La cataract is one of the leading causes of blindness in the world. In fact, the most recent World Health Organisation (WHO) estimates report that 47.8% of blindness globally is due to cataracts. This is a form of 'avoidable blindness', as vision loss can be counteracted by surgical removal of the opacified lens. Cataract surgery is, in fact, the main therapeutic approach. To date, it is a safe and effective intervention, however, in rare cases complications such as iris prolapse, increased intraocular pressure, infections, cystoid macular oedema and posterior capsular opacification can occur. For these reasons and as a general approach, prevention is the first weapon available against cataracts. One of the factors that can play an important role in preventing cataracts, as is the case for many other eye diseases, is certainly diet. In particular, certain naturally occurring substances, called nutraceuticals, can help prevent the onset of cataracts.Â
Nutraceuticals: what are they?
The term 'nutraceutical' was coined in 1979 and indicates a 'food, or part of the diet, with medical or health benefits, including disease prevention and treatment'. Nutraceuticals can be isolated or processed nutrients and dietary supplements. One of the main characteristics of nutraceuticals is that, being substances taken through the diet, they are virtually risk-free for health.Â
Nutraceuticals and cataract prevention
Over time, the studies on cataracts have revealed the presence of mechanisms linked to glucose toxicity, in particular oxidative stress, non-enzymatic glycation processes and the polyol pathway, which are significantly involved in the development of lens opacity.
Consequently, certain phytochemicals, which can be taken through the diet, may help in the prevention of cataracts. In particular, these are:
1. Antioxidants or scavengers of ROS (Reactive Oxygen Species);
2. Aldose reductase inhibitors;
3. Anti-glycating agents;
4. Inhibitors of apoptosis of lens epithelial cells.
- Antioxidants
Classes of antioxidants that may be useful for cataract prevention include flavonoids, carotenoids, ascorbic acid, tocopherol, caffeine and pyruvate. Flavonoids can be obtained from eating fruits such as apples, grapes, bananas, cherries and berries and from green leafy vegetables. Vitamin C and vitamin E are among the most important antioxidants. Corn oil and wheat germ oil are the main sources of vitamin E, while vitamin C, i.e. ascorbic acid, is mainly found in citrus fruits. Finally, carotenoids are a family of about 700 compounds of which 20 have been detected in human plasma and tissue. Carotenoids are contained in green leafy fruits and vegetables. In particular, lutein and zeaxanthin are the two carotenoids found in the lens of the human eye. It has been reported that these two carotenoids may also be useful in the prevention of cataracts because they have the potential ability to filter short-wave blue light radiation, reducing photo-oxidative damage to proteins, lipids and DNA in the lens
- Aldose reductase inhibitors
The accumulation of the polyol sorbitol in the lens is the basis of diabetic cataract formation. The enzyme aldose reductase, within the natural lens, converts glucose to sorbitol and is responsible for the accumulation of sorbitol in the lens. Therefore, aldose reductase inhibitors may be used as potential agents to prevent the onset or progression of diabetic cataract.
- Anti-glycating agents
The non-enzymatic glycation process is one of the well-known mechanisms involved in the pathogenesis of diabetic cataracts. With age, the products of advanced glycation tend to accumulate and may contribute to opacification of the crystalline lens. Therefore, the use of anti-glycation agents is a clinical option for cataract prevention. Polyphenols, in addition to having antioxidant properties, also have strong antiglycating activity and are common constituents of fruits, vegetables, cereals, seeds, nuts, chocolate and beverages such as coffee and tea. Similarly, phenolic acids, found in many foods such as vegetables and fruit, coffee, rice, wheat and oats, are useful. Another important anti-glycating agent is citrate, a natural food constituent found in citrus fruits which, when administered orally, has the potential ability to help delay cataract development. Other anti-glycating agents are by-products of rice and barley fermentation, which can inhibit glycation. Finally, also pyridoxamine and acid α-lipoic have shown an inhibitory effect on the formation of glycation products.Â
- Inhibitors of lens epithelial cell apoptosis
Apoptosis is a physiological process of cell death that provides an important molecular basis for both the initiation and progression of cataracts. Oxidative stress has been recognised as an important mediator of apoptosis in lens epithelial cells and plays a key role in the pathogenesis of cataracts. Epigallocatechin gallate (EGCG), the most abundant component of green tea, and other flavonoids, have potent antioxidant activity and prevent apoptosis. Grape seed extract and resveratrol are also able to reduce ROS generation and inhibit apoptosis. Finally, coenzyme Q10 (ubiquinone) is also a benzoquinone compound that acts as a free radical scavenger and prevents light-induced apoptosis in the human lens epithelium.