Thermodynamics, Fat and Low Carbohydrate Diets

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27 Οκτ 2013 (πριν από 3 χρόνια και 8 μήνες)

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Thermodynamics, Fat
and Low Carbohydrate Diets
Richard D. Feinman, PhD.
Department of Biochemistry,
State University of New York Downstate Medical Center,
Brooklyn, NY 11203
Email: I read with interest your article in
Nutrition Journal. I think what I find most
curious is that we nutritionists have been
citing the laws of thermodynamics as proof
that "a calorie is a calorie" for AGES and no
physicist has shut us up! It's not like the
second law of thermodynamics was just
invented, so why hasn't a fat, frustrated
physicist written an op-ed on this?
The Atkins Diet -actually, 4 diets:
1. Induction. 20 g. of CHO: 2 weeks
2. Ongoing Weight Loss: Add back CHO
in 5 g. increments
3. At 5-10 lbs of goal weight: add carbs to
to find maintenance level of CHO
4. Lifetime maintenance.
Where does the low carb revolution come from? Many
factors. We asked medical students the following
question.
In the past thirty years the per cent fat in the American
Diet has:
_____ Increased
_____ Decreased
_____ Stayed about the same.
the obesity epidemic
•obesity up
•fat down
•CHO up
Exercise down too,
but makes you wonder.
Medical Students thought
% fat has:
Increased: 76 %
Decreased 22 %
Same 3 %
1. The diet component that is most likely to raise triglyceridesis
_____ Fat
_____ Carbohydrate
_____ Protein
2. In general, what effect does a low-fat diet have on triglycerides?
_____ Increase
_____ Decrease
_____ No change
3. In general, what effect does a low-fat diet have on HDL
(high density lipoprotein, “good,”cholesterol)?
_____ Increase
_____ Decrease
_____ No change
1. The diet component that is most likely to raise triglycerides is
_____ Fat
X Carbohydrate
_____ Protein
2. In general, low-fat diets
X Increase triglycerides
_____ Decrease
_____ No change
3. In general, low-fat diets
_____ Increase
X Decrease HDL (“good”cholesterol)
_____ No change
•We use low carbohydrate diets to teach
metabolism to medical students.
•Traditional: feed-fast cycles
(postprandial-postabsorptive cycles)
•We look at low carbohydrate diets as an
extension of fasting
•When you wake up in the morning,
12 hours after a meal...
Lipolysis is stimulated.
Fatty acids are oxidized.
The purpose of metabolism is
1. To provide energy (ATP) and
2. To maintain blood glucose ...
Gluconeogenesis is stimulated.
Glycogen is hydrolyzed.
Bottom line:
•TAG is primary fuel.
ATP comes from fat oxidation.
•Glucose comes from glycogen and
gluconeogenesis.
Isn’t this a good weight loss diet ?
You burn fat for energy.
Glucose is provided by glycogen and
gluconeogenesis.
Low carbohydrate (high protein) diet as
Protein-sparing fast
Low Carbohydrate diets can help us
understand metabolism.
But, everybody wants to know...
What is the best diet ?
$ 8 a pound?
Why so much?
Across the street
they’re only
$2 a pound.
The best diet
is the one
your patient
can stay on.
Of course, they may stay on it if it works.
Do low carbohydrate diets really work
better, calorie for calorie. Is there a
“metabolic advantage?”
To test the idea you have to compare
isocaloric diets of different macronutrient
composition.
“It violates the laws of thermodynamics. No
one has ever found any miraculous metabolic
effects.”
“It violates the laws of thermodynamics. No one
has ever found any miraculous metabolic effects.”
Metabolic advantage, if not actually
miraculous, was, in fact what was
shown in the data.
Of course, there can be experimental
error, but this must be identified. You
can’t just make the data go away.
Thermodynamics
Violated !
What does it take to establish metabolic
advantage?
1. Can it happen? Is it consistent with
the laws of thermodynamics?
2. If it is possible, does it actually occur?
3. If so, how does it happen?
What mechanisms would account for
greater weight loss on
low carbohydrate diets?
What are the laws of thermodynamics?
Where do they come from?
Julius Robert Mayer (1842)
Energy is
conserved
Mayer
If two bodies find themselves in a given difference,
then they could remain in a state of rest after the
annihilation of [that] difference if the forces that
were communicated to them as a result of the
leveling of the difference could cease to exist; but
if they are assumed to be indestructible, then the
still persisting forces, as causes of changes in
relationship, will again reestablish the original
present difference.
First Law of Thermodynamics
Change in energy
∆E = q -w
work done
Enthalpy:
∆H = q at constant T, P
heat added
Caloric value of food is heat, ∆H, from
complete oxidation. But, what is heat?
