Leafy Greens for Vision

Have you ever wondered if there is any validity to the old wives' tale claiming that eating carrots enables you to see in the dark?

Well, it certainly isn't the case that eating a bag of carrots is going to give you night vision, however, the underlying message that eating certain vegetables is of benefit to your vision has been heavily researched and it has been concluded that the nutrients of certain vegetables are good for our eyes. What's more, carrots are not the only vegetable to offer this health advantage. The leafy greens, a group of vegetables unique for their green colour and leafy texture, have also been shown to contain specific nutrients that play an important role in protecting the health of your eyes.

Read on to learn more about leafy green vegetables, the nutrients they contain, and why they're so important for our eyes.

Introduction

Protecting the health of our eyes is, for most people, of the utmost importance. Visual loss comes with consequences on both a person's ability to function as well an increased risk of developing certain mental health conditions. It also has wide-reaching effects on all those close to them as well as on society as a whole. 

Some eye conditions we have no control over, either through genetics or unexpected events we may one day find ourselves impacted by them. There are however other ocular disorders that we can protect against, While that doesn’t necessarily mean that we can say for certain they will never affect us, it does mean that we can lower our risk of developing them. This risk reduction often takes the form of lifestyle changes, and the key among them is making sure we are consuming enough of the nutrients that are imperative to maintaining good eye health.

What are leafy green vegetables?

You may have heard the term ‘leafy greens’ before when it comes to healthy eating but not be familiar with exactly which vegetables the ‘leafy greens’ encompass.

Here is a list of some of the most commonly consumed leafy green vegetables:

• Spinach contains vitamins K, A, and folate. Often recommended for pregnant women and those with anaemia.
• Kale is considered the most nutrient-dense vegetable on the planet.
• Broccoli is one of the most well-known and commonly consumed leafy greens.
• Cabbage, although it can come in green, white, or purple, is a vegetable that is widely available around the world and can be included in many different dishes.
• Watercress is known in many different cultures for its healing properties; this leafy green is not just a pretty garnish.
• Romaine lettuce is a common lettuce found in salads and is a good source of vitamins A and K. It is also extremely calorie-light.
• Swiss chard belongs to the same family as spinach and is often found in mediterranean dishes which is universally known as one of the healthiest diets on the planet.
• Arugula is a leaf also goes by the name ‘rocket’ and is a popular addition in many salads with its unique ‘peppery’ taste.
• Endive is a slightly bitter less well-known leafy green that can be eaten both raw and cooked.

Structure of the eye

Before we discuss the nutrients found in leafy greens that benefit the eye it is important to understand the basic structure of the eye.

This image is by the National Eye Institute and is taken from Wikimedia Commons. 

Front of the eye

The front of the eye consists of the parts of the eye that you can see. The white covering of the surface of the eye is called the conjunctiva, the coloured part of the eye is called the ‘Iris’ and the dark hole in the middle is called the ‘pupil’, muscles in the iris contract and relax to dilate or constrict the pupil and consequently control the amount of light that passes into the eye. Overlying the iris and pupil is a see-through, dome-shaped membrane called the ‘cornea’, and directly under the pupil is a structure called the lens. The lens is responsible for focusing light on the back of the eye and can change shape allowing us to focus on objects close by. This is the part of the eye where cataracts can form.

Back of the eye

The back of the eye is the part of the eye which requires a special instrument, called an ophthalmoscope to view. This will be done by someone who specialises in looking at the structures of the eye, usually an optician or an ophthalmologist depending on your need. The most important structures in the back of the eye are the retina, macula, and optic nerve. The space between the lens and the back of the eye is called the vitreous cavity and it is filled with a gel-like substance called ‘vitreous humour’ whose purpose is to maintain the rounded shape of the eye. The retina is the tissue that lines the back of the eye and contains photoreceptors which are special cells called ‘rod and cone’ cells that detect the light that passes through the eye and turn it into electrical energy which is transmitted through the optic nerve to the visual cortex in the brain enabling you to see. The macula is a small part of the retina that has a very high concentration of cone cells and is responsible for our central, detailed vision which allows us to read, drive and recognise faces. The rest of the retina is responsible for our blurrier peripheral vision. 

