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Gut reaction

Poor nutrition may be contributing to poor ocular health

Roibeard O’hEineachain

Posted: Tuesday, October 1, 2019

Monique Hope-Ross (left) and Sinead Corr

Western lifestyles are leading to an increasing incidence of diabetes. Poor nutrition is contributing to an unhealthy gut microbiome, with loss of microbial diversity. This in turn may be contributing to poor ocular health. This can have serious long-term consequences for vision, according to speakers at a symposium held at the Annual Conference of the Irish Society of Ophthalmologists in Galway, Ireland.

Since the beginning of the industrial revolution, social progress has meant that fewer people are dying from communicable diseases and more people are instead dying of non-communicable diseases. Such diseases primarily include cancer, diabetes and cardiovascular diseases. There is much research that implicates lifestyle choices and poor diet in all these conditions. The accompanying adverse changes to the gut microbiota may be one of the mechanisms implicated in the development of these diseases, said Monique Hope-Ross FRCP, FRCS, FRCOphth, UK.

“Diet is not only a means of preventing disease, it is also a way of treating disease,” she added. This has now formerly entered clinical practice, where low-carbohydrate programmes for people with diabetes are showing remission of diabetes.

She noted that obesity has reached epidemic proportions in modern industrialised societies around the world, where an estimated 55% of people are overweight. Diet affects the gut microbiome and so-called western diets with high proportions of refined carbohydrates and fast foods have adverse effects on the gut microbiota.

EXTREME EXAMPLE
As an extreme example, Dr Hope-Ross cited the case of a celebrated young Chinese man who attained a weight of 175kg on a diet that consisted primarily of fast food. In addition, he had a voracious appetite, which he could never satisfy. An analysis of his gut microbiome revealed that it was composed of predominantly one species, compared to the average of 1,500 species in healthy people. An expert in obesity and the microbiome moved him to a plant-based diet; not only did he lose weight, but his microbiome greatly increased in bacterial diversity and he was not hungry all the time.

At the opposite extreme are the Hadza people in north-central Tanzania, modern-day hunter gatherers whose way of life is likely similar to the ancestral culture of all of humanity. Death from non-communicable diseases is rare in this indigenous ethnic group. Their diet is composed entirely of plants that they obtain through foraging and hunting such as wild berries, tubers, honey and wild meat, when available. Their gut microbiome has the greatest bacterial diversity of any population anywhere.

Dr Hope-Ross noted that some species of bacteria appear to be especially beneficial to health. However, in order to thrive in the gut, they need a hospitable environment, which in turn depends on a healthy diet and other lifestyle factors.

There are very few studies concerning the impact of the microbiome and ocular disease. Although some observations have been made, this area of research is very much in its infancy and trials of interventions in humans have up to now involved only small numbers of patients. However, it is a reasonable hypothesis that microbiota-driven disease may be associated with eye conditions, said Sinead Corr PhD, Trinity College Dublin, Dublin, Ireland.

The dysbiosis of the gut microbiome that is associated with obesity and diabetes may also contribute to diabetic retinopathy. The well-established role of diet in the prevention of age-related macular degeneration (AMD) also points to a potential role of gut biome imbalance in that condition. In addition, around 10% of IBD cases are accompanied by manifestations in the eye such as uveitis and conjunctivitis.

In regard to AMD, she noted that studies using a mouse model of the disease have shown that animals receiving oral antibiotics and germ-free mice had a reduced disease severity. Furthermore, in mice fed a high-fat diet and patients with AMD there are increased phylum amounts of Firmicutes species, but reduced Bacteroides species, and a lower number of fatty-acid digesting bacteria. Gut microbiome dysbiosis may contribute to elevated complement levels and chronic overactivation of complement cascade, Dr Corr said.

UNBALANCED GUT MICROBIOME
Research has also shown that autoimmune uveitis has a gut microbial signature associated with disease severity of uveitis in mice and that oral antibiotics reduce severity, while germ-free mice are protected from the condition. An unbalanced gut microbiome may provoke or worsen autoimmune uveitis, in that the microbiota activate T cells, which migrate to the eye and react to retinal proteins, she said.

The ocular surface appears to have its own microbiome, Dr Corr noted. In 2009, the Bascom Palmer Eye Institute, University of Miami, Florida, initiated the Ocular Microbiome Project. The researchers found that eyes with infections of the cornea had reduced bacterial diversity.

She noted that manipulation of the gut microbiome can, in experimental settings, reduce and eliminate the gut’s response to inflammatory stimuli. It may therefore also be possible to improve disease resistance by manipulating the ocular biome.

One approach is to apply commensal bacteria directly to the surface of the eye. In one study involving seven patients, treatment with probiotic lactobacillus acidophilus eye drops for four weeks resulted in a modest reduction in signs and symptoms of conjunctivitis.

Another approach is to treat the eye indirectly by improving the gut biome. In a study involving 62 patients, oral administration of lactobacillus over a period of eight weeks appeared to alleviate eye fatigue caused by blue light exposure.

“Where we go next with microbiome research is to determine the specific impacts of individual microbiota, elucidate the mechanisms that mediate microbe-host interaction and apply that knowledge to therapeutic aims,” Dr Corr concluded.

Monique Hope-Ross: monique.hopeross@icloud.com
Sinéad Corr: corrsc@tcd.ie