Modern
Calorimeter
Enter Count Rumford:
The determination of the specific heats of two
bodies ... attained by friction and heat flow ...
is an example of a crucial experiment. This
experiment was performed…by Rumford, and
dealt a death blow to the substance theory of
heat.
--Einstein A & Infeld L(1938).
“A calorie is a calorie”is a little bit like
the caloric. It treats the energy value of
food as if it were a thing instead of a
process.
This mistake is easy to make because
the first law allows very powerful
calculations.
Hess’s Law
Mathematically, we say that Enthalpy
and Energy are State Functions. They
do not depend on how you do the
process. All paths must give the same
value. But, you have to be careful...
What makes the reaction go?
Old idea: exothermic reactions are
spontaneous. But ...
The second law:
The entropy, S,
relates to probability,
spontaneity,
reversibility.
For any real process,
∆S > 0
People are not
storage lockers.
People are
(metabolic)
machines.
Sadi Carnot (1823)
“The motive power of heat is...fixed solely by
the temperatures of the bodies between which
is effected, finally, the transfer of the caloric.”
Rudolf Clausius (1865)
The Laws of
Thermodynamics
1. Die Energie der Welt ist Konstant.
2. Die Entropie strebt einem Maximum zu.
Email: Oh, re understanding the second law of
thermodynamics...am I correct in thinking that the energy "lost"
due to the thermic effect of metabolizing various nutrients is not
really LOST --just not conserved in the place it started out (i.e.,
the body). In other words --it's out in the world SOMEWHERE
and could be harnessed to power cars, windmills, etc. Have you
given any thought to "selling" the idea of low-carb diets as an
alternative energy source?!!
My answer: Wow! You just described the whole history of the
second law in that paragraph. You are exactly half right! The
energy is not really lost in that it is somewhere, it is in heator in a
hidden form of heat in some chemical bonds. That’s what the
FIRST law says. It can’t be destroyed. It is out there ( or IN
there, remember your body still stores energy). But NO, you can’t
use it. That is the whole idea of entropy.
First and Second Law of Thermodynamics
Change in freeenergy
∆G = ∆H
-T∆S
heat lost
useful part
of energy
entropy
changes
Free energy is like the potential energy
that water has at the top of a waterfall.
If a “calorie is a calorie,”(1)=(2) and (3)=0.
Fallacy: Energy in metabolism is not equal
to the calorimeter value. (1) and (3)+(2)
are not the same process in metabolism.
Inefficiency in a living system
has the same effect as lower
calories. Less ATP, more fat
lost: also usually more heat.
Thermogenesis (Thermic
effect of feeding)varies with
food: Protein >> CHO>Fat
Effective calories = 0.975 x 9 x (g. FAT) +
0.93 x 4 x(g. CHO) + 0.725 x 4 x(g. Protein)
1. OK. Metabolic advantage is consistent
with the laws of thermodynamics.
It couldhappen.
1. OK. Metabolic advantage is consistent
with the laws of thermodynamics.
It could happen.
2. But, does it actually occur?
3. If so, how does it happen? What
mechanisms would account for greater
weight loss on low carbohydrate diets?
Look at more isocaloric diet comparisons...
1. OK. Metabolic advantage is consistent
with the laws of thermodynamics.
It could happen.
2. OK, it actually occurs.
3. How does it happen? What
mechanisms would account for greater
weight loss on low carbohydrate diets?
Look at requirements on glucose
production ... Look at substrate cycles...
So how does it happen ?
MACRONUTRIENT / PATH
Yield
( kcal / g )
inefficiency
( % )
GLUCOSE ➞CO2
1.54
-
GLUCOSE ➞GLYCOGEN ➞GLUCOSE ➞CO
2
1.46
5.3
2 ALANINE ➞GLUCOSE ➞CO2
1.23
20.2
The effect of path.
Multiple cycles waste energy
Summary: Low carbohydrate diets:
1. If anything, improve lipid profile.
2. Frequently show a metabolic advantage,
greater weight loss calorie-for-calorie.
3. There is no violation of thermodynamics;
no reason not to accept published data.
4. Plausible metabolic mechanisms explain
how this could occur: substrate cycles,
energetically costly pathways, e.g., GNG.
5. The best diet is the diet your patient
can stay on. How do you choose?
6. Low fat diets are still effective for many
people, but it is increasingly difficult to
say they are an established standard.
7. In terms of using thermodynamics for
a dieter’s advantage, low carbohydrate
diets may be the “default diet,”the one
to try first.
Illustrations by Robin Feinman. Not only the artist.
Also a client
2002Today
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