Cataracts are caused by the breakdown and clumping of proteins inside the lens of your eye. The lens is normally clear because its proteins are arranged in a precise structure that lets light pass through. When those proteins lose their shape and stick together, they form cloudy patches that scatter light instead of transmitting it. This process happens for several reasons, with aging being the most common.
How Lens Proteins Break Down
Your eye’s lens is made largely of specialized proteins called crystallins, which are arranged in tight, orderly patterns that keep the lens transparent. Unlike most cells in your body, the lens has no blood supply and very little ability to replace damaged proteins. The crystallins you’re born with are essentially the same ones you carry your entire life.
Over time, these proteins begin to unfold. The process starts at one end of the protein, exposing inner surfaces that are normally hidden. Those newly exposed surfaces are sticky, chemically speaking, and they latch onto neighboring proteins. One damaged protein links to another, then another, forming clumps large enough to scatter light. This chain reaction of protein aggregation is the fundamental mechanism behind age-related cataracts, which account for the vast majority of cases worldwide. By age 80, more than half of all Americans either have a cataract or have had one surgically removed.
Why Aging Is the Primary Cause
Decades of exposure to ultraviolet light, oxidative stress, and the body’s own metabolic byproducts gradually damage lens proteins. The lens also grows throughout life, adding new layers of fiber cells on top of old ones (similar to tree rings). The oldest cells get compressed into the center of the lens, where they become increasingly dense and stiff. This is why many cataracts start in the center of the lens and slowly spread outward.
The lens has built-in defenses against protein damage, including antioxidant molecules that neutralize harmful free radicals. But these defenses weaken with age. As the balance tips, protein damage outpaces repair, and clouding begins. The process is so gradual that most people don’t notice vision changes until the cataract has been developing for years.
Diabetes and High Blood Sugar
People with diabetes develop cataracts earlier and more frequently than the general population. The reason is a sugar-processing problem inside the lens itself. When blood sugar is high, excess glucose enters the lens. Normally, the lens metabolizes glucose through its standard energy pathway, but when glucose levels spike, an alternate pathway kicks in that converts glucose into a sugar alcohol called sorbitol.
Sorbitol doesn’t move out of the lens easily. It builds up, along with fructose, and both sugars draw water into the lens through osmotic pressure. The result is swelling of the lens fibers, disruption of their carefully organized structure, and eventually clouding. This is why good blood sugar control matters for eye health, not just for preventing diabetic retinopathy but for slowing cataract formation as well.
Smoking and UV Exposure
Smokers are two to three times more likely to develop cataracts than nonsmokers. Cigarette smoke floods the body with free radicals, reactive molecules that damage proteins throughout the body, including in the lens. Smoking also depletes the antioxidants that normally protect the lens, creating a double hit: more damage and less defense.
Ultraviolet radiation works similarly. UV-B rays are absorbed directly by lens proteins, causing chemical changes that accelerate unfolding and clumping. This is one reason cataracts are more common in populations living closer to the equator and in people who spend significant time outdoors without UV-blocking sunglasses.
Medications That Increase Risk
Corticosteroids are the most well-established medication-related cause of cataracts. Oral corticosteroids increase the odds of developing a specific type of cataract (posterior subcapsular) by roughly four times compared to non-users. Even inhaled corticosteroids, commonly used for asthma, raise the risk about two and a half times. The cataracts caused by steroids tend to form in the center of the back surface of the lens, which is particularly disruptive to vision because it sits right in the path of incoming light.
The risk increases with higher doses and longer duration of use. If you take corticosteroids regularly for a chronic condition, periodic eye exams can catch steroid-related lens changes early.
Eye Injuries and Trauma
A direct blow to the eye, a puncture wound, or even intense heat or radiation exposure can cause what’s called a traumatic cataract. The mechanism is straightforward: physical force disrupts the lens fibers or tears the thin capsule that surrounds the lens. If the capsule ruptures, fluid from inside the eye rushes in and causes the lens fibers to swell and turn opaque. This can happen within minutes to hours of the injury.
Even without a capsule tear, blunt trauma can cause a characteristic star-shaped or rosette-patterned cataract that may develop weeks or months after the injury. The lens can also be knocked partially or completely out of position, a condition called subluxation or luxation. Traumatic cataracts can occur at any age, and anyone with a history of significant eye injury should be aware of the possibility.
Cataracts Present at Birth
Some babies are born with cataracts or develop them in the first few months of life. Congenital cataracts have two main causes: genetic mutations and prenatal infections.
On the genetic side, researchers have identified mutations in more than 100 genes linked to hereditary cataracts. Many of these genes code for the same crystallin proteins that break down in age-related cataracts, but the mutations cause them to misfold from the start rather than degrading over decades. Congenital cataracts can run in families or appear as part of broader genetic syndromes.
Prenatal infections are the other major cause. The TORCH group of pathogens (toxoplasma, rubella, cytomegalovirus, herpes simplex, and others) can all damage the developing lens. Rubella and herpes simplex are the most common infectious causes of congenital cataracts. Widespread rubella vaccination has dramatically reduced this cause in many countries, but it remains a concern in regions with lower vaccination coverage.
Other Contributing Factors
Several additional factors raise your risk of cataracts beyond the major causes listed above:
- Alcohol use. Heavy drinking is associated with earlier cataract development, likely through oxidative stress and nutritional deficiencies.
- Obesity. Higher body mass index correlates with increased cataract risk, possibly through the same metabolic pathways involved in diabetes.
- Previous eye surgery. Procedures for other eye conditions, such as retinal surgery, can accelerate cataract formation in the affected eye.
- Radiation exposure. Both ionizing radiation (such as radiation therapy for head and neck cancers) and prolonged infrared exposure (historically seen in glassblowers) can cause lens damage.
Can You Prevent Cataracts?
You can’t eliminate the risk entirely, since aging is the dominant factor, but you can meaningfully slow the process. Quitting smoking, wearing UV-blocking sunglasses, managing blood sugar if you have diabetes, and limiting alcohol are the best-supported strategies.
You may have heard that antioxidant supplements, particularly lutein and zeaxanthin, can prevent cataracts. Despite their popularity, the evidence doesn’t hold up well. After a systematic review, the FDA concluded that no credible evidence supports a health claim linking lutein or zeaxanthin intake to reduced cataract risk. One large study found that eating cooked spinach was associated with fewer cataract surgeries, but when researchers isolated the individual nutrients, none of them, including lutein, zeaxanthin, or beta-carotene, showed an independent protective effect. Eating a diet rich in fruits and vegetables is still a good idea for overall eye health, but specific supplements aren’t a proven cataract prevention tool.
Global Scale of the Problem
Cataracts remain the leading cause of blindness worldwide. In 2020, an estimated 17 million people were blind due to cataracts, representing about 40% of all blindness globally. Another 83.5 million people had moderate to severe vision impairment from cataracts. The burden falls disproportionately on lower-income countries where access to cataract surgery is limited. In wealthier nations, cataract removal is one of the most commonly performed surgeries, with high success rates and quick recovery times.