Neurogenesis

Neurogenesis is a sequence, not an event: a neural stem cell divides, a progenitor proliferates, a daughter cell commits to a neuronal fate, and either integrates into an existing circuit or dies. In adult mammals it is largely confined to the subgranular zone of the hippocampal dentate gyrus and the subventricular zone feeding the olfactory bulb, and attrition dominates, since most newborn cells are lost within weeks unless activity recruits them. The staging matters, because an intervention can act at any one step, and raising proliferation while survival is unchanged adds no functional neurons.

The human evidence is real but contested. Bromodeoxyuridine labelling in patients who had received it clinically showed dividing cells in the adult dentate gyrus in the late 1990s, and carbon-14 birth dating using atmospheric nuclear test fallout later estimated a turnover on the order of several hundred dentate neurons a day. Then two studies published in 2018 reached opposite conclusions from post-mortem tissue, one finding essentially no immature neurons in the adult human hippocampus and the other finding them preserved into old age.

Even at the higher human estimates, the numbers cap what can be claimed. A few hundred new cells a day in one small structure cannot replace a lost circuit; the plausible function is a contribution to pattern separation and hippocampal flexibility, a modulatory role rather than regeneration.

The marketing failure follows the measurement. Supplement claims typically rest on a rodent count of bromodeoxyuridine-positive or doublecortin-positive cells, and doublecortin marks immature neurons, so a higher count can equally reflect slower maturation or better survival of cells that already existed. The remaining leap, from a cell count in a rodent dentate gyrus to human mood or memory, is asserted rather than demonstrated.