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Harvard Commentaries
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Reviewed by the Faculty of Harvard Medical School

Food for Thought Food for Thought

Phytonutrients: Revisiting These Nutritional Giants

January 16, 2013

By Anne Chiavacci, R.D., M.S., M.A.
Brigham and Women's Hospital

For generations our parents have said, "An apple a day keeps the doctor away." Compelling scientific research during the last several decades suggests that they may be right. The simple message, "Eat 5 to 10 servings of fruits and vegetables daily" has been shown to pack a powerful payoff for disease prevention.

In two large Harvard-based studies of about 110,000 people, those who averaged 8 or more servings of fruits and vegetables a day were 30% less likely to have a heart attack or stroke than those who ate less than 1.5 servings daily.

What Counts as a Serving?

One serving equals

  • 1 cup leafy greens, berries or melon chunks
  • ½ cup other fruits and vegetables
  • 1 medium fruit/vegetable (such as apple, orange or tomato)
  • ¼ cup dried fruit

Eating fruits and vegetables has been shown to lower cholesterol and blood pressure, improve bowel regularity and colon health, aid in weight and blood sugar control, reduce the risk of developing macular degeneration, and may guard against certain cancers.

Despite the resounding benefits of consuming plant foods, Americans eat only about 1.5 servings of vegetables and less than 1 serving of fruit per day, on average. Americans spend billions of dollars each year on dietary supplements such as antioxidants despite inconclusive scientific backup for their effectiveness in disease prevention and treatment.

What's So Unique About Fruits and Vegetables?

Fruits, vegetables, whole grains and spices are the sole source of phytonutrients. (Phyto is the Greek word for plant.) These are natural compounds that give plants their color, flavor, smell and texture. Phytonutrients are the immune system of a plant. They protect plants from disease and they can protect you too!

There are as many as 2,000 known phytonutrients. Just one serving of vegetable or fruit may possess more than 100 different types.

Let's look at phytonutrient families and their role in health promotion.

Broccoli, cabbage, cauliflower, Brussels sprouts, bok choy, dark leafy greens, watercress

Cruciferous vegetables contain phytonutrients that may prevent cancer and interfere with the growth of cancer cells.

Indole-3-Carbinol (I3C), (converted to diindolylmethane in the stomach) has properties that could decrease a woman's risk of developing breast cancer. Test-tube and animal studies have shown I3C to interfere with prostate cancer cell growth. It also may be effective against growth of tumors of the lung and colon.

I3C and another phytonutrient found in cruciferous vegetables, sulforaphane, help the liver "detoxify." This means that they turn on the cleaning agents in the liver, called enzymes, to change harmful, cancer-causing chemicals to forms that the body can get rid of.

Berries, cherries, red and purple grapes, currants, pomegranates, walnuts, apples with skin, citrus, red onions, tomatoes, bell peppers, red wine, grape juice

Researchers are interested in ellagic acid as a potential cancer-fighting agent. Most of the studies to date have been done in test tubes and animals. Ellagic acid interferes with steps that allow cancer cells to keep multiplying. It also causes apoptosis, which is cancer cell death. Ellagic acid also stimulates the detoxification enzymes in the liver.

Glucarate neutralizes several cancer-causing compounds such as heterocyclic amines, which form when cooking animal protein at high temperatures. It also may play a role in estrogen-related cancer. Glucarate blocks the enzyme that allows estrogen to be reabsorbed into the blood from the bile in the digestive tract.

Flavonoids resveratrol and quercetin may provide protection against heart disease by decreasing inflammation, hindering the clumping of platelets, and protecting cholesterol from being changed to unsafe oxide compounds.

Anthocyanins interfere with the enzyme in the liver that makes cholesterol. These flavonoids also have antitumor actions.

Among flavonoid-rich foods, apple consumption appears to be associated with decreased risk of diabetes in a Harvard-based study of 40,000 women. Those who consumed at least one apple a day showed a significant 28% reduced risk of type 2 diabetes compared with those who did not eat apples.

Red, green, yellow and orange vegetables and fruits, such as pumpkin, carrots, sweet potato, squash, broccoli, dark leafy greens, tomatoes, corn, peppers, mango, guava, apricots, peaches, cantaloupe, watermelon, red grapefruit, oranges, tangerines

Beta-carotene is a familiar carotenoid to many, yet it is only one of more than 600 compounds in the carotenoid family that have been discovered to date. Alpha-carotene, cryptoxanthin, lycopene, lutein and zeaxanthin also have notable health benefits.

Carotenoids have been linked with prevention of colon, prostate, breast and lung cancer. Two large Harvard studies of more than 124,000 people showed a 32% reduction in risk of lung cancer for those who consumed a variety of carotenoid-rich foods. However, carotenoid supplements in pill form have not demonstrated the same protective benefits. In fact, two studies found beta-carotene supplements to be associated with higher risk of lung cancer and death in smokers.

The Women's Healthy Eating and Living (WHEL) study evaluated the concentration of total carotenoids in the blood of more than 1,500 women who had completed treatment for early-stage breast cancer. Women with the highest carotenoid concentration had a 43% reduction in risk of cancer recurrence compared with those who had the lowest level of blood carotenoids.

Lutein and zeaxanthin are the two primary carotenoid pigments in the eyes that protect the macula and retina from photo damage. Studies suggest that approximately 10 milligrams of dietary lutein daily may reduce the risk of developing age-related macular degeneration and cataracts. One cup of cooked kale or spinach has three to four times this recommended dose!

Citrus fruits

D-limonene, limonin and nomilin are among about 40 limonoids found in the peel, membranes, seeds, flesh and juice of citrus fruits. These powerful compounds have anticancer activity and turn on the liver's detoxification processes to rid the body of cancer-causing agents.

The Bottom Line

Distinctive health benefits are often related to the intensity of color of the plant. Most phytonutrients are not lost in cooking because they are heat-stable.

Aim to consume a complete color spectrum of fruits and vegetables daily. Make it your goal to eat at least two servings by noon each day. Retrain your thinking to create a meal around vegetables and fruits and fill half your plate with color. Savor the skins and add orange and lemon zest to breads, casseroles and desserts.

In this case, more is better! Be mindful that medicine is in your vegetable garden!

Anne Chiavacci, R.D., M.S., M.A. is a senior nutritionist at Brigham and Women's Hospital and Dana-Farber Cancer Institute. She received her Bachelor of Science in nutrition from the University of Massachusetts at Amherst and her Master of Science in nutrition at Tufts University. Chiavacci completed her dietetic internship at Frances Stern Nutrition Center, New England Medical Center in Boston. She has a Master of Arts in counseling from the Biblical Theological Seminary in Hatfield, Pa.

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