Phytochemical

Definition

Phytochemicals are chemicals produced by plants through primary or secondary metabolism. They generally have biological activity in the plant host and play a role in plant growth or defense against competitors, pathogens, or predators. As components of plants, all individual phytochemicals make up the whole plant as it exists in nature.

Phytochemicals are generally regarded as research compounds rather than essential nutrients because proof of their possible health effects has not been established yet. Phytochemicals under research may be classified into major categories, such as carotenoids and polyphenols, which include phenolic acids, flavonoids, stilbenes, or lignans. Flavonoids may be further divided into groups based on their similar chemical structure, such as anthocyanins, flavones, flavanones, isoflavones, and flavanols. Flavanols are further classified as catechins, epicatechins, and proanthocyanidins. In total, between 50,000 and 130,000 phytochemicals have been discovered.

Phytochemists study phytochemicals by first extracting and isolating compounds from the origin plant, followed by defining their structure or testing in laboratory model systems, such as in vitro studies or in vivo studies using laboratory animals. Scientific challenges in that field include isolating specific compounds and determining their structures, which are often complex, and identifying what specific phytochemical is primarily responsible for any given biological activity.

Further, upon consuming phytochemicals in a food entering the digestion process, the fate of individual phytochemicals in the body is unknown due to extensive metabolism in the gastrointestinal tract, producing smaller phytochemical metabolites with different biological properties from those of the parent compound, and with low bioavailability and rapid excretion. Other than for dietary fiber, no non-nutrient phytochemical has sufficient scientific evidence in humans for an approved health claim.

History of uses

Without specific knowledge of their cellular actions or mechanisms, phytochemicals are used in traditional medicine (although some may be toxic). For example, salicin, having anti-inflammatory and pain-relieving properties, was originally extracted from the bark of the white willow tree that was prescribed in folk medicine. Later, it was synthetically produced to become the common, over-the-counter drug, aspirin.

The tropane alkaloids of Atropa belladonna were used as poisons, and early humans made poisonous arrows from the plant. Other uses include perfumes, such as the sesquiterpene santolols, from sandalwood.

The English yew tree was long known to be extremely and immediately toxic to animals that grazed on its leaves or to children who ate its berries; however, in 1971, paclitaxel was isolated from it, subsequently becoming a cancer drug.

Functions

The biological activities for most phytochemicals are unknown or poorly understood, in isolation or as part of foods. Phytochemicals with established roles in the body are classified as essential nutrients.

The phytochemical category includes compounds recognized as essential nutrients that are naturally contained in plants and are required for normal physiological functions, so must be obtained from the diet in humans.

Some phytochemicals are known phytotoxins that are toxic to humans; for example, aristolochic acid is carcinogenic at low doses.

Some phytochemicals are antinutrients that interfere with the absorption of nutrients. Others, such as some polyphenols and flavonoids, may be pro-oxidants in high ingested amounts. Non-digestible dietary fibers from plant foods, often considered as a phytochemical, are generally regarded as a nutrient group having approved health claims for reducing the risk of some types of cancer and coronary heart disease.

Phytochemical dietary supplements are neither recommended by health authorities for improving health, nor are they approved by regulatory agencies for health claims on product labels.

Consumer and industry guidance

While health authorities encourage consumers to eat diets rich in fruit, vegetables, whole grains, legumes, and nuts to improve and maintain health, evidence that such effects result from specific, non-nutrient phytochemicals is limited or absent. For example, systematic reviews and/or meta-analyses indicate weak or no evidence for phytochemicals from plant food consumption having an effect on breast, lung, or bladder cancers. Further, in the United States, regulations exist to limit the language on product labels for how plant food consumption may affect cancers, excluding mention of any phytochemical or nutrient except for those with established health benefits against cancer, such as dietary fiber, vitamin A, and vitamin C.

Phytochemicals, such as polyphenols, have been specifically discouraged from food labeling in Europe and the United States because there is no scientifically proven evidence for a cause-and-effect relationship between dietary polyphenols and inhibition or prevention of any disease.

Among carotenoids such as the tomato phytochemical, lycopene, the U.S. Food and Drug Administration found insufficient evidence for its effects on any of several cancer types, resulting in limited language for how products containing lycopene may be described on labels.

Effects of food processing

Phytochemicals in freshly harvested plant foods may be degraded by processing techniques, including cooking. The main cause of phytochemical loss from cooking is thermal decomposition.

The opposite exists in the case of carotenoids, such as lycopene present in tomatoes, which may remain stable or increase in content from cooking, due to liberation from cellular membranes in the cooked food.

Food processing techniques such as mechanical processing also may free carotenoids and other phytochemicals from the food matrix, increasing dietary intake.

In some cases, processing of food is necessary to remove phytotoxins or antinutrients. For example, societies that use cassava as a staple have traditional preparation practices that involve some processing (soaking, cooking, fermentation), which are necessary to avoid illness from cyanogenic glycosides present in unprocessed cassava.