Despite all of the controversy surrounding the optimal components of a healthy diet, there is little disagreement among scientists regarding the importance of fruits and vegetables. The results of numerous epidemiological studies and recent clinical trials provide consistent evidence that diets rich in fruits and vegetables can reduce the risk of chronic disease. On the other hand, evidence that very high doses of individual micronutrients or phytochemicals can do the same is inconsistent and relatively weak. Fruits and vegetables contain thousands of biologically active phytochemicals that are likely to interact in a number of ways to prevent disease and promote health. The best way to take advantage of these complex interactions is to eat a variety of fruits and vegetables.
Disease Prevention
Cardiovascular Diseases (Heart Disease and Stroke)
Dietary patterns characterized by relatively high intakes of fruits and vegetables are consistently associated with decreased risk of coronary heart disease (CHD) and stroke. A meta-analysis that combined the results of 11 prospective cohort studies found that people in the 90th percentile of fruit or vegetable intake (about five servings/day) had a risk of myocardial infarction (heart attack) that was approximately 15% lower than those in the 10th percentile of intake (1). Among more than 126,000 men and women participating in the Health Professionals Follow-up Study and the Nurses’ Health Study, those who consumed eight or more servings of fruits or vegetables daily had a risk of developing CHD over the next 8-14 years that was 20% lower than those who consumed less than three servings daily (2). In the same men and women, the risk of ischemic stroke (stroke caused by a reduction in blood flow to part of the brain) was 30% lower in those who consumed at least five servings of fruits or vegetables daily than in those who consumed less than three servings daily (3). Based on the results of the Health Professionals Follow-up Study and the Nurses’ Health Study, eating just one extra serving of fruits or vegetables daily would decrease the risk of coronary heart disease by about 4% and the risk of ischemic stroke by 6%.
Although high blood pressure (hypertension) often has no symptoms, people with hypertension are at increased risk of heart disease and stroke. Adding more fruits and vegetables to a sensible diet is one potential way to lower blood pressure. In the DASH (Dietary Approaches to Stop Hypertension) study, 459 people with and without hypertension were randomly assigned to one of three diets: 1) a typical American diet that provided about three servings/day of fruits and vegetables and one serving/day of a low-fat dairy product, 2) a fruit and vegetable diet that provided eight servings/day of fruits and vegetables and one serving/day of a low-fat dairy product, or 3) a combination diet (now called the DASH diet) that provided nine servings/day of fruits and vegetables and three servings/day of low-fat dairy products (4). After eight weeks, the blood pressures of those on the fruit and vegetable diet were significantly lower than those on the typical American diet, while blood pressures of those on the combination (DASH) diet were lower still. For more information on the DASH eating plan, go to the National Heart, Lung, and Blood Institute Web site.
A number of compounds may contribute to the cardioprotective effects of fruits and vegetables, including antioxidant vitamins and minerals, folate, potassium, fiber, and phenolic compounds like flavonoids. However, it is important to note that supplementation of individual micronutrients or phytochemicals has not generally resulted in significantly less cardiovascular events (heart attacks or strokes) in controlled trials. Thus, in the case of fruits and vegetables, the benefit of the whole package may be greater than the sum of its parts.
Type 2 Diabetes
In addition to its other complications, type 2 diabetes is associated with an increased risk of cardiovascular diseases, which are the leading cause of death in diabetic individuals. Although the evidence for a beneficial effect of a diet rich in fruits and vegetables on diabetes is not as strong as it is for heart disease, the results of a small number of studies suggest that higher intakes of fruits and vegetables are associated with improved control of blood glucose levels and lower risk of developing type 2 diabetes. In a cohort of almost 10,000 adults in the U.S., the risk of developing type 2 diabetes over the next 20 years was approximately 20% lower in those who reported consuming at least five servings/day of fruits and vegetables compared to those who reported consuming none (5). In a cross-sectional study of more than 6,000 nondiabetic adults in the U.K., those with higher fruit and vegetable intakes had significantly lower levels of glycosylated hemoglobin (HbA1c), a measure of long-term blood glucose control (6). Although the relationships between fruit and vegetable intake and diabetes remain to be clarified, possible compounds in fruits and vegetables that may enhance glucose control include fiber and magnesium (7).
Cancer
Although initial evidence derived from numerous case-control studies indicated that fruit and vegetable-rich diets were protective against a variety of cancers, the results of recent prospective cohort studies suggest that the relationships between fruit and vegetable intakes and risks of different types of cancer may be more complex (8). In general, the results of prospective cohort studies have not been as strong in their support of protective effects for fruit and vegetable intake on the risk of lung, bladder, breast, stomach, and colorectal cancers as were previous case-control studies (9). In case-control studies, the past diets of people who have been diagnosed with a particular disease are compared to the diets of people without that disease. Because they are retrospective, the results of case-control studies are more likely to be biased than the results of prospective cohort studies, which collect information on the diets of large groups (cohorts) of healthy people and follow the development of disease over time. A recent study that pooled the results of eight prospective cohort studies on lung cancer risk found that only those with the lowest fruit and vegetable intakes were at increased risk for lung cancer and that most of the increase in risk was related to low fruit intake(10). Because the beneficial effects of higher fruit and vegetable intakes were modest compared to the adverse effect of smoking on lung cancer risk, smoking cessation should remain the highest priority in lung cancer risk reduction. When scientists pooled data from prospective cohort studies that examined relationships between fruit and vegetable intakes and the risks of breast cancer (11) and bladder cancer (12), no significant benefits were found.
