July 2015 Issue
Celebrate Summer Fruits
By Judith C. Thalheimer, RD, LDN
Vol. 17 No. 7 P. 20
From blueberries and strawberries to peaches and watermelon, Today's Dietitian reviews the health benefits of your favorite North American summer fruits.
A diet rich in fruits and vegetables confers a multitude of health benefits, but getting clients and patients to increase consumption can be a challenge. Summer, when many fruits are ripe, available, least expensive, and most delicious, is the perfect time to encourage more fruit intake. And often, locally grown summer fruits are bursting with flavor and nutrition.
Today's Dietitian explores the health-promoting nutrients and dietary factors in 10 favorite North American summer fruits, as well as buying and storage tips to help inspire clients and patients to make the most of these nutritious powerhouses.
Fiber-rich, nutrient-dense, and full of antioxidants, berries are a sweet summer treat bursting with health benefits. Colleen Doyle, MS, RD, managing director of nutrition and physical activity for the American Cancer Society, says that berries are thought to offer powerful cancer-preventive effects. "Berries contain antioxidants like polyphenols—including ellagic acid—and anthocyanins that counteract, reduce, and repair damage to cells."
These deep-purple gems are an excellent source of vitamin C and manganese and pack an impressive amount of fiber in a tiny package.1 Most significantly, blueberries have high levels of antioxidants, which have the potential to decrease risk of many illnesses and diseases caused by oxidative stress. Anthocyanins, flavonoid phytochemicals that have antioxidant and anti-inflammatory effects, give blueberries their distinctive color.2,3 Their phytochemical content is why blueberries have been studied in relation to everything from aging, bone health, brain function, and cancer to cardiovascular disease (CVD), blood lipids, diabetes, insulin resistance, eye health, urinary tract health, kidney and liver function, and obesity.2 In the March 2015 issue of the Journal of the Academy of Nutrition and Dietetics, Johnson and colleagues found that daily blueberry consumption may help reduce blood pressure and arterial stiffness.4 Doyle says blueberries are thought to offer powerful cancer-preventive effects.
"It's no surprise that blueberries are antioxidant standouts," says Sarah Krieger, MPH, RDN, LD/N, a spokesperson for the Academy of Nutrition and Dietetics. "The darker the color of a fruit or vegetable, the higher the concentration of phytochemicals."
Fresh US-grown blueberries are available across the country from April through October.2 When ripe, they should be firm, plump, and dry with a dusty blue color. Blueberries stay fresh in the refrigerator for 10 to 14 days.5
Like blueberries, the rich, dark color of blackberries indicates that they have high levels of anthocyanins and other phenolic compounds and all of the health benefits that come with being among the top fruits for antioxidant strength.6 They also are an excellent source of fiber, vitamin C, manganese, and vitamin K, and provide plenty of vitamin E, folate, magnesium, potassium, and copper.1
Of the fruits discussed here, blackberries have the highest folate content, although strawberries are a close second.1 Folate is essential for brain development and function and critical in DNA and RNA synthesis and methylation.7 Deficiency can lead to megaloblastic anemia, and low intake is linked to increased cancer risk.7 When seen in combination with elevated homocysteine levels, folate deficiency is associated with increased CVD risk.7 In addition, inadequate folate intake in early pregnancy raises the risk of congenital anomalies such as neural tube defects.7 One cup of blackberries provides 9% of the DV for folate.1 "Foods that are high in folate are great for pregnant women and women of childbearing age," Krieger says. "Eating foods such as blackberries can help ensure women are getting enough folate, especially if they're not taking prenatal vitamins before conceiving."
Since the late 1990s, folic acid has been added to products made with refined wheat flour to prevent folate-related birth defects.8 People following a gluten-free or low-FODMAP diet, or Hispanics or others whose cultures traditionally involve eating a cornmeal-based diet, may be at risk of low intake because these products generally aren't enriched or fortified.8,9 People in these population groups should be encouraged to add folate-containing foods such as blackberries to their diets.
