Proposed Mechanisms of Photobiomodulation or Low-Level Light Therapy. de Freitas LF, Hamblin MR. IEEE Journal of Selected Topics in Quantum Electronics, 2016; 22(3). DOI: 10.1109/JSTQE.2016.2561201.
Why a mechanism paper belongs here
Every claim in this field ultimately rests on a physical question: how does shining light on tissue change anything? This review is the standard modern answer, the paper other papers cite for the biochemistry. Note its own title: proposed mechanisms. The honesty is in the adjective.
The central chain
- The chromophore. Cytochrome c oxidase, the final enzyme of the mitochondrial respiratory chain, contains heme and copper centers that absorb red and near-infrared light. It is the leading candidate for the primary photoacceptor.
- The immediate effects. Light absorption is proposed to increase mitochondrial ATP production, transiently shift reactive oxygen species, and release nitric oxide from the enzyme, three signals with wide downstream reach.
- The downstream cascade. Those signals activate transcription factors and gene-expression changes that outlast the light itself, which is the proposed explanation for why minutes of exposure could matter days later.
Two ideas that discipline the whole field
The optical window: tissue transmits light best between roughly 600 and 1,100nm, below it, melanin and hemoglobin absorb; far above it, water does. Effective wavelengths live in this window, which is why serious devices cluster there.
The biphasic dose response: the review consolidates evidence that dose curves in this field rise, plateau and reverse, too little does nothing, too much undoes the benefit. Every \"more power\" marketing instinct runs directly into this curve.
Limits worth stating
Much of the mechanistic evidence comes from cell culture and animal work; connecting a molecular chain to a clinical endpoint in humans is exactly the gap the trials elsewhere in this section attempt to close. Mechanism makes claims plausible, only trials make them supported.