Obese people have double the risk of losing their sight because their weight increases their chance of developing certain conditions, a report has said.
The Royal National Institute of the Blind said the obese risk cataracts and age-related macular degeneration - the most common cause of sight loss.
It warned that millions of obese adults and children in the UK are also at risk from other eye conditions.
The RNIB said people did not do enough to look after their eyes.
Latest government estimates predict that 13m adults and children could be obese by 2010, if no action is taken.
The RNIB says being obese is linked to a risk of developing four main conditions linked to sight loss.
Age-related macular degeneration (AMD) is the UK's leading cause of sight loss - with around 500,000 people affected.
People with a body mass index of over 30, who are classed as obese, have double the risk of the most common form of the condition - dry AMD.
Obesity has also been found to increase the rate of progression of wet AMD, which can lead to severe sight loss and blindness in as little as three months.
People who have a genetic predisposition to AMD must be particularly careful about their weight as their risk of the condition increases from a four-fold risk if they are a normal weight to an 11-fold risk if they are obese.
Those who are obese also have double the risk of cataracts, which are linked to one in four cases of sight loss in the over-75s.
Obesity also significantly increases the risk of type 2 diabetes, which is linked to diabetic retinopathy.
The risk of sight loss through diabetes is also particularly high in people from Asian, African and African/Caribbean backgrounds, who are four to five times more likely to develop diabetes.
There is also a link between being severely overweight and glaucoma, caused by pressure within the eye.
Barbara McLaughlan, an eye health consultant who wrote the RNIB report, said: "With a staggering one in five adults and children in the UK now considered to be obese, RNIB believes it is vital that people are made aware of the risks to their sight.
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"Sight is the sense we most fear losing, but most people don't do nearly enough to look after their eyes.
"To safeguard sight we recommend people maintain a healthy weight, eat lots of fresh fruit and vegetables, take exercise to improve general health, don't smoke and most importantly have regular eye tests.
"Many thousands of people lose their sight each year through conditions that could have been treated if picked up early enough through an eye test."