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A cataract is a clouding of the lens in the eye leading to a decrease in vision. It can affect one or both eyes, it is more common with age, and can even lead to blindness. About 20 million people globally are blind due to cataracts. Vitamin supplements have failed to find an effect in numerous studies.

But in this study, eating foods rich in vitamin C and to a smaller degree manganese had the beneficial effect of slowing cataract progression over the course of 9 1/2 years. Manganese is a micronutrient that is necessary in small amounts, but it is rare to be deficient in manganese. However manganese has numerous negative effects if too much is ingested or if there is too much exposure. Bottom line: increased intake of fruit and vegetables (for vitamin C) could help prevent the development or progression of cataracts.  From Medical Xpress:

Increased vitamin C in the diet could help protect against cataracts

Higher dietary intake of vitamin C has been found to have a potentially preventative effect on cataract progression in the first twin study of cataracts to examine to what degree genetic and environmental factors influence their progression with age. Cataract is a common condition in which the lens of the eye becomes cloudy as a result of oxidation over time. Whilst this is a natural part of ageing for many, for others it is more severe and causes blurred vision, glare and dazzle that cannot be corrected by glasses or contact lenses.

The study, led by King's College London and published in the journal Ophthalmology, looked at the progression of cataracts in the eyes of 324 pairs of female twins .... They found that those participants who had a higher intake of vitamin C were associated with a 33 per cent risk reduction of cataract progression and had 'clearer' lenses after the 10 years than those who had consumed less vitamin C as part of their diet.

The study, funded by the Wellcome Trust and Guide Dogs for the Blind, also found that environmental factors (including diet) influenced cataract more than genetic factors, which only explained a third of the change in lens opacity. The fluid in the eye that bathes the lens is high in vitamin C, which helps to stop the lens from oxidising and protects it from becoming cloudy. It is thought that increased intake of vitamin C has a protective effect on cataract progression by increasing the vitamin C available in the eye fluid.

Kate Yonova-Doing, the study's first author said: 'The human body cannot manufacture vitamin C, so we depend on vitamins in the food we eat. We did not find a significantly reduced risk in people who took vitamin tablets, so it seems that a healthy diet is better than supplements.'

This study, even though done on mice, reinforces previous research that taking antioxidant supplements are linked to higher rates of cancer or the spread of cancer (metastases). Here it is linked to the spread of metastatic skin cancer. The researchers say that the overall conclusion from the various studies is that antioxidants protect healthy cells from free radicals that can turn them into malignancies, but may also protect a tumor once it has developed. They also suggest that suntan lotions and skin lotions containing vitamin E and beta-carotene may have the same negative effect and are now studying that possibility. However, note that many, many studies find that eating foods does NOT find negative effects, but only beneficial ones for health. The October 5, 2015 post also discussed why supplements containing large doses of antioxidants (such as beta-carotene) don't work, and can even cause harm. Bottom line: Eat fruits and vegetables daily for numerous health benefits, but skip the antioxidant supplements. From Science Daily:

Antioxidants cause malignant melanoma to metastasize faster

Antioxidants can double the rate of melanoma metastasis in mice, new research shows. The results reinforce previous findings that antioxidants hasten the progression of lung cancer. People with cancer or an elevated risk of developing the disease should avoid nutritional supplements that contain antioxidants, the researchers say.

Researchers at Sahlgrenska Academy, University of Gothenburg, demonstrated in January 2014 that antioxidants hastened and aggravated the progression of lung cancer. Mice that were given antioxidants developed additional and more aggressive tumors. Experiments on human lung cancer cells confirmed the results. Given well-established evidence that free radicals can cause cancer, the research community had simply assumed that antioxidants, which destroy them, provide protection against the disease. Found in many nutritional supplements, antioxidants are widely marketed as a means of preventing cancer. Because the lung cancer studies called the collective wisdom into question, they attracted a great deal of attention.

The follow-up studies at Sahlgrenska Academy have now found that antioxidants double the rate of metastasis in malignant melanoma, the most perilous type of skin cancer. Science Translational Medicine published the findings on October 7. "As opposed to the lung cancer studies, the primary melanoma tumor was not affected," Professor Bergö says. "But the antioxidant boosted the ability of the tumor cells to metastasize, an even more serious problem because metastasis is the cause of death in the case of melanoma. The primary tumor is not dangerous per se and is usually removed."

Experiments on cell cultures from patients with malignant melanoma confirmed the new results. "We have demonstrated that antioxidants promote the progression of cancer in at least two different ways," Professor Bergö says. The overall conclusion from the various studies is that antioxidants protect healthy cells from free radicals that can turn them into malignancies but may also protect a tumor once it has developed.

Avoid supplements: Taking nutritional supplements containing antioxidants may unintentionally hasten the progression of a small tumor or premalignant lesion, neither of which is possible to detect. "Previous research at Sahlgrenska Academy has indicated that cancer patients are particularly prone to take supplements containing antioxidants," Dr. Bergö says. Our current research combined with information from large clinical trials with antioxidants suggests that people who have been recently diagnosed with cancer should avoid such supplements."

The role of antioxidants is particularly relevant in the case of melanoma, not only because melanoma cells are known to be sensitive to free radicals but because the cells can be exposed to antioxidants by non-dietary means as well. "Skin and suntan lotions sometimes contain beta carotene or vitamin E, both of which could potentially affect malignant melanoma cells in the same way as antioxidants in nutritional supplements," Professor Bergö says. How antioxidants in lotions affect the course of malignant melanoma is currently being explored. "We are testing whether antioxidants applied directly to malignant melanoma cells in mice hasten the progression of cancer in the same way as their dietary counterparts," Professor Bergö says.