What Really Happens to Your Brain and Body When You Fast

The effects of extreme dieting are extremely variable.

A plate on a wooden table with a glass of water on it

In Silicon Valley, where optimizing physical and mental health is paramount to achieving professional success, some people will do anything to get ahead — including giving up food for days a time. Intermittent fasting is hot, largely because of its promise to promote rapid weight loss and, as one Bay Area company called WeFast puts it, create “an enhanced sense of mental agility.”

But we live in a world where dangerous fasting behaviors are considered an epidemic medical problem, and it’s not entirely clear that fasting can deliver on its promises — let alone in a safe way.

There is some evidence that intermittent fasting can have real medical benefits. A study published in PNAS in 2003 showed that — in mice at least — intermittent fasting is correlated with better resistance to stress and a longer lifespan. A program of periodic, controlled fasting may slow down the aging process and help people lose weight, said the authors of a review on fasting studies published in Ageing Research Reviews in 2006.

Just skip some meals, the theory goes. Space out your intake you that you regularly go 12 hours or so without food, then eat full, healthy meals in the interim, and you should be golden. It isn’t just Silicon Valley that makes it work: Hugh Jackman does it. So does New Jersey Senator Cory Booker, and those guys look great for their age (they’re both 48). Must be something to it, right?

Would you eat like this man?

Getty Images / Mark Wilson

The best available research, published in the form of a sweeping review in Cell in 2016, suggests that a carefully controlled program of intermittent fasting may be one of the few interventions that likely slows down aging. It works pretty well in rats, at least. And humans at risk for brain diseases like Alzheimer’s or Parkinson’s may experience some real benefits, according to a 2007 study published in the journal Neurobiology of Disease. However, other research has shown that extreme calorie-cutting is also a powerful anti-aging intervention, calling into question whether all-out fasting is really necessary.

Human beings, after all, aren’t built to fast. There’s a vast array of chemical signals in our bodies and minds that push us toward food when we’re hungry, refined as our early primate ancestors learned to stock up on food when it was plentiful so they’d be prepared when it was scarce.

The effects of fasting on the brain are even less well defined. There’s a significant body of research that shows that fasting when your body doesn’t want to can alter your mental approach to food for the worse, as a 1996 review published in the Journal of the American Dietetic Association showed. Further research in the Journal of Nutrition shows that folks who fast are more likely to develop an addictive relationship to food, which manifests as harmful cycles of fasting and binge eating that can really harm their bodies. And even short-term fasting seems to bias faster toward higher-calorie, less-healthy foods than they’d select otherwise.

On top of that, certain kinds of fasting can disrupt your brain’s usual systems, leading to unhealthy outcomes like delayed and reduced sleep.

Still, the persistence of this trend in Silicon Valley suggests that it has enough of an effect that people are willing to sacrifice their meals for it. If you’re still interested in intermittent fasting, it’s probably best not to just set out on your own. Talk to your doctor about it, develop a system, and make sure you’ve got someone in your life to keep you accountable and not let you slip into the danger zone.

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