In the United States, blackberry season typically peaks in June in the South and July in the North.10 Choose shiny berries that aren't bruised or leaking. Blackberries will remain fresh three to six days in the refrigerator. Wash just before using.5
Strawberries are an excellent source of manganese, and provide a good amount of folate and potassium. But where strawberries really stand out nutrient-wise is vitamin C.1
One cup of halved strawberries has more than 149% of the recommended DV of vitamin C.1 This important vitamin is a potent antioxidant in its own right, and an essential cofactor in numerous enzymatic reactions.7 Prospective cohort studies indicate that higher intakes of vitamin C from diet or supplements are associated with a reduced CVD risk, and people with higher blood levels of vitamin C have a lower risk of death from all causes, including CVD and cancer.7 Vitamin C also is known to play a role in immune function and may shorten the duration of colds.7
"Levels of water-soluble vitamins [such as vitamin C] are highest in freshly picked fruits," Krieger says, "so fresh summer strawberries are a great choice."
The same phytochemicals responsible for the deep color in blueberries and blackberries—anthocyanins—are responsible for the tempting red of strawberries. Strawberries also are high in other phenolic compounds such as flavonoids, ellagitannins, and ellagic acid, which have antioxidant, anti-inflammatory, anticancer, and antimutagenic properties.11 In 2011, a small double-blind, randomized, crossover trial found that strawberries (in the form of freeze-dried strawberry powder) reduced risk factors for CVD, stroke, and diabetes in obese participants, suggesting that strawberries may help to decrease obesity-related diseases.12
Strawberries should be shiny, with a bright red color and intact green caps. They can be stored in the refrigerator for one to three days and shouldn't be washed until right before they're consumed.5
Like blueberries, blackberries, and strawberries, raspberries get their color from anthocyanins.13 Raspberries are an excellent source of vitamin C and manganese, a good source of vitamin K, and a dietary fiber standout. One cup of raspberries has more than twice the fiber as the same amount of blueberries or halved strawberries, and just edges out blackberries at 32% of the DV.1 Fiber promotes bowel health by preventing constipation and reducing risk of diverticular disease. Current data no longer support a link between fiber intake and colorectal cancer.7 However, diets rich in fiber can reduce risk of CVD and type 2 diabetes.7 "We know that fiber from fruits helps lower blood cholesterol levels, which helps decrease risk of CVD," says Rachel Johnson, PhD, MPH, RD, a professor of nutrition at the University of Vermont and a spokesperson for the American Heart Association. "The fiber in whole fruits like raspberries also provides a feeling of fullness with fewer calories, helping to maintain a healthful weight, reducing the risk of heart disease."
A 2015 study in rats showed that gut bacteria ferment raspberry seeds, leading to the creation of beneficial short-chain fatty acids. The rats fed the diet that included raspberry seeds had reduced serum triglycerides, and therefore lower CVD risk.14
In the Pacific Northwest, locally grown black raspberries are available in July. These less-common native fruits are reported to have health benefits from phytochemical content that top even the blueberry.15
Choose plump, firm berries. Even refrigerated, raspberries are best if used in one or two days, and shouldn't be washed until ready to eat.5
Other Summer Favorites
Berries get good press, but increasing intake of all fruits is beneficial. The following favorites also can provide plenty of disease-fighting nutrients this season.
Cherries aren't native to North America but they're grown in most parts of the United States.5,16 The dark, rich color of sweet red cherries reflects their high levels of anthocyanin pigments and phenolic compounds and all the health benefits that go along with them.13 Sweet cherries also are a good source of vitamin C and have a healthful helping of satiating fiber.5 "What I like about sweet cherries," Krieger says, "is that they're very sweet for the number of calories they contain. There are only 50 calories in 10 good-sized cherries and almost 2 g of fiber."
