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Stephen Phinney, MD, PhD
Dr. Phinney is Professor Emeritus at University of California, Davis and an expert on the use of low-carbohydrate diets by endurance athletes. He works with Dr. Jeff Volek in studying endurance athletes’ performance on these diets. Dr. Phinney has spent 35 years studying diet and exercise. He received his MD, PhD, and post-doc at Stanford, MIT, and Harvard.
Dr. Jeff Volek (left) and Dr. Stephen Phinney. Photo courtesy of Volek and Phinney.
Before I start this article, I have to thank Zach Bitter (the American 100-mile record holder) and Casper Wakefield of Denmark (the 2013 record breaker of the Yukon Artic Ultra) for introducing me to the concept of increased fat metabolism after they both had such success with altering their diet to achieve this.
Dr. Phinney apparently knew this diet could work a long time ago. But, speaking of Denmark, in 1939 Christensen and Hansen came out with the first study that showed the opposite, that endurance performance tended to be about twice as good on a high-carb diet. And Bergström of Sweden showed in 1967 that athletes who swtiched to a low-carb, high-protein diet performed poorly.
But Dr. Phinney was interested in a book he found on his father’s bookshelf about the Inuits who survived on essentially a no-carb diet. Not only did they survive, but they were nomads and their lifestyle was described in a collection of diary stories entitled The Long Arctic Search by Dr. Schwatka in 1878. Dr. Schwatka lived and traveled with the Inuits for an extended period of time and noted it took him two to three weeks to adapt to their diet.
Dr. Phinney decided to study what he interpreted to be close to the Inuit diet in highly trained cylists in 1983. The diet was composed of 80% fat, 15% protein, and <2% carbs.
At first, Dr. Phinney was surprised that he could suddenly sprint faster than these elite cyclists. But after a period of four weeks adaptation, he saw that the cyclists’ performance returned to where it started, but importantly, they had a significant drop in their respiratory quotient and they were burning over 90% of their calories from fat at 65% VO2 max. This meant that he could see on their very long rides that their glycogen use was reduced by 3.5 times and their glycogen reserves were not depleted.
There have been a host of ultrarunners of late who have claimed they have had success with a low-carb, high-fat diet. If you are like me, you have been anxiously awaiting the results of Dr. Volek and Dr. Phinney’s FASTER case-matched control trial study in elite male ultrarunners on low-carb, high-fat versus high-carb, low-fat diets. Dr. Phinney pointed out that the process for publishing data is long and, thus, this may be your one and only opportunity to hear/read about the results for a while.
FASTER stands for “Fat Adapted Substrate oxidation in Trail Elite Runners.” Runners were matched with controls on the basis of competitive record, running times, and age. It is important to note that they were told to continue their “habitual diet” and placed into one of the two categories based on what they ate before the start of the study.
Here is how the study subjects compared on after their first set of tests (there were 10 runners in each group):
HCD: High-carb diet. LCD: Low-carb diet. Photo: iRunFar/Tracy Høeg
The “Low-Carb Diet” (LCD) runners had a lower body-fat percentage and a slightly higher VO2 max on average than the “High-Carb Diet” (HCD).
HCD and LCD composition. Photo: iRunFar/Tracy Høeg
Dr. Phinney made continual note of the low-protein intake in both of the diets. He said neither of these diets is akin to high-meat-content Paleo diets and this is especially important for the low-carb dieters because if you eat low carb but not enough fat, you will not make enough ketones and you may go into a starvation state and experience decreases in your performance.
Results from a three-hour treadmill run in all participants. Red dots are those on the LCD and they were significantly better at oxidizing fat and there was no overlap with the HCD (blue dots). Photo: iRunFar/Tracy Høeg
Fuel usage in the HCD versus the LCD groups during three hours on a treadmill at 65% of VO2 max.
The LCD group has less than 10% reliance on carbohydrates. Dr. Phinney says this is important because they are better at using their fat as fuel–and this was what they expected to see.
He says athletes on a high-carb diet who “bonk” during a race are akin to a gasoline truck filled with a ton of gasoline in the back, but an empty fuel tank. After adaptation to a low-carb, high-fat diet, you are able to access all of your fat reserves and there is at least 20 times more energy stored in our fat than in our glycogen.
What does this mean in terms of performance? In the LCD athletes, if indeed 85% of the fuel comes from fat while these athletes are racing at 900 kilocalories/hour, then they will only need an additional 150 kcal/hour, which can either come from oral intake or glycogen stores. This is a very low reliance on additional energy and should decrease nausea and vomiting and the sensation of low energy.
[Author’s Note: LCD athletes do not necessarily burn 900 kcal/hour. That number was used as a random example and the number is in actuality dependent upon their weight, sex, speed, and the conditions of the course. These things being equal, the LCD and HCD runners are expected to burn the same amount of calories, but the LCD runners are able to meet a much higher percentage of their caloric needs with their own fat stores.]
Beyond this, there is evidence that:
There is less insulin resistance in low-carb runners.
A low-carb diet appears to be protective against oxidative stress because the “nutritional ketogenic diet” which these runners are on increases, among other things, arachadonic acid, that protects our cells’ membranes from oxidative damage. There is a ton of oxidative stress involved in running 100 miles because of the enormous amount of oxygen that is required to go through the body. (Dr. Phinney says there may be no better place to study oxidative stress in humans than Western States).
Increased levels of arachadonic acid may also help protect the stomach from damage.
At the end of the talk, there was a question about women runners who develop eating disorders on this diet and have decreased performance. Dr. Phinney stressed again that if you are going to decrease your carb intake, you need to increase your fat intake otherwise the diet is unhealthy and won’t meet nutritional needs. He also said that he doesn’t place a lot of importance on the particular types of fat a runner gets.
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