Cataracts form when proteins inside the eye’s lens clump together and block light from passing through clearly. The lens sits behind the iris and is made almost entirely of water and specialized proteins called crystallins, which are arranged in a precise structure that keeps the lens transparent. When that structure breaks down, the lens clouds over. About 25% of people develop cataracts by their 70s, and that number climbs to over 31% by age 80.
How Lens Proteins Break Down
The crystallin proteins in your lens are unique in the body. They’re produced before birth and are never replaced. That means they need to last your entire life without being swapped out, which is an extraordinary demand on any biological structure. Over decades, these proteins accumulate damage from chemical modifications: oxidation, glycation (sugar molecules binding to proteins), and most commonly, deamidation, where small chemical groups on the protein surface change shape.
These modifications cause crystallin proteins to unfold, exposing sticky surfaces that were previously tucked inside. Once exposed, those surfaces latch onto neighboring proteins, forming insoluble clumps. These clumps scatter incoming light instead of letting it pass through, producing the characteristic clouding of a cataract. The process is gradual. Most people notice it first as slightly blurred vision or increased glare from headlights at night, and it worsens over years.
Aging: The Primary Driver
Age is the single biggest risk factor. Because crystallin proteins are never replaced, the older you are, the more cumulative damage those proteins have absorbed. Cataract prevalence roughly doubles with each decade of life after 60: about 14% of people aged 60 to 69 have cataracts, rising to 25% in the 70 to 79 group and 31% in those 80 and older. The natural antioxidant defenses inside the lens, particularly a molecule called glutathione that protects proteins from oxidative damage, also decline with age, leaving the lens increasingly vulnerable.
UV Radiation and Sunlight Exposure
Ultraviolet light from the sun directly damages lens proteins. UV-B radiation, which makes up about 3% of the UV light reaching Earth’s surface, is linked to cortical cataracts (those forming at the outer edge of the lens) and posterior subcapsular cataracts (forming at the back). UV-A radiation, which accounts for the remaining 97%, is associated with nuclear cataracts that develop in the center of the lens.
The damage mechanism is straightforward. UV exposure triggers oxidative stress in the lens, which causes protein building blocks to form abnormal chemical bonds with each other. These cross-linked proteins aggregate and become insoluble. Over time, the lens also loses its built-in UV-filtering compounds, making the inner regions even more susceptible to further light damage. This is why consistent sunglasses use, particularly lenses that block both UV-A and UV-B, matters over a lifetime rather than just on bright days.
Smoking
Smoking increases cataract risk in a clear dose-dependent pattern. A large prospective study of men found that those smoking more than 15 cigarettes per day had a 42% higher rate of cataract surgery compared to people who never smoked. Even light smoking carried measurable risk: 1 to 5 cigarettes per day raised the rate by 11%, 6 to 10 by 14%, and 11 to 15 by 18%. The mechanism involves the same oxidative stress pathway as UV damage. Cigarette smoke introduces free radicals directly into the bloodstream, which reach the lens and accelerate protein damage. Quitting reduces your risk over time, though the lens proteins that were already damaged remain.
Diabetes and High Blood Sugar
People with diabetes develop cataracts earlier and more frequently than the general population, and the reason is well understood. When blood sugar is elevated, excess glucose floods into the lens. The lens processes this glucose through an alternative chemical pathway that produces sorbitol, a sugar alcohol that gets trapped inside the lens because it can’t easily cross cell membranes. Fructose also accumulates through the same process.
Both sugars raise the osmotic pressure inside the lens, pulling water in. The lens fibers swell, their internal scaffolding breaks apart, and the lens becomes opaque. This is why good blood sugar control matters for eye health specifically, not just for the commonly discussed complications like nerve or kidney damage.
Steroid Medications
Long-term corticosteroid use is one of the most significant medication-related causes of cataracts. Over 50% of patients taking systemic corticosteroids for more than 60 days develop some kind of eye complication, with cataracts accounting for the majority at 36%. The risk depends on three factors: total cumulative dose, duration of treatment, and how the medication is delivered. Oral steroids carry the highest risk, followed by topical applications, with localized injections carrying the least (under 2% in some studies).
The risk is real even for inhaled corticosteroids used for asthma or COPD. High-dose inhaled steroids (above a certain threshold) produced cataracts in nearly 40% of users after more than 12 months, while low-dose inhalers showed no significant increase. If you’ve been on corticosteroids for months or years, regular eye exams can catch lens changes early.
Eye Injuries
A hard blow to the eye or a penetrating injury can cause a cataract to form rapidly. In children who experienced blunt eye trauma, 13% developed a cataract, and the median time to onset was just 14 days after injury. The range spanned from 2 days to nearly 4 months. The mechanism is different from age-related cataracts: physical force disrupts the lens capsule or damages the lens fibers directly, causing immediate or near-immediate clouding rather than the slow protein degradation of aging.
Genetics and Congenital Cataracts
Some cataracts are inherited. Researchers have identified at least 34 genetic locations linked to cataract formation, many involving genes that code for the crystallin proteins themselves. When mutations make crystallin proteins inherently prone to clumping, cataracts can appear at birth or in early childhood rather than in old age. These hereditary cataracts are most often passed down in an autosomal dominant pattern, meaning a child needs to inherit only one copy of the affected gene from one parent to develop the condition.
Congenital cataracts are relatively rare compared to age-related ones, but they’re clinically important because they can interfere with visual development in children if not treated promptly.
Nutrition and Antioxidants
Because oxidative damage is central to cataract formation, there has been significant interest in whether antioxidant-rich diets or supplements can slow the process. Lutein and zeaxanthin, pigments found in leafy greens and eggs, concentrate naturally in the eye and act as both light filters and antioxidants. However, clinical trials have not shown that supplementing with these compounds prevents cataracts. A large trial of over 3,000 participants found no difference in the need for cataract surgery between groups receiving lutein and zeaxanthin supplements and those receiving a placebo over five years.
This doesn’t mean diet is irrelevant. The lens depends on antioxidant defenses that are built and maintained over a lifetime. A diet consistently rich in fruits and vegetables supports those defenses, but taking a supplement once damage has already accumulated doesn’t appear to reverse it.
When Cataracts Need Treatment
Not every cataract requires surgery. The decision is based primarily on how much the cataract interferes with your daily life: difficulty reading, trouble driving (especially at night), problems watching television, or inability to meet job requirements. There is no universal visual acuity cutoff that triggers surgery for most insurers. The standard is functional impairment, meaning the cataract is making it hard for you to do things that matter to you. Glare testing, where a bright light is shone toward your eye during a vision test, can reveal how much the cataract is scattering light even when your standard eye chart reading seems adequate.