8 ways that metabolic health and glucose levels affect eye health
Metabolic health and blood glucose levels can affect almost every aspect of the eye, and improving metabolic fitness may reduce the risk of developing these conditions.
As our primary sense, vision is critical. In a 2016 survey of more than 2,000 adults, participants ranked loss of eyesight as the worst possible health condition they could develop---worse than loss of limb, loss of memory, loss of hearing or speech, or having H.I.V./AIDS. The consequences of vision loss can be profound. Studies in older adults with visual impairment (low vision and/or blindness) have reported difficulty functioning in daily activities, along with a reduced health-related quality of life and increased rates of depression and anxiety.
A common misconception is that all eye problems noticeably affect vision and are thus easy to detect based on visual changes alone. But many eye problems, especially in their early stages, can exist without symptoms. Because most visual changes progress slowly and gradually over time, it is unlikely that an individual would detect a change in vision until the condition is much more advanced. In one study, approximately 1 out of 4 people who had an eye exam for refractive complaints only (i.e., seeking a prescription for glasses or contact lenses because of blurred vision, due to the natural shape of their eyes) also had some form of ocular condition. This is a testament to the importance of a regular eye exam.
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An eye exam not only detects undiagnosed eye conditions but sometimes even undiagnosed systemic health conditions. Our eye health is intimately connected to our overall health, especially our metabolic health, though few people know about this connection. Among people with diabetes, about 26% have diabetic retinopathy---10% of whom are unaware they have it. In a study including 183 Hispanic people who had been diagnosed with diabetes for at least a year, only half knew that diabetes can cause eye problems and that dilated eye exams are essential. Only a third knew that strict diabetes control can prevent eye problems.
Eye exams are important not only for people with diabetes but also for the general population, especially as a spectrum of metabolic conditions is rising. As of 2018, 42% of American adults had metabolic syndrome, and 65% had elevated glucose, in particular, both of which could affect their eye health.
This article explores how metabolic health and blood glucose levels can affect almost every aspect of the eye, from front to back, and how metabolic control and awareness can protect your eyes.
1. Increased risk of dry eye disease
Dry eye is a disease of the ocular surface that occurs due to abnormalities of the tear film---the liquid layer that covers the surface of the eye---either in quantity or quality of the tears. This can cause dryness, irritation, and eventually damage to the eye's front surface (the cornea). Dry eye disease can be due to multiple factors, including vitamin A deficiency, autoimmune conditions, hormonal conditions, medication side effects, inadequate blinking, and inadequate production of one or more components of the tear film.
The consequences of dry eye disease go beyond mild discomfort. Dry eye disease can significantly affect vision due to an increased risk of painful corneal ulcers, infection, scarring, irreversible corneal damage, and vision loss. Additionally, patients with dry eye disease are sometimes in chronic pain.
Although there are no studies regarding dry eye and blood sugar levels in people without diabetes, dry eye is linked to diabetes, especially in people with poor glucose control. Studies in people with diabetes show that those with higher glucose levels measured via HbA1c are at a higher risk for dry eye. A small study also suggests that people with diabetes who have higher HbA1c levels are more likely to lose the oil glands in their eyelids, which are critical for tear production and preventing dry eye, and another suggests that higher glucose levels are linked to greater oil gland atrophy.
Similarly, another study found that people with diabetes have decreased tear production, decreased corneal sensitivity, and decreased tear stability (the tear film staying intact between blinks), especially if they also had peripheral neuropathy. Those with poor metabolic control have less tear production and the loss of specific types of eye cells, and the study authors suggest that metabolic control may be more influential than duration of diabetes.
Bottom line: Metabolic health and strict control of glucose levels could play a role in producing healthy tears and maintaining the health of the front of the eye.
