What Makes Fruits and Veggies So Healthy?
If there is one mainstay in nutrition recommendations, it’s that increasing your intake of fruits and vegetables is key to maintaining good health. But what’s remained a mystery is exactly why that is – is it nutrients like vitamin C that are present in fruits and veggies? Something else? This is a surprisingly complex question and it requires smart approaches to answer.
Enter Kobylecki et al., who followed a large (n=97, 203) cohort of Danish adults and used a Mendelian randomization approach to find the association between genetically high concentrations of plasma vitamin C, dietary intake of fruits and vegetables, and the risk of ischemic heart disease and all-cause mortality. What’s a Mendelian randomization approach? Put simply, it’s a way to use genetics to determine whether an environmental factor (like vitamin C intake, and subsequently vitamin C status) is linked to an outcome in a causal manner, making it a powerful tool in epidemiology. They used the genetic marker SLC23A1, gene which is linked to plasma vitamin C concentrations, as a way to determine chronic exposure to high vitamin C status (those with the GG genotype have a 25% higher plasma vitamin C status).
In their analysis, higher intake of fruits and vegetables was significantly associated with a lower risk of both ischemic heart disease and all-cause mortality. Genetically high plasma vitamin C concentrations had a similar effect, but failed to reach statistical significance. However, the investigators were quick to remind the reader that the effect sizes observed were remarkably similar between both fruit & veggie intake and genetically high plasma vitamin C, and “the data cannot exclude that a favorable effect of high intake of fruit and vegetables could in part be driven by high vitamin C concentrations”.
Even with this favorable interpretation, why was there no significant effect of vitamin C? Two thoughts: maybe the 25% difference in plasma vitamin C concentrations wasn’t sufficient to demonstrate the effect. Option 2, the fact that the observed mean vitamin C concentration even in the “high exposure” group (~28 umol/L) was well below what is expected when one meets the RDA for vitamin C (50 umol/L) makes it difficult to observe a statistically significant difference.
Bottom line, I think this is a smart approach to try and tackle this complex question. However, we have to remember the context of status measurements and not just assume “high vs low” is always the proper test. My guess is that if the high vitamin C group was just a bit higher, then that difference might have been statistically significant. Once again, nutrient status matters.
Kobylecki CJ, Afzal S, Davey-Smith G, Nordestgaard BG. Genetically high plasma vitamin C, intake of fruit and vegetables, and risk of ischemic heart disease and all-cause mortality: a Mendelian randomization study. Am J Clin Nutr 2015; 101: 1135-1143.
Lawlor DA, Harbord RM, Sterne JAC, et al. Mendelian randomization: using genes as instruments for making causal inferences in epidemiology. Stat Med 2008; 27(8): 1133-1163.