Phytonutrients, also called phytochemicals, are chemicals produced by plants. Plants use phytonutrients to stay healthy. For example, some phytonutrients protect plants from insect attacks, while others protect against radiation from UV rays.
Phytonutrients can also provide significant benefits for humans who eat plant foods. Phytonutrient-rich foods include colorful fruits and vegetables, legumes, nuts, tea, whole grains and many spices. They affect human health but are not considered nutrients that are essential for life, like carbohydrates, protein, fats, vitamins and minerals.
Among the benefits of phytonutrients are antioxidant and anti-inflammatory activities. Phytonutrients may also enhance immunity and intercellular communication, repair DNA damage from exposure to toxins, detoxify carcinogens and alter estrogen metabolism. The U.S. Department of Agriculture (USDA) notes that consuming a phytonutrient-rich diet seems to be an “effective strategy” for reducing cancer and heart disease risks.
Many phytonutrients give plants their pigments, so a good way to tell if a fruit or vegetable is rich in phytonutrients can be by its color, according to Louis Premkumar, a professor of pharmacology at Southern Illinois University School of Medicine and author of "Fascinating Facts about Phytonutrients in Spices and Healthy Food (opens in new tab)" (Xlibris, 2014). Look for deep-hued foods like berries, dark greens, melons and spices. These foods also are rich in flavor and aroma, which makes them more palatable. But some phytonutrient-rich foods have little color, like onions and garlic, and you don’t want to discount them.
Understanding the effectiveness of phytonutrients
There is ample evidence that a diet high in phytonutrient-rich plant foods is good for humans. “For centuries, there have been implications that healthy food garnished with exotic spices and condiments provides vital ingredients that help ward off diseases and promote longevity," Premukar said. "Obviously, it appears to be true based on the evidence that people who consume healthy, wholesome food as individuals or as a group in certain parts of the world, have enjoyed health benefits, using longevity as a metric.
"For example, Seventh-day Adventists, with their pure vegetarian diet, have a lower incidence of heart disease, diabetes, and certain types of cancers; Kuna Indians in Panama, who consume large quantities of unprocessed cocoa-containing beverages, show lower incidence of heart disease," he continued. "More recently, the Mediterranean diet, which consists of olive oil, fresh produce, fish, and wine, has been shown to reduce the incidence of grave diseases.”
Though population studies demonstrate the effectiveness of a diet full of phytonutrient-rich foods, scientists are still working to understand the specific mechanisms of how phytonutrients work. Premkumar said that not enough randomized, large-scale clinical trials have been undertaken, and even when they are it can sometimes be difficult to quantify the results. Furthermore, many trials have been done on phytonutrient or antioxidant supplements, which have returned fairly poor results when it comes to disease prevention, according to the National Institutes of Health(NIH). But this is likely because supplements interact with the body differently than whole foods.
It can sometimes be hard for scientists to link health benefits to specific phytonutrients. All plants contain complicated mixtures of bioactive compounds and effects like antioxidant activity can be difficult to quantify. Furthermore, each individual plant possesses a unique biochemical makeup. “The levels of active ingredients [in each plant] can vary, depending upon where the plant is grown, the amount of fertilizers used, whether they are cooked on uncooked, and so on,” Premkumar said.
Additionally, it can be difficult to infer the levels at which the phytonutrients act in each individual body, said Premkumar. Phytonutrients are diverse in nature and affect multiple areas of the body, which sometimes makes it challenging to know precisely which phytonutrient is acting on which part of the body, and if the phytonutrients are helping temporary symptoms or systemic problems.
Despite these challenges, Premkumar said that the health benefits from phytonutrients, even if they cannot be demonstrated readily, “have to be taken seriously.” Government agencies like the USDA, NIH and several health organizations seem to agree, and encourage people to eat more fruits and vegetables and take advantage of the potential benefits from these foods.
According to Premkumar, one thing we do know is that the beneficial effects of phytonutrients will not be seen immediately, but over months or years. He pointed out that phytonutrients and other healthy compounds like vitamins and minerals can only prevent or delay developing diseases. “Once a disease has manifested, the only option to combat is to consult with a physician and take appropriate medications that have been unequivocally proven to be effective in large clinical trials,” he said.
Types of phytonutrients
There are classes of phytonutrients, determined by chemical structure. Phytonutrient classes include:
Within these classes are dozens of phytonutrient groups, which in turn contain hundreds of phytonutrients.
Phytonutrient groups include:
- Hydroxycinnamic acids
There are more than 25,000 types of phytonutrients. Scientists consider these six phytonutrients or phytonutrient groups to be of particular note.
Lignans can mimic the effects of estrogen, so lignans are considered phytoestrogens, though they can also affect the body through non-estrogenic means, according to the Linus Pauling Institute at Oregon State University.
Like all phytonutrients, lignans are found in fruits and vegetables, especially kale, broccoli, apricots and strawberries, according to Premkumar. They are particularly abundant in seeds and whole grains, including sesame seeds, poppy seeds, rye and oat bran. Flaxseeds are the richest source of lignans.
Lignans are associated with preventing hormone-related cancers because of their estrogen-like activity, according to the Linus Pauling Institute. Research is mixed about their effectiveness in preventing breast cancer, but studies have shown positive results in terms of endometrial and ovarian cancers. A Journal of the National Cancer Institute study looking at lignans and endometrial cancer showed a reduced risk in postmenopausal women with high lignin intake, while another study, published in the Journal of Nutrition, saw women with the highest lignin intake had the lowest ovarian cancer rates, regardless of age or menopausal status.
