Diet and your eye sight

Recently one of my clients was diagnosed with age-related macular degeneration and was advised by a health professional to take supplements of the carotenoids lutein and zeaxanthin to prevent progression of this condition. Given that these supplements are expensive, she wanted to know if her investment would pay off.

As discussed below, the bottom line is that, to date, there is little scientific evidence to support the use of antioxidant supplements either to prevent or delay the progression of age-related macular degeneration. Indeed, there is evidence that such supplements may be harmful.

In contrast, there is scientific evidence that consumption of a plant-based diet with a variety of whole grains, vegetables and fruit has many benefits with regard to preventing and treating diseases like macular degeneration, and does no harm.  Further research is required in this area.

The best way to protect the health of your eyes is to eat plenty of different coloured vegetables and fruit; for example, broccoli, cabbage, kale, collard greens, green peas, spinach, courgettes, red and yellow peppers, winter squash, kiwi fruit, blueberries, oranges and mango.

What is age-related macular degeneration?

Age-related macular degeneration is an eye condition affecting older people, and involves the loss of the person’s central field of vision. It occurs when the macular (or central) retina in the eye develops degenerative lesions. The retina is the light-detecting membrane at the back of the eye. The macula contains a particularly high density of light receptors, especially cones (the receptors that detect colour) and so the macula is particularly important for seeing fine detail and objects directly in front of us. It plays a vital role in helping people to read, write, drive and perform other detailed tasks. It also enables us to recognise faces and see colour.

According to the World Health Organisation (1), globally, age-related macular degeneration ranks third as a cause of blindness after cataract and glaucoma; it is the primary cause in industrialized countries. Several forms of age-related macular degeneration exist.

Carotenoid pigments in the eye – lutein, zeaxanthin and meso-zeaxanthin

Pigments known as carotenoids are found within the eye. There are over 600 known carotenoids in nature but in the human eye, just three of these pigments – lutein, zeaxanthin and meso-zeaxanthin – form a concentrated ‘yellow spot’ in the macula, known as the macular pigment. This pigment has two main functions: it acts as an antioxidant and it filters light.

As short wavelength (blue) light passes through the retina to the photoreceptors and pigmented epithelial cells, reactive oxygen species are generated. The conversion of the light energy into a nerve impulse by the photoreceptors generates free radicals – unstable molecules which damage a variety 
of tissues – resulting in many of the diseases and conditions associated with ageing. Antioxidants in the eye are able to quench these free radicals, thereby protecting the eye from oxidative damage. Lutein, zeaxanthin and particularly meso- zeaxanthin are very effective antioxidants.

The pigments lutein and zeaxanthin can be found naturally in a number of food sources (2). For example, lutein can be found in yellow peppers, mango, bilberries and green leafy vegetables such as kale, spinach, chard and broccoli. Zeaxanthin can be found in winter squash, orange sweet peppers, broccoli, corn, lettuce, spinach, tangerines, oranges and eggs.

Blue light, because of its relatively high photon energy, more readily damages the retina than yellow or red light, which is less energetic. The macular pigment acts as a filter, particularly to blue light, and therefore protects against this damaging effect.

Given that the development of age-related macular degeneration is likely to involve a complex interaction of cellular and vascular factors, which may be promoted by light damage, oxidative stress, and inflammation, it is biologically plausible that a number of dietary components may be protective.

Effect of diet on age-related macular degeneration

The results of several epidemiological studies and clinical trials in the peer-reviewed scientific literature suggest that diets high in antioxidant nutrients (vitamins C and E, carotenoids such as lutein and zeaxanthin, fruit and vegetables that contain these nutrients, and non-nutritive antioxidants) or zinc are associated with a decreased occurrence of early or late age-related macular degeneration (3) (4). A high dietary intake of fat was associated with a higher prevalence or incidence of early or late age-related macular degeneration in numerous studies (5), whereas higher intakes of fish or omega 3 fatty acids were associated with lower rates of age-related macular degeneration (5).

Despite nutritional advice being available, awareness of the link between diet and eye health is poor. A survey conducted by the Eyecare Trust (6) found that Britons are oblivious to the fact that unhealthy lifestyles and obesity can substantially increase the risk of macular degeneration and ultimately blindness.

