Plasma vitamin C level as well as fruit and vegetable intake shows a “striking” inverse relation with the risk of developing diabetes, according to a prospective study.
Compared with men and women in the lowest quintile of plasma vitamin C, those in the highest quintile have a 62% lower chance of developing diabetes, said Dr. Anne-Helen Harding of the Institute of Metabolic Science at Addenbrooke's Hospital, Cambridge (England), and her associates. People who were in the top quintile of fruit and vegetable consumption had a 22% lower chance of developing diabetes.
“Plasma vitamin C level is a good candidate to act as a biomarker for fruit and vegetable consumption because in Western diets, fruit and vegetable consumption is the main source of vitamin C,” the investigators wrote (Arch. Intern. Med. 2008;168:1493–9).
They concluded that, “the findings suggest that eating even a small quantity of fruit and vegetables may be beneficial and that the protection against diabetes increases progressively” with the quantity consumed.
To the investigators' knowledge, “no published studies have examined the association of plasma vitamin C level and incident diabetes.” While previous studies found lower plasma vitamin C levels in people with diabetes compared to people without diabetes, those studies “cannot establish the temporal sequence of events” whether low plasma vitamin C precedes diabetes or is a consequence of the condition.
The aim of the current study was “to investigate the prospective association between plasma vitamin C level and the risk of developing type 2 diabetes in a population of middle-aged men and women,” the researchers stated. “An additional aim was to examine the relationship between fruit and vegetable intake and incident diabetes.”
The investigators in the European Prospective Investigation of Cancer-Norfolk Prospective Study assessed the relationship between plasma vitamin C level and diabetes risk in middle-aged men and women using data from a cohort study of 21,831 subjects aged 40–75 years who attended 35 medical practices.
A total of 735 incident cases of diabetes developed during 12 years of follow-up (3.2% incidence). “The cases of diabetes in this study were diagnosed in the community and are likely to be typical of newly diagnosed cases in the general population,” the researchers noted.
The mean plasma vitamin C concentration in people who developed diabetes (0.76 mg/dL) was lower than that in subjects without diabetes (0.95 mg/dL). Moreover, vitamin C level inversely correlated with diabetes risk in a logistic regression analysis.
In a further analysis of the data that adjusted for subject age, sex, family history of diabetes, alcohol intake, physical activity level, smoking status, education level, and social class, this strong inverse association “was materially unchanged” and remained statistically significant, Dr. Harding and her associates said.
In the subset of subjects who had hemoglobin A1c levels of less than 7%, a similar magnitude of association was seen. This analysis effectively excluded cases of diabetes that may have been present but undiagnosed at baseline, the investigators said.
“Our findings re-endorse the public health message of the beneficial effect of increasing total fruit and vegetable intake,” they said.
“Whether the association is causal or a marker for other factors associated with fruit and vegetable intake, signifying a clustering of healthier lifestyles and residual confounding, needs to be addressed in specifically designed studies,” Dr. Harding and her associates added.
The investigators reported no conflicts of interest.