Unfortunately, plants have become the latest foods to be demonized in the integrative health community, primarily due to the growing influence of outspoken carnivore diet advocates. While humans may prefer meat and fat, phytochemical-rich plant foods are a natural part of our omnivorous diet and offer us numerous health benefits. Read on to learn about the myriad health benefits of phytochemicals, the foods in which these substances can be found, and specific situations in which phytochemicals may act as a double-edged sword.
Many advocates of the carnivore diet say that phytochemicals are harmful for your health—but that’s not the whole story. Check out this article from nutritionist Lindsay Christensen for an in-depth look at the role phytochemicals play in your health. #paleo #nutrition #wellness
Phytochemicals: An Evolutionary Perspective
Phytochemicals are chemical compounds produced by plants that help the plants defend themselves against pathogens and hungry herbivores, disease, and ultraviolet light from the sun, while also regulating critical aspects of plant health such as metabolism and pollinator attraction. As such, many phytochemicals could be considered mild “toxins.” Carnivore diet advocates have grasped this idea with fervor, using it to argue why we shouldn’t eat plants. However, there’s a lot more to this story!
It turns out that, throughout our history, we humans have evolved to experience certain health benefits through the consumption of plants. The process by which the consumption of slightly “toxic” plant compounds bolsters our health and resilience is known as xenohormesis.
Xenohormesis is a subcategory of “hormesis,” which is a favorable biological response initiated by low exposures to toxins or other stressors. The phytochemicals our ancestors consumed via plant foods, and the resultant xenohormesis they experienced, may have played a vital role in shaping our physiology. (2) For reference, exercise, heat, and cold exposure are also forms of hormesis; the proper dose and frequency of these stressors have net benefits for our health, just as ingesting an appropriate amount and frequency of phytochemical-containing foods supports our health. (3)
When we look at the diets of remaining hunter–gatherers and horticultural groups worldwide, we see that each consumes a mixture of plant and animal foods. Even the Inuit occasionally eat plant matter, in the form of berries and lichen during the warmer months. The meat- and milk-reliant Maasai also incorporate plants by seasoning their meat-based soups with bitter herbs. Bitter herbs are among some of the most potent phytochemical-rich plants in our diet. (4)
Based on the concept of xenohormesis and the extant anthropological literature on the dietary habits of traditional cultures, there is no question that plant foods have been a part of our dietary subsistence for an extended period. Exciting modern-day research indicates that dietary phytochemicals are powerful health-promoting allies, making them a natural part of a diet based on ancestral health principles. Let’s discuss some of the most notable phytochemicals found in our diet and how these compounds can support our health.
16 Superstar Phytochemicals
Allicin is a sulfur-based phytochemical found in Allium species vegetables, including:
In plants, allicin acts as a defense molecule, defending plants against pests. It is released when vegetable tissues are damaged, such as through chopping or chewing; the tissue damage stimulates the transformation of the amino acid alliin, which is naturally present in the plant, into allicin via the catalytic enzyme alliinase. (5)
In animals, including humans, allicin demonstrates potent antimicrobial properties, including antifungal and antibacterial activity. It even exerts activity against methicillin-resistant Staphylococcus aureus, a notorious antibiotic-resistant pathogen. (6) It also demonstrates beneficial effects on metabolic health, including blood lipid levels. I will discuss these physiological health benefits of allicin and other phytochemicals in more detail in the following section, “How Phytochemicals Promote Health.”
Anthocyanins are flavonoids that impart plants with deep red, purple, and blue colors. In the natural food industry, they are frequently used as non-toxic, organic food colorants. Foods rich in anthocyanins include:
- Purple sweet potatoes
Anthocyanins appear to exert antioxidant activity in tissues throughout the body, ranging from the cardiovascular system to the brain. (7)
Apigenin is a flavonoid found in various edible plants, including oranges, celery, onions, and the herbs chamomile, parsley, thyme, and oregano. It offers an array of health benefits ranging from antidiabetic to neuroprotective properties. (8) Dried parsley is reported to have one of the highest apigenin contents of any edible plant. (9)
Berberine is a bitter compound found in the roots of Berberis species plants, including Oregon grape and barberry. Berberine-containing plants are not typically consumed as part of the diet, but rather, are used for their medicinal properties. Berberine is one of the most extensively researched phytochemicals and exerts beneficial effects largely on cardiometabolic health. (10)
While you may not immediately think of caffeine as a plant-derived compound, it is actually one of the most widely consumed phytochemicals in the world. As Michael Pollan posits in his book Caffeine, caffeine may have played a vital role in the cultural evolution of our modern-day society, making it a pretty unique phytochemical!
