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Difference Between a 16 Hour Fast and a 20 Hour Fast
The Timeline of Fasting
To understand the difference between different lengths of fast we need to examine charts which describes how is our body fueled during a fast. Such a chart was published in Annual Reviews of Nutrition and as you can see, it identifies an important point in 16th hour of a fast.
At hour 16 our body utilizes more glucose from GNG than it does from glycogen. Glycogen is the storage of glucose in the liver. GNG is a process of creating glucose from proteins, lactate or fat.
Now we will get a little heavy on biochemistry but a lot of content on this channel is focused on ketones, so it is worth it. We generate energy by a process called the Krebs cycle. In a fed state, Acetyl-CoA binds to oxaloacetate in the first reaction of the cycle. However, when we fast, oxaloacetate has to be used for GNG, so there available glucose for the brain. Therefore, acetyl-CoA can’t bind to oxaloacetate and it is instead converted into ketones by an enzyme called HMG CoA synthase.
In simple terms, normally acetyl CoA binds with oxaloacetate but during fasting, the oxaloacetate is "stolen" by GNG. Acetyl CoA starts to accumulate and it starts to be converted into ketones by HMH CoA synthase.
Ketones are the molecule that brings a lot of benefits of fasting like improving cognition via stimulating BDNF, reducing inflammation via blocking NLRP3 or possibly prolonging lifespan through acting as HDAC inhibitors. As seen in the first chart, after the 16th hour we are already generating more than half of all glucose in the body by GNG. Oxaloacetate is "stolen", acetyl CoA accumulates and production of ketones can increase. In a study published in Metabolism, it was reported that the difference in concentrations of ketone bodies after 15h and 20h can be two-fold (0.7umole/ml vs 1.5umole/ml).
However, it also must be noted that these increases might be highly individual. It is expected that a fat-adapted individual will see a big difference in ketone concentration in hours 16 and 20. In contrast, someone whose diet was 50-60% carbohydrates with minimal breaks between meals might need even longer time to experience the spike in ketones. This is influence by levels of ketogenic enzymes (like HMG CoA synthase). Those who fast regularly and restrict carbs will have higher levels and they will be able to produce ketones sooner. Others might need to fast longer than 24 hours to surpass ketones blood levels of 1 μmole/ml.
The increasing rate of GNG after 16th hour of fast also has important implication for fat loss, another important goal of fasting. Firstly, decreasing insulin allows for greater activity of HSL which is an essential enzyme for breaking down our fat reserves.
According to research, it seems that deeper adaptations to fasting begin after the 16th hour. You might know that at the moment you are not able to fast for 36 or 48 hours but you would like to get there in future. You would like to get the full benefit of these long fasts associated with ketone levels being as high as 3 μmole/ml. Then the 20h fast is a better choice for you. Thanks to those 4 hours, you will get higher levels of aforementioned enzymes like HSL and HMG CoA synthase. These will make it easier for your body to gradually deal with longer and longer fasts.