There is some evidence for near-infrared light to stimulate mitochondrial function and thus improve cell and tissue function where mitochondrial function has been impaired, such as in aging. It has been tested in models of Parkinson's disease, in which mitochondrial dysfunction is known to be important, and here researchers provide evidence for it to help compensate for failing photoreceptor function in age-related retinal degeneration. It is worth noting that improved mitochondrial function is a hypothesis to explain the observed benefits - which are quite modest in the grand scheme of things. It is a plausible hypothesis, but how exactly near-infrared light is producing this outcome is not yet fully understood.
In humans around 40 years-old, cells in the eye's retina begin to age, and the pace of this ageing is caused, in part, when the cell's mitochondria, whose role is to produce energy (known as ATP) and boost cell function, also start to decline. Mitochondrial density is greatest in the retina's photoreceptor cells, which have high energy demands. As a result, the retina ages faster than other organs, with a 70% ATP reduction over life, causing a significant decline in photoreceptor function as they lack the energy to perform their normal role.
Researchers built on their previous findings in mice, bumblebees, and fruit flies, which all found significant improvements in the function of the retina's photoreceptors when their eyes were exposed to 670 nanometre (long wavelength) deep red light. "Mitochondria have specific light absorbance characteristics influencing their performance: longer wavelengths spanning 650 to 1000nm are absorbed and improve mitochondrial performance to increase energy production."
For the study, 24 people (12 male, 12 female), aged between 28 and 72, who had no ocular disease, were recruited. All participants' eyes were tested for the sensitivity of their rods and cones at the start of the study. Rod sensitivity was measured in dark adapted eyes (with pupils dilated) by asking participants to detect dim light signals in the dark, and cone function was tested by subjects identifying coloured letters that had very low contrast and appeared increasingly blurred, a process called colour contrast.
Researchers found the 670nm light had no impact in younger individuals, but in those around 40 years and over, significant improvements were obtained. Cone colour contrast sensitivity (the ability to detect colours) improved by up to 20% in some people aged around 40 and over. Improvements were more significant in the blue part of the colour spectrum that is more vulnerable in ageing. Rod sensitivity (the ability to see in low light) also improved significantly in those aged around 40 and over, though less than colour contrast.