Research on the relationships between fruit and vegetable intakes and gastric cancer risk is complicated by other known risk factors. Chronic infection with a bacteria called Helicobacter pylori is a known risk factor for gastric cancer, and the risk of gastric cancer is much greater in some parts of the world (e.g., Asia and South America) than others (13). Although a prospective study in a high-risk Japanese cohort found that relatively low fruit and vegetable intakes were associated with higher gastric cancer risk, studies of low-risk U.S. and Dutch cohorts did not find significant associations between fruit or vegetable intake and gastric cancer risk (14, 15). Results from prospective cohort studies of colorectal cancer have also been mixed. While no associations between fruit and vegetable intakes and colorectal cancer risk were found in very large cohorts of female nurses and male health professionals in the U.S. (16), Swedish women with very low fruit and vegetable intakes were found to be at significantly increased risk of colorectal cancer (17).
The term “cancer” includes many different types of cancer with different etiologies. Similarly, the terms “fruit” and “vegetable” include many different plant-based foods that vary in their nutrient and phytochemical content (8). The idea that higher intakes of some classes of fruit or vegetable may affect the risk of certain types of cancers more than others is supported by the results of several prospective studies. Higher intakes of cruciferous vegetables, such as broccoli and kale, were associated with significantly lower risks of colon cancer (18) and bladder cancer (19), and higher intakes of tomato products were associated with lower risks of prostate cancer (20). Although prospective studies provide some evidence that very low fruit or vegetable intakes may increase the risk of some types of cancer, many questions remain regarding the intake of specific classes of fruits and vegetables and specific types of cancer.
Osteoporosis
Several cross-sectional studies reported that higher intakes of fruits and vegetables were significantly associated with higher bone mineral density (BMD) and lower levels of bone resorption (loss) in men and women (21-23). In a study that followed BMD over 4 years, higher fruit and vegetable intakes were associated with significantly less decline in BMD at the hip in elderly men but not elderly women (21). Fruits and vegetables are rich in precursors to bicarbonate ions, which serve to buffer acids in the body. When the quantity of bicarbonate ions is insufficient to maintain normal pH, the body is capable of mobilizing alkaline calcium salts from bone in order to neutralize acids consumed in the diet and generated by metabolism (24). Increased consumption of fruits and vegetables reduces the net acid content of the diet and may preserve calcium in bones, which might otherwise be mobilized to maintain normal pH. Results from the DASH study also support a beneficial link between fruit and vegetable intake and bone health. In addition to decreasing blood pressure, increasing fruit and vegetable intakes from about 3 to 9 servings daily decreased urinary calcium loss by almost 50 mg/day in men and women (4).
Age-related eye diseases
Cataracts
Cataracts are thought to be caused by oxidative damage of proteins in the eye’s lens induced by long-term exposure to ultraviolet (UV) light. The resulting cloudiness and discoloration of the lens leads to vision loss that becomes more severe with age. The results of several large prospective cohort studies suggest that diets rich in fruits and vegetables, especially carotenoid- and vitamin C-rich vegetables, are associated with decreased incidence and severity of cataracts (25-27). In a study of male U.S. health professionals, high intakes of both broccoli and spinach were associated with fewer cataract extractions (26).
Macular degeneration
Degeneration of the macula, the center of the retina, is the leading cause of blindness in people over the age of 65 in the U.S (28). The carotenoids, lutein and zeaxanthin, are found in relatively high concentrations in the retina, and may play a role in preventing damage to the retina caused by light or oxidants (29). In two case-control studies, high intakes of carotenoid-rich vegetables, especially those rich in lutein and zeaxanthin, were associated with a significantly lower risk of developing age-related macular degeneration (30, 31).
Chronic Obstructive Pulmonary Disease
Chronic obstructive pulmonary disease (COPD) is a term that includes emphysema and chronic bronchitis, two chronic lung diseases that are characterized by airway obstruction. Although smoking is by far the most important risk factor for COPD, the results of several epidemiological studies suggest beneficial associations between vegetable and, particularly, fruit intakes and COPD risk (32). The results of several cross-sectional studies in Europe indicate that higher fruit intakes, especially apple intakes, are associated with higher forced expiratory volume (FEV1) values, indicative of better lung function (33-35). In a study of 2,500 middle-aged Welsh men, those who ate at least five apples weekly had significantly slower declines in lung function than those who did not eat apples over a five-year period (34). In a study of 2,917 European men followed over 20 years, each 100 gram (3.5 ounce) increase in daily fruit consumption was associated with a 24% decrease in the risk of death from COPD (36). The reasons for these beneficial associations between fruit intake and lung health are not yet known. Because oxidative stress is thought to play a role in the etiology of chronic obstructive lung disease, scientists are currently investigating the possibility that antioxidants found in fruits, such as vitamin C or flavonoids, could play a protective role.
Neurodegenerative Diseases
Although it’s not yet clear whether a diet rich in fruits and vegetables will decrease the risk of neurodegenerative diseases like Alzheimer’s disease and Parkinson’s disease in humans, recent studies in animal models of these diseases suggest that diets high in fruits like blueberries (37) or tomatoes may be protective (38).
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The Linus Pauling Institute Micronutrient Information Center provides scientific information on health aspects of micronutrients and phytochemicals for the general public. The information is made available with the understanding that the author and publisher are not providing medical, psychological, or nutritional counseling services on this site. The information should not be used in place of a consultation with a competent health care or nutrition professional.
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