Importance of leafy greens

The leafy greens have, for many years, been universally known for their goodness. In some cultures, they are used as both a source of nutrition as well as natural medicines, with recipes and formulations passed down through generations. 

This group of food is nutrient-dense containing several very important vitamins, minerals, and phytochemicals as well as being a good source of dietary fibers.¹ Whilst studies have shown that leafy green vegetables have a beneficial effect on many aspects of health, including reducing your risk of heart disease, type 2 diabetes, and all-cause mortality¹ this article will be focusing on the health benefits they provide specifically related to your eye-sight.

Nutrients found in leafy greens crucial for eye health

There are several nutrients found in leafy green vegetables that have been scientifically proven to have a positive effect on your vision. 

Lutein and zeaxanthin

Lutein and zeaxanthin are compounds belonging to the carotenoid classification. Carotenoids are phytochemicals that are found in all photosynthetic organisms (organisms that use light as an energy source).² They have strong photo-protective and antioxidant properties and are found in the macula within the eye.² There are two classes of carotenoids, lutein, and zeaxanthin belong to the class called xanthophyll (meaning they contain oxygen).²

Humans cannot produce their own carotenoids so we must get them from dietary sources and one of the best sources of carotenoids is leafy green vegetables.³ These phytochemicals are made up of hydrogen and carbon molecules and their unique structure means that they are very efficient at absorbing free radicals which prevents harmful inflammation occurring within tissues as a result of oxidation.³ They are also very good at absorbing potentially damaging excess light and are found at all levels of the food chain albeit in varying amounts.³ 

Recent evidence has indicated that carotenoid pigments are responsible for the bright colours of some birds, fish, shrimp, and sea sponges as well as the diverse colours of many fruits and vegetables, including the dark green colour that many leafy greens possess.^3,5 With regard to the human body, carotenoids are thought to be responsible for the yellow colour of the macula found in the back of the eye.³ The macula, as discussed above, plays an extremely important role in our ability to see and damage to it can be catastrophic to our vision.

It is of no surprise then that research has been directed towards understanding the role that lutein, zeaxanthin and its derivative meso-zeaxanthin play in the macula and whether dietary levels of ingestion have an effect on vision.³ These two carotenoids are also found in the lens of the eye.⁴

Beta-carotene and vitamin A 

Beta-carotene is another carotenoid that is found in high quantities within leafy green vegetables and has been shown to play an important role in the health of the eye. It is part of the class of carotenoid called ‘carotenes’ and unlike the xanthophylls (lutein and zeaxanthin) these do not contain any oxygen molecules. Beta-carotene, which is the most commonly found carotene in the human diet, is a precursor to vitamin A meaning, in the body we convert beta-carotene into vitamin A, which is also called retinol.⁴

Vitamin A, also referred to as retinol, is an essential micronutrient. This means that our bodies cannot manufacture it themselves and therefore it has to be ingested as a part of our diets.⁵ Vitamin A can be absorbed from either animal or plant sources. The vitamin A found in animal sources is ‘active vitamin A’ whereas vitamin A from plant sources comes in the form of carotenoids which are converted into retinol during digestion.⁵ Vitamin A helps form the light-sensing pigment in the retina which helps us to see in dim light.⁶

Vitamin C (ascorbic acid)

Vitamin C, also called ascorbic acid, is an antioxidant found in a variety of different foods including in leafy greens and in particular Kale. Antioxidants protect our body against free radicals which can damage cells leading to the irreversible damage of tissues. There is some evidence to suggest that it is beneficial to human eye health and prevents the development of eye diseases in later life.⁷

Leafy greens and eye disease

Macular degeneration

Macular degeneration, an age-related eye disease, is the leading cause of blindness in older adults in the developed world.⁸ It is estimated that the global prevalence of age-related macular degeneration will reach 300 million by 2040.⁹ This chronic degenerative disease occurs as a result of damage to our macular which is the region of the eye responsible for our central, detailed vision that allows us to perform many everyday tasks such as reading and driving.⁹

The functional disability that arises from macular degeneration also increases the risk of mental health conditions such as depression and anxiety.⁹ Age-related macular degeneration places an enormous burden on the affected individual, their family, and the health system. There are two forms of macular degeneration, the ‘wet’ type, which can be managed with anti-vascular endothelial growth factor injections, and the ‘dry’ type which is more severe, irreversible, and currently untreatable.⁹

It has been hypothesised that the carotenoids lutein and zeaxanthin play a protective role against age-related macular degeneration. They are present in the centre of the macula (also called the fovea) in far higher quantities than anywhere else in the body so it is surmised that they play an important role in visual performance.⁸ This is thought to occur through two different mechanisms, firstly, they are thought to have blue light filtering properties.

