Fit in my 40s: will intermittent fasting boost my energy levels?

Zoe Williams
·3 min read

In my youth, fasting was considered a fantastically bad for you fad. The cod-scientific wisdom was that all you’d do was lower your metabolism, so when you did start eating again, your weight would go roaring back up.

Since the success of the 5:2 diet, in which you eat normally for five days, then restrict calories to 500-800 daily for two days, the thinking on weight loss has shifted. Fasting intermittently is incredibly effective when it comes to losing weight, and in fact giving your digestive system a break may do wonders for your health.

Forget weight loss, or what you look like, says Adam Collins, principal teaching fellow in nutrition at the University of Surrey: many of us are simply too full, too much of the time. “The reality is you’ve got a lot of clinically obese people who are metabolically healthy and a proportion of lean people who are metabolically unhealthy,” he says. Metabolic ill-health is linked to cardiovascular disease, diabetes and all-round sluggishness, which deters physical activity, which intensifies the problem. “You can almost always root down to the fact that we’re replete and always have a layer of food for our guts to deal with. So your appetite suppression signals stop working,” Collins says. As the nutritionist Alice Mackintosh once told me, “There is basic housekeeping your gut has to do, which it can’t do when it’s full.”

Related: Prebiotics are hard to stomach, but will your gut thank you for trying? | Zoe Williams

Probably the simplest change to make, Collins explains, is to lengthen your overnight fast, which is a fancy way of saying skip breakfast. “Even intermittent breakfast-skipping could have an impact,” he says. You don’t need to restrict your calories after that; just avoid bingeing. If you want to get experimental with the rest of your diet, you could try “carb cycling”, alternating between high- and low-carb periods. Collins follows a 4:3 diet, restricting calories to 800 daily for three days a week. You have to jettison the idea that you’re aiming to consume less food; it may be a side-effect, but it’s not the important thing.

It is not easy to stick to 500 calories, I found: eating even minuscule amounts of food makes you long for more. It was all fine until my daughter made me taste the boba (tapioca pearls) at the bottom of her bubble tea (this is an 11-year-old girl thing). They were disgusting, but I finished them, her dumplings and her sushi, then ordered it all again for me, like The Tiger Who Came To Tea.

What I can guarantee about intermittent fasting is that, when you do it right, you’ll feel it, immediately. On the days when I was fasting, I often had more energy and slept better. I wonder if there’s something about appetite, that if you thwart it for a day, it comes back as something stronger, like lust for life.

What I learned

This is not a recent discovery: there are studies going back decades, which found that energy-restricted mice live longer.