Cataracts form when proteins inside the lens of your eye clump together, scattering light instead of letting it pass through clearly. Over 100 million people worldwide had cataracts in 2021, and nearly 90% of cases occur in people over 40. The process is gradual, driven by a combination of aging, oxidative damage, and in some cases specific triggers like diabetes or medication use.
What Happens Inside the Lens
Your eye’s lens is made mostly of water and specialized proteins called crystallins, arranged in a precise, orderly structure that keeps the lens transparent. A built-in protection system, alpha-crystallin, works like a chaperone: it recognizes damaged or misfolded proteins and grabs onto them before they can clump together. In a young, healthy lens, this system handles the steady trickle of protein damage that comes with normal life.
The problem is that alpha-crystallin is finite. Your lens doesn’t produce new supplies the way other tissues regenerate. The proteins you’re born with are largely the same ones you carry for life. Over decades, more and more crystallin proteins accumulate damage and begin to unfold. Alpha-crystallin binds to these damaged proteins one by one until it becomes saturated. Once it’s overwhelmed, the damaged proteins are free to stick to each other, forming larger and larger clumps. These aggregates scatter incoming light instead of transmitting it, and the lens gradually turns cloudy. Eventually, the saturated alpha-crystallin itself gets pulled into the growing clumps, accelerating the process.
How Oxidative Damage Drives the Process
The lens sits in a uniquely vulnerable position. It’s constantly exposed to light and oxygen, both of which generate free radicals, unstable molecules that damage proteins and cell membranes. Under normal conditions, the lens has strong defenses. Glutathione, the lens’s primary antioxidant, is so effective that under normal conditions less than 1% of it exists in its used-up (oxidized) form. Even after an oxidative attack, the lens can regenerate its glutathione supply within minutes.
But decades of chronic exposure slowly erode these defenses. As glutathione and other antioxidants deplete, oxidized protein residues accumulate in the long-lived crystallins. Free radicals also attack the fatty acids in lens cell membranes through a chain reaction called lipid peroxidation, which produces toxic byproducts that cross-link proteins together. This cross-linking further destabilizes the orderly protein structure the lens depends on for clarity. The result is a slow, compounding loss of transparency that typically takes years to become noticeable.
Where Cataracts Develop in the Lens
Not all cataracts are the same. Where the clouding forms determines what symptoms you notice first and how quickly they progress.
- Nuclear cataracts form in the center of the lens. They often start by making distant objects blurry while close-up vision actually improves temporarily, sometimes enough to read without glasses. Over time the lens yellows or browns, distorting color perception and worsening overall vision. These are the most common age-related type.
- Cortical cataracts start at the outer edges of the lens as white, wedge-shaped streaks. As they grow inward toward the center, they increasingly interfere with light passing through the lens, causing glare and difficulty with contrast.
- Posterior subcapsular cataracts form at the back surface of the lens. They tend to progress faster than other types and are the kind most associated with steroid use and diabetes. They often cause problems with reading and bright-light glare early on.
Why Diabetes Accelerates Cataract Formation
High blood sugar creates a separate, faster pathway to lens damage. When glucose levels are elevated, an enzyme in the lens converts the excess glucose into a sugar alcohol called sorbitol. Sorbitol is produced faster than it can be broken down, and its chemical properties prevent it from simply diffusing out of cells. It accumulates inside the lens fibers, creating an osmotic imbalance: water rushes in to dilute the concentrated sorbitol, and the lens fibers swell.
This swelling doesn’t just distort the lens mechanically. It triggers a cascade of biochemical damage, ultimately causing the swollen fibers to collapse and liquefy. The osmotic stress also kills lens epithelial cells through programmed cell death. This mechanism is especially aggressive in young people with type 1 diabetes, where rapid swelling of the outer lens fibers can cause cataracts to develop much faster than the slow, decades-long progression typical of age-related cataracts.
UV Light and Lens Damage
The lens absorbs UV radiation in the 295 to 400 nanometer range, which includes both UVA and the portion of UVB that reaches the eye. This radiation boosts free radical production inside the lens, accelerating the same oxidative damage that happens with aging. The UV-triggered oxidative stress modifies crystallin proteins and promotes the formation of large molecular aggregates. Cumulative UV exposure over a lifetime is one reason cataracts are more common in regions with high sun exposure and in people who spend significant time outdoors without eye protection.
Medications That Increase Risk
Long-term corticosteroid use is one of the best-documented medication-related causes of cataracts, specifically the posterior subcapsular type. The risk climbs with both dose and duration. Studies of inhaled corticosteroids for asthma and COPD found that daily doses exceeding 1,000 micrograms were associated with a substantial increase in cataract risk. For oral corticosteroids, five or more years of use tripled the odds of developing posterior subcapsular cataracts. This is relevant for anyone managing a chronic condition with ongoing steroid treatment, whether inhaled, oral, or applied as eye drops.
Smoking and Cataract Risk
Smoking accelerates cataract formation through multiple routes: it increases oxidative stress, depletes the lens’s antioxidant reserves, and introduces heavy metals that damage lens proteins. The encouraging finding is that quitting helps. Compared to people who keep smoking, those who quit see roughly a 20% reduction in cataract risk within the first 10 years. The benefit plateaus at about a 26% to 27% reduction for those who quit 10 or more years earlier, suggesting that while quitting doesn’t erase all the accumulated damage, it meaningfully slows future progression.
Can Diet Slow Cataract Formation?
Lutein and zeaxanthin, two pigments concentrated in leafy greens, egg yolks, and corn, accumulate in the lens and act as both antioxidants and light filters. Their potential to prevent cataracts has been studied extensively, and the results are nuanced. In the large AREDS2 trial, lutein and zeaxanthin supplements did not reduce cataract risk across the full study population.
However, for people whose diets were already lowest in these nutrients, supplementation reduced the risk of developing any cataract by 30% and the need for cataract surgery by 32%. For people already eating plenty of lutein-rich foods, supplements offered no additional benefit. The practical takeaway: if your diet is low in colorful fruits and vegetables, increasing your intake of leafy greens and other lutein-rich foods may offer real protective value. If you’re already eating well, adding a supplement on top is unlikely to help.
How Quickly Cataracts Progress
Most age-related cataracts develop over years, often so gradually that you don’t notice the change until it’s well advanced. Early cataracts may cause no symptoms at all and are only detected during a routine eye exam. Over time, you might notice increasing glare from headlights at night, colors looking faded or yellowish, or needing brighter light to read. The pace varies widely: some cataracts take a decade or more to reach the point where they meaningfully affect daily life, while cataracts linked to diabetes or steroid use can progress much faster. Surgery is typically recommended when the vision loss starts interfering with activities like driving, reading, or watching television, not at any fixed stage of the cataract itself.