Blue light can be harmful especially when overly bright and xanthophylls filtrate and absorb excess blue light, protecting the retina from light-related damage as well as reducing light scatter and thereby improving visual acuity.¹⁰  Secondly, lutein and zeaxanthin are potent antioxidants. The retina of the eye has the highest rate of metabolism of anywhere in the body, this can cause oxidative stress leading to chronic inflammation and irreversible damage to cells and tissue.¹⁰

Xanthophylls are very good at neutralising free radicals and reactive oxygen species produced from oxidation and dispelling the excess energy they produce without harming nearby cells.

Vitamin C has also been found to protect against the development of age-related macular degeneration. It is a strong antioxidant that is also thought to protect against damage caused by oxidation in the retina thus protecting against macular degeneration.⁷

Cataracts

Cataracts, a disease of the lens, are the world's leading cause of blindness.¹¹ They occur when the lens becomes clouded and treatment involves surgical removal of the lens and replacement with a synthetic lens. Age and diabetes are significant risk factors for the development of cataracts and with both of these things increasing globally there is a need for alternative treatments or effective preventative measures.¹² 

Vitamin C is present within the lens at a concentration 50 times higher than that found in the plasma.¹² It is thought that it acts as a physiological ‘sunscreen’, protecting the lens from UV light-induced oxidative damage.¹² There is evidence that the incidence of cataracts is higher in people who have a low plasma concentration and since vitamin C levels within the lens can be increased through dietary supplementation,¹² there is reason to believe that eating leafy greens and other foods with high levels of vitamin C can protect against the development of cataracts as we age.¹²                                                                  

The xanthophylls lutein and zeaxanthin as well as vitamin A and its precursors are also potent antioxidants and evidence has found a correlation between increased lutein consumption and the reduction of cataract formation although the exact mechanism by which this happens currently remains unknown.^{11, 13}

Incorporating leafy greens into your diet

Now that we have looked at what exactly it is about leafy greens that make them so good for you and your vision, here are a few tips and tricks to get them into your diet.

• There is such a huge variety of leafy greens that chances are you will find a couple that you like, don’t be put off if you don’t like spinach or kale, a little bit of exploration is bound to help you find some that you enjoy.
• Add a handful of spinach or a couple of stems of broccoli to a morning smoothie, this can give you all the nutrients you need without sacrificing flavour. 
• Hide them in pasta doughs, on pizza, or in sauces it doesn't matter where. There are many ways to benefit from the nutrients that leafy greens provide without having to eat them solo and raw.

Other vision-boosting foods

It is common knowledge that a diverse and balanced diet is the healthiest model of consumption and many different varieties of foods also contain vision-boosting nutrients. One of the most commonly known and eaten is of course the carrot, but here is a list of other foods that contain nutrients proven to be beneficial for your eyes:

• Avocados
• Eggs
• Sweet potatoes
• Red peppers 
• Apricots
• Peas
• Oranges 
• Strawberries

Summary

It has been well known for generations that certain foods are good for the health of our eyes, however, it has not been until more recent years that we have been able to pinpoint what exactly it is about certain foods that benefit our eyes. Carotenoids, vitamin A, and vitamin C have all been shown to be present in higher-than-average concentrations within the structures of the eye leading researchers and doctors alike to conclude that they play an important role in protecting our eyes. Their potent antioxidant function as well as the xanthophyll's unique photo-protective properties suggests that they play a role in protecting our eyes against damage caused by excessive amounts of light as well as against the effects of oxidative stress. The wide variety of leafy green vegetables and their availability all over the world and through different seasons makes them an excellent nutritional addition to a healthy balanced diet.