Do Certain Fruits Decrease the Risk of Type 2 Diabetes?
By Melissa Quick, DO, and David Kiefer, MD
Dr. Quick is a third-year resident in New York at the Beth Israel Residency in Urban Family Medicine.
Dr. Quick reports no financial relationships relevant to this field of study.
Synopsis: The daily whole fruit and fruit juice consumption of 187,382 participants who were free of major chronic diseases at baseline was analyzed over 3,464,641 person-years of follow-up to determine the association between certain fruits and the risk of type 2 diabetes. Specific whole fruits, particularly blueberries, were significantly associated with a lower risk of type 2 diabetes. Fruit juice was associated with a greater risk.
Source:Muraki I, et al. Fruit consumption and risk of type 2 diabetes: Results from three prospective longitudinal cohort studies. BMJ 2013;347:f5001.
Do certain fruits put you at risk for diabetes? The authors of this article reviewed three prospective longitudinal cohort studies evaluating the relationship between individual fruit consumption and the risk of developing type 2 diabetes. The subjects of the study were 66,105 women from the Nurses’ Health Study (established in 1976; timeframe evaluated in this study: 1984-2008; n = 121,700), 85,104 women from the Nurses’ Health Study II (established in 1989; time frame evaluated in this study: 1991-2009; n = 116,671), and 36,173 men from the Health Professionals Follow-Up Study (established in 1986; timeframe evaluated in this study: 1986-2008; n = 51,529).
Participants were excluded from the study if they reported any baseline diagnosis of any form of diabetes, cardiovascular disease, or cancer. Additional exclusion criteria included if missing data existed for the consumption of individual fruit or fruit juice or if daily energy intake was unusual (< 500 or > 3500 kcal/day for the Nurses’ Health Study and the Nurses’ Health Study II and < 800 or > 4200 kcal/day for the Health Professional Follow-Up Study).
The original questionnaire for this study was developed in 1984 and was sent to the women in the Nurses’ Health Study to assess the frequency of consumption of 118 food items over the past year. In 1986, a similar but more extensive questionnaire was developed and this version was sent every 4 years to participants in each study (starting in 1986 for the Nurses’ Health Study II and in 1991 for the Health Professionals Follow-Up Study.) To account for covariates, a follow-up questionnaire administered every 2 years assessed for updates on anthropometric and lifestyle factors, including body height, weight, cigarette use, physical activity, and family history of diabetes. The follow-up rate of all three cohorts was approximately 90%.
The frequency of individual food consumption was gauged by asking how often, on average, participants consumed food in a standard portion size. Ten individual fruits were consistently evaluated: grapes or raisins; peaches, plums, or apricots; prunes; bananas; cantaloupe; apples or pears; oranges; grapefruit; strawberries; and blueberries.
There were nine possible responses to how often the participant ingested these fruits, ranging from "never, or less than once per month" to "six or more times a day."
The authors validated food frequency questionnaires against diet records of a handful of the participants (n = 173, Nurses’ Health Study; and n = 127, Health Professionals Follow-Up Study). Corrected correlation coefficients between the questionnaire and diet records were not available for all individual fruits, but in general ranged from 0.38 (strawberries in men) to 0.95 (bananas). "Total whole fruit" consumption was calculated by the sum of the consumption levels of the 10 individual fruits and watermelon, a fruit that was sporadically inquired about during follow-up.
The main outcome of the study — incident cases of type 2 diabetes — was evaluated by sending supplementary questionnaires to participants who reported physician-diagnosed diabetes in their routine follow-up questionnaires. A type 2 diabetes diagnosis was confirmed if at least one of the following National Diabetes Data Group criteria were met (see Table 1). Sixty-two self-reported cases of type 2 diabetes were randomly selected from the Nurses’ Health Study and 61 (98%) were confirmed after an endocrinologist reviewed the information. In the Health Professionals Follow-Up Study, 57 of 59 (97%) of self-reported type 2 diabetes cases were confirmed by medical record review.
