Does fasting improve your eyesight?

The claim

The claim is that fasting makes vision sharper, or that it improves a glasses prescription.

Where it comes from

The question usually starts from a person's own eyes. Someone fasts, distance vision seems different, or the next eye test does not match the old glasses, and the fast is the change they can point to.

A Ramadan daytime fast, a 16:8 window, one meal a day (OMAD), and a multi-day water fast are different practices. In the Ramadan studies below, people took no food and no water from sunrise until sunset, and then they ate and drank. In a 16:8 window, water is still allowed and food returns every day. A steep rise or fall in blood sugar is a different event from any of those fasts. This note keeps them apart.

What the evidence shows

The fasting studies this note opened measured Ramadan daytime fasting.

In 2011, Majid Assadi and colleagues measured healthy men who were fasting for Ramadan in Bushehr, Iran. The abstract reports 58 men, with a mean age of 40.7 years. The full paper enrolled 65 men and says 7 did not finish. One sentence in the results still says 65. The eye measurements were taken in the morning and the afternoon on the first day, and again on the last day. Intraocular pressure, visual acuity, and refractive error did not differ between the first and last day, or between morning and afternoon.

In 2019, Zarife Ekici Gok, Abuzer Gunduz, and Cem Cankaya measured 45 healthy men, ages 20 to 40, mean age 28.8 years, who fasted through Ramadan. Women were not included. The fast in that study lasted about 16.5 hours, still with no food and no water until sunset, and the fasting measurements were taken at least 12 hours after it began. Best corrected vision stayed 20/20 in both eyes before Ramadan and in the first and fourth weeks. Refractive error and visual acuity did not change significantly. Contrast sensitivity, a test of faint stripes rather than the letter chart, was higher than before Ramadan at one stripe size in the first week and at two stripe sizes in the fourth week. The first and fourth weeks did not differ from each other. The authors say the fast had no negative effect on the letter chart or on contrast sensitivity, and that the stripe-test rises likely make no practical difference. They had no separate group of men who were not fasting.

In 2026, Seyed-Hashem Daryabari and colleagues reviewed 21 studies of the front of the eye. Every study was Ramadan fasting. Tear breakup time fell, and basal tear secretion fell. Dry eyes can cause blurred vision, according to the NHS, so a less stable tear film is one possible reason vision can feel blurry during a fast.

Two further studies measured a large swing in blood sugar while people were eating or being treated for diabetes.

In 1999, Masatoshi Furushima, Masamoto Imaizumi, and Kazuo Nakatsuka gave an oral glucose load to seven healthy young adults, five men and two women, mean age 23.2 years. At 150 minutes the measured focus had shifted about 1.93 diopters toward nearsightedness. Sharpness on the chart, once the new measurement was used, did not change. The authors report that the shift had reversed by about six hours, when blood sugar had come back down. A shift of that size blurs vision through the usual glasses until the reading moves back.

In 2000, Fumiki Okamoto, Hirohito Sone, Tomohito Nonoyama, and Sachiko Hommura followed 28 eyes of people with diabetes while treatment brought high blood glucose down. A temporary shift toward farsightedness occurred in every patient. The largest shift in an eye averaged 1.47 diopters, and the range was 0.50 to 3.75 diopters. It began about 3 days after treatment started and peaked at about 10 days. The shift was larger when blood glucose fell faster. The previous refraction returned between 14 and 84 days. Lens thickness did not change significantly during the shift. Those figures are the authors' abstract. The usual glasses would not match during the shift, and they matched again when the previous refraction returned.

Furushima's volunteers reached about 280 mg/dL after a large sugar drink with insulin held down, and Okamoto's patients had diabetes. Neither study describes a fast in healthy people. This mostly matters for people with diabetes, especially anyone fasting while taking medicine that lowers blood sugar.

What isn't known

This note found no study of a 16:8 window, one meal a day, or a multi-day water fast that measured eyesight or a glasses prescription. A PubMed search on 19 October 2026 for intermittent fasting, time-restricted eating, alternate-day fasting, water fasting, prolonged fasting, or one meal a day, together with visual acuity, refraction, refractive error, contrast sensitivity, or a glasses prescription, returned five records. None of those five measured eyesight or a glasses prescription during a 16:8 window, one meal a day, or a multi-day water fast.

Assadi and Gok studied healthy men during one month of Ramadan. Gok's group had no comparison group of men who were not fasting. Neither study followed people for years.

Okamoto's patients were being treated for high blood glucose, and the previous refraction returned over weeks. A change in your own prescription can still be real. These studies can't tell you which kind yours is; a repeat test can.

Bottom line

Studies of fasting found no change in the prescription or in chart sharpness. The studies this note opened were Ramadan daytime fasts, with no food and no water until sunset. The blood sugar studies involved diabetes or a forced sugar spike, not a fast in healthy people. They mostly matter for people with diabetes, especially anyone fasting while taking medicine that lowers blood sugar. Have the eyes checked again after a few weeks of ordinary eating before you buy new lenses. In Okamoto's patients the previous refraction returned somewhere between two weeks and about three months, so repeat the test if the numbers are still moving.

If vision goes blurry during a fast, tell an eye doctor. Blur with a headache that is severe or new for you, or with weakness or confusion, means stop and get help. That stop sign is written for the first weeks in Common snags in the first weeks, for the second day in After the first day, and for a stretch past 72 hours in Signs on a very long stretch.