Thursday, 13 December 2012

Role of Mediterranean diet, tropical vegetables rich in antioxidants, and sunlight exposure in blindness, cataract and glaucoma among African type 2 diabetics

Int J Ophthalmol. 2012; 5(2): 231–237.
Published online 2012 April 18. doi: 10.3980/j.issn.2222-3959.2012.02.23
PMCID: PMC3359045

Role of Mediterranean diet, tropical vegetables rich in antioxidants, and sunlight exposure in blindness, cataract and glaucoma among African type 2 diabetics



To assess whether regular Mediterranean diet and regular intake of vegetables may reduce the risk of blindness, cataract, and glaucoma in these type 2 diabetics.
Fruits and vegetables contain a vast array of antioxidant components, mainly polyphenols and flavonoids such as anthocyanins[13],[14]. Flavonoids possess several physiological properties: antioxidant, antibacterial, antiviral, anti-inflammatory, anti mutagenic and antitumor activity, as well as the activation or inactivation of certain enzymes[15]. The researchers found that several flavonoids can prevent cell death at low concentrations; other flavonoids have limited or no effect. Quercetin, a flavonoid present at higher concentrations in yellow onions, prevents oxidative stress-induced cell death in retinal pigmentary epithelium (RPE) cells and has no cellular toxicity over a wide range of concentrations.
Dietary antioxidants prevent cataract formation by preventing oxidation of proteins or lipids within the lens in general and during aging also related to oxidative stress and cataract genesis[16],[17].


A cross-sectional design was carried out among known black diabetics admitted at the diabetic clinics of Kinshasa, between October 2008 and March 2009. The Mediterranean-style dietary score (MSDPS) was used to characterize a Mediterranean-style dietary pattern in the study population using the Harvard semi quantitative FFQ adapted for Africa.