2. Increased risk of developing cataracts
A cataract is the clouding of the lens inside the eye. This lens is aptly named the intraocular lens and is located behind the front of your eye (the cornea) and the colored part of your eye (the iris). Its job is to bend the light that enters the eye to focus it on the back of the eye (the retina). The retina is the neural tissue in the back of the eye that captures light and sends signals to the brain, which are then interpreted and perceived as vision. This process and focusing light on the retina are key for creating clear sight.
At birth, the intraocular lens is clear and inevitably will slowly become more opaque or cloudy over time due to the natural processes of aging. With a cataract, the lens is no longer clear, and the light is not bent as cleanly; instead, it scatters in many directions. Without a clear focus on the retina, this translates to blurry and cloudy vision that could require cataract removal.
Our metabolic health and blood sugar levels can affect the intraocular lens. Metabolic syndrome, in particular, has been linked to cataracts. Metabolic syndrome is a combination of multiple components that indicate poor metabolic health, including high blood sugar, high blood pressure, abnormal cholesterol levels, and obesity.
One study found that the prevalence of cataracts increases with an increasing number of metabolic syndrome components. Additionally, high blood glucose and systolic blood pressure were individually associated with cataracts.
This study also found:
- The combination of diabetes, high blood pressure, obesity, and low high-density lipid (HDL) cholesterol levels is associated with a 7.2 times higher odds of cataracts
- The combination of diabetes and high blood pressure is associated with 4.73 times the odds of cataracts
Another study found that people with metabolic syndrome have double the odds of needing cataract-removal surgery compared to people without metabolic syndrome.
Diabetes is also linked to cataracts. People with diabetes are 3 to 5 times more likely to develop cataracts than those without diabetes, and they do so at an earlier age---on average, 20 years earlier. The duration of diabetes and poor metabolic control are associated with a higher likelihood of developing cataracts. Additionally, recovery after cataract surgery for people with diabetes can also be more complicated with fluctuations in blood glucose levels.
Bottom line: Factors of metabolic health, including weight, blood sugar, blood pressure, and cholesterol levels can increase the risk of developing cataracts. High or unstable blood sugar levels could cause earlier cataracts and possible complications in cataract surgery.
3. Temporary fluctuations in vision due to blood sugar levels
It is well known that high blood glucose levels can cause temporary near-sightedness in people with diabetes. In some cases, these temporary changes may last for months in people with uncontrolled diabetes, causing them to temporarily need a different prescription for glasses or contacts. High blood sugar levels are thought to cause transient thickening or swelling of the lens, causing temporary fluctuations in vision. However, studies show that a spike in blood sugar could cause transient nearsightedness by affecting other parts of the eye as well. Some evidence also indicates that rapidly correcting hyperglycemia may result in temporary farsightedness. Other studies suggest that hyperglycemia itself can also cause temporary farsightedness.
Bottom line: Blood glucose levels can cause fluctuations in vision, particularly nearsightedness.
4. Changes to the retinal microvasculature and circulation
As previously stated, the retina is the neural tissue in the back of the eye that converts light into electrical signals, which are then sent to the brain via the optic nerve to create the perception of vision. Retinal health is critical, and without a properly functioning retina to translate light into vision, blindness occurs.
A dilated retinal exam is the only time a physician can directly observe blood vessels in the body in vivo without an incision. The retina has its own circulatory system to supply and remove blood and nutrients, consisting of microvasculature such as the retinal venules, arterioles, and capillaries. Subtle retinal microvascular changes in these vessels (i.e., retinal microaneurysms, arteriovenous nicking, focal arteriolar narrowing) can indicate a problem with microvascular health, not only in the retina but all over the body.
Metabolic syndrome has been associated with large and small vessel inflammation and damage, and there is evidence that metabolic health can affect the retinal microvasculature. In one study, advanced imaging of the retinal microvascular circulation found altered blood flow in people with impaired glucose tolerance before they developed diabetes or cardiovascular disease.