Like their sweet relatives, sour red cherries have high antioxidant and vitamin C content, but they're also an excellent source of vitamin A, which is involved in regulating growth and differentiation of virtually all the cells in the body.5,7
Because of vitamin A's role in immune function and vision, deficiency is a major cause of preventable blindness in children worldwide and increases susceptibility to infections.7,17 "There's been much research on sour cherries," Krieger says. "In one study, participants who drank 100 g of tart cherry juice showed a decrease in arthritis pain and inflammation. Still, I'd rather see people tossing cherries on a salad or oatmeal than buying gallons of cherry juice."
Choose firm red cherries with stems attached. Sweet cherries can be stored in the refrigerator for up to 10 days, but sour cherries are best if used in two to three days. Cherries can be frozen for later use with the pits removed.5
Peaches and Nectarines
Peaches are native to China but grow well in most states in America.16 Nectarines are developed from a natural mutation of peaches, and grow in California.18 Both are at their peak in July and August.19
Peaches and nectarines are good sources of vitamin C, dietary fiber, vitamin A, potassium, and niacin.1 More than 400 enzymes in the human body require niacin coenzymes to function.7 Niacin's involvement in mechanisms that maintain gene stability make it useful in cancer prevention.7 Peaches have been found to inhibit LDL oxidation, particularly when eaten with the skin.20 In addition, "Orange fruits and vegetables are loaded with beta-carotene, an antioxidant thought to protect cell membranes from damage," Doyle says.
For a fruit, nectarines are high in lutein and zeaxanthin, carotenoids that are found in the macula at the center of the retina. Many studies have shown that lutein and zeaxanthin reduce the risk of age-related macular degeneration, a leading cause of blindness.21 Peaches also are fairly high on the list of lutein- and zeaxanthin-containing fruits.1 Since cooking and heating make these carotenoids more available, canned peaches contain even more than their fresh counterparts.1
"Fruits like peaches and nectarines are great because they're easy to portion control and they only have 50 to 70 kcal in one whole fruit," Krieger says.
Peaches and nectarines will get softer after picking, but they won't get sweeter. Smell is a good indicator of flavor, so choose fruit that smells fragrant and is neither green nor overly soft. Keeping them in a paper bag speeds ripening.19
Watermelon is the quintessential summer fruit. It's originally from Africa but grows across the Southern United States and in the Northeast.16 Peak watermelon season is August, just when the high water content is most needed to help keep hydration levels up on hot summer days.22 Watermelon also is an excellent source of vitamin C and a very good source of vitamin A.1 "Watermelon has more beta-carotene than berries, and it has a lot of iron for a fruit," Krieger says. But it's the carotenoid phytochemicals, such as lycopene, that make watermelon stand out. Diets high in carotenoid-rich fruits and vegetables such as watermelon are associated with reduced risk of CVD and some cancers.7
"Lycopene is the shining star here," Krieger says. "The lycopene levels in watermelon are similar to the levels in cooked tomatoes," and a serving of watermelon has three times as much lycopene as a raw tomato.7 "Lycopene is an antioxidant that gives foods their red color," Doyle says. "It's thought to be associated with reduced risk of prostate cancer."
Choose symmetrical melons that are heavy for their size (yellowish undersides are normal). Store whole watermelons at room temperature, and refrigerate cut watermelon in an airtight container for up to five days.5
Native fruits that grow wild or in home gardens offer an opportunity for locally sourced, nutrient-packed food ready to be eaten right off the plant.
The largest edible fruit native to the United States, the pawpaw is indigenous to 26 states, from Florida to as far north as Canada and as far west as Nebraska.
