In this large prospective cohort of middle-aged American women, overall fruit and vegetable intake was not associated with the development of type 2 diabetes. Intake of fruit juices was positively associated with incidence of type 2 diabetes, whereas intake of whole fruits and green leafy vegetables was inversely associated. These associations were independent of known risk factors for type 2 diabetes, including age, BMI, family history, smoking, postmenopausal hormone use, alcohol intake, physical activity, smoking, total energy intake, and consumption of whole grains, nuts, processed meats, coffee, and potatoes. This study is one of the first to prospectively examine fruit juice intake and the risk of type 2 diabetes.
The positive association between fruit juice consumption and diabetes risk may relate to the relative lack of fiber and other phytochemicals, the liquid state, and the high sugar load. The rapid delivery of a large sugar load, without many other components that are a part of whole fruits, may be an important mechanism by which fruit juices could contribute to the development of diabetes. Fructose consumption has also been implicated in the development of many manifestations of the insulin resistance syndrome (25
). Frequent consumption of fruit juices may contribute to a higher dietary glycemic load, which has been positively associated with diabetes in this cohort (27
). Fruit and green leafy vegetables may contribute to a decreased incidence of type 2 diabetes through their low energy density, low glycemic load, and high fiber and micronutrient content (28
). In particular, green leafy vegetables may supply magnesium, which has been inversely linked to the development of type 2 diabetes in women (8
We searched MEDLINE to January 2008 to identify prospective studies of fruit and vegetable intake and risk of type 2 diabetes. In all, we identified six studies that are summarized in (5
). Many of these studies had small sample sizes, combined fruit juice intake with whole fruit intake, and did not include updated measures of dietary intake during the study.
Prospective cohort studies reporting measures of association between intake of fruits and vegetables and diabetes
Other investigations have related the consumption of sugar-sweetened or nondiet colas, other sodas, and fruit punches to development of type 2 diabetes (23
). In the Nurses' Health Study II cohort, which comprised 91,249 women followed for 8 years from 1991–1999, women consuming at least 1 sugar-sweetened soft drink/day were 1.83 times more likely (95% CI 1.42–2.36; Ptrend
< 0.001) to develop type 2 diabetes compared with those who consumed this type of beverage less than once per month, after adjustment for potential confounders. Consumption of fruit punches was also associated with increased diabetes risk (multivariate-adjusted HR 2.00 [95% CI 1.33–3.03]; P
= 0.001). In that study, fruit juice consumption was not associated with diabetes risk; however, increased consumption of fruit juices in the first 4 years from baseline was associated with a significantly greater weight gain among women over the course of follow-up (4.03 kg) compared with decreased fruit juice consumption (2.32 kg) during the same period (P
< 0.001). Possible reasons for the discrepancy between results in these two cohorts include misclassification of fruit juice intake in the Nurses' Health Study II, which incorporated only two dietary assessments (1991 and 1995). In the present study, five dietary assessments were available (1984, 1986, 1990, 1994, and 1998) to classify fruit juice intake. In addition, the Nurses' Health Study II cohort is younger and over the course of 9 years of follow-up developed only 741 cases of incident type 2 diabetes. In the present study, 18 years of follow-up were available, with 4,529 cases of incident type 2 diabetes.
The primary limitation of our study was the potential for bias due to measurement error. We attempted to reduce measurement error in assessing long-term diet by using the average of all available measurements of diet up to the start of each 2-year follow-up interval (18
). In addition, although our results for fruit juice consumption and type 2 diabetes are a relatively new finding, those for green leafy vegetable consumption have been replicated in at least one large study using different dietary assessment methods that should have differently structured measurement errors (10
). The possibility of unknown confounding, which cannot be ruled out in any observational study, must also be acknowledged. The FFQ used in this study does not distinguish between canned and fresh fruits, which have different nutrient profiles and may be associated with different food habits. Moreover, the food supply has changed significantly over the past decades, whereas our FFQ has not; nevertheless, the most common foods eaten in the U.S. population are encompassed in our instrument. There may be underestimation of type 2 diabetes by self-report; however, our population is highly educated about medical conditions, so self-report error should be substantially less than that in a general population. Fasting glucose criteria for diabetes were lowered in 1997, possibly contributing to underestimation in this study. Also, it is possible that women may have misreported fruit punches as juices. Fruit punches have been associated with an increased incidence of diabetes in U.S. women (23
). Because of the homogeneity of our population, generalizability of these results to women of other race and ethnicity bears further examination.
Our findings of a positive association of fruit juice intake with hazard of diabetes suggest that caution should be observed in replacing some beverages with fruit juices in an effort to provide healthier options. Moreover, the same caution applies to the recommendation that 100% fruit juice be considered a serving of fruit as it is in the present national dietary guidelines (30
). In general, the observed associations between fruits and vegetables are weaker than those for cardiovascular disease (31
). However, if fruits and vegetables are used to replace refined grains and white potatoes, both of which have been shown to be associated with increased risk of diabetes (20
), the benefits of regular consumption of fruits and vegetables should be substantial.