|Table 1. National Diabetes Data Group Criteria — Diagnosis of Diabetes|
* In June 1998, the diagnostic criteria changed from fasting level ≥ 140 mg/dL to ≥ 126 mg/dL
The results of this study show that during 3,464,641 person-years of follow-up, 12,198 participants developed type 2 diabetes. After adjusting for personal, lifestyle, and dietary risk factors for diabetes, the pooled hazard ratio of type 2 diabetes for every three servings/week of total whole fruit consumption was 0.98 (95% confidence interval [CI], 0.96-0.99). For individual fruits, the pooled hazard ratios of type 2 diabetes for every three servings/week were 0.74 (CI, 0.66-0.83) for blueberries; 0.88 (CI, 0.83-0.93) for grapes and raisins; 0.89 (CI, 0.79-1.01) for prunes; 0.93 (CI, 0.9-0.96) for apples and pears; 0.95 (CI, 0.91-0.98) for bananas; 0.95 (CI, 0.91-0.99) for grapefruit; 0.97 (CI, 0.92-1.02) for peaches, plums, and apricots; 0.99 (CI, 0.95-1.03) for oranges; 1.03 (CI, 0.96-1.1) for strawberries; and 1.1 (CI, 1.02-1.18) for cantaloupe. To summarize, the authors found the risk of type 2 diabetes differed significantly between individual fruits: blueberries, grapes, apples, bananas, and grapefruit consumption were significantly associated with a reduced risk of type 2 diabetes.
Another endpoint of this study demonstrated no association between the glycemic index or glycemic load of individual fruit consumption and the risk of type 2 diabetes. Additionally, the consumption of fruit juice was analyzed (juices evaluated include apple, orange, grapefruit, and others) and the pooled hazard ratio for three servings/week of fruit juice was 1.08 (CI, 1.05-1.11). Lastly, the authors found that substitution of whole fruits for fruit juice was also associated with a lower risk of type 2 diabetes.
In 2010, 25.8 million people (8.3% of the U.S. population) were affected by diabetes.1 Approximately 18.8 million of those affected had an official "diabetes" diagnosis, and 90-95% of these diagnosed cases were type 2 diabetes. (See Table 2.) We know the sequelae of diabetes can be devastating: Heart disease, stroke, kidney failure, new cases of blindness, and non-traumatic limb amputation are commonly seen in uncontrolled diabetics. Furthermore, prediabetes, or an impaired glucose tolerance or impaired fasting glucose level, can place individuals at a greater risk of developing type 2 diabetes.1 As such, it is critical for medical providers to be well versed in not only the treatment of diabetes, but perhaps even more crucially, strategies to prevent diabetes’ development. Current management guidelines for prediabetes recommend a multifactorial approach — from weight loss to lifestyle and dietary changes to metformin use — in order to reduce individual risk.2
|Table 2. Criteria for Testing for Diabetes in Asymptomatic Adult Individuals from the ADA Standards of Medical Care in Diabetes — 2013|
1. Testing should be considered in all adults who are overweight (BMI ≥ 25 kg/m2) and have additional risk factors:
2. In the absence of the above criteria, testing for diabetes should begin at age 45 years.
3. If results are normal, testing should be repeated at least at 3-year intervals, with consideration of more frequent testing depending on initial results (e.g., those with prediabetes should be tested yearly) and risk status.
Increasing the amount of whole fruits and vegetables in our diets — an all too familiar edict from parents to physicians nationwide — has been associated with decreased incidence of and mortality from various health outcomes including obesity, hypertension, and cardiovascular disease.3 We know that fruits provide a plethora of phytonutrients (a group of at least 5000 bioactive compounds) that have been linked to reductions in the risk of major chronic diseases.4 Additionally, the soluble fiber found in fruits can decrease postprandial glucose levels.5 Furthermore, the antioxidants in fruits, such as carotenoids, vitamin C, vitamin E, and flavonoids, may reduce diabetes risk by relieving oxidative stress that interferes with the glucose uptake by cells.6
Interestingly, despite the known benefits of consuming whole fruits and vegetables, prior epidemiological research has inconsistently shown a link between increased fruit consumption and a decreased risk of diabetes.7 Thus, the authors of the above reviewed study sought to explain these inconsistencies by postulating that perhaps it is not enough just to increase the generic amount of fruit one eats, but rather, specific fruits may have more positive effects than others.
