> >> 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).
> What you have to do is look at the fuels being burned. This can be done
> using the respiratory quotent (RQ). The RQ is 1 for glucose and 0.7 for
> fatty acids. Protein gives a RQ of 0.8. Unless you actually measure what
> is being burned, you can't make any conclusions about the relative amounts
> of fat and protein being used to provide energy during starvation (as far
> as I can tell).
I'm slightly confused. You say that the figures that protein is spared is based upon measurements of glucose
oxidation rates. But as you then point out from the RQ you cannot tell what fuels are being oxidised. So a
decrease in RQ only means means that less glucose is being oxidised noit necessilarily the same thing as
saying that less protein is being catabolised. Are there other direct measures of glucose oxidation ? Coold it
be that protein is being catabolised directly and not being converted into glucose first ? For example as I
understand it some amino acids can be directly catabolised via pyruvate (e.g. alanine) which then feeds into
the krebbs cycle ? So decreases in the RQ do not necessarily support the hypothesis that protein is spared.
>> Calorie restriction causes about a 12% reduction in BMR so the reduction in
> glucose use is not solely the result of a lowered BMR. The brain is one
> tissue whose energy requirement does not appear to decrease during
> starvation (as far as I can tell from everything that I've read). Under
> normal conditions, brain requires about 140 grams of glucose per day.
> Brain use of glucose results in production of CO2 and H20 (complete
> destruction of glucose) and this glucose must be replaced each day. The
> estimates that I've seen for maximum gluconeogenesis indicate that about
> 130 grams of glucose can be produced each day (but this is based on liver
> enzyme activities and levels). So even here there is a real problem, brain
> needs 140 grams of glucose per day but gluconeogenesis can only produce 130
> grams per day.
>> When actual glucose oxidation rates are examined, things get even stranger,
> the maximum glucose oxidation rate is about 240 grams per day (10 grams per
> hour) and even at 6 grams per hour, the 24 hour requirement is 144 grams of
> glucose which is still higher than what gluconeogenesis is supposed to be
> able to produce.
>> Unless the brain starts using large amounts of ketone bodies, there is going
> to be a big problem since gluconeogenesis can not apparently produce the
> amount of glucose that the brain normally oxidizes each day. Just
> about every biochemistry textbook will state that the brain uses mostly
> ketone bodies for energy during starvation but I'm not sure that this has
> ever been proven (it's mostly by inference). Ketone body levels go up in
> both blood and brain tissue. Brain mitochondria can't burn fatty acids but
> they can burn ketone bodies so they must be using them to spare glucose
> use. The drop in glucose use during starvation is always attributed to
> increased ketone body use. If less glucose is needed, then gluconeogenesis
> must also decrease.
>> Gluconeogenesis is a very complicated process and it's really not as simple
> as looking at urea production. Glycerol from fat breakdown in adipose
> tissue can become an important source of glucose and this can spare the use of
> amino acids. Having brain and muscle both use primarily ketone bodies for
> energy can also help spare some amino acids.
I think that some of the confusion might be because protein is catabolised by means other than gluconeogenesis.
Therefore estimating protein catabolism based upon glucose oxidation rates might be misleading.
Thnaks for the continuing discussion, I'm finding it useful and stimulating. It certainly would appear to make
sense that a protein sparing response to starvation has evolved but I'm still not convinced that the evidence
that I have seen necessarily bears that out.
Yours
Steve collins
