The human eye is an interesting design. The design of the eye is related to the function it must play in allowing vision in the presence of damaging ultraviolet light. In this regard, comparison to the octopus eye is interesting, as some have claimed that the design of the octopus eye is superior to that of the human eye. However, this argument is disingenuous because it fails to address the quite different and obvious habitats of octopuses and humans. Octopuses live in the water and water is a poor conductor of ultraviolet light. As a result the octopus eye is not exposed to damaging levels of ultraviolet light and so the retina of the octopus eye has the blood vessels behind it, improving the vision of the animal. In contrast, humans live in the air and air is a good conductor of ultraviolet light. As a result the humans have to contend with direct exposure to ultraviolet light which could damage the retina. To address this humans have their blood vessels in front of their retina, and this is to the detriment of vision.
However, the human eye is a good design because it works well in the environment in which humans live. Taking an octopus out of water and exposing it to ultraviolet light would result in serious damage to its eyes. Humans and other animals also concentrate antioxidants in the lense of their eyes and this adds further protective effects against the damaging effects of ultraviolet light. Such accumulation of antioxidants in the lense of the eye is an important protective effect in humans against the development of oxidative stress, that may for example increase the risk of developing cataracts and other diseases of oxidative stress. It has been reported for example, that higher plasma levels of antioxidants such as ascorbic acid (vitamin C), tocopherols (vitamin E) and carotenoids is associated with a lower risk of cataracts. Consumption of spinach has also been shown to be associated with protection from cataracts and this may relate to the antioxidant carotenoids it contains.
Spinach is a good source of a number of carotenoids and oxidised carotenoids, the latter being referred to as xanthophylls. In particular, spinach contains lutein and zeaxanthin, that may accumulate in the lens of the eye and aid in protecting it from oxidation caused by ultraviolet light. For example, one study investigated the antioxidant content of human eye lenses1. The results of the study showed that the lenses contained 11 to 25 ng of xanthophylls in the form of lutein and zeaxanthin per gram of tissue. In addition the lenses contained other antioxidants including 31 to 50 ng retinol (vitamin A), 21 to 25 ng retinyl palmitate, 1575 to 2500 ng α-tocopherol and 257 to 501 ng or γ-tocopherol per gram of tissue. The ratio of lutein to zeaxanthin in the lenses was between 1.6 and 2.2 and considerable variation in antioxidant in the lenses of eye were detected. Interestingly, both β-carotene and lycopene, both major carotenoids in human plasma, were not detected in the lenses.
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