So, with respect to the reported results in the above study, did blueberries, grapes, apples, bananas, and grapefruit reduce the risk of developing type 2 diabetes more than peaches, plums, apricots, oranges, and strawberries? More to the point, does eating cantaloupe three times a week actually increase your risk of type 2 diabetes? Upon careful examination of the results — specifically reviewing hazard ratios and confidence intervals among individual cohorts — it appears that the only individual fruit with consistent significant findings among each cohort are blueberries when consumed five or more times a week (Nurses’ Health Study: 0.82 [CI, 0.69-0.98]; Nurses’ Health Study II: 0.69 [CI, 0.55-0.87]; and Health Professionals Follow-Up Study: 0.74 [CI, 0.55-1.00]). (See Table 3).
|Table 3. Hazard Ratios and Glycemic Load and Glycemic Index Values for Studied Fruits|
|Adjusted Hazard Ratios for 2-4 servings/week|
|Nurses's Health Study||Nurse's Health Study II||Health Professionals Follow-up Study||Glycemic Load||Glycemic Index|
|Grapes and raisins||0.80 (0.72-0.88)||0.83 (0.72-0.97)||0.87 (0.76-1.01)||High||High|
|Peaches, plums,and apricots||1.04 (0.94-1.14)||0.99 (0.86-1.14)||0.88 (0.75-1.04)||Low||Low|
|Prunes||0.89 (0.75-1.06)||1.16 (0.88-1.53)||0.86 (0.66-1.12)||High||Moderate|
|Bananas||1.04 (0.94-1.15)||0.82 (0.72-0.94)||
|Cantaloupe||Cantaloupe1.07 (0.96-1.19)||1.11 (0.94-1.30)||1.19 (1.01-1.40)||Moderate||High|
|Apples and pears||0.85 (0.77-0.95)||0.79 (0.68-0.91)||0.91 (0.77-1.07)||High||Low|
|Oranges||0.96 (0.87-1.05)||0.93 (0.81-1.07)||0.89 (0.78-1.03)||Moderate||Low|
|Grapefruit||0.88 (0.80-0.96)||0.97 (0.83-1.14)||0.93 (0.81-1.06)||Low||Moderate|
|Strawberries||0.87 (0.77-0.98)||1.09 (0.93-1.27)||1.16 (0.95-1.42)||Low||Low|
|Blueberries||0.82 (0.69-0.98)||0.69 (0.55-0.87)||0.74 (0.55-1.00)||Moderate||Moderate|
Furthermore, cantaloupe does seem to increase risk of type 2 diabetes, but only among the men’s cohort when consumed more than once a month (Health Professional Follow-Up Study — adjusted hazard ratio: 1-3 servings/month = 1.15 [CI, 1.03-1.27]; 1 serving/week = 1.08 [CI, 1.02-1.14]; and 2-4 servings/week = 1.19 [CI, 1.01-1.40]). Of note, the authors mention that they "pooled" their hazard ratios and this explains the results that they list are significant.
Another significant and surprising finding in this study is the lack of association between the glycemic index of individual fruits and their effect on the risk of type 2 diabetes. In traditional diabetes management, patients are often taught to count carbohydrates. Additionally, many patients are taught that elevated glycemic indices (measure of the incremental glucose response per gram of carbohydrate) and glycemic loads (the amount of carbohydrate multiplied by its glycemic index) may increase the risk and progression of type 2 diabetes.8,9 However, many studies have shown that the total amount of carbohydrate in food is generally a better predictor of blood glucose response than the glycemic index.10,11 Interestingly, and as noted in the above study, the glycemic index/glycemic load of individual fruits does not correlate to their abilities to reduce diabetes risk.
This study also found that fruit juice consumption does not prevent the risk of type 2 diabetes. Evidence from this and other studies shows that the consumption of whole fruit has profound positive health benefits that fruit juice lacks. Although fruit juices may have a portion of the antioxidant activity found in whole fruits, juices have minimal fiber, are less satiating, and often contain large amounts of sugar.3
There are several potential limitations of this study. The authors admit that their surveying system was imperfect and that there were "inevitable errors" in the estimates of fruit consumption and that some individual fruits were often combined in food-frequency questionnaires, complicating accurate individual fruit to diabetes associations. Additionally, all study participants were either female nurses or male health professionals (dentists, veterinarians, pharmacists, optometrists, osteopathic physicians, and podiatrists).12 It could be argued that this educated population may make healthier dietary and lifestyle decisions than those not in the health care profession. These study participants were also overwhelmingly white (95-98% in each study), which makes it difficult to connect these results with the much more diverse U.S. population. Also, the average body mass index (BMI) of both female and male participants overall ranged from 24.3-25.2 kg/m2 — less than the average U.S. adult man (BMI of 26.6 kg/m2) and the average U.S. adult woman (BMI of 26.5 kg/m2).13 Lastly, recall bias cannot be excluded in this study, as is inherent in most questionnaire-based studies and has potential confounding implications.
