Reviewed: Guo X-F, Yang B, Tang J, Jiang J-J, Li D. Apple and pear consumption and type 2 diabetes mellitus risk: A meta-analysis of prospective cohort studies. Food Funct. 2017;8(3):927-934.

Incidence of type 2 diabetes mellitus (T2DM) is increasing worldwide, but changes in diet and lifestyle may help prevent this chronic disease. Fruits and vegetables contain a variety of beneficial constituents, including dietary fiber, minerals, phytochemicals, and vitamins. Apples (Malus spp., Rosaceae) and pears (Pyrus spp., Rosaceae) are rich sources of polyphenols, and they are widely consumed as fresh fruits, jams, and juices. The purpose of this meta-analysis was to determine the relationship between apple and pear consumption and risk of T2DM.

The authors searched the Cochrane Library, Embase, and PubMed databases from inception through November 2016 using the terms “fruit,” “vegetable,” “apple,” or “pear” in combination with “type 2 diabetes.” Citations from more recent literature reviews and meta-analyses also were searched. Only prospective cohort studies that evaluated apple and/or pear consumption in relation to T2DM were included. All included studies used a validated food frequency questionnaire.

To conduct the meta-analysis, the authors converted all consumption data into servings per week using a standard portion size of 106 g per week. (On average, a small apple weighs approximately 100 g, and a small pear weighs 150 g.) The meta-analysis was conducted according to the criteria for Meta-analysis of Observational Studies in Epidemiology (MOOSE), and individual study quality was assessed using the Newcastle-Ottawa Scale.

Of the 3,971 articles that were located, only three met all inclusion criteria. However, one of the three articles comprised three separate prospective cohort studies, so data from five studies were reviewed. According to the Newcastle-Ottawa Scale, one study was of moderate quality and the other four studies were of high quality.

The included studies had a total of 228,315 participants, of which 14,120 (6.2%) developed T2DM. As detailed in Table 1 of the article, each of the included studies controlled for various confounding factors (e.g., age, gender, ethnicity, weight, smoking status). The meta-analysis revealed that consumption of apples and pears was associated with a significant 18% reduction in T2DM risk (P < 0.001). The authors also found a dose-response relationship between apple and pear consumption and T2DM risk: One, two, three, four, and five servings per week corresponded to respective reductions of 3%, 8%, 12%, 15%, and 19% in T2DM risk (P < 0.001).

The authors conclude that apple and pear consumption was significantly associated with a decreased risk of developing T2DM. This finding is of value because apples and pears are popular fruits consumed worldwide and provide numerous beneficial compounds to the diet. In the United States, apple consumption contributes approximately 22% of the total polyphenol intake of an average person.

The authors hypothesize that the decrease in T2DM risk from consuming apples and pears could be related to the fruits’ contents of (1) polyphenols, which have favorable effects on antioxidant capacity; (2) soluble fibers, such as pectin, which slow gastric emptying, reduce postprandial glucose spikes, and are associated with reduced hyperinsulinemia; or (3) various other phytochemicals that may reduce inflammation.

The strengths of this study are its large sample size, strong statistical power, and inclusion of studies that were of moderate and high quality. Limitations of this analysis are that all the studies were conducted in the United States and the varieties of apples and pears were not addressed.

—Heather S. Oliff, PhD