Despite its controversial reputation, research indicates that caffeine offers beneficial health effects, including neuroprotective effects; just keep in mind that individual tolerance of caffeine varies based on several factors, ranging from genetics to your current stress level. (11)
Carotenoids are bright pigments that impart plants with yellow, orange, and red hues. Three types of carotenoids found in plants are beta-carotene, lutein, and zeaxanthin. Carrots, sweet potatoes, and red, orange, and yellow bell peppers are among the vegetables richest in carotenoids. Wild-caught salmon, shellfish, and certain types of phytoplankton are rich in an animal-based carotenoid called astaxanthin.
Carotenoids attracted some controversy after a study revealed that carotenoid supplementation increased the risk of cancer in smokers. However, it is still up for debate whether this is a valid association or impacted by other confounding variables such as vitamin C and E status. (14) This finding suggests that isolated, long-term supplementation with certain phytochemicals may have unintended consequences and that we may be better off obtaining the majority of our phytochemicals from whole food sources.
Curcumin is a polyphenol found in the bright orange root of the turmeric (Curcuma longa) native to Southeast Asia. It has a plethora of biological activities in the body, ranging from antioxidant to anti-inflammatory actions. (15) Curcumin absorption in the gastrointestinal tract is quite limited; combining curcumin with another phytochemical from black pepper, piperine, increases curcumin bioavailability by 2,000 percent. (16) In the nutraceutical world, a variety of strategies have been utilized to enhance curcumin bioavailability, ranging from the mixing of curcumin and piperine extracts in capsules to the high-tech encapsulation of curcumin into liposomes. (17)
8. Ellagic Acid
Ellagic acid is a polyphenol found in numerous fruits and vegetables, including:
Fisetin is a flavonoid found in a wide variety of fruits and vegetables, including: (20)
It has received attention for its impact on sirtuins, proteins that regulate cellular health, metabolic health, and longevity. (21)
10. Glucosinolates, Isothiocyanates, and Sulforaphane
Glucosinolates are sulfur-containing chemical groups found primarily in cruciferous vegetables such as:
Isothiocyanates are produced through the hydrolysis of glucosinolates, a process initiated by cell damage induced through chopping or chewing of isothiocyanate-containing vegetables. (22)
Sulforaphane is one type of isothiocyanate created when the enzyme myrosinase acts on the glucosinolate glucoraphanin. It is one of the most well-studied isothiocyanates, with over 3,000 publications describing its efficacy in animal models of disease and over 50 clinical trials examining its pharmacokinetics, pharmacodynamics, and effects on disease mitigation in humans. Broccoli sprouts, the immature or “baby” form of broccoli, offer the richest dietary source of glucoraphanin, which can subsequently be converted into sulforaphane. (23) Lightly steaming cruciferous vegetables, rather than using higher-heat cooking temperatures, preserves their sulforaphane content. (24)
11. Green Tea Polyphenols
Green tea has been a part of the human diet for at least 4,000 years. It is one of the most popular beverages consumed worldwide (except, perhaps, for coffee) and thus provides a readily available source of phytochemicals. Green tea contains several polyphenols, including:
- Epigallocatechin gallate
The polyphenols have effects ranging from hormone-balancing to neuroprotective properties.
Luteolin is a flavone, a subtype of flavonoid, found in:
- Green peppers
- Chamomile tea
It has received attention for its neuroprotective properties in animal models. It also demonstrates anti-inflammatory activity. (25)
13. Plant MicroRNAs
Plant microRNAs are a group of phytochemicals that you’ve likely never heard of, but nonetheless may have powerful effects on our health. MicroRNAs are small groups of non-coding RNA molecules that play essential roles in cell signaling and other plant functions. Emerging research indicates that plant microRNAs have cross-kingdom effects, meaning that they influence plant health and, upon human ingestion of plants, our own health. (26)
Upon ingestion, these substances are absorbed intact by intestinal cells. They can subsequently enter the systemic circulation and bind to cell receptors involved in physiological processes ranging from immunity to lipid metabolism.
