Various iterations of fasting are becoming increasingly popular. Whether it’s time-restricted eating (consuming all of one’s calories within a defined period of time each day), alternate-day fasting, fasting-mimicking diets, or extended fasting, researchers and the general public alike are catching on to the idea that going some length of time without eating may be beneficial for overall health and possibly weight control. A paper published recently in Cell Metabolism adds to this growing body of research, suggesting that alternate-day fasting may be an effective alternative to across-the-board caloric restriction for supporting overall health and potentially increasing longevity.

The paper, “Alternate Day Fasting Improves Physiological and Molecular Markers of Aging in Healthy, Non-obese Humans,” was based on a clinical trial called The Impact of Intermittent Fasting on Human Metabolism and Cell Autophagy (InterFast). In the trial, healthy, normal-weight adults followed a strict alternate-day fasting (ADF) protocol for four weeks. The protocol called for alternating 36 hours of zero-calorie intake with 12 hours of unlimited eating. (Zero-calorie fluids were permitted on fasting days.) After four weeks, subjects showed improvements in cardiovascular risk markers, reduced fat mass (especially trunk fat), and increased levels of the ketone β-hydroxybutyrate, even on non-fasting days.

In a separate part of the study, researchers evaluated 30 people who had already practiced ADF for more than six months in order to assess the long-term safety of this type of strategy. They found reduced levels of sICAM-1 (an inflammatory marker associated with aging), LDL-cholesterol, and triiodothyronine (T3). Whether or not lower LDL-C is beneficial is debatable, but it’s well-established that lower T3 is associated with increased lifespan in numerous animals and lower order organisms. (Low T3, however, is not always a reason to celebrate! When T3 is lowered as a response to drastic caloric reduction combined with overzealous exercise, the metabolic slowdown and negative physical, emotional, and psychological effects that result may not be worth a presumed but as-yet unproven extension of human lifespan.)

Outside of research studies, many individuals are now taking it upon themselves to implement a fasting practice, be it for fat loss, improving blood glucose and insulin levels, or for some other reason. Fat loss is a common but not guaranteed result of ADF or intermittent fasting. The InterFast study supports a role for fasting in weight loss: during the four week intervention, subjects had a mean caloric reduction of approximately 35% and lost an average of 7.7 pounds (3.5 kg). Even though food intake was unrestricted during the 12-hour feeding periods, subjects did not completely compensate for the previous 36-hour period of zero calorie intake. This is an important point, because some patients find it’s easier to skip food entirely than it is to limit their intake to a predetermined number of calories, or grams of fat or carbs. For the purpose of weight loss, these individuals may be more successful implementing a fasting strategy than trying to adhere to a low-fat, low-carb, or low-calorie diet.

Another positive aspect of ADF is that it’s simple and straightforward. According to study co-author Thomas Pieber, head of endocrinology at the Medical University of Graz in Austria, “The elegant thing about strict ADF is that it doesn't require participants to count their meals and calories: they just don’t eat anything for one day.” It doesn’t get much simpler than that! ADF might be a good way to go for those who prefer not to mess with apps, spreadsheets, and other trackers every time they sit down to eat.

Looking beyond this particular study, which was conducted in healthy adults, ADF has been shown to be helpful for those with type 2 diabetes as well. In fact, fasting may be so powerful in these cases that, under physician supervision, individuals who’ve been living with type 2 diabetes for twenty years or more and who are taking over 70 units of insulin daily may be able to discontinue insulin injections and yet achieve better glycemic control. (See here for our coverage of this important research.)

For individuals who prefer not to fast, however, adopting a ketogenic or very low-carbohydrate diet can go most of the way toward achieving the same benefits with regard to improvement in glycemic control. A study comparing the short-term effects of a carbohydrate-free diet to a 3-day fast in people with type 2 diabetes found that carbohydrate restriction was responsible for 71% of the reduction in 24-hour integrated glucose concentration. A full-on fast might have a slight edge over merely cutting carbs, but cutting carbs even without fasting can go a long way toward improving blood sugar.

There’s also a lot of debate about when to fast, particularly for those who prefer time-restricted eating rather than a full 24-hour fast or longer. Conventional thinking has most people believing that it’s better to consume the bulk of one’s food earlier in the day: “Breakfast like a king, lunch like a prince, dinner like a pauper,” as the saying goes. But research published earlier this year showed that, in men at risk for type 2 diabetes, a 9-hour eating window (consuming all one’s food within a 9-hour timeframe and fasting for the other 15 hours of the day) resulted in similar improvements regardless of whether the window was early or late in the day (8 a.m. to 5 p.m. versus 12 p.m. to 9 p.m.). The earlier eating window resulted in lower mean fasting glucose while the later window did not but other parameters were not statistically different.

Clearly, fasting imparts numerous benefits in both healthy subjects and those with type 2 diabetes. But fasting may not be appropriate for everyone. The decision to adopt a fasting practice is one best made between a patient and their doctor or nutritionist. Certain situations make fasting contraindicated and use of certain medications (particularly those used for diabetes and hypertension) means that patients must have medical supervision in order to fast safely and have dosages adjusted as needed. 

Individuals considering incorporating fasting into their lives should be educated about the importance of maintaining healthy electrolyte levels. This may be less of an issue for short-term intermittent fasting and time-restricted eating, but for longer periods without food, supplemental sodium, magnesium, and potassium may be warranted.