In another study, people with metabolic syndrome were more likely to have retinopathy and retinal microvascular changes than people without metabolic syndrome, independent of age, gender, race, education, smoking, or alcohol consumption. Associations with retinal microvascular changes were significant even in people with metabolic syndrome without diabetes and high blood pressure.
Bottom line: Optimizing blood sugar levels and metabolic health (including blood pressure, weight, and cholesterol) could help maintain proper circulation within the small vessels of your eyes and the rest of your body.
5. Increased risk of retinopathy
Systemic metabolic and vascular disease can lead to progression to frank retinopathy (retinal disease) with retinal hemorrhages (bleeding in the back of the eye) and signs of retinal ischemia (inadequate blood and oxygen supply).
The process of retinopathy begins with microvascular abnormalities that lead to reduced blood flow to the retina. The oxygen-starved retina then sends signals that start constructing new blood vessels to try and bring more blood (and thus more oxygen). However, these blood vessels are more fragile and prone to break and leak. After a bleed, the risk is not over. Blood is reabsorbed, but leaves scar tissue that can pull on the retina and lead to retinal detachment and subsequent blindness.
Retinopathy is a common complication of diabetes, and blood sugar levels in people with and without diabetes have been found to correlate with retinopathy in several studies. In people with Type 2 diabetes, strict control of blood sugar reduces the progression of retinopathy by 21%. Another study found that increased fasting glucose, diastolic blood pressure (the bottom number), and body mass index (BMI) were higher in people without diabetes who had retinopathy compared to those without retinopathy. In this same study, retinopathy was found in 7.3%, 13.6%, and 17.5% of people with normal, impaired, and diabetic glucose metabolism, respectively, suggesting that those with better glucose control had less chance of having retinopathy. A more recent study also found that people with diabetes are more likely to have retinopathy if they have high blood pressure, BMI, fasting glucose, total cholesterol, triglycerides, and a longer duration of diabetes.
Beyond glucose levels, overall metabolic health is important for preventing retinopathy. For instance, high blood pressure is the best-known systemic condition associated with non-diabetic retinopathy, and poor blood pressure control is tied to worsening microvascular damage. An independent association between retinopathy and other metabolic health factors, such as high blood pressure and larger waist circumference, has been found. Studies in people with diabetes have found that the more factors of metabolic syndrome an individual has, the more likely they are to have diabetic retinopathy. A recent review study found that intensive blood pressure control reduces the 5-year risk of people with diabetes developing retinopathy by 18%.
Bottom line: Optimizing metabolic health, both in people with and without diabetes, can reduce the risk of development or progression of retinopathy.
6. Increased risk of retinal vascular occlusions (stroke in the eye)
Permanent, significant, and sudden vision loss can occur due to large vessel involvement in the retina via occlusion of the retinal arteries or veins. This essentially constitutes a stroke in the eye and could mean that a systemic stroke is impending.
Metabolic health can be connected with vascular occlusive events. Diabetes and high blood pressure, in particular, are associated with vascular damage and an increased risk of large vessel atherosclerosis. Central retinal artery occlusion has a known association with high blood lipid levels, high blood pressure, and diabetes. Additionally, high blood pressure and obesity are significant risk factors for central retinal vein occlusion.
In a study of non-diabetic people with a history of retinal vein occlusion, 53% of people had abnormal results on a glucose tolerance test, along with reduced retinal arterial blood flow. Notably, this indicates that poor glucose metabolism, even without diabetes, may be correlated with retinal vein occlusions.
Bottom line: Controlling metabolic health can reduce your risk of a vascular occlusive event (stroke) in the eye. Some evidence suggests that impaired glucose metabolism could be associated with these occlusive events.
7. Increased risk of macular disease
One area of the retina is crucial as it is responsible for our clearest, central vision. This area is known as the macula. When the macula is damaged, it interferes with the central vision that we rely upon to function in our daily lives.
