Response to Letter by Lim et al
We thank Lim and colleagues for their interesting letter regarding potential dietary effects on the microvasculature in healthy children. Their approach is based on the ability to limit both socioeconomic and disease confounders by examining the retinal microcirculation in children, which has been useful to examine the early course of conditions like hypertension.1 For the reasons listed below, however, we believe that the findings by Lim et al are not directly comparable to our reported findings.
We specifically examined the glycemic index of foods consumed and cereal fiber rather than separate intakes of carbohydrates, sugar and total fiber: food component variables which we also found were not significantly related to microvascular caliber. This distinction is important as the concept of glycemic index relates to the physiological glucose and insulin response to carbohydrates by the body, and in this, carbohydrates from different food sources are not equal.2
Cereal fiber is more relevant and related to glycemic index, as it may belong to the wholegrain family, which has shown stronger associations with reduced risk of cardiovascular and stroke mortality than other fiber components or total fiber.3 Lim et al’s study reported neither glycemic index nor cereal fiber. Childhood data linking retinal vessel caliber and these 2 dietary variables would be of considerable interest.
The population characteristics of Lim et al’s study are not stated. For example, the number of children born prematurely or with low birthweight may be an important potential confounding factor in studying retinal vessel diameter.4 Singapore Chinese children may have different dietary patterns than our older white population sample. For example, most Chinese consume rice daily. The study sample used by Lim et al could therefore be clumped at the higher end of the glycemic index distribution with a narrower spectrum of glycemic index. This type of sample may not be ideal to assess the effects of low glycemic index food groups.
As Lim et al alluded, it is likely that dietary effects on the microcirculation accumulate over a long period of time. Children clearly have a much shorter duration of exposure to dietary factors than adults.
Many differences exist in relation to study and statistical analysis methods used in the 2 studies. It is unclear whether Lim et al energy-adjusted their dietary variables. As carbohydrates are a major dietary macronutrient, energy-adjustment is important to separate their specific effects from the effects of other macronutrients and total energy intake. The food frequency questionnaire used in the study by Lim et al has been validated in adults, but not in children5 and its ability to separately assess carbohydrate fractions is therefore unknown. We also used an approach that enabled us to analyze the retinal arteriolar and venular caliber associations independently of each other (ie, eliminating influences from the fellow vessel caliber, or from their shared variance).
To conclude, we believe that the findings reported by Lim et al should not be used to compare with our study findings, as the 2 studies did not investigate the same research question.
Mitchell P, Cheung N, de Haseth K, Taylor B, Rochtchina E, Islam FM, Wang JJ, Saw SM, Wong TY. Blood pressure and retinal arteriolar narrowing in children. Hypertension. 2007; 49: 1156–1162.
Mitchell P, Liew G, Rochtchina E, Wang JJ, Robaei D, Cheung N, Wong TY. Evidence of arteriolar narrowing in low-birth-weight children. Circulation. 2008; 118: 518–524.