Intermittent fasting is currently salient on the shortlist of generally short-lived dietary fixations. The first thing to say about this is that there should be no such list.
Where diet reliably contributes most to vitality, longevity, and, yes, weight control, it is because of cultural traditions, heritage, and the time-honored practices of generations, not the vagaries of news cycles and hyperbolic headlines. But because dietary fads perennially supplant science and sense, there is always a shortlist of fleeting fixations. Intermittent fasting is currently parked there, so let’s talk about it.
The value proposition for intermittent fasting is all about weight loss beyond the expectations of mere calorie restriction. Inevitably, the arguments for this approach assert or imply novelty and the discovery of something new and therefore shiny.
This is a standard approach to the marketing of any dietary tactic. The ketogenic diet, for instance, popular at present under that rubric, was the induction (and, thus, most impactful) phase of the Atkins diet. While popularized as if newly discovered, the diet has been with us through a sequence of temporary infatuations spanning nearly 50 years. Where attention spans are short, and news cycles frequent, it’s never long before what’s old is dusted off and introduced as if new again. But if these diets were so terrific in the first place, why did we fall out of love with them?
Throughout human pre-history, people ate when they could and starved when they must. Those days of want were periods of intermittent fasting not by choice, but chance, with weight control not an objective, but a constraint imposed by the challenges of foraging for survival.
The topic of old being new may be truer of intermittent fasting than anything else in the dietary realm because this approach is almost certainly the most ancient means of weight control, predating all others by many millennia. Throughout human pre-history, people ate when they could and starved when they must. Those days of want were periods of intermittent fasting not by choice, but chance, with weight control not an objective, but a constraint imposed by the challenges of foraging for survival.
Today, we have data on the weight-loss benefits of that ancient way of life: A 2017 study published in Science Translational Medicine assigned a group of overweight people to either their usual diet or fasting five days per month for three months. Those who fasted lost weight and showed improvement in an array of metabolic markers spanning lipids, glucose, and measures of inflammation. The study authors suggested that was a benefit of fasting, and the media was predictably inclined to eat that up. The result was a great deal of breathless hype in the blogosphere about the magical benefits of fasting.
But short-term weight loss among those with an excess of body fat improves metabolic markers — temporarily, at least — no matter how it’s achieved. Intermittent fasting pokes intermittent holes in calorie intake. Reduced calorie intake is the key to weight loss — always.
Eating some of the time generally leads to weight loss relative to eating all of the time (although — warning! — a sequence of self-deprivation followed by binges can increase rather than decrease calories). Weight loss, in turn, produces short-term improvement in all of the biomarkers that weight loss always improves, whatever good or bad, sustainable or fleeting thing is causing the weight loss. Fasting has not been shown to have much of anything that cabbage soup or grapefruit didn’t have before.
Whether fasting offers any metabolic advantage over other approaches to calorie restriction is unresolved. Some studies in animal models hint at benefits beyond the effects of calorie restriction. A recent paper in Nutrition reported that intermittent fasting increased energy expenditure and fostered the development of brown fat (which is involved in thermal regulation and inversely correlated with obesity) in mice. The same paper, however, reported that some effects seen in mice were not replicated in humans.
For the most part, reviews of fasting find effects that are comparable with those of calorie restriction by any other means. Informed speculation continues to point toward the hope of enhanced weight loss and accelerated health enhancement. Still, the weight of evidence to date suggests that fasting intermittently is not reliably better, surer, or faster than weight loss and health improvement by other dietary means.
That said, for some people, sequenced days of fasting, or partial fasting, may be easier and more comfortable than portion control every day. A weekly sequence of shifting dietary rules also focuses the mind on dietary intake, and that mindfulness of choices and timing may be advantageous in its own right. All diets work by imposing some discipline where formerly there was none, and intermittent fasting does that as well as any other. For some, fasting days are somewhat spiritual, representing a psychological reboot of sorts. The benefits of this are hard to tabulate, but may be considerable.
For others, the balance tips the other way. Fasting may produce headaches, fatigue, and irritability. The consistency of daily rules may be more comfortable than rules that alternate. The tactics of applied dietary discipline need not be one-size-fits-all.
I believe the best approach to calorie control is the selection of wholesome, high-quality foods. I think there is a danger in intermittent fasting, namely the idea that as long as you don’t eat some days, you can eat whatever you want on other days. The quality of what you eat matters, whether you eat it every day, or just some days.
For now, the only reliable conclusion is that intermittent fasting is a viable tactic for calorie control. It should always be combined with good food choices on the non-fasting days. If tempted to try it in that context, it’s generally best to confer with a doctor first if you have any medical conditions or take medication.
Be aware, though, that claims of metabolic magic with intermittent fasting have been oversold. Expect weight loss and health improvement commensurate with your calorie restrictions, and you’ll spare yourself the fury of disappointment.
Dr. David L. Katz is the founder and president of the True Health Initiative and the founder and CEO of DietID. You can follow him on Twitter @DrDavidKatz.