Simply put, a fast is a period of time for which one abstains from eating. In the most general sense, we all engage in a form of intermittent fasting: the hours between our last meal of the previous day and the first meal of the current day. Hence the term breakfast, which literally means to break the overnight fast.
Intermittent fasting as a dietary strategy simply extends the fast a little longer, in some instances from 12-16 hours, but possibly as long as 24 hours. And while there are those who recommend fasting longer, it should be noted that as the length of the fast increases, so do the potential risks. The one possible exception is for those with diabetes/prediabetes/metabolic syndrome or the morbidly obese. For these individuals longer fasts of two or more days may offer a significant metabolic payoff. Think gastric bypass, which is in some respects little more than a forced fasting state. Benefits aside, prolonged fasts (greater than 24 hours) particularly in the presence of metabolic diseases should always be approved and monitored by your doctor.
For the average person, however, short-term fasts (12-24 hours) appear to be relatively safe.
What are the benefits of intermittent fasting?
There are several benefits attributed to intermittent fasting and perhaps why historically it has been practiced by so many religions.
1. It is believed that intermittent fasts give the digestive tract a much needed break.
2. Short fasts may help alleviate inflammation associated with a number of disorders that are impacted by the gut and the microbiome.
3. Over the long-term, fasting may help to improve insulin sensitivity and thus improve glucose control. Again, it's important to note that those with diabetes or another metabolic disorder should be cautious when attempting fasts, particularly if they are taking insulin as there may be unwanted side effects in this special population even with short-term fasts.
4. Intermittent fasting may assist in weight loss as it not only restricts calories but also encourages the body to burn more fat for fuel.
5. Fasting can reacquaint us with hunger. Too often we eat out of habit. In fact, I often have clients tell me that they don't even know what it feels like to be hungry. In that sense, short fasts may allow people to experience hunger on a low level, which can ultimately help to reduce non-hungry eating.
What is a safe approach to intermittent fasting?
An intermittent fast as a dietary strategy generally lasts between 12-16 hours as mentioned above. I personally like the idea of an overnight fast since you are working with your body, which naturally prepares for a state of fasting during your sleeping hours. Typically, I tell clients who are interested in experimenting with intermittent fasting to eat nothing after 6, 7 or 8 p.m. and then not eat again until 6, 7 or 8 a.m. the next morning. If all goes well, and they want to up the ante, they can increase the fasting period by eating their last meal at 4 or 6 p.m. and not eating again until 8 a.m. the next morning. I usually have clients start with one day and then progress to two non-consecutive days if desired.
I don't generally recommend fasting beyond 16 hours to my clients because I'm conservative. I also believe that for most, fasting from 12-16 hours is long enough to get many of the benefits, but short enough to minimize any significant risk. Finally, as I've mentioned several times already, as a precaution, anyone who currently has a metabolic disorder like diabetes should probably discuss fasting, even short-term fasting, with their doctor.
During fasts, you want to continue to drink water or unsweetened tea or coffee. Some more extended fasting approaches allow consumption of bone broth or other fillers during the fasting period that do not provided any substantial calories.
Intermittent fasting when practiced over short periods (12-16 hours) is generally safe for most individuals. Benefits include weightloss, improved insulin sensitivity, and increased awareness of true hunger. Fasts that last longer than 24 hours may convey additional benefits to certain special populations like those who are obese or have a metabolic disorder. However, those with a previously known metabolic condition like diabetes should always consult a physican before making any major dietary changes as even small changes like short fasts may have unwanted and even dangerous side effects.