Polysaccharides derived from plants are a significant component of the human diet. Polysaccharides are carbohydrate molecules and, while they are a considerable component of dietary fiber, also offer beneficial effects beyond fiber. For example, edible mushrooms contain polysaccharides that are immunomodulatory and anti-inflammatory. (27, 28)
Quercetin is a polyphenol found in:
Like curcumin, it offers pleiotropic health benefits, ranging from antioxidant to anti-allergy activities. Its absorption is also quite limited in the gut. However, quercetin bioavailability is also subject to interindividual variability and may depend on genetics and the types of microbes present in an individual’s gut microbiome. (29)
Resveratrol is another polyphenol but is found primarily in:
- Red grapes
- Red wine
Supplemental forms of resveratrol are typically made from a naturally resveratrol-rich plant called Japanese knotweed. Resveratrol has effects ranging from antioxidant activity to anti-obesity effects mediated through modulation of the “gut–adipose tissue” axis. (30, 31)
The dose of food-based resveratrol necessary to induce health benefits remains uncertain. In one study in healthy humans, a single 100 mg dose of resveratrol was sufficient to induce anti-inflammatory effects; for reference, the average glass of red wine contains a mere 1.9 mg of resveratrol. (32) Blueberries, another source of resveratrol, have only a few micrograms of resveratrol per berry, meaning you would potentially need to eat 100,000 berries to get a 100 mg dose of resveratrol! Resveratrol is thus one phytochemical that people may be better off supplementing with for specific therapeutic purposes since it is hard to come by in food.
While xenohormesis is one mechanism of action of phytochemicals, it is certainly not the only one. Phytochemicals beneficially impact health through numerous pathways, ranging from interactions with the gut microbiome to impacts on proteins involved in healthy aging and longevity.
Numerous phytochemicals demonstrate varying degrees of antimicrobial activity. Allicin exerts antimicrobial activity against a wide range of Gram-negative and Gram-positive bacteria, fungi such as Candida albicans, and parasites such as Entamoeba histolytica. (33) Quercetin, apigenin, luteolin, and fisetin demonstrate antiviral properties by inhibiting viral entry into host cells and interfering with viral RNA replication. (34)
The impact of phytochemicals on brain health is an exciting area of research. Studies suggest that diets rich in phytochemicals support neuroplasticity and may aid in the prevention of neurodegenerative diseases. Certain phytochemicals have even been called “neurohormetic” for the site-specific hormetic effects they exert on the brain, inducing a mild degree of stress that leads to overall improved brain function. (35)
Sulforaphane may protect the brain and nervous system from oxidative stress and aging by upregulating the Nrf2 pathway, an antioxidant response pathway that culminates in the generation of glutathione, a neuroprotective antioxidant. (36) Resveratrol attenuates hypoxia-induced neuron death and brain inflammation, while apigenin promotes the growth of new, healthy neurons (a process known as neurogenesis) in animal models. (37, 38)
Interestingly, quercetin and apigenin are under preclinical investigation for their mood-regulating effects due to their ability to cross the blood–brain barrier and interact with monoamine oxidase (MAO), an enzyme involved in the degradation of various monoamine neurotransmitters including dopamine, serotonin, and norepinephrine. (39) Synthetic MAO inhibitors are used in the treatment of various neuropsychiatric disorders, such as depression and anxiety, and other neurological conditions with impaired MAO activity, such as Alzheimer’s and Parkinson’s diseases. These lines of research suggest that diets rich in these phytochemicals could be beneficial for mood. (40)
“Cardiometabolic risk” is a term used to describe the common set of interrelated risk factors that influence the progression of heart disease and diabetes, including:
- Elevated fasting blood sugar
- Abdominal obesity
- Elevated triglycerides
In the scientific literature, plant-rich diets and specific phytochemicals are repeatedly associated with improvements in cardiometabolic risk factors.