Swelling and leaking fluid in the macula (known as macular edema) can cause irreversible vision loss. In a meta-analysis, some studies suggested that significantly elevated triglycerides, low-density lipoproteins, and total cholesterol were correlated with diabetic macular edema. However, this remains to be further examined.
Additionally, one study found a higher risk of age-related macular degeneration in people with high blood pressure or higher BMI.
Bottom line: Metabolic health, including BMI and cholesterol and triglyceride levels, could impact the part of your eye responsible for your clearest, central vision.
8. Increased risk of elevated eye pressure and glaucoma
Glaucoma is a condition that affects the nerve fibers of the eyes and their pathway of communication with the brain, known as the optic nerve. Damage to the optic nerve causes irreversible vision loss. The full mechanism of glaucoma is not yet fully understood; however, it has been shown that elevated pressure in the eyes, known as intraocular pressure (IOP), is associated with glaucoma progression. One theory is that elevated IOP could cause microvascular damage around the optic nerve, causing chronic nerve damage and eventual nerve death. The only known effective treatment for glaucoma is lowering intraocular pressure, which has been shown to slow disease progression, although more research is needed.
There is some evidence that supports an association between elevated IOP and metabolic syndrome. One study found an association between IOP and BMI, glucose levels, blood pressure, and cholesterol. Another study found an increase in IOP in subjects with metabolic syndrome compared to those without---and an increase in IOP with each additional component of metabolic syndrome that a person has. It also found a link between elevated IOP and features of insulin resistance.
However, having elevated IOP doesn't automatically mean you have glaucoma, as glaucoma is defined by structural and visual changes, not by eye pressure alone. A 2020 review on metabolic syndrome and glaucoma highlights a lack of prospective patient studies. One of only two prospective studies that have been conducted found that patients with higher BMI or more components of metabolic syndrome were more likely to develop open-angle glaucoma. But a second prospective study found that higher BMI is linked to a reduced risk of open-angle glaucoma, and that there was no link to metabolic syndrome.
A recent meta-analysis of observational studies found that metabolic syndrome is associated with a 34% higher odds of having glaucoma. Breaking down the components of metabolic syndrome, hyperglycemia, and hypertension, were individually associated with increased risk of glaucoma.
Additionally, in people with diabetes, a unique form of glaucoma can occur due to the growth of new blood vessels in the eye's drainage system. These blood vessels back up fluid in the eye, leading to a subsequent increase in eye pressure. This form of glaucoma is known as neovascular glaucoma, and its incidence increases with the duration of diabetes and poor blood sugar control.
Bottom Line: High blood glucose levels and poor metabolic health may be associated with increased eye pressure that could result in glaucoma.
Conclusion
Evidence suggests that metabolic health and blood glucose control could be key in maintaining eye health, especially in those with metabolic syndrome and diabetes. Continuous glucose monitoring and the ability to make educated decisions regarding our metabolic health promote better overall health and eye health.
The good news is that many of the conditions mentioned above are easily diagnosed in a dilated eye exam, and effective treatments do exist. Early detection is key, and early treatment often slows or stops disease progression. Don't wait until you have vision problems to see your eye doctor.
About the Author
Dr. Kellen Riccobono is a fellowship-trained optometrist and a Fellow of the American Academy of Optometry. She completed her undergraduate coursework with honors at the University of Texas at Austin. Afterward, she received her Doctor of Optometry degree from The University of Houston College of Optometry, graduating summa cum laude. Dr. Riccobono completed a post-graduate fellowship at the same institution, specializing in corneal disease and medically necessary contact lenses. During her fellowship, Dr. Riccobono developed an interest in both writing and clinical research. She has experience in medical writing, ranging from research protocols to articles in Review of Optometry and co-authoring publications in journals such as the American Journal of Ophthalmology & Contact Lens and Anterior Eye. Kellen is passionate about medical optometry and believes in the importance of providing compassionate and comprehensive care to her patients and spreading awareness of the importance of ocular health.
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