Fasting is enjoying a surge in popularity, thanks to proponents who claim it activates weight loss. Everyone these days, it seems, is skipping their breakfasts or drinking only water every third day. But is fasting actually good for you? And (more importantly to some) will it make you lose weight?
The short answer is yes, not eating will make you lose weight — for a while. Intermittent fasting involves severely restricting food intake for periods of time, which does cause weight loss. Until relatively recently, it was largely a behaviour associated with religious ceremonies or other spiritual works. Now it’s become a mainstream fitness craze.
Writer Julia Belluz did an explainer on how it works for Vox, breaking down the basic types of fasting routines that practitioners adhere to:
Intermittent fasts involve eating no food or massively cutting back on calorie intake (e.g., 500 calories per day) only intermittently (like the very popular 5:2 diet).
Time-restricted feeding involves consuming calories only for a four- to six-hour window each day (for instance, skipping breakfast and only eating lunch and an early supper).
Periodic fasts, the most extreme, typically last several days or longer. These diets involve drinking only calorie-free fluids or very few calories for long stretches to get the body into full fasting mode (instead of switching back and forth between fasts and feeding).
Fasting-mimicking diet, a plant-based diet that involves eating very few calories — through light foods like soups, energy bars, and energy drinks — for several days each month.
There is no denying that eating less functions for weight loss for some, but before you decide fasting is the best way for you to eat less, here are some basic misunderstandings about the “diet.”
There’s Not Much Research on Weight Loss and Fasting
There is actually not much science on fasting as a weight loss tool. As Belluz explains, most research is focused on the potential health and longevity benefits of eating less. Proponents of fasting say that eating three square meals a day is a relatively new thing for humanity, since we lived without agriculture for quite some time.
The scientists who research fasting are mostly focusing on whether it improves the outcomes for health markers for diabetes, cancer, and cardiovascular disease. And it does, according to Valter Longo, director of the Longevity Institute at the University of Southern California:
“A lot of organs start shrinking [during a periodic fast],” Longo explained. “A lot of cells start dying and we have evidence a lot of the cells killed by this process are the bad cells. Then the stem cells get turned on, and we see the body starts regenerating itself.”
But this research only covers a few months of study—a fairly brief period in a person’s lifespan. Other studies regarding the benefits of fasting for long-term health have been inconclusive.
It’s Hard to Do
The best diets aren’t diets at all; they’re changes to your lifestyle that are manageable and maintainable. It’s hard to describe a more difficult diet to maintain than “don’t eat.” Many people dropped out of Longo’s study because they were unable to stick with the fasting demand, even for science.
You can lose weight, but whether or not it’s a sustainable behaviour for you is important. Belluz shared this observation from a fasting researcher who conducted another study from 2018:
“Dropout rates have been as high as 40 per cent. Thus, despite the statistical significance of weight loss results, the clinical significance and practicality of sustaining an [intermittent fasting] regimen are questionable.”
If you can’t fast, you won’t lose weight and you will probably feel pretty lousy the whole time, too.
It Can Be Unhealthy
The research on fasting hasn’t extended to every group. They don’t know what kind of negative impact it can have on the elderly, children, or people who are underweight. It can also be dangerous for people with a history of disordered eating, according to psychologist Debra Safer:
“The research evidence generally shows that patients with eating disorders do best when they eat regular meals and snacks,” Safer said. “Intermittent restriction of intake is often one of the behaviours that people with eating disorders engage in as part of their eating disorder — and it often sets them up to binge and/or purge.”
So in people with eating disorders, fasting is “potentially risky in that it disrupts attempts to build and maintain hard-won normalized eating patterns.”
Longo also added that fasting can increase the risk of gallstones when done incorrectly, and should not be attempted by people who are “diabetic and taking insulin or any other drugs, or if you have metabolic disorders.”
Other Forms Of Calorie Restriction Are More Effective
If you’re determined to cut calories from your diet, fasting is still not the most effective way to do so. Instead, try regular ol’ everyday restriction.
The researchers looked at randomised controlled trials of intermittent fasting and found that the people who fasted lost about 4 to 8 per cent of their original bodyweight, on average. So fasting worked, but, interestingly, it didn’t outperform regular, continuous calorie restriction (“eat less every day” dieting), and it didn’t lead to dramatic weight loss.
There are people (those who don’t fall into any of the groups who could potentially suffer dangerous health consequences) who find fasting beneficial. They prefer to just restrict the time in which they eat rather than count calories or worry about what they consume. Fair enough. Just understand the costs and benefits of not eating regularly, because there may not be too many of the latter.
This article has been updated since its original publication.