Interesting article,
American Journal of Clinical Nutrition, Vol. 75, No. 5,
951-953, May 2002 © 2002 American Society for Clinical
Nutrition
Letter to the Editor
"Is dietary carbohydrate essential for human nutrition?"
Eric C Westman
Department of Medicine Duke University Medical Center Suite
200-B Wing Box 50, 2200 West Main Street Durham, NC 27705
Email: ewestman~duke.edu
Dear Sir:
I read with interest the article by Dewailly et al (1)
regarding diet and cardiovascular disease in the Inuit of
Nunavik, but I was disappointed that no information
regarding macronutrient intake was presented or considered
in the estimation of cardiovascular risk.
The traditional Inuit diet consists primarily of protein and
fat, somewhat similar to the low-carbohydrate diets promoted
in popular weight-reducing diets (2). These diets have
caused concern among nutritionists because of the metabolic
changes and health risks associated with limited
carbohydrate consumption (3). However, in exploring the
risks and benefits of carbohydrate restriction, I was
surprised to find little evidence that exogenous
carbohydrate is needed for human function.
The currently established human essential nutrients are
water, energy, amino acids (histidine, isoleucine, leucine,
lysine, methionine, phenylalanine, threonine, tryptophan,
and valine), essential fatty acids (linoleic and -linolenic
acids), vitamins (ascorbic acid, vitamin A, vitamin D,
vitamin E, vitamin K, thiamine, riboflavin, niacin, vitamin
B-6, pantothenic acid, folic acid, biotin, and vitamin
B-12), minerals (calcium, phosphorus, magnesium, and iron),
trace minerals (zinc, copper, manganese, iodine, selenium,
molybdenum, and chromium), electrolytes (sodium, potassium,
and chloride), and ultratrace minerals (4). (Note the
absence of specific carbohydrates from this list.)
Although one current recommended dietary carbohydrate intake
for adults is 150 g/d, it is interesting to examine how this
recommendation was determined at a recent international
conference (5):
"The theoretical minimal level of carbohydrate (CHO) intake
is zero, but CHO is a universal fuel for all cells, the
cheapest source of dietary energy, and also the source of
plant fiber. In addition, the complete absence of dietary
CHO entails the breakdown of fat to supply energy [glycerol
as a gluconeogenic substrate, and ketone bodies as an
alternative fuel for the central nervous system (CNS)],
resulting in symptomatic ketosis.
Data in childhood are unavailable, but ketosis in adults can
be prevented by a daily CHO intake of about 50 g. This value
appears to approximate the quantity of glucose required to
satisfy minimal glucose needs of the CNS and during
starvation. The Group therefore concluded that the
theoretical minimum intake of zero should not be recommended
as a practical minimum....about 100 g of glucose/d are
irreversibly oxidized by the brain from the age of 3_4 y
onward. However, this excludes recycled carbon,
gluconeogenic carbon, for example from glycerol, and it does
not account for glucose used by other non-CNS tissues.
For example, in the adult, muscle and other non-CNS account
for an additional 20_30 g of glucose daily. For this reason
a safety margin of 50 g/d is arbitrarily added to the value
of 100 g/d and the practical minimal CHO intake set at 150
g/d beyond the ages of 3_4 y."
Thus, although carbohydrate could theoretically be
eliminated from the diet, the recommended intake of 150 g/d
ensures an adequate supply of glucose for the CNS. However,
it appears that during starvation (a condition in which the
intakes of carbohydrate, protein, and fat are eliminated),
an adequate amount of substrate for the CNS is provided
through gluconeogenesis and ketogenesis (6).
The elimination of dietary carbohydrate did not diminish the
energy supply to the CNS under the conditions of these
experiments. Second, carbohydrate is recommended to avert
symptomatic ketosis. In the largest published series on
carbohydrate-restricted diets, ketosis was not typically
symptomatic (7).
The most direct way to determine whether carbohydrate is an
essential nutrient is to eliminate it from the diet in
controlled laboratory studies. In studies involving rats and
chicks, the elimination of dietary carbohydrate caused no
obvious problems (8_12). It was only when carbohydrate
restriction was combined with glycerol restriction (by
substituting fatty acids for triacylglycerol) that chicks
did not develop normally (13).
Thus, it appears that some minimum amount of a gluconeogenic
precursor is essential - for example, glycerol obtained from
fat (triacylglycerol) consumption. More subtle abnormalities
from carbohydrate elimination might not have been observed
in these studies.
In addition, the essentiality of some nutrients is
species-specific; therefore, these studies do not provide
convincing evidence that elimination of dietary carbohydrate
is safe in humans (4).
The usual way to discover the essentiality of nutrients is
through the identification of specific deficiency syndromes
(4). I found no evidence of a carbohydrate deficiency
syndrome in humans. Protein deprivation leads to
kwashiorkor, and energy deprivation leads to marasmus;
however, there is no specific carbohydrate deficiency
syndrome.
Few contemporary human cultures eat low-carbohydrate diets,
but the traditional Eskimo diet is very low (50 g/d) in
carbohydrate (2). It is possible that if more humans
consumed diets severely restricted in carbohydrate, a
carbohydrate deficiency syndrome might become apparent.
When carbohydrates are eliminated from the diet, there is a
risk that intakes of vitamins, minerals, and perhaps yet
unidentified beneficial nutrients provided by
carbohydrate-rich foodstuffs (eg, fiber) will be inadequate.
There are case reports of extreme dieters who probably
developed deficiencies. One dieter who only ate cheese,
meat, and eggs (no vegetables) was reported to have
developed thiamine-deficient optic neuropathy (14). Another
dieter may have developed a relapse of acute variegate
porphyria (15). However, most of the current
low-carbohydrate, weight-reducing diets advocate the
consumption of low-carbohydrate vegetables and vitamin
supplements.
Although there is certainly no evidence from which to
conclude that extreme restriction of dietary carbohydrate is
harmless, I was surprised to find that there is similarly
little evidence to conclude that extreme restriction of
carbohydrate is harmful. In fact, the consequential
breakdown of fat as a result of carbohydrate restriction may
be beneficial in the treatment of obesity (7). Perhaps it is
time to carefully examine the issue of whether carbohydrate
is an essential component of human nutrition.
REFERENCES
1.Dewailly E, Blanchet C, Lemieux S, et al. n-3 Fatty acids
and cardiovascular disease risk factors among the Inuit of
Nunavik. Am J Clin Nutr 2001;74:464_73.[Abstract/Full Text]
2.Shaffer PA. Antiketogenesis. II. The ketogenic
antiketogenic balance in man. J Biol Chem 1921;47:463_73.
3.Westman EC. A review of very low carbohydrate diets for
weight loss. J Clin Outcomes Manage 1999;6:36_40. 4.Harper
AE. Defining the essentiality of nutrients. In: Shils MD,
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the utilization of "carbohydrate-free" diets by the chick.
II. Level of glycerol. J Nutr 1964;84:327_30. 14.Hoyt CS,
Billson FA. Low-carbohydrate diet optic neuropathy. Med J
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