Moderate exercise tracked 21% less cataract, more did not

Cataract is filed under things that happen to you. A cataract is a clouding of the clear lens inside your eye, most develop as part of the normal aging process, and by age 80, more than half of all Americans either have a cataract or have had cataract surgery.

The treatment is good and the prevention list is short. Surgery is the only way to remove a cataract.

A UK Biobank study in the journal Eye adds a modifiable factor to that short list, using activity measured by wrist device rather than recalled on a form. The people meeting the standard exercise guidelines developed fewer cataracts.

The people doing far more than the guidelines did not do better.

Why a lens might respond to exercise

The clouding is a protein problem. It happens when the protein in the lens of your eye clumps together, gradually, over decades.

Two of the things that accelerate protein damage in tissue generally are oxidative stress and high blood sugar, and exercise moves both. That is the hypothesis, and this study did not test it, so treat the mechanism as plausible rather than shown.

What has been missing is decent exposure data. Previously, most work on lifestyle and eye disease asked people how much they exercise, and people are unreliable narrators about that. Physical activity (PA) here was captured by a wrist device instead.

How the activity was measured

Triaxial accelerometer data from wearable devices, collected over a continuous 7-day wear period (accounting for potential seasonal variation), were used to define PA at light (LPA), moderate (MPA), and vigorous (VPA) intensities.

A week of device data is a snapshot, and it is a considerably better one than a questionnaire. It captures the walking people forget and discounts the exercise they overstate.

The study ultimately enrolled 77,123 participants with a mean age of 55.67 years. During a mean follow-up of 6.3 years, 1194 incident senile cataract cases were identified, meaning new diagnoses during the study rather than cataracts people already had.

For reference, the World Health Organization recommends that adults do at least 150 minutes of moderate-intensity physical activity per week.

What the activity numbers showed

Compared with insufficient activity, achieving 150-600 min of moderate to vigorous-intensity PA (MVPA) per week was associated with a 21% lower risk of SC, where SC is senile cataract, the age-related kind.

That window starts exactly where the WHO guideline starts. Meeting the standard target was where the benefit sat.

Above it, the line turned. Dose-response analyses revealed U-shaped associations for MPA and MVPA, with the lowest risk observed at moderate volumes. More was not better past a point, on this data.

Intensity mattered too, and not in the direction people assume. For other intensities, engaging in 300 min/week or more of MPA was associated with reduced risk, while no significant associations were observed for LPA or VPA alone, meaning nothing the data could separate from chance. Moderate activity carried the finding; light activity and vigorous activity on their own carried nothing.

What the genetic analysis adds on cataract

Cohort studies of exercise and disease have a standing problem: people who are already unwell exercise less, so the arrow can point backward.

This study tries to get around it, using a method called Mendelian randomization, which stands inherited gene variants in for a behavior. Two-sample Mendelian randomization analyses were conducted using genome-wide association data from multiple large-scale prospective study cohorts (UK Biobank for PA and FinnGen for SC).

The method substitutes inherited gene variants for the behavior. Because your genes were set before you were born, an eye disease at 60 cannot have changed them, which removes the reverse-causation route.

It agreed with the cohort. The genetic analysis supported a causal relationship, indicating that genetically proxied higher MVPA was associated with a lower risk of SC, and by a larger margin than the observational estimate.

Two different designs, with different weaknesses, pointing the same way is a stronger position than either alone.

What this cataract study cannot show

The activity measurement is one week, treated as though it describes years. People change.

Mendelian randomization is not proof either. It rests on assumptions that cannot be fully checked, chiefly that the gene variants affect cataract only through activity, and genes linked to exercise are also linked to body weight, mood and much else.

UK Biobank volunteers are healthier and less deprived than the general population, and the accelerometer subgroup is healthier still.

The paper is paywalled, so the authors’ own limitations section was not available for this piece.

What to do about cataract and your week

The authors’ framing is modest and reasonable: adhering to the PA guidelines established by the World Health Organization emerges as a novel and modifiable protective measure to delay the progression of SC.

Which is to say: hit the target you were already being asked to hit, and eyes may be one more thing that benefits. Nobody needs a new reason to do 150 minutes a week, but a possible one for cataract is a reason more.

What the study argues against is the assumption that more is always better. Past the guideline window the curve stopped falling, and the extra hours bought nothing for the lens.

Movement gets studied for the eye in another way too: Baduanjin, a slow Chinese exercise form, has been tested in glaucoma patients, though for their distress rather than their sight.

At the other end of life the eye problem is growth rather than clouding. In children, spectacle lenses with highly aspherical lenslets slowed both the prescription and the axial elongation that drives it.