Pick up any popular magazine or click on a health headline and you’re more likely than not to find something about fasting. Fasting is all the rage right now, being touted as having anti-aging effects, facilitating fat loss, boosting mood and providing mental clarity and sharp cognition. Research even supports the use of intermittent fasting for potential “prevention and treatment of cardiometabolic disorders.” But even if fasting does have these positive effects, does that mean it’s appropriate for everyone? Let’s take a closer look at fasting, some of the benefits, and who might be better off avoiding it.

The fact that there’s a fasting craze at all is interesting. Less than a hundred years ago, during the Great Depression, getting enough food was a challenge. We went from scarcity and worrying about having too little to eat to being awash in abundant food year-round, thanks to people who dedicate their lives to farming as well to advances in food production, transportation and agricultural and supply chain technology in general. We’re so fortunate to have constant access to inexpensive food, in fact, that we now have to impose scarcity upon ourselves in the form of deliberate abstinence from food.

It’s typically assumed in the ancestral and Paleo health communities that pre-agricultural humans and our hominid ancestors experienced long periods of famine, or, at the very least, weren’t grazing and snacking all day. It’s more likely that they feasted and fasted, going some length of time between successful hunts of large animals. We don’t know for certain, though. Unfortunately, Homo habilis and Homo erectus didn’t leave behind their food logs for us to read, and any diet tracking apps they might have used are long since obsolete. Joking aside, we don’t know for sure how much or how often our ancestors ate, but we can say confidently they weren’t eating pastries and popcorn, and they weren’t slaking their thirst with sugar-sweetened soft drinks. 

As a practice, though, fasting is nothing new. Humans have been fasting for thousands of years for religious or cultural purposes, so it can hardly be considered a “fad.” In the modern era, it’s commonplace for people to fast before having blood tests done or before surgical procedures. The difference now is fasting when there’s no absolute necessity for it, but rather, to potentially reap the many reputed benefits, particularly for the brain and neurological function, but also for weight loss and general metabolic health. We can’t say fasting is novel, though. The word breakfast, itself, hints at the fact that it’s a meal that breaks a fast. In the 21^st Century, this “fast” might be just a few hours since last night’s dinner (or midnight snack!) or it could be a full day or more since the last meal. Whether it happens at 7am or 4pm, the first meal of the day is technically breakfast.  

There are different ways to implement fasting, making fasting highly customizable to an individual’s lifestyle and health goals. Some people choose to simply skip one meal a day, eating only breakfast and lunch, or lunch and dinner. Some people choose to eat only one meal per day, giving them nearly 23 hours in a fasted state, although many include coffee, tea, bone broth or even MCT oil in their “fast.” (Sipping on these items or adding MCT oil to coffee or tea can make it easier to abstain from more substantial foods for a longer period.) This kind of meal skipping is usually called “intermittent fasting,” “time restricted eating” or “time restricted feeding.” Others may opt for an extended fast, going multiple days without solid food, which may confer additional benefits but it also presents risks not typically seen with intermittent fasting.

Fasting appears to have numerous beneficial effects, but that doesn’t mean it’s appropriate for everyone. Individuals with type 2 diabetes can fast safely but under only medical supervision. Individuals taking certain medications may be advised to avoid fasting or to be sure to work with their doctor to make any necessary adjustments to medications that are typically taken with food, or whose potency or pharmacology may be affected by fasting. Fasting is not recommended for women who are pregnant or breastfeeding, nor for adolescents in an active growth stage. It’s also not advised that anyone with an eating disorder or a history of eating disorder adopt a fasting practice.

Another group of people for whom fasting is not recommended is the elderly, except under medical supervision. Many older people are already under-consuming protein. It wouldn’t be wise for an individual already experiencing sarcopenia or dynapenia to become even more protein deficient or malnourished overall. 

For people who prefer not to fast or for whom fasting is not recommended, certain practices and various nutraceutical compounds and may help induce some of the benefits of fasting even in the absence of caloric restriction. For example, the very low-carb, high-fat ketogenic diet is a powerful nutritional strategy for lowering blood glucose and insulin levels. For individuals who experience benefits from elevated ketones even when carbs aren’t restricted, medium-chain triglyceride (MCT) oil can elevate ketone levels regardless of carb intake, as can exogenous ketones, which are usually provided as powdered beta-hydroxybutyrate salts that can be consumed in a beverage.

Regarding the purported longevity benefits of fasting, the drug metformin may have some fasting-mimicking effects, some of which may be accomplished by berberine, sometimes called “herbal metformin.” Studies have shown berberine to rival the potency of metformin in lowering HbA1c, fasting insulin, triglycerides, and fasting and postprandial blood glucose. The antioxidant polyphenol resveratrol is also known for its anti-aging benefits. Regarding the metabolic effects of fasting, particularly improvements in glycemic control and insulin sensitivity, alpha-lipoic acid may be helpful in these areas.

Fasting, like any other potentially therapeutic intervention, is simply one tool among many available to health care practitioners and their patients. For those who should not fast, other strategies may help accomplish some of the same effects.