Garlic demonstrates antihyperlipidemic effects in human subjects, lowering total cholesterol and low-density lipoprotein (LDL) cholesterol. (41, 42) The antihyperlipidemic effects of garlic may be mediated, in part, by allicin, which activates a metabolic pathway called AMP-activated protein kinase, or AMPK, to reduce triglyceride and cholesterol production. (43) Allicin also inhibits platelet aggregation, another contributing factor in cardiovascular disease. (44)
Resveratrol also attenuates cardiometabolic disease risk factors. It decreases arterial stiffness, produces dose-dependent reductions in blood pressure, reduces fasting blood glucose, at least in the short term, and reduces pro-inflammatory cytokine signaling, a phenomenon that underlies the development of cardiometabolic disease. (45)
Berberine is another superstar phytochemical for cardiometabolic health. It reduces blood glucose and decreases LDL cholesterol and triglycerides quite potently. (46) Luteolin and fisetin protect against the pro-inflammatory effects of hyperglycemia and oxidative stress. (47, 48)
Many phytochemicals reduce the glycemic response from ingested carbohydrate, a phenomenon referred to as “postprandial glycemia.” (49) Postprandial glycemia has significant effects on cardiometabolic health, with excessive postprandial hyperglycemia contributing significantly to the development of cardiometabolic conditions. Phytochemicals may inhibit postprandial glycemia by inhibiting the activity of digestive enzymes involved in carbohydrate digestion or by delaying gastric emptying, which reduces the rate at which glucose is absorbed in the gut and enters the systemic circulation.
Uninhibited cellular growth and proliferation is an intrinsic process in cancer development and progression. Several dietary phytochemicals demonstrate inhibitory effects on abnormal cell growth and proliferation and may thus offer chemopreventive properties. In the lab, apigenin induces cell cycle arrest and cellular apoptosis in cancerous cell lines. (50, 51) Sulforaphane, through activation of the Nrf2 antioxidant pathway and effects on cell cycle arrest, also may offer chemopreventive properties. (52) Curcumin, resveratrol, lycopene, and green tea polyphenols also demonstrate inhibitory effects on cell growth and proliferation and epigenetically regulate genes involved in cancerous cell growth. (53)
By mildly stressing our bodies, dietary phytochemicals upregulate our endogenous detoxification pathways, helping our bodies metabolize and eliminate the numerous toxins to which we are exposed in our environment.
For example, sulforaphane upregulates the Nrf2 pathway, which culminates in glutathione production, helping the body detoxify airborne pollutants. (54) Other phytochemicals, such as curcumin and ellagic acid, balance Phase 1 and Phase 2 liver detoxification, preventing “backups” in the body’s detoxification pathways. (55)
Most phytochemicals are poorly absorbed in the gastrointestinal tract, an observation that has led scientists to wonder how exactly these compounds exert such broad-ranging beneficial health effects. One mechanism through which these poorly absorbed compounds may impact systemic health is by affecting the health of your gut!
Ellagic acid, found in pomegranates and berries, is metabolized by gut bacteria to create urolithin, a compound with gastroprotective properties. (56) Sulforaphane modulates the gut microbiota, pruning back Helicobacter pylori. (57, 58) Quercetin may improve intestinal barrier integrity, repair “leaky gut,” and increase microbial diversity in the large intestine. (59, 60)
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Several types of phytochemicals exert hormone-like actions. Isoflavones are phytochemicals with mild estrogen properties and are thus often referred to as “phytoestrogens.” Soy isoflavones have long been touted as a treatment for menopausal symptoms, such as hot flashes. (61) Soybeans are the richest dietary source of isoflavones, followed by other legumes such as chickpeas.
Certain phytochemicals help balance hormones without exerting direct hormone-mimicking effects. Diindolylmethane, a metabolite of indole-3-carbinol, which is a glucosinolate found in cruciferous vegetables, modulates estrogen levels; it increases the ratio of 2-hydroxyestrone, the “good” form of estrogen, to 16-alpha-hydroxyestrone, a more potent form of estrogen with genotoxic and proliferative activity. (62)
Isoflavones, however, are one example of a phytochemical that acts as a double-edged sword. While the estrogenic effects of isoflavones may be beneficial for menopausal women, they may exert hormone-disrupting effects in children and adversely impact thyroid function. (63)
Many phytochemicals upregulate the body’s anti-inflammatory signaling pathways, reducing chronic inflammation. Sulforaphane is broadly anti-inflammatory, reducing inflammation in tissues ranging from the gut to the brain. (64) Resveratrol reduces inflammation by activating the activity of AMPK, an anti-inflammatory nutrient-sensing, and signaling pathway. (65) Quercetin stabilizes mast cells, critical cells within the immune system, inhibiting them from releasing pro-inflammatory mediators such as histamine. (66) Apigenin, curcumin, luteolin, and countless other phytochemicals found throughout our diet also reduce inflammation. (67, 68, 69)
Scientific interest in the biology of aging is growing, and dietary phytochemicals are emerging as a valuable tool for stemming the tide of age-related physiological decline.
