Posted: 3/20/2012 9:28:18 PM
There was a resurgence of interest in very low calorie ketogenic diets for weight
loss in the 1970's, followed closely by the complications (including sudden death)
associated with the Liquid Protein diet popularized in 1976. However, the fatigue
and apparent cardiac dysfunction caused by this collagen-based fad diet stood in stark
contrast to the published experience of arctic explorers such as Schwatka and Stefansson.
In addition, physicians who monitored patients following very low calorie diets observed
wide variations between the exercise-tolerance of these individuals.
Given that the elegant research on the metabolism of total fasting by Dr. George Cahill
and colleagues had demonstrated that full adaptation of nitrogen, fat, and carbohydrate
metabolism required a number of weeks 12], it seemed reasonable to hypothesize that exercise tolerance would take more than
a week to recover after removal of carbohydrate from the diet. This view was supported
by the subsequent discovery of the prescient adaptation quote from Schwatka's diary
6] noted above.
To test this hypothesis, the current author (under the mentorship of Drs. Ethan Sims
and Edward Horton at the University of Vermont) undertook a study of subjects given
a very low calorie ketogenic diet for 6 weeks in a metabolic research ward 13]. The protein for this diet, along with a modicum of inherent fat, was provided by
lean meat, fish, and poultry providing 1.2 grams of protein per kg of reference ("ideal")
body weight daily. In addition, mindful that the natriuresis of fasting could reduce
circulating blood volume and cause secondary renal potassium wasting, the subjects
were prescribed 3 grams of supplemental sodium as bouillion and 25 mEq (1 g) of potassium
as bicarbonate daily.
Treadmill performance testing of these subjects included determinations of peak aerobic
power (VO2max) after a 2-week weight maintenance baseline diet, and again after 6 weeks of the
ketogenic weight loss diet. Endurance time to exhaustion was quantitated at 75% of
the baseline VO2max. This endurance test was repeated again after one week of weight loss and finally
after 6 weeks of weight loss. Other than these tests, the subjects did no training
exercise during their participation in this study. To compensate for the fact that
the average subject had lost over10 kg, the final endurance treadmill test was performed
with the subject carrying a backpack equivalent in weight to the amount lost.
The energy expenditure data (expressed as oxygen consumption) and exercise times across
this 8-week inpatient study are shown in Table 1. That these subjects'peak aerobic power did not decline despite 6 weeks of a carbohydrate-free,
severely hypocaloric diet implies that the protein and mineral contents of the diet
were adequate to preserve functional tissue. As can be noted, endurance time to exhaustion
was reduced after one week of the ketogenic diet, but it was significantly increased
over the baseline value by the 6-week time point. However the interpretation of this
endurance test is confounded by the fact that the oxygen cost (ie, energy cost) of
the treadmill exercise had significantly decreased following the weight loss, and
this occurred despite the subjects being made to carry a backpack loaded to bring
them back to their initial exercise test weight.