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Answers
to Critics of Low Carbohydrate Diets
There have been many widely-reported criticisms of the low carbohydrate
diet in the past year, and these still surface now and then in various
news shows and publications. I have seen news reports that have absolutely
no scientific substance despite a great fanfare, and have also seen comments
by nutrition experts and others that are based on scientific misinterpretation.
I believe
it is important for those individuals who want to use a low carbohydrate
diet as a maintenance plan to understand enough of the science behind
the frequent criticisms to recognize whether they are valid or not. I
present some of the more common ones I have heard here:
Too
much protein can cause kidney disease, especially in diabetics
This
belief arises from incorrectly reversing a known medical fact. It is known
that with certain types of kidney disease protein should be restricted
in the diet because the unhealthy kidneys cannot dispose of the nitrogen
break-down product efficiently. However, the reverse assumption that protein
in the diet will cause this condition is totally incorrect. Healthy kidneys
should be able to deal with any amount of nitrogen that is delivered to
them.
Diabetics
are prone to kidney disease, which is more likely to occur if their blood
sugar levels are too high. However, if there is no evidence of current
kidney disease, a low carbohydrate diet is an ideal way to help control
blood sugar levels and avoid the onset of kidney problems.
A low carb diet is hazardous because it is high
in fat, which can cause heart disease and cancer
This
is the statement I have seen most commonly from physicians and other health
experts. It comes from considering studies showing negative effects of
fat intake on cholesterol, heart disease and cancer without recognizing
that the studies being quoted were performed on diets that were not
limited in carbohydrates.
In reality,
studies that were performed on diets low in carbohydrate reveal that dietary
fat promotes the production of HDL cholesterol - the one that prevents
heart disease. At the same time, limiting carbohydrate in the diet results
in decreased levels of LDL cholesterol, the one which produces atherosclerosis.
(This is discussed more in detail in the section on cholesterol.)
As for
cancer, I know of no studies that examine whether the amount of fat in
a low carbohydrate diet has the same effect on cancer risk as in a diet
that obtains 50% of calories from carbohydrate. Based on the metabolic
processes in the human body, I suspect we would see a very different picture.
So when
you see statements about the high fat content of a low carbohydrate diet,
remember that the other side of the equation - the low carbohydrate content
- must be considered also in order to be scientifically valid.
A
low carb diet causes weight loss because your calories are so limited
that you are in a state of starvation. You do not feel hungry because
of ketones created by the starvation.
I must
admit, this criticism is correct for some of the popular low carb plans
promoted by various authors, because they restrict the total caloric intake.
(For details, see the comparison of popular
low carb plans.)
Ketones
are the breakdown products of fat when it is used for energy. One of the
major principles of a low carbohydrate diet is that the body is required
to use fat for energy because not enough glucose is supplied by dietary
carbohydrate. This same situation occurs every night while we sleep and
are not eating anything, so having ketones in the bloodstream is a natural
state. The difference is the quantity of ketones in the natural
state versus a state of ongoing ketosis.
Let us
consider a low carb plan where only carbohydrate is limited, and both
protein and fat are eaten as desired to satisfy hunger. This would still
lead to some ketone production when fat is used for fuel, however, there
is no scientific evidence that this level of ketones would inhibit the
appetite enough to prevent ingestion of adequate calories to meet the
body's needs.
In other
words, starvation = ketones, but ketones do not = starvation. Additionally,
it is likely that the the major appetite control in an unrestricted low
carb diet is the hormonal effect of dietary fat on brain neurotransmitters.
The
only reason you lose weight on a low carb diet is because of limited calories
and food boredom.
As noted
above, it is likely that an individual on a low carb diet will consume
fewer calories because of the appetite control of the fat intake, and
of the ketones to some degree. But there are other factors at work that
are equally important.
In a
low carb diet, the body's glucose needs are partially met by converting
dietary protein to glucose. It is a known scientific fact that some of
the energy of the protein is lost in this conversion process. It actually
requires 1.6 grams of protein to produce 1 gram of glucose.
Here
is an example:
Suppose
you eat a meal that contains 28 grams of carbohydrate (like a baked potato)
and 56 grams of protein (like an 8 oz lean steak). Both protein and carbohydrate
are known to provide 4 calories per gram, so the total for this meal would
be 336 calories.
After
digestion, the body converts the 28 grams of carbohydrate to 28 grams
of glucose, for 112 calories. Let's assume you get a full 4 calories
per gram of protein when it is used as energy (it is actually somewhat
less), which means you get 224 calories from the steak. That is a total
of 336 calories.
Now
suppose instead you ate only a 12 oz steak and no potato. This would
be 84 grams of protein (7 grams of protein per ounce of meat), or the
same total of 336 calories for the meal.
After
digestion, the body will convert some of the protein to glucose to meet
its baseline needs (mainly for brain activity). Let's assume it produces
the same 112 calories as we got in the previous meal, or 28 grams of
glucose. Since it requires 1.6 grams of protein for each 1 gram of glucose
produced in this process, it will require 44 grams of protein to accomplish
this. This leaves 40 grams of protein to be used as energy. Again, assuming
we get the full 4 calories per gram, this gives us 160 calories of energy
from the protein. The final effect is that we have now obtained only
272 calories of energy from this 336-calorie meal. The rest of the energy
was used in converting the protein to glucose.
An additional
factor involves the ketones that are produced when fat is used for energy.
When excess amounts of ketones are present in the bloodstream, they are
excreted by the kidney. Because ketones represent a portion of the 9 calories
per gram of energy that is stored in the fatty acid molecules, this also
represents a loss of calories.
These
scientific principles explain why it is possible to lose weight on a low
carb diet without a restriction in calories, and even with an increase
in caloric intake!
A word
of warning - over the years numerous dietary supplements or specific foods
have been advertised as being "fat-burning", which sounds like
the process described above. Be aware that there is no single food, "natural"
product or vitamin supplement that can take advantage of this process.
It is a result of the composition of the complete dietary intake.
Additional explanations
about these concepts are provided in "DIET
TRUTHS REVEALED: The IDEAL DIET for Human Health." This
142-page book reveals the known scientific facts about human nutrition
and metabolism in layman's terms, explaining why low carbohydrate diets
are the healthiest for most people. More
information about the book
When
you understand the science, you can interpret these and other criticisms
(and there will undoubtedly be more) with the security that comes with
knowledge.
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(c) copyright 2000 Jan McBride, M.D.
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