Efficacy of nutrient supplements for age-related macular degeneration

Some clinicians argue that it is difficult to obtain adequate amounts of the protective nutrients required for the eyes from a ‘normal’ diet. They thus prescribe high doses of nutrient supplements. But are these nutritional supplements effective?

A recently published Cochrane Review (7) identified four large, high-quality randomised controlled trials which included 62,520 people. The trials were conducted in Australia, Finland and the USA and investigated the effects of vitamin E and beta-carotene supplementation. These trials provide evidence that taking vitamin E and beta-carotene supplements is unlikely to prevent the onset of age-related macular degeneration. There was no evidence for other antioxidant supplements and commonly marketed combinations.

Another Cochrane Review (8) investigated whether progression of age-related macular degeneration may be slowed down in people who take antioxidant supplements (carotenoids, vitamins C and E) or minerals (selenium and zinc). The authors identified 13 randomised controlled trials which included 6150 participants; five trials based in the USA, two in the UK, two trials in Austria, and one trial in each of a further four countries (Australia, China, Italy and Switzerland). The review of trials found that supplementation with antioxidants and zinc may be of modest benefit in people with age-related macular degeneration. This was seen mainly in one large trial that followed up participants for an average of six years. The other smaller trials with shorter follow-up do not provide evidence of any benefit. Large well-conducted trials in a range of populations and with different nutritional status are required.

Safety of antioxidant nutrient supplements

Although generally regarded as safe, there is evidence that antioxidant supplements may have harmful effects. A Cochrane Review in 2008 (9) demonstrated that antioxidant supplements seem to increase mortality in sufferers of cancer and cardiovascular disease. This review was updated in 2012 (10) and found no evidence to support antioxidant supplements for primary or secondary prevention. Beta-carotene and vitamin E seem to increase mortality, and so may higher doses of vitamin A. The authors concluded that current evidence does not support the use of antioxidant supplements in the general population or in patients with various diseases in a stable phase, including gastrointestinal, cardiovascular, neurological, ocular, dermatological, rheumatoid, renal, endocrinological, or unspecified diseases.

Conclusion

The bottom line from all of this work is that, to date, there is little evidence to support the use of antioxidant supplements either to prevent or delay the progression of macular degeneration. Indeed, there is evidence that such supplements may be harmful.

In contrast, consumption of a plant-based diet with a variety of whole grains, vegetables and fruit has many benefits with regard to preventing and treating disease, and none of the disadvantages associated with nutritional supplements.  Further research and analysis of the literature is required in this area.

Further information and help

If you are suffering from a specific health problem or would just like to improve your general health and well-being, you will benefit from a personal nutrition consultation. Simple changes to diet and lifestyle can lead to significant improvements in the way you feel.  For a delicious recipe full of the carotenoids lutein and zeaxanthin, please click here.

References

(1) http://www.who.int/blindness/causes/priority/en/index8.html

(2) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1722697/

(3) http://www.ncbi.nlm.nih.gov/pubmed/16380590?dopt=Abstract

(4) http://informahealthcare.com/doi/abs/10.3109/09286580903450353

(5) http://www.ncbi.nlm.nih.gov/pubmed/14662593?dopt=Abstract

(6) http://www.eyecaretrust.org.uk/view.php?item_id=563

(7) http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD000253.pub3/abstract

(8) http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD000254.pub3/abstract

(9) http://www.ncbi.nlm.nih.gov/pubmed/18425980

(10) http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD007176.pub2/abstract

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Soybean product may be of benefit in preventing Alzheimer’s disease

Scientists in Taiwan have published a paper in the February 2009 edition of the Journal of Agricultural and Food Chemistry indicating that an enzyme, nattokinase, which is found in a fermented soybean product called natto, has powerful ability in lab experiments to prevent formation of the clumps of tangled protein (amyloid fibrils) observed in Alzheimer’s disease.

More than 20 unrelated proteins can form amyloid fibrils in the body, which are related to various diseases, such as Alzheimer’s disease, prion disease, and systematic amyloidosis.  Enhancing amyloid clearance is one of the targets of the therapy of these amyloid-related diseases. Although there is debate on whether the toxicity is due to amyloids or their precursors, research on the degradation of amyloids may shed light on the prevention or alleviation of these diseases.