Five hundred Type 2 diabetic patients were included in this study (48% of males; 40% aged ≥60 years). There was a significant association between blindness, cataract and aging; between blindness (P<0.05), cataract (P<0.05), glaucoma (P<0.05), and physical inactivity; between blindness (P<0.05), cataract (P<0.0001), glaucoma (P<0.01) and high SES, and a very significant association between blindness (P<0.0001), cataract (P<0.0001), glaucoma (P<0.0001) and exposure to sunlight. There was also a significant association between blindness, glaucoma, and male sex. Regular intake of Mediterranean diet, Brassica Rapa, beans, Abelmoschus, Musa acuminata reduced significantly the risk of blindness, cataract and glaucoma.
The present study showed that regular Mediterranean Diet as well as regular intake of vegetables reduced the risk of blindness, cataract, and glaucoma in these type 2 diabetics. These findings highlighted a very low rate of consumption of vegetables in Africa[27]. Indeed, fruits are only eaten by children in general and diabetics are always discouraged to eat fruits by popular believes.
The present study is, to our knowledge, the first survey focussed on the effects of Mediterranean diet among African type 2 diabetics. The protection against chronic diabetes complications such as cataract, blindness, and glaucoma observed in this study may be explained by the significant reduction in cellular lipids level, and LDL oxidation (oxidative stress) reported by many researchers on Mediterranean diet[9],[28]. This protective action has also been attributed to the higher level in antioxidants in Mediterranean diet[29],[30]. The antioxidant capacity from olive oil, flavonoids, fruits, vegetables, and red wine, the main components of Mediterranean diet[9],[13],[14],[20] are supposed to counter the pro-oxidant of aging, age-related ocular diseases (age-related macular degeneration, diabetic retinopathy, glaucoma, cataract). Indeed, the pathogenetic mechanisms in this diabetic eye complication are the deficiency of antioxidant system, and the chronic hyperglycaemia-related overproduction of free radicals precursors[10]-[12]. The antioxidant effects of the Mediterranean diet have also been reported to prevent atherosclerosis[30], cancer, and other mortalities.
The other significant protective factors against blindness, cataract, and glaucoma were regular intake of Brassica rapa (pointe noire), Phaseolus vulgaris (dry beans), Abelmoschus esculentus (gombo), and Musa acuminata (plantains). Brassica rapa is rich in polysaccharides (BRP1-1, BRPé-1, BRP3-1) which are known with a novel potential anti-hypoxia agent[31]. The antiradical activity of dry beans (Phaseolus vulgaris) is well known[32]. Dry beans considered to have the highest total antioxidant capacity[33]-[37] were recently identified as significant and independent protective factor against Metabolic syndrome (MetS) in African type 2 diabetics[27]. Abelmoschus esculentus Moench (gombo), often eaten in the North of Nigeria is characterized by antioxidant activities[38]. The ferric-reducing antioxidant power FRAP), and oxygen radical absorbance capacity (ORAC), markers of antioxidant capacity are reported in different varieties of Musa acuminata[39].
However, despite the intake of antioxidants from Mediterranean diet and locally grown vegetables, aging, usually inducing oxidative stress, was identified as a significant risk factor of both blindness and cataract among these diabetics.
Physical inactivity accompanied by a low grade inflammatory status was identified as a significant risk factor of blindness, cataract, and glaucoma. And a very significant association was shown between blindness, cataract, glaucoma, and exposure to sunlight by professions. Those results confirm the role of environmental factors in inducing the DNA damage, heart shock, apoptosis and oxidative stress. Indeed, after rural-urban migration, these type 2 diabetics are exposed to epidemiologic, demographic, and nutrition transition, with subsequent consequences such as obesity, inappropriate diet (high salt, fat, sugar, cigarette, and alcohol consumption)[40]-[44].
In general, primary metabolites of plants rich in antioxidants (carbohydrates, proteins, amino acids, lipids) are essential for cells survival and propagation[45]. The secondary metabolites from plants function in defence against predators and abiotic stresses like UV sunlight.
Indeed, we know that free radical-driven, molecular and cellular processes modulated by phenylpropanoid (antioxidants) in human cells cultures in vitro and in the in vivo animal models of tumours, inflammation, and cellular damage are reported in the literature[45]-[46].
These findings have several strengths such as new insights on oxidative stress and ocular diabetic complications that may lead to a casual antioxidant therapy[47] and health promotion. Thus, exercise, smoking cessation, moderate alcohol intake, and protection against sunlight are the background to curve the rising epidemic of type 2 diabetes and ocular complications in these Africans.
Health professionals could educate African diabetics in general and African male diabetics in particular, and prescribe them supplements rich in antioxidants[48] in terms of primary prevention. Globally, and in sub-Saharan Africa, after adjusting for age, the female sex is significantly associated with blindness and major causes of blindness because of low coverage and poverty[49]-[51]. In the present study and in other African studies[1],[2],[8],[18],[41],[52]-[56], the diabetic African men are more susceptible to blindness and glaucoma and their causes such as hypertension, diabetic retinopathy, cataracts, cigarette smoking, low intake of vegetables-fruits, overrepresentation in hospital series because of income, and late admissions.
In the poor African context, both men and women with blindness lack awareness of the potential to cure these conditions, lack access to services, or do not accept services for a variety of cultural and social reasons. These issues prevent both women and men from availing the services they need but affect them differently[49]. The issue of late presentation has not improved over the last three decades[56].
The present study was limited because of its cross-sectional design which is not capable to demonstrate a casual association between the identified determinants and ocular diabetes complications. Indeed, only prospective studies can do that.
Endogenous markers of oxidative stress and antioxidants were not measured in blood samples. However, potential bias and confounding factors were reduced by the large size of participants and the use of the multivariate logistic regression.
In conclusion, both regular intake of the Mediterranean Diet and the consumption of locally grown vegetables including Brassica Rapa, dry beans, Abelmoschus esculentus, and Musa acuminata rich in antioxidants could exert significant protective effects on blindness, cataract, and glaucoma development in type 2 diabetics. Thus, the present data provide evidence to recommend supplement in antioxidants as useful tools against type 2 diabetes mellitus and its ocular complications. Aging, higher SES, male sex, physical inactivity, and exposure to sunlight were identified as risk factors of ocular diabetic complications.