Time-Restricted Eating (TRE: Eating only within a certain window of time each day. Noted as "TIME FASTING:TIME EATING" - so for example, if you only eat from 12:00 noon to 8pm on any given day, you would say you do 16:8 TRE. Typically 16:8 is thought of as the bare minimum, other common regimens include 18:6, 20:4, or OMAD, which is "one meal per day."
Note that IF and TRE are NOT the same thing.
IF and TRE are extremely easy to do (just don't eat anything!), free, and exceptionally safe for most patients. Obviously, speak to your health care provider to make sure it's safe for you before trying. (For example, if you are on insulin for diabetes, please don't try this before speaking with your doctor.)
You are allowed NO calories within your fasting window. Yes, there are exceptions, but it's easier to think of this way.
It has been shown in some studies that restricting food intake over a lifetime has benefits on aging and lifespan in animals. This principle very probably holds for humans too, based on what we know about obesity-related illness and based on cross-sectional studies. However, we don't know the optimal "dose" or frequency of fasting to reap optimal benefits.
Those benefits, though, are not only from decreased free-radial formation (thought to damage DNA leading to increased error rate in replication, and therefore more rapid aging) or from weight loss itself (in fact a study in JAMA in 2020 did not see weight loss benefit of 16:8 time-restricted eating compared to standard 3 meals per day). According to peer-reviewed studies, additional benefits come from:
Activation of adaptive cellular pathways, which improve glucose regulation, improve stress resistance, and suppress inflammation. (Note: The terms "stress" and "inflammation" are defined more carefully in the NEJM paper, however decreasing both of these things should be thought of as beneficial in this context.)
Improved blood pressure and lower heart rate.
Improved endurance training.
Fat loss specific to the abdomen.
Increased autophagy via proteasome activation. (Essentially, this is selective degradation of misfolded and regulatory proteins, but not the bulk of normal healthy proteins. This has implications in decreased oncogenesis/cancer risk, while typically maintaining muscle mass.)
In mouse models, a 20-hour fast is typically used, and has resulted in significant benefits in longevity. Given the faster basal metabolic rate of mice, the equivalent in humans is probably on the order of days, not hours. The point remains, however, that we don't know the optimal fasting time for humans, however we DO know that a typical breakfast-lunch-dinner timing of meals does not trigger any fasting response, and can be thought of as a constant "IV drip" of calories. (In fact, the notion of 3 meals per day is a relatively modern thing, and has no basis on our physiology. An interesting history of modern meals can be found HERE.)
Beyond longevity, fasting has benefits on many chronic disorders (in animal models), including obesity, type 2 diabetes, cardiovascular disease, cancer, and neurodegenerative disorders. Fasting may also "bolster mental and physical performance, as well as disease resistance."
Change your expectation. Don't expect to eat 3 meals and tons of snacks every day. Expect to feel hungry and to feel cravings for food.
Start with something manageable. Try skipping all food/calories after dinner until lunch the next day (16:8) for a few days. Slowly increase your fasting window and see how you feel.
Anticipate being HANGRY (more irritable when you're hungry) for the first month or so.
When feeling hungry in your fasting window - drink water! Avoid the temptation to snack.
Don't count calories within your feeding window. Try to simply eat as healthy as possible (see the "Diet" blog post and abide by the tenets outlined therein).
4-5 days per week, I only eat between noon and 8pm. This gives me a 16:8 TRE window, which I sometimes push to 18:6 or even OMAD. This is easier to do if there are no good options for lunch or I get too busy, but typically I do 16:8. On the weekends, I will often cook and eat breakfast with my family, so I break the 16:8 window there.
I will do an intermittent fast a few times per year, most often when I feel like I need a reset. The longest fast I have done is 72 hours (3 days), taking in nothing but water with salt and black coffee to stay sane. These fasts can be miserable, but the key is starting off on the right foot. Author Tim Ferriss has a good 3-day fasting protocol (go to the section 3-day fasting protocol by day) that I've used and liked. I usually track my longer fasts with the Zero app.
Last, I don't get down on myself if I break fasting or eat when I'm not supposed to. I'm human and imperfect like everyone else.
Cabo and Mattson, N Engl J Med 2019; 381:2541-2551
Lowe et. al., JAMA Intern Med. 2020 Sep 28:e204153
Verplank et al, PNAS 116; 10:4228-4237
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