Muscle Glycogen and Exercise – Phinney Revisited

Stephen Phinney did an interesting study which put elite bicycle races on a ketogenic diet  (Metabolism. 1983 Aug;32(8):769-76. The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation. Phinney SD, Bistrian BR, Evans WJ, Gervino E, Blackburn GL).

The study measured the amount of muscle glycogen in the subjects before they went on the ketogenic diet and after then they had been in the diet for 4 weeks and before/after exercise in both diets. Here’s the mean values.

Before Exercise
After Exercise
Keto Wk 4
Before Exercise
Keto Wk 4
After Exercise
Mean 143 53 76 56

After the four weeks the subjects had approximately half the muscle glycogen in the ketogenic state. The fact that the post exercise numbers were similar pre and post keto appears to show the glycogen sparing effects of the diet. The study states that the reason that the muscle glycogen doesn’t go to zero is that there is muscle glycogen stored in fast and slow twitch muscles. The slow twitch muscles were used during this exercise and depleted by this test. Hence, glycogen use was 90 pre-keto and only 20 post keto.

RER Shift from Carbs to Fat

The difference between the two diets  is also seen in the shift down of the RQ (RER) of the subjects during exercise from a mean of 0.83 to 0.72. This represents a significant shift towards fat oxidation in place of carbohydrate oxidation.

My Own Numbers on Low Carb Diet

Adding in my VO2max numbers to the chart from a previous post:

HR RER %Fat %Carbs %VO2max
120 0.70 99% 1% 56%
125 0.74 88% 12% 61%
130 0.77 77% 23% 66%
135 0.80 66% 34% 71%
140 0.84 55% 45% 77%
145 0.87 44% 56% 82%
150 0.90 33% 67% 87%
155 0.94 21% 79% 92%
160 0.97 10% 90% 97%
165 1.00 0% 100% 102%

For me, 65% of my VO2max corresponds to a heart rate of about 129 or fat as about 79% of calories. My RER was about 0.75 at that rate which is not as good as the elite bicyclists in this study who were at an RER or 0.72 at that level of activity. Hopefully with additional cardio training my RER will be reduced further during exercise.

Energy Used

The study showed that the athletes used about 950 kcals/hr. Before keto 28% of that came from carbs and the rest from fat. After keto, the carb oxidation rate dropped to 9%.

The energy from fat when in keto was 864 kcal/hr which is 1.6 g/min. This is a high level of fat oxidation and is more than my own fat oxidation rate (of around 1.2 g/min).


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