Phytochemicals, chemical compounds that occur naturally in plants (phyto means "plant" in Greek). Some are responsible for color and other organoleptic properties, such as the deep purple of blueberries and the smell of garlic. The term is generally used to refer to those chemicals that may have biological significance, for example antioxidants, but are not established as essential nutrients. Scientists estimate that there may be as many as 10,000 different phytochemicals having the potential to affect diseases such as cancer, stroke or metabolic syndrome.
 Phytochemicals as candidate nutrients
Without specific knowledge of their cellular actions or mechanisms, phytochemicals have been considered as drugs for millennia. For example, Hippocrates may have prescribed willow tree leaves to abate fever. Salicin, having anti-inflammatory and pain-relieving properties, was originally extracted from the bark of the white willow tree and later synthetically produced became the staple over-the-counter drug aspirin.
Sometimes they can be harmful and sometimes they can be very helpful. There is evidence from laboratory studies that phytochemicals in fruits and vegetables may reduce the risk of cancer, possibly due to dietary fibers, polyphenol antioxidants and anti-inflammatory effects. Specific phytochemicals, such as fermentable dietary fibers, are allowed limited health claims by the US Food and Drug Administration (FDA).
Some phytochemicals with physiological properties may be elements rather than complex organic molecules. For example, selenium, which is abundant in many fruits and vegetables, is involved with major metabolic pathways, including thyroid hormone metabolism and immune function. Particularly, it is an essential nutrient and cofactor for the enzymatic synthesis of glutathione, an endogenous antioxidant.
 Clinical trials and health claim status
There are currently many phytochemicals in clinical trials for a variety of diseases. Lycopene from tomatoes, for example, has been tested in human studies for cardiovascular diseases and prostate cancer. These studies, however, did not attain sufficient scientific agreement to conclude an effect on any disease. The FDA position reads:
"Very limited and preliminary scientific research suggests that eating one-half to one cup of tomatoes and/or tomato sauce a week may reduce the risk of prostate cancer. The United States Food and Drug Administration concludes that there is little scientific evidence supporting this claim."
Likewise, although lutein and zeaxanthin are suspected to inhibit macular degeneration and cataracts, there was insufficient scientific evidence from clinical trials for such specific effects or health claims.
 Food processing and phytochemicals
Phytochemicals in freshly harvested plant foods may be destroyed or removed by modern processing techniques, including cooking. For this reason, industrially processed foods likely contain fewer phytochemicals and may thus be less beneficial than unprocessed foods. Absence or deficiency of phytochemicals in processed foods may contribute to increased risk of preventable diseases.
A converse example may exist in which lycopene, a phytochemical present in tomatoes, is either unchanged in content or made more concentrated by processing to juice or paste, maintaining good levels for bioavailability.
 See also
- List of antioxidants in food
- List of phytochemicals in