In this Taiwanese study, nattokinase degraded several kinds of amyloid fibrils suggesting its possible use in the treatment of amyloid-related diseases.

natto

Natto is a traditional Japanese food made from fermented soybeans and is a popular breakfast dish.  For some, natto is an acquired taste due to its powerful smell, strong flavour and sticky consistency.

Natto is made from soybeans, typically a special type called natto soybeans.  Smaller beans are preferred as the fermentation can progress to the centre of the bean more easily.  The beans are washed and soaked in water for 12 to 20 hours, which causes the beans to swell.  Next, the soybeans are steamed for 6 hours, although a pressure cooker can be used to reduce the time.  The beans are then mixed with the bacterium Bacillus subtilis natto, known as natto-kin in Japanese.  From this point on, care has to be taken to keep the ingredients away from impurities and other bacteria.  The mixture is fermented at 40°C for up to 24 hours.  Afterwards the natto is cooled, then aged in a refrigerator for up to one week to add stringiness.  During the ageing process at a temperature of about 0°C, the bacteria develop spores, and enzymes break down the soybean protein into its constituent amino acids.  For this reason, the high protein content of the soybean is in a very digestible form. 

In addition to its high protein content, natto is rich in fibre, vitamin C, vitamin K, calcium, iron, potassium, phosphorus and magnesium.  In common with other soybean products, natto contains significant quantities of the isoflavone phytonutrients including genistein and daidzein, which are believed to have cancer-protective properties.

Natto is believed to have numerous health benefits and there is some medical research to support this.  The enzyme nattokinase is a serine protease which may reduce blood clotting by direct fibrinolysis of clots and inhibition of the plasma protein plasminogen activator inhibitor 1[i].  Clinical trials are needed to confirm laboratory studies.  An extract from natto containing nattokinase is available as a dietary supplement.

Vitamin K, which is present in significant amounts in natto, is involved in the formation of calcium-binding groups in proteins, assisting the formation of bones and preventing osteoporosis.  Vitamin K1 is found naturally in seaweed, liver and some vegetables, while vitamin K2 is found in fermented food products such as cheese and miso.  Natto has very large amounts of vitamin K2, approximately 870 mg per 100 g natto.

Natto is reported to contain substantial levels of a natural product called pyrroloquinoline quinone, which has been shown to stimulate DNA synthesis in cultured human fibroblasts, modulate immune response, and reduce liver injury, cataract formation and lipid peroxidation[ii].   

 A study reported in 1996 suggested that natto may have benefits in reducing cholesterol levels in people whose cholesterol and triglyceride levels are high[iii].

In January 1997, a Japanese television programme called Revealed! Encyclopaedia of Living recommended two portions of natto per day as a means of losing weight in only two weeks.  With the Japanese struggling with overweight and obesity this hit a nerve, and by lunchtime the next day national stocks of natto had sold out.    Whilst it is the case that natto has a relatively low number of calories per g of protein and a high nutrient density, it will only contribute to weight loss if consumed as part of a healthy diet, high in whole grains, vegetables, fruits and unsaturated fats, and low in saturated fats, salt and sugar, combined with plenty of exercise.

The most popular way to eat natto is to put it in a small bowl, add a little soy sauce and some finely-chopped spring onion and/or some mustard, mix the ingredients together and serve on some steamed rice.  Natto can also be added to miso soup to create a rich and nourishing dish, which smells a little like capuccino.

Natto can be purchased in the UK from specialist suppliers of Japanese food, such as the Japan Centre  and Japanese Kitchen.

Learn how to cook with natto and other soybean products such tempeh and tofu at popular Cooking for Health classes  held throughout the year in Somerset, UK .

Jane Philpott

 

References

[i]  Fujita M et al (December 1993). “Purification and characterization of a strong fibrinolytic enzyme (nattokinase) in the vegetable cheese natto, a popular soybean fermented food in Japan”. Biochemical and Biophysical Research Communications 197 (3): 1340–1347.

[ii] Kumazawa, T.  et al. Levels of pyrroloquinoline quinone in various foods.  Biochem J. (1995) 307: 331-333

[iii] National Cardiovascular Center, Osake, Japan (April 2006). “Examining the effects of natto consumption on lifestyle-related disease prevention