I listened to Andrew Huberman's podcast titled:
"Effects of Fasting and Time Restricted Eating on Fat Loss and Health" It is dense with information from the most recent studies on how fasting affects different parts of the body. I would highly recommend listening for those interested in fasting.
He also links
to this study that has a table of ~40 different studies that looked at fasting (of various lengths) and their conclusions. Much of the podcast references and expands on the results from the studies within this table.
The following are my notes from the episode:
When you eat is just as important as what you eat. In most cases, you should eat during the period when we are most active (during the day). Meals during the nocturnal cycle can mess up hormone balance in the brain and negatively affect the circadian rhythm. Regular fasting windows during the day help regulate circadian rhythm as well as organ health. By eating unconstrained/around the clock you are making yourself sicker.
8 hour feeding window was studied based on human constraints. Maintaining these types of lab studies are very intensive and require the grad student/post doc to be present and ensure feeding occurs at the right time and that the subjects (in this case mice) do not have any food. Well, in the original landmark study the grad student’s SO told him that she didn’t want him away from home all day so they decided to put the study at 8 hours to keep healthy relations with his SO.
However, the 8 hour feeding window is not completely arbitrary and has been tested on humans in multiple studies and shown to be very beneficial.
First pillar of intermittent fasting is not to eat any food for an hour after you wake up and possibly longer.
Second pillar is for 2-3 hours prior to bedtime, you do not ingest any food or liquid calories.
Extending the daily fast past the first hour of the day extends the benefits of the deep fasted state from sleep. Someone who is eating within 2-3 hrs of bed time is not fasting during sleep. Huberman says that a schedule starting from 10 AM-12 PM and ending sometime between 6 PM-8 PM will work best for most people to achieve this purpose (8 hour window). Some studies suggest that eating (specifically protein) earlier in the day is more beneficial for maintaining muscle composition and/or growth, regardless of when during the day resistance training occurs. Data from Satchin Panda (
my circadian clock) says that almost everyone underestimates their eating window by an hour or 2. So if you want a 8 hour eating window, you should focus on a 6 or 7 hour feeding window.
4-6 hrs improve insulin sensitivity, pancreas function, decrease evening appetite and oxidated stress etc. However they produce no change in body weight or tend to produce increase in body weight. Suggests that for many people this results in overeating. Not true with one meal a day, people tend to lose weight.
Huberman recommends an intermittent fasting schedule of 8 hrs and does not recommend below 8 hours, especially for people who train hard (physically or mentally). Below 8 hours seems to run the risk of poor hormonal health especially when one is faced with increased stress.
If you are shifting your feeding window, it takes about 7-10 days for your body to become accustomed to it. Dr. Panda recommends changing only about one hour at a time.
A feeding window that drifts around a lot during the week offsets a lot of the positive benefits of intermittent fasting, even with a short feeding window. For example, if you are strict about your 8-hour feeding window during the week but on the weekends it is longer and extends later, it is as if you are going to bed at a later time to your metabolic clock.
Things we can do to accelerate from fed to unfed (fasted) state:
-Glucose clearing- ie light movement or activity after feeding. Activities such as light walking for 20-30 min or even calisthenics such as air squats or pushups.
-HIIT ideally later in the day. Some studies have shown that glucose levels increase when performed earlier in the day. This is provided you do not ingest something after the training
-Some supplements such as metformin or berberine . Huberman says these are a double edged blade and if used should be used with caution.
Things that help reduce blood glucose (but do not sweep it out of the system like the above): cinnamon, lemon, lime, and sea salt.
Intermittent fasting has been shown to have a direct positive effect on the mucosal lining which improves the gut microbiome.
There is no black and white answer to what constitutes breaking of a fast. Much of it depends on when you ate and what you ate.
Drinking water, tea or black coffee will not break your fast. Eating one peanut when in a deep fasted state won't break your fast, perhaps even a handful of peanuts wouldn't break your fast. However, if you just ate recently, one peanut may break your fast. The technical answer would be to wear a continuous glucose monitor. The simple rule is that anything that involves sugar can break your fast. Some studies indicate that even 1 g of sugar can break your fast. If you eat purely fat, you are likely not breaking your fast.