Resveratrol is perhaps the most well-studied phytochemical with implications for anti-aging and longevity. Resveratrol activates sirtuins, a family of seven proteins that play important roles in cellular health and aging. (70) Resveratrol-induced sirtuin activation has been found to improve healthspan, defined as the period of one’s life lived in good health, in aging mice. (71) Healthspan improvement may be mediated by positive changes in glucose metabolism, inflammation levels, and gene expression. Resveratrol also supports the aging brain through sirtuin activation. (72)
Quercetin beneficially influences the aging process by acting as a senolytic, or a compound that selectively kills senescent cells, which have ceased to divide and instead secrete an array of pro-inflammatory mediators that harm neighboring cells. (73)
Promotion of Healthy Gene Expression
A variety of phytochemicals impact the body at the DNA level by influencing gene expression, or the process by which the instructions in our DNA are converted into instructions for making proteins that regulate our physiology. (74)
Our bodies use many complex mechanisms to regulate gene expression, and phytochemicals can assist in this regulatory process. For example, quercetin regulates gene expression by acting as a histone deacetylase inhibitor; histones are proteins around which our DNA is wound; by influencing this DNA-winding process, quercetin modulates the expression of genes involved in processes ranging from aging to cardiovascular health. (75)
Are Phytochemicals a Double-Edged Sword?
While phytochemicals offer numerous health benefits, there are some potential drawbacks to consuming isolated forms of these compounds instead of the whole food version. For example, high intakes of curcumin and green tea catechins can inhibit iron absorption in the small intestine; while this may be beneficial for someone with iron overload, this could have significant drawbacks for someone trying to boost their iron status. (76, 77) High doses of berberine may mitigate the beneficial effects of exercise by impacting metabolic and inflammatory pathways involved in exercise-induced adaptations. (78)
The topic of isothiocyanates, such as sulforaphane, and thyroid function is a point of contention in the Functional Medicine community. It has been suggested that consuming large amounts of isothiocyanates could adversely impact thyroid function. However, a randomized clinical trial (albeit small) contradicts this belief, showing that 12 weeks of supplementation with a sulforaphane-rich broccoli sprout beverage had no adverse effects on serum levels of thyroid-stimulating hormone, free thyroxine, thyroglobulin, or thyroid autoimmunity. (79) While these results are encouraging, the study was small and short-term, so I still recommend that you optimize your iodine status before consuming significant amounts of sulforaphane, or other isothiocyanates, either from broccoli sprouts or a supplement.
If you’re experiencing a health issue and you need extra support, our practitioners at the California Center for Functional Medicine can help. Our team can come up with a plan to specifically help you—tailored to your specific health history, diet, and lifestyle. Find out more about what CCFM can offer.
Food Preparation Methods Impact the Phytochemical Content of Foods
Different food preparation methods, including chopping, cooking, and fermentation, differentially affect foods’ phytochemical content. For example, low-heat cooking methods may be best for preserving isothiocyanate and sulforaphane content of foods. (80) Conversely, cooking enhances the bioavailability of lycopene, a carotenoid, in tomatoes; the addition of olive oil to cooked tomatoes may render lycopene even more bioavailable. (81) Fermentation may enhance the bioavailability of some, but not all, phytochemicals; for example, lactic acid bacteria fermentation of a mixture of apple, pear, and carrot juice was found to increase the content of quercetin. (82) Ultimately, consuming a combination of raw, cooked, and fermented fruits and vegetables will provide you with a diverse, bioavailable array of phytochemicals.
Final Thoughts on Phytochemicals
Humans are omnivores; we have evolved to eat a wide variety of plant matter alongside animal foods. Fruits, vegetables, and herbs supply our bodies with a rich array of phytochemicals with health-promoting properties. By “eating the rainbow,” you can provide your body with a wide range of phytochemicals and enjoy the numerous health benefits these compounds have to offer.