What Is NAD+ and Why Does It Decline After 40?

If you've spent any time reading about longevity science, you've probably encountered NAD+. It comes up in discussions about energy, ageing, cellular repair, and even cancer prevention. But what exactly is it — and why does it matter so much?

NAD+ is your cellular fuel

NAD+ stands for nicotinamide adenine dinucleotide. It's a coenzyme — a helper molecule — that is involved in hundreds of metabolic reactions in every cell in your body. Without it, cells can't produce energy efficiently. They can't repair DNA. They can't maintain the epigenetic information that keeps them functioning as young, healthy cells.

Think of NAD+ as the fuel that powers your longevity engines. The sirtuins — the proteins that act as the conductors of your cellular orchestra — can't do their job without NAD+. It's the catalyst for nearly everything that keeps your cells young.

The problem: NAD+ falls dramatically after 40

Here's the issue. By the time you reach your 50s, your NAD+ levels may have fallen by as much as 50% compared to your younger years. Your body is both producing less of it and destroying it faster than before.

The consequences are significant. Lower NAD+ means less efficient cellular energy production — that persistent tiredness that doesn't fully resolve with sleep. It means slower DNA repair, leaving cells more vulnerable to the kind of damage that drives ageing. And it means the sirtuins that protect your epigenetic information become less effective, accelerating the cellular identity crisis that underlies ageing.

"When we're young, we have lots of NAD. So it works well," explains Sinclair. "As we get older, by the time you're 50, you have about half the levels. That's a problem."

How to restore NAD+ levels

You can't simply swallow NAD+ directly in large amounts — it doesn't absorb efficiently. What works is taking precursors: molecules that the body converts into NAD+. The two most studied are NMN (nicotinamide mononucleotide) and NR (nicotinamide riboside).

When you take a gram of NMN, clinical studies have shown it can roughly double your NAD+ levels. Fasting also raises NAD+ naturally — which is one of the reasons why intermittent fasting has such profound effects on longevity. The two approaches together are more powerful than either alone.