There is so much literature on fat adaptation I am not sure where to start but here are some studies list lower down that you might want to look at. Many studies involving shifting macro nutrient proportions are conducted on obese and sedentary subjects so don’t really cross over well to what we do.
I would say that our position comes, first of all, from empirical evidence gathered over 30 some years of training and coaching. Then from reading the scientific literature to see if what we have observed can be explained with science. This allows us to place a theoretical basis to our practical observations and connect some of the dots a bit better.
For what we advocate we can can’t and don’t separate dietary manipulation from the exercise. We see them as working synergistically. Ketogenic diets work to increase you body’s ability to utilize fat for fuel at all intensities. There are many studies and a huge amount of anecdotal evidence to support this. The problem with ketogenic diets is that they can be hard to stick to. Tat’s why we recommend a diet with lowered carbohydrate content than the traditional USDA food pyramid but not take to the extreme of ketogenic levels. This will be easier to live with and will still result in good fat adaptation when coupled with a high volume of low to moderate intensity aerobic work. Including fasted AM workouts seems to enhance the fat adaptation process.
My observations suggest that the higher the volume of training the athlete is doing the less he/she needs to restrict carbs to get a fat adaptation effect. Folks doing Z1-2 aerobic training for 8-10 hours/week may need to restrict carbs more to get a similar effect to someone doing 15+ hours of Z1-2 per week. One of the pros I coach is a vegetarian and eats a high carb diet but frequently trains 20 hours a week and does many fasted workouts. He seems to be very well fat adapted and can go for many hours in the mountains with no food and no drop in energy or performance.
For decades the nutrition experts told us that fat maxed out at about 60% of maxVO2 or close to ones anaerobic threshold. This falsehood persisted up until recently by bing passed down from one generation of sport scientists and nutritionist/dietitians to the next. It was codified in text books and taken as gospel. Coaches cold see that this could not possibly be true but all the peer reviewed studies showed so the science was settled. Recent studies have completely debunked this dogma and verified what the old coaches knew. Why: All the old studies were done on untrained subjects. Volek’s F.A.S.T.E.R. and Seiler’s studies (below) used well trained endurance athletes the results showed that fat can be used at high rates right up to maximal intensity and rates twice as high as previously thought. One of my XC skiers was once tested in a lab and found to be getting 10% of his energy from fat when operating at his maxVO2. The lab folks told him that he had a messed up metabolism and needed to fix it. We had worked hard to get him this way so we ignored their recommendation.
When Steve, Mark Twight and Scott Backes climbed the Slovak direct on Denali in 60 hours consuming only a few GU packets each, Steve was accused of lying about this by a dietitian. She told him it was physically impossible to do such a feat without taking in more calories. She had learned the dogma I referred to above and didn’t know about fat adaptation.
The scientists do not always have all the answers.
As for satiety question: Fat seems to make us feel more full for longer so we feel less hungry. I do not advocate caloric restriction for training athletes. You need calories to fuel your work. As mentioned above, I believe it is the combination of high volume work and restricted carbs that give s the best fat adaptation effect.
Fat Adaptation References
Study of 20 endurance trained 11 on HC and 9 on LC/HF diet for 12 weeks doing the same training. LC/HF body fat dropped 5.2% vs .7% for HC group. 100km time trial improved 4min in the LC/HF group vs 1 min in the HC group. Fat oxidation (use as fuel increased dramatically) during the 100km time trial LC/HF group vs the HC group.
Jeff Volek’s F.A.S.T.E.R. study of 20 elite ultra runners. Showing that fat oxidation rates during submax intensity exercise by well fat adapted elite ultra runners was 2.3 time higher than those with a high carb diet.
For those who struggle to pick out the important points of peer reviewed science studies. A summary of the findings of the Volek F.A.S.T.E.R. study with easy to read graphics and explanations.
Study on high intensity interval protocol comparing well trained and fat adapted athletes vs recreationally trained athletes showed a much greater fat use at high intensity among the fat adapted athletes.
This study shows the vast increase in fat oxidation (use as fuel) in fat adapted athletes and how this spares glycogen.