Natto, and the ingredient that came out of it

Natto divides a room. Fermented soybeans, eaten at breakfast, notorious for the smell and for the long sticky threads that follow the chopsticks out of the bowl. Those threads are the reason it is in this note.

The stickiness is polyglutamic acid — a chain of glutamic acid units produced by Bacillus subtilis as it ferments the soy. Isolated, purified and stripped of everything that makes natto smell like natto, it is one of the better humectants available to a formulator.

It binds water, like hyaluronic acid does, and it holds it in a film that is more cohesive and slower to give it up. In side-by-side use the skin surface looks smoother for longer, and it stays comfortable in dry air where a thin humectant film would have flashed off.

It also works with hyaluronic acid rather than instead of it. Different molecular sizes settling into different parts of the surface hold water at different depths and rates, which is why the Overnight Recovery Cream contains both rather than picking a side.

Two humectants of different sizes beat twice as much of one. The layer is more even, and it lasts through the night rather than the first hour of it.

• It is a humectant, nothing more. It holds water, and the improvements in the look of the skin follow from that. It is not an active and it does not change structure.
• The evidence base is thinner than hyaluronic acid’s. It is a newer cosmetic ingredient and much of the published work is small or supplier-funded. We use it because it performs in our own testing, not because the literature is settled.
• Use levels are low — typically well under one percent, because more turns the texture tacky, which is the exact fault we spent a revision removing from the 500x.
• Eating natto will not do this for your skin. Different route, different destination. Eat it because it is good for you, if you can face it.

It is not sentimentality about tradition. It is that an ingredient with a thousand-year food history has an unusually long safety record before anyone thought to put it in a jar, and that production is a fermentation of soybeans rather than a long petrochemical route. Neither of those facts makes it work better. Both make it easier to defend.