3 Weekly Servings of These Fruits Can Lower Type 2 Diabetes Risk

Frank J. Domino, MD

Review

Muraki I, et al. Fruit consumption and risk of type 2 diabetes: results from three prospective longitudinal cohort studies. BMJ 2013; 347:f5001. http://www.bmj.com/content/347/bmj.f5001.

Study Methods

This prospective longitudinal cohort study of 151,000 women and 36,000 men from the Nurses’ Health Study: I and II and the Health Professionals Follow-Up Study measured new diagnoses of type 2 diabetes and self-reported food intake through food-frequency questionnaires. The data was then combined and evaluated in a multivariate model.

Results and Outcomes

Over the roughly 3.5 million person-years of follow-up, there were 12,198 new cases of type 2 diabetes, representing about 6.5% of the study population. After adjusting for the personal, lifestyle, and dietary risk factors of diabetes, the pooled hazard ratio of type 2 diabetes for every 3 servings of whole fruit consumed per week was found to be 0.98 (95% confidence interval [CI] 0.96-0.99).

When the data was adjusted for individual fruits, the pooled hazard ratios of type 2 diabetes for every 3 servings of the fruits per week were 0.74 for blueberries (0.66-0.83); 0.88 for grapes and raisins (0.83-0.93); 0.93 for apples and pears (0.90-0.96); 0.95 for bananas (0.91-0.98); 0.95 for grapefruit (0.91-0.99); 0.89 for prunes (0.79-1.01); 0.97 for peaches, plums, apricots (0.92 to 1.02); 0.99 for oranges (0.95-1.03); 1.03 for strawberries (0.96-1.10); and 1.10 for cantaloupe (1.02-1.18).

Additionally, the risk of developing type 2 diabetes for every 3 servings of fruit juice per week was 1.08 (1.05-1.11).

Conclusion

Consuming 3 weekly servings of certain whole fruits — especially blueberries, grapes, apples, and potentially grapefruit and bananas — was associated with a statistically significant lower risk of type 2 diabetes development, while fruit juice consumption was associated with an increased risk.

Commentary

This wonderful cohort study used well-defined and well-studied health professional datasets to determine which fruits might decrease the risk of type 2 diabetes. Because this was an observational study, a cause-and-effect relationship cannot be established; however, in a population as closely defined as this one, important clinical actions can be suggested.

The method of dietary evaluation used in this study was an extensive food-frequency questionnaire that initially included 118 food-related questions, yet was expanded as the study went on. The questionnaire queried how often subjects consumed each food in a standardized portion size on average — a process that was validated against diet records among a subset of the Nurses’ Health Study, as well as among participants in the Health Professionals Follow-Up Study.

To address the diagnosis of type 2 diabetes, all 3 study cohorts were sent supplementary questionnaires to determine new illnesses and diagnostic criteria. A diabetes diagnosis was confirmed if participants met one of the National Diabetes Data Group criteria.

Like the food-frequency questionnaire, data analysis was quite extensive and found positive correlations of reduced risk of type 2 diabetes for blueberries, grapes, apples, and possibly other fruits. The authors also conducted a secondary analysis to determine whether glycemic load (GL) or glycemic index (GI) had an influence.* With regards to GL, a greater weekly consumption of high GL fruits, but not moderate or low GL fruits, was associated with a lower type 2 diabetes risk. Looking at GI, a greater weekly consumption of moderate GI fruits, but not high or low GI fruits, was inversely associated with diabetes risk.

As fruit juice consumption was uniformly associated with an increased risk for type 2 diabetes, the authors speculated patients who replaced 3 servings of fruit juice with 3 servings of individual fruits per week would see a drop in their risk of developing type 2 diabetes by 7% for all whole fruits; 33% for blueberries; 19% for grapes and raisins; 14% for apples and pears; 13% for bananas; 12% for grapefruit; 18% for prunes; 11% for peaches, plums, and apricots; and 8% for oranges.

This observational study has significant strengths, as well as some limitations. The strengths include the large number of participants and the validity of the large cohort study protocols, while the limitations involve the retrospective analysis of patients’ food-frequency questionnaires. As a patient’s memory is not an empirically measured variable, it is potentially fraught with bias. However, working against this limitation is that most of the associations discovered in the combined dataset were individually found across all 3 cohorts, and the strongest associations between type 2 diabetes risk and the consumption of blueberries, grapes, and apples remained statistically significant even after more conservative analysis.

Another limitation is the assortment of lifestyle and dietary factors included in the multivariate analysis, which can confound datasets due to inconsistency in the method used to describe the factors, as well as in how participants interpreted the definitions. Additionally, this study population was made up of healthcare professionals, most of whom had European ancestry, so the data might not apply to the general population.

Could 3 weekly servings of fruits really have that great of an influence on diabetes risk? Or are they just markers for a better lifestyle? The range of fruit intake varied among the study populations, from many portions of fruit per day to none. While this study hints fruit intake is a major factor in type 2 diabetes risk, a multivariate analysis can only determine so much.

Nevertheless, this evaluation provides some useful take-home points. First and foremost, family physicians should encourage all patients to stop consuming fruit juices on a regular basis. The only healthy aspect of a large glass of orange juice in the morning is the small amount of fiber it contains. Unless a patient is at risk for scurvy, the vitamin C in that glass has no benefit. In fact, orange juice may potentially induce diabetes in the patient.

Second, clinicians should encourage patients at risk for type 2 diabetes to consume more whole fruits, particularly blueberries, grapes, and apples. Though fruits like strawberries and cantaloupe did not lower type 2 diabetes risk, it is important to consider the results were derived from an intake of only 3 servings per week. Other cohort studies found consuming 3-5 servings of fruits per day, rather than per week, lowered patients’ risk of developing type 2 diabetes, as well as increased weight loss and decreased cancer risk.

While you have your patients’ attention, include a mild to moderate exercise plan in their fruit recommendation to further reduce their risk of developing type 2 diabetes. It is incumbent upon healthcare professionals to rely on non-pharmacologic treatments whenever possible, since reliance on medication may undermine patients’ autonomy and confidence in their ability to determine their future health outcomes. Thus, family physicians should deliver a simple message on the elimination of fruit juice and the inclusion of appropriate fruits, which will likely have a significant impact on their patients’ risk of type 2 diabetes.

_{*GL is serving-size dependent and represents how much 1 serving will increase blood glucose levels; a GL of 20 is high, while a GL of <10 is low. GI measures how quickly blood glucose levels rise after eating a particular type of food; the lower the number, the less rapid the rise in serum glucose levels.}

About the Author

Frank J. Domino, MD, is Professor and Pre-Doctoral Education Director for the Department of Family Medicine and Community Health at the University of Massachusetts Medical School in Worcester, MA. Domino is Editor-in-Chief of the 5-Minute Clinical Consult series (Lippincott Williams & Wilkins). Additionally, he is Co-Author and Editor of the Epocrates LAB database, and author and editor to the MedPearls smartphone app. He presents nationally for the American Academy of Family Medicine and serves as the Family Physician Representative to the Harvard Medical School’s Continuing Education Committee.