You thought eyes were just for seeing? Maybe not! Laboratory studies, currently in flies, reveal that they may influence how many years we live, and diet plays a role in this relationship too.
There are studies that have shown that there is a correlation between eye disorders and poor health in humans. The latest was published in 2021. It states, among other things, that more than half a billion people worldwide are blind or have problems with distance vision.
Blindness and visual impairment are most common among adults aged 50 years and older, who account for more than 80% of people with vision loss.
«As populations continue to age, the prevalence of visual impairment and blindness is projected to more than double in the next 30 years,» her findings state. «The consequences of visual impairment and blindness are wide-ranging, including increased risk of falls, cognitive impairment and dementia, depression, disability and loss of independence. Some studies have reported that visual impairment and blindness are associated with an increased risk of mortality.
The most recent study, published in early June 2022, is the first to link circadian rhythm (see biological clock), diet and life expectancy to eye health. The research was conducted in drosophila (flies) and revealed processes in the fly's eye that appear to drive the aging process.
«Our study argues that it is a more than correlation: eye dysfunction can actually cause problems in other tissues» said Pankaj Kapahi, senior writer and professor at the Buck Institute, PhD, said.
The Buck Institute is a pioneer in research on ageing. It is the first biomedical research institution on the planet dedicated exclusively to the study of the biology of ageing. It has been around for 20 years and everything it does revolves «around our commitment to help people live better and longer».
Dr. Kapahi's laboratory has shown for years that fasting and caloric restriction can improve many functions in the body. «We now show that not only does fasting improve vision, but it also improves the ability to see.the eye plays an essential role in determining lifespan».
Lead author Brian Hodge, PhD, who did his postdoctoral studies in Dr. Kapahi's lab, added that «the finding that the eye itself can directly regulate lifespan, at least in the fruit fly, was a surprise to us.».
According to scientists, the explanation for this connection lies in the circadian «clock» or molecular mechanism that exists in every cell of every organism and has evolved to adapt to the challenges of everyday life (e.g. changes in light and temperature caused by sunrise and sunset). Circadian rhythms influence complex behaviours in animals, from predator-prey interactions and sleep-wake cycles, to the fine-tuning of the molecular functions of gene transcription and protein translation.
In 2016, Dr. Kapahi's lab published a study in Cell Betabolism showing that fruit flies, on a restricted diet, had significant changes in circadian rhythms and extension in lifetime.
When he joined the lab, Dr. Hodge wanted to do further study to understand which of the processes that enhance circadian functions were affected by the change in diet and whether circadian processes were required for the longer lifespan seen with dietary restriction.
«The fly has such a short lifespan, which makes it a really beautiful model that allows us to control many things at once,» explained Dr. Hodge, whose study began with a broad survey to see which genes swing in a circadian fashion. He compared what happened in flies on an unrestricted diet with those fed only the 10% protein.
The scientist noticed that many genes responded to the diet. They also showed ups and downs at different time points (or «rhythmic»). Subsequently, he discovered that the rhythmic genes that were most activated by dietary restrictions seemed to originate from the eye. Specifically by photoreceptors, which are the specialized neurons in the retina of the eye that respond to light.
The finding led to a series of experiments designed to understand how eye function fits into the narrative that dietary restriction can extend lifespan. The next step involved bioinformatics that would answer the question: Do rhythmic eye genes that respond to dietary restrictions affect lifespan? or not? The answer was that they do.
«We always think of the eye as something that serves us for vision. We don't think of it as something that needs to be protected in order to protect the whole body,» explained Dr. Kapahi. He added that since the eyes are exposed to the outside world, immune defenses are extremely active. Which can lead to inflammation. When this is present for long periods of time, it can cause or exacerbate a variety of common chronic conditions.
In addition, light alone can cause degeneration of photoreceptors, which can cause inflammation.
«Looking at computer and phone screens, and exposure to light pollution at night are conditions of great concern for circadian clocks. They are habits that damage eye protection and can affect other functions besides vision. They can damage the rest of the body and the brain.».
There is much to understand about the role that the eyes play in the overall health and lifespan of an organism. Such as how they regulate lifespan and, of course, whether the effect found in flies applies to other organisms.
As far as mammals - and therefore humans - are concerned, the big question is whether it is true for humans that photoreceptors affect longevity.
In the first instance, Dr. Hodge estimated that «probably not as much as fruit flies,» in which the majority of what they do depends on the eye. But because photoreceptors are specialized neurons, «I would argue that the strongest link (to humans) is the role that circadian function plays in neurons in general and particularly with dietary restrictions - along with how they can be used to maintain neuronal function during aging.».
Dr Hodge explained that once researchers understand how these processes «work», they can begin to study how the molecular clock can slow down ageing. Until then, «humans can preserve our vision by activating the clocks inside the eyes. This can be done through diet, medication or lifestyle changes. We have some very interesting research ahead of us.».