Despite the restraints discussed above, there are also many positive aspects of this study. The sheer number of participants evaluated in this study along with the huge amount of person-years analyzed is impressive. Additionally, though the population studied were all medical professionals, it could be argued that if this potential "healthier" subset of the population still developed diabetes despite their increased knowledge of diabetes and diabetes prevention, then the results of this study would be even more pronounced in a more diverse and "lay" population.
How should we counsel our patients about the relationship between fruit and diabetes? The World Health Organization recommends a minimum intake of 400 g or five portions of combined fruits and vegetables daily for the prevention of type 2 diabetes and other common noncommunicable diseases.14 Also, a study published in 2012 suggests that a greater variety, not quantity, of fruit and vegetable consumption is associated with a lower risk of type 2 diabetes.15 These two arguments, along with the study reviewed in this article, suggest that perhaps reiterating the common "5-a-Day" anthem to our patients may be tremendously high-yield guidance. It may even be most useful to emphasize vegetable consumption more than fruit by reversing this tagline to "eat 5 vegetables and fruits a day." It is important to keep in mind that not all fruits and vegetables have equivalent safety profiles in terms of risk of pesticides (See Table 4 for the "dirty dozen" and the "clean fifteen"). Of course, consuming vegetables and fruits regardless of farming techniques is certainly preferred to avoidance if more expensive organic versions are unavailable. Finally, discouraging patients from substituting fruit juice for whole fruits and limiting overall fruit juice consumption will surely reduce patient’s risk of developing type 2 diabetes.
|Table 4. Pesticide Exposure of Common Fruits and Vegetables|
|Dirty Dozen: Best to eat organic if possible||Clean Fifteen: Safer non-organic choices|
Sweet bell peppers
From the Environmental Working Group.
- enters for Disease Control and Prevention. National Diabetes Fact Sheet: National Estimates and General Information on Diabetes and Prediabetes in the United States; 2011. Atlanta, GA.
- merican Diabetes Association. Standards of medical care in diabetes — 2013. Diabetes Care 2013;36(Suppl 1):S11-S66.
- azzano LA, et al. Intake of fruit, vegetables, and fruit juices and risk of diabetes. Diabetes Care 2008;31:1311-1317.
- iu RH. Health benefits of fruit and vegetables are from additive and synergistic combinations of phytochemicals. Am J Clin Nutr 2003;78:517S-520S.
- lavin JL, et al. Plausible mechanisms for the protectiveness of whole grains. Am J Clin Nutr 1999;70:459S-463S.
- ontonen J, et al. Food consumption and the incidence of type II diabetes mellitus. Eur J Clin Nutr 2005;59:441-448.
- urotani K, et al. Vegetable and fruit intake and risk of type 2 diabetes: Japan Public Health Center-based Prospective Study. Br J Nutr 2012:1-9.
- u H, et al. Glycemic index and glycemic load in relation to food and nutrient intake and metabolic risk factors in a Dutch population. Am J Clin Nutr 2008:87:655-661.
- illet W, et al. Glycemic index, glycemic load, and risk of type 2 diabetes. Am J Clin Nutr 2002;75:274S-289S.
- American Diabetes Association. Glycemic index and diabetes. Available at: www.diabetes.org/food-and-fitness/food/planning-meals/glycemic-index-and-diabetes.html. Accessed Oct. 30, 2013.
- Evert AB, et al. Position Statement: Nutrition Therapy Recommendations for the Management of Adults with Diabetes. Diabetes Care 2013;36;3812-3842.
- Van Dam RM, et al. Dietary fat and meat intake in relation to risk of type 2 diabetes in men. Diabetes Care 2002;25:417-424.
- National Health And Nutrition Examination Survey. Healthy weight, overweight, and obesity among U.S. adults. Available at: www.cdc.gov/nchs/data/nhanes/databriefs/adultweight.pdf. Accessed Oct. 30, 2013.
- World Health Organization/UN Food and Agriculture Organization. Diet, Nutrition and the Prevention of Chronic Diseases: Report of a Joint FAO/WHO Expert Consultation. Geneva: World Health Organization; 2003 (Tech. Rep. Ser., no. 916).
- Cooper AJ, et al. A prospective study of the association between quantity and variety of fruit and vegetable intake and incident type 2 diabetes. Diabetes Care 2012;35:1293-1300.