A) Relationship between glycemic index, dietary protein and myopia.

The Cordain theory [3] proposes that myopia occurs as a result of environmental conditions associated with modern civilisation, which came about after the Palaeolithic era. The prevalence of myopia is very low in hunter-gatherer populations compared to more "advanced" populations (less than 2% vs. greater than 20%) [1]. In Eskimos, increasing acculturation during the 1960's led to a marked increase in myopia prevalence [2]. The typical Palaeolithic diet is characterised as being high in protein, low in fat and low in carbohydrates, particularly low glycaemic index carbohydrates: which are slowly absorbed in the blood stream and produce a gradual and minimal rise in plasma glucose and insulin levels when compared to higher glycaemic foods . As well as being implicated in metabolic syndrome, the modern - lower protein/higher glycaemic index - diet has also been suggested as a potential risk factor for myopia [3].

Dietary protein has also been considered as a possible risk factor for myopia, because protein generally results in lower rises in both plasma glucose and insulin when compared to carbohydrate.

In a number of studies, Gardiner [4,5], indicated that myopic individuals consumed significantly lower amounts of animal protein than non-myopes. Further, in an intervention study (albeit a non-rigorous one) he was able to show that increasing the level of animal protein in the diets of myopic children slowed progression of their myopia when compared to a control group receiving no dietary modification [6].

B) Exploratory analysis: Dietary copper

Normal copper metabolism is essential to ocular tissue health and is associated with myopic refractive error development in some studies [7,8]. A recent study found that mutations in the SCO2 gene were associated with autosomal-dominant high-grade myopia. SCO2 encodes for a copper homeostasis protein influential in mitochondrial cytochrome c oxidase activity. Mutations of SCO2 were associated with a destabilisation of protein structures which in turn may result in modulation of oxidative toxicity-particularly in the retina, leading to retinal neuronal thinning. In addition, mutations can affect copper metabolism, which may result in an imbalance of copper enzymatic support activity and oxidative levels within eye tissues [9]. Proper copper metabolism is essential for cell differentiation, development, and maintenance. One study demonstrated the protective effect of copper supplements in individuals with myopia, restoration by capsule injection of copper compounds resulted in increased scleral copper concentration and improved scleral tissue elasticity with cessation of myopic refractive error development [10].

C) Exploratory analysis: Breastfeeding

Observational studies have suggested that breast-feeding benefits the visual development of infants, which has been attributed to the presence of DHA in breast milk but not most formula milks.

In a cross-sectional study of 797 children (ages 10-12 years) in Singapore, the prevalence of myopia was significantly lower in children who had been breastfed than in those who had not (62% vs. 69.1%; p=0.04). The duration of breastfeeding (three months or less versus more than three months) was not associated with myopia risk. However, since the incidence of myopia in children in Singapore is among the highest in the world, it is not clear whether these findings can be generalised to other populations [11].

Analyses of data from three large British birth cohorts showed that there were no differences in visual outcome in childhood or adolescence among those initially breastfed for greater than 1 mo compared with those formula fed. Rates of breastfeeding fell across successive cohorts considerably, but there was no evidence of a subsequent increase in adverse visual outcome in childhood, although, there was a small increase in adverse visual outcome in adolescence from 1946 to 1970. However, the latter did not take into consideration important confounding factors, especially the age at which vision was assessed. (12)