Scientists are beginning to study the relationship between lignans and prostate cancer and osteoporosis. So far, results are inconclusive.
Resveratrol has gotten a good deal of buzz in recent years because large concentrations of it are found in red wine. It is a member of the stilbenoid phytonutrient group.
The best-known source of resveratrol is grapes. Resveratrol has particularly high concentration in grape skin and red wine. It is also found in peanuts, grape juice, cocoa, blueberries and cranberries. The presence of resveratrol in red wine may explain what Premkumar called the “French paradox,” in which French people who drink a good deal of red wine enjoy long and healthy lives despite eating saturated fats and smoking. Premkumar said this is likely because of resveratrol’s ability to reduce the risk of heart disease through antioxidant and anti-inflammatory activities.
According to the Linus Pauling Institute, resveratrol may help slow cognitive decline. In animal studies, resveratrol has shown neuroprotective activities and promotion of healthy peptides. A 2010 human study published in the American Journal of Clinical Nutrition found that resveratrol increased cerebral blood flow but did not lead to improved performance in cognitively difficult tasks. Scientists will continue to study resveratrol’s effects in this area.
Resveratrol is also being studied as a possible treatment for type 2 diabetes because in animal studies, it has been shown to improve insulin sensitivity and glucose tolerance, according to a 2015 article in Biochimica Biophysica Acta.
There are more than 600 carotenoids. They are yellow, orange and red pigments in plants. The most common carotenoids in a Western diet are alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein, zeaxanthin and lycopene. Each of these carotenoids carries a distinct set of actions, benefits and originating fruits and vegetables.
Premkumar listed carrots, yams, sweet potatoes, papaya, watermelon, cantaloupe, mangos, spinach, kale, tomatoes, bell peppers and oranges among the fruits and vegetables in which carotenoids can be found. In order to be properly absorbed, carotenoids should be consumed with a fat.
Carotenoids are associated with antioxidant activity, eye health, immune system activity, intercellular communication and reduced risk of cancer and cardiovascular disease, according to the Linus Pauling Institute.
The body can covert alpha-carotene, beta-carotene and beta-cryptoxanthin into vitamin A (retinol), which is associated with anti-aging and immune system function. Lutein and zeaxanthin are the only carotenoids found in the retina and are associated with lower risks of macular degeneration, according to the Linus Pauling Institute.
Curcumin is found primarily in turmeric, a member of the ginger family. It gives turmeric its distinctive yellow color. Because of its curcumin, turmeric has been used as a medicinal remedy in India for centuries, said Premkumar.
“The claimed effects of curcumin range from relieving flatulence to curing Alzheimer’s disease and cancer,” said Premkumar. Animal studies have shown good results when looking at oral administration of curcumin and to inhibit the spread of mouth, stomach, liver and colon cancer. Studies are under way to investigate this effect in humans.
Curcumin is an effective anti-inflammatory agent and antioxidant. It may also affect carcinogen metabolism, helping the body get rid of toxic compounds, and aid in combating cancer cell growth and tumors, according to the Linus Pauling Institute. All of these factors contribute to its being a potentially effective cancer-prevention agent.
Based on successful animal trials, it has been suggested that curcumin could aid in inflammatory diseases like rheumatoid arthritis as well as cystic fibrosis and Alzheimer’s disease, but studies are either not yet under way or are inconclusive, according to the Linus Pauling Institute.
According to Premkumar, curcumin can also be helpful in cardiovascular protection by lowering LDL cholesterol levels and increasing HDL (good) cholesterol levels. “Treatment with curcumin selectively increases the expression of low-density lipoprotein (LDL) receptor levels and is able to clear LDL, which is bad or lousy cholesterol,” he said.
Ellagic acid is also called a tannin. It is found in raspberries, strawberries, blackberries, cranberries, grapes, pomegranates and walnuts, according to Premkumar. It can also be produced during the body’s process of breaking down larger phytonutrients called ellagitannins. It is absorbed rapidly.
Ellagic acid is associated with antioxidant, anti-inflammatory and anti-cancer activity, as well as reducing blood pressure and arterial plaque, said Premkumar.
According to the Memorial Sloan Kettering Cancer Center, scientists hypothesize that ellagic acid enhances detoxing enzymes in the liver while also inhibiting liver enzymes that encourage metabolism. Combined, these actions cause carcinogens to be removed before they can be metabolized. Another hypothesis is that ellagic acid changes the cellular structure of tumor cells. Both of these hypotheses have been supported by animal studies but have not been proven in humans.
Another potential benefit from ellagic acid is improved glucose metabolism. According to a 2010 article in the Journal of Medicinal Food, ellagic acid may block the intestinal enzyme alpha-glucosidase, which triggers glucose absorption. This means that less glucose enters the bloodstream, which could be beneficial for type 2 diabetics and hyperglycemics.
Flavonoids are a very large group of phytonutrients. Well-known flavonoids include quercetin and kaempferol. There are several significant subgroups of flavonoids, including flavones, anthocyanins, flavonones, isoflavones, flavonols and flavanols.
Because they are so diverse, flavonoids are found across a large range of foods. Premkumar listed apples, onions, coffee, grapefruit, tea, berries, chocolate, legumes, red wine, broccoli, cabbage, kale, leeks, tomatoes, ginger, lemons, parsley, carrots and buckwheat as a sampling.
Flavonoids are associated with longevity and reduced risk of cardiovascular disease and cancer. One large-scale, 25-year study, published in Archives of Internal Medicine, looked at men across seven countries and found that flavonoid consumption was significantly associated with longevity.