NAD+ therapies need better cell-level targets to fight aging

NAD+ is a molecule cells depend on for energy production and repair. Its levels decline with age, which seems to suggest a simple solution: take more of it. But the reviewers publishing in the journal Biology explain why that picture is incomplete.

Availability is not the same as function

The issue is not just how much NAD+ is inside a cell, but whether that NAD+ reaches the protein that needs it: SIRT1. SIRT1 is a sirtuin (a class of proteins involved in cell maintenance, stress response, and energy regulation). Among other functions, it helps cells manage reactive oxygen species (harmful byproducts of energy metabolism that accumulate with age).

The reviewers introduce the concept of ‘SIRT1 functional reserve’: a measure of how much capacity SIRT1 still has to respond to cellular stress. That reserve can be low even when total NAD+ in the cell appears adequate. This is because NAD+ is distributed across different compartments within the cell and is consumed by multiple competing processes. More total NAD+ does not guarantee more SIRT1 activity.

Why existing therapies often disappoint

SIRT1 also exists in several variants (isoforms). Two of these (v2 and v3) accumulate with age and function less efficiently than the original variant (v1). Measuring only total SIRT1 protein misses this distinction entirely. The same logic applies to NAD+: a measurement of total cellular NAD+ says little about how much is actually available to SIRT1.

The analysis calls for more precise measurement tools as a foundation for better therapies. This is a review article, not a clinical trial. The recommendations are theoretically grounded but have not yet been tested in large-scale human studies.

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