase about four hours after the last meal and it
peaks at about 10 grams of glucose produced per hour about 2-3 days after
the last meal is consumed. Gluconeogenesis then decreases to about 6 grams
of glucose produced per hour until about 12 days after the last meal at
which time it increases very slowly upto 40 days (the longest time period
that I have) where it's producing about 8 grams of glucose per hour.
The source of the carbon for the glucose production is amino acids (from
the breakdown of skeletal muscle protein and also from internal organs if
the fast lasts long enough), lactate, glycerol and odd chain
fatty acids (only the last 3-carbon fragment).
There is adaptation to decrease the breakdown of muscle protein during
starvation (fasting) but the process never stops. It is possible to loose
enough skeletal muscle mass to become too weak to move and therefore find
food (or more importantly water) and die before all of the fat energy
reserve is gone. In addition, a prolonged fast also decreases the weight
of all internal organs.
The main sparing element for protein is the production of ketone bodies
from stored fat. As the stored fat reserve gets depleted, ketone body
production decreases and more protein has to be used. That data I quoted
is for non-obese individuals. For obese individuals, the fat reserve is
large enough to go more than 12 days without seeing a reduction in ketone
body formation (meaning that the production of glucose via gluconeogenesis
would not start to increase at about 12 days and the protein would be
"spared" for a longer time period).
--
Marty B. "You are what you eat"
