In episode 124 of his video series, Dr. Larson discusses:

Is it good to exercise in a fasted state? There was an article published just a few days ago that provides an answer to that question. A recent study gives us a little slice of the answer. This particular study evaluated a hormone called Ghrelin, which is a hormone that is produced by the stomach in a fasted state. It is responsible for telling your brain that you are hungry and should eat. But the study found that there were other things that it does and reasons why it is produced.

The study was performed on mice, and the researchers evaluated Ghrelin in a couple of different ways. Ghrelin is a hormone that is responsible for appetite regulation, and it gets released when we are in a fasted state. Sometimes the appetite process is oversimplified by talking about just Leptin and Ghrelin as being responsible for the appetite process. Basically, Leptin gets released when we are full, and Ghrelin gets released when we are hungry. But there are many other hormones involved in the process. It is a very complicated hormonal cascade with a lot of different moving parts. In essence, both fat accumulation and fat burning are hormonally regulated.

The researchers evaluated mice in three different groups. One group was allowed to eat whenever they wanted. The second group of mice was on a time-restricted diet, which is similar to intermittent fasting. In intermittent fasting, you go without food for 14-16 hours, and you have a restricted window to eat. The second group of mice was placed on time-restricted eating. And then there was a third group, which was the GKO group or the Ghrelin knock-out group.

The mice in the third group were genetically engineered to not produce Ghrelin at all. Fasting for humans is considered 10-12 hours. But you are always either in a fasted state or you are in a fed state. Researchers found that in a fasted state, Ghrelin in mice went up, which would be expected. What was interesting, however, is that they had a sophisticated way to measure voluntary activity (“exercise”). And in the fasted state, the researchers found that the mice’s voluntary activity went up as Ghrelin went up. But in the group that was allowed to eat whenever they wanted, they did not see a spike of Ghrelin and did not see an increase in voluntary activity.

Being that this study was done on mice, researchers went one step further, and they looked at parts of the brain. What they found was that as Ghrelin went up, and voluntary activity went up, the brain gave the mice a little shot of dopamine. Dopamine is a motivation and reward neurotransmitter. So those mice brains that were in a fasted state and exercised more released dopamine to reward and motivate them.


Why would the body do that? For evolutionary reasons, when you are in a hunger state, the body motivates you for food exploratory behavior. Your brain wants to encourage you to seek food when you are hungry. So, when you are fasting, Ghrelin helps to promote more activity, and then it rewards you for that activity. The no Ghrelin (GKO) mice did not have a release of Ghrelin, did not exercise more and did not produce dopamine. But, interestingly, the researchers injected them with it, and they found that the GKO group’s voluntary activity, likewise, went up.



This study demonstrates how Ghrelin is a key component of the whole cascade of hormonal reactions that help in a fasted state. The study supports the theory that not only is it a good idea to exercise in a fasted state, but it actually built into your DNA to do so. Ghrelin helps to increase movement, and it promotes the release of dopamine, which motivates you to go further.

It is a good confirmation that fasting is a good idea before you exercise, especially if you are trying to fat burn and boost your motivation. The fact that it was done on mice will probably not negate its extrapolation to humans. When it comes to hormones and animal studies, what we find in animals, we tend to see similar reactions in humans. So the answer is if you want to fat burn and boost your motivation to work out, do it in a fasted state.


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