The Science of Fasting: How Eating Less Often Slows Ageing at the Cellular Level

Intermittent fasting has become one of the most popular dietary approaches of the last decade. But most people who practice it are focused on weight loss, not longevity. What they may not realise is that fasting's most profound benefits have nothing to do with the scale — and everything to do with how fast their cells are ageing.

Adversity is the signal your body needs

The key concept here is hormesis — the principle that mild stress, applied to a biological system, triggers protective responses that make the system stronger and more resilient. What doesn't kill you, as the saying goes, genuinely does make you stronger — at least at the cellular level.

Fasting is adversity. When the body goes without food, it reads this as a signal: times are hard, resources are scarce, we need to be efficient and resilient. In response, it activates a cascade of repair mechanisms — the same mechanisms that, in nature, help animals survive periods of famine.

In modern life, we almost never experience this kind of beneficial adversity. Three meals a day, abundant food, total comfort. The body's repair systems stay largely dormant. And we age faster because of it.

What fasting does to your NAD+ and sirtuins

When you fast, NAD+ levels in your cells rise. This matters enormously, because NAD+ is the fuel for your sirtuins — the proteins that maintain epigenetic information and repair DNA. With more NAD+ available, your sirtuins become more active, doing their job of preserving the cellular information that keeps you young.

"Fasting raises NAD+ and makes the sirtuins young again essentially," explains Sinclair. "And that preserves the epigenome and it also repairs the DNA better."

The 14–16 hour window

You don't need to fast for days to get these benefits. A daily fasting window of 14–16 hours — achievable simply by skipping breakfast or eating an early dinner — is enough to trigger most of the beneficial cellular responses. Think of it as giving your body 14 hours to clean house, repair itself, and reset for the next day.

The extended 3-day fast that Sinclair practices once a month goes even deeper, triggering a type of cellular recycling called chaperone-mediated autophagy that doesn't fully activate in shorter fasts — but this isn't necessary for most people to see significant benefits.