After studying nutrition and naturopathic medicine in the 90s, and competiting in ultra endurance sports since then - I was shocked at what happened when I made the switch to Run more on Fat...
From 2007-2012 while training for a number of ultras between 50km and 250km my weight hovered between 76 and 81kg...and three problems remained constant for me,
1) muscle tightness and fatigue after 30km (glucose dependence),
2) Joint/muscle aches - ibuprofen post training (inflammation),
3) digestive issues and some resistant belly-fat. (gut irritation)
Then in the 2013 off-season I reduced my miles down by about 90%, and drastically reduced grains, sugar-bars, breads, pastas, rice, juices and sports drinks where ever possible - increased more quality fats and low GI carbs in my training diet.
Within around 4 weeks my weight dropped to 74kgs. As I entered the racing season, I increased my weekly mileage - and I found my muscles could endure 8 plus hour training sessions, no longer needing any anti-inflammatory meds after training, and my recovery was notably faster.
Conversely, running UTMB (Ultra Trail Mont Blanc) I was forced to use simple high-carb foods at checkpoints, only to experience bloating, joint/foot pain-inflammation, fluid-swelling and blistering - which dramaticaly slowed my finishing time, over 40 hours.
"Reducing the triggers of an up-regulated blood sugar/insulin metabolism, is THE key to turning on my fat-burning metabolism, lowering inflammation and enabling more consistent training and improved endurance-speed."