Depending on where they're growing, pawpaw trees bear fruit from late August into the fall. The pawpaw has a custardy texture and a surprisingly tropical flavor said to be reminiscent of bananas, pineapples, and mangos. This remarkably nutritious fruit has as much potassium and fiber as bananas. "We know that diets rich in potassium can help to maintain healthy blood pressure, and can even have a blood pressure lowering effect," Johnson says. But the pawpaw has twice as much vitamin C and niacin as a banana, about 10 times as much calcium, and significantly more phosphorus, magnesium, zinc, copper, and manganese. One serving of pawpaw has 7 g of iron. This fruit contains all of the essential amino acids, and the fatty acid profile is preferable to that of a banana, containing 40% monounsaturated fatty acids.23
A ripe pawpaw will give slightly when pressed, similar to a peach. The skin color ranges from green to yellow, and the skin speckles and darkens like a banana. The fruit should have a strong, pleasant aroma. Remove the skin and seeds before eating.23
The purple passionflower is a fast-growing native North American vine that grows from Pennsylvania to as far south as Florida, and from the Atlantic coast to as far west as Texas.16 After the stunning, large purple blossoms emerge, yellowish fleshy fruits about the size of a hen's egg begin to mature in late July.24,25 The maypop is an excellent source of fiber (41% of the DV per 100 g) and vitamin C (36%), and a good source of phosphorus (10%), iron (12%), riboflavin (11%), and niacin (10%).22 Maypops also have 8% of the DV of vitamin B6, 4% of folate, and other B-complex vitamins.22 These B vitamins support the body's metabolism rate and help fight disease and infection.26
To eat a maypop, the Edible Plant Project website recommends simply tearing open the skin, sucking out the flesh, and eating it along with the crunchy seeds.25
"The key thing is that there's no one food that will reduce [disease] risk," Doyle says. "It's likely that it's the synergy between many nutrients—vitamins, minerals, phytochemicals, antioxidants—that give you the most protection against cancer and other diseases. Eating a diet that includes plenty of colorful vegetables and fruits, and whole grains, with limited amounts of red and processed meats, and maintaining a healthy weight is our best bet to help reduce cancer risk through the foods you eat."
Susan Adams, MS, RD, LDN, FAND, an assistant professor of nutrition in the department of urban public health and nutrition in the school of nursing and health sciences at La Salle University in Philadelphia, offers the following tips for helping clients incorporate summer fruits into their diets:
• Include fruits (and vegetables) in every meal.
• Keep a fruit bowl in the kitchen.
• Toss fruit onto oatmeal or dry cereal.
• Make whole fruit smoothies.
• When preparing dessert, make it fruit-based, such as a fruit salad or fruit-based pie (with a lattice top to cut the fat).
• Slice fruits into salads.
• Make elegant cold fruit soups by pureeing fruits such as strawberries, raspberries, or peaches; thinning with a little fruit juice or milk; and adding spices, such as cinnamon for peaches or a dash of pepper for strawberries.
• Place peeled, sliced fresh fruit in the freezer. Put frozen fruit in a blender with a dab of lemon juice and a little spice to make a sorbet.
• Grill fruits such as pineapple and peaches. Grilling softens the fruit and gives it a more complex flavor, making it an ideal accompaniment to meats.
• Use chopped peaches or watermelon for salsas.
"While we know that fruits are loaded with phytochemicals and antioxidants, we don't know which of them are most protective," Doyle says. "So encourage people to eat a variety of them throughout the day, and focus on the ones with the most color, since those (in general) are the ones most packed with phytochemicals and antioxidants."
Summer fruits are delicious, nutritious, and versatile. By knowing when fruits are at their peak and understanding the health benefits associated with them, nutrition professionals can help clients and patients improve their dietary intake and prevent chronic disease.
— Judith C. Thalheimer, RD, LDN, is a freelance nutrition writer, a community educator, and the principal of JTRD Nutrition Education Services.
1. Nutrition facts. SELF Nutrition Data website. http://nutritiondata.self.com
2. Health Research. U.S. Highbush Blueberry Council website. http://www.blueberrycouncil.org/health-professionals/health-research/
3. Krikorian R, Shidler MD, Nash TA, et al. Blueberry supplementation improves memory in older adults. J Agric Food Chem. 2010;58(7):3996-4000.
4. Johnson SA, Figueroa A, Navaei N, et al. Daily blueberry consumption improves blood pressure and arterial stiffness in postmenopausal women with pre- and stage 1-hypertension: a randomized, double-blind, placebo-controlled clinical trial. J Acad Nutr Diet. 2015;115(3):369-377.
5. Fruit and vegetable nutrition database. Fruits and Veggies More Matters website. http://www.fruitsandveggiesmorematters.org/fruit-vegetable-nutrition-database
6. Huang WY, Zhang HC, Liu WX, Li CY. Survey of antioxidant capacity and phenolic composition of blueberry, blackberry, and strawberry in Nanjing. J Zhejiang Univ Sci B. 2012;13(2):94-102.
7. Dietary factors. Oregon State University Linus Pauling Institute Micronutrient Information Center website. http://lpi.oregonstate.edu/mic/dietary-factors
8. Issue brief: status of folic acid fortification in the United States. Spina Bifida Association website. http://www.spinabifidaassociation.org/site/c.knKLINNkEiG/b.5712303/k.3079/Status
9. Thompson T. Folate, iron and dietary fiber contents of the gluten free diet. J Am Diet Assoc. 2000;100(11):1389-1396.
10. How to can, freeze, dry and preserve any fruit or vegetable at home: fruit/veg guides and picking tips. Pick Your Own website. http://pickyourown.org/allaboutcanning.htm. Updated May 20, 2015.
11. California strawberries: guardians of health. California Strawberries website. www.californiastrawberries.com/health_and_nutrition/for_health_professionals
12. Zunino SJ, Parelman MA, Freytag TL, et al. Effects of dietary strawberry powder on blood lipids and inflammatory markers in obese human subjects. Br J Nutr. 2012;108(5):900-909.
13. Fact sheets. Berry Health Benefits Network website. http://berryhealth.fst.oregonstate.edu/health_healing/fact_sheets/
14. Kosmala M, Zduńczyk, Juśkiewicz, J, et al. Chemical composition of defatted strawberry and raspberry seeds and the effect of these dietary ingredients on polyphenol metabolites, intestinal function, and selected serum parameters in rats. J Agric Food Chem. 2015;63(11):2989-2996.
15. Health and healing fact sheets: black raspberries. Berry Health Benefits Network website. http://berryhealth.fst.oregonstate.edu/health_healing/fact_sheets/black_raspberry_facts.htm
16. USDA Natural Resources Conservation Service website. http://plants.usda.gov/java/. Updated May 15, 2015.
17. Micronutrient deficiencies. World Health Organization website. www.who.int/nutrition/topics/vad/en/
18. Nectarine. Wisconsin Department of Public Instruction website. http://fns.dpi.wi.gov/sites/default/files/imce/fns/pdf/ffvp_fs_nc.pdf
19. Find a food. Eat the Seasons website. http://eattheseasons.com
20. Chang S, Tan C, Frankel EN, Barret DM. Low-density lipoprotein antioxidant activity of phenolic compounds and polyphenol oxidase activity in selected clingstone peach cultivars. J Agric Food Chem. 2000;48(2):147-151.
21. Lutein & zeaxanthin. American Optometric Association website. http://www.aoa.org/patients-and-public/caring-for-your-vision/diet-and-nutrition/lutein?sso=y
22. Bosley E. In season: watermelon. Cooking Light website. http://www.cookinglight.com/food/in-season/in-season-watermelon-summer-produce
23. Frequently asked questions about pawpaw. Kentucky State University website. http://www.pawpaw.kysu.edu/faq.htm. Updated October 22, 2009.
24. Maypop fruit. Fruitsinfo website. http://www.fruitsinfo.com/maypop-fruit.php
25. Maypop. Edible Plant Project website. http://edibleplantproject.org/maypop/
26. What are B-vitamins and folate? Academy of Nutrition and Dietetics website. http://www.eatright.org/resource/food/vitamins-and-supplements/types-of-vitamins-and-nutrients/b-vitamins-and-folate. Updated January 28, 2014.