Exercising while fasted burns fat
You’ll notice that the amount of calories being burned are roughly the same (about 2000 Kcals), however during fasting a lot more of these calories are coming from fat. And, since the subjects were not eating any fat while fasting, this fat must have been body fat.
Remarkably good at regularting blood sugar
In clinical research there generally isn’t a concern that exercising while fasting will cause an ill effects to a persons blood sugar. The body is remarkably good at regulating blood sugar levels, even in the face of intense exercise.
One IF experiment
What’s going on here? I don’t know for sure. But going back to insulin again, there’s a good chance levels of this hormone are lower than before. Insulin encourages fat storage. Put another way, lower insulin means for efficient release of fat, which can be used to fuel the body. The reason that I’m not hungry is possibly because the body is ‘feeding’ off my fat. That might also explain how I’ve lost about 5 lbs and an inch has gone from my waist.
Leptin and Ghrelin
Leptin levels seem to go down significantly after 12 h of fasting, leading to increased body fat catabolism and leptin sensitivity. This is a good thing, since leptin resistance seems to frequently precede insulin resistance.
Insulin *resistance*? …no, the opposite!
Over time, however, you will very likely become more insulin sensitive. What is happening is compensatory adaptation, with different short-term and long-term responses. In the short term, your body is trying to become a more efficient fat-burning machine, and GH is involved in this adaptation. But in the short term, GH leads to insulin resistance, probably via actions on muscle and fat cells. This gradually improves in the long term, possibly through a concomitant increase in liver insulin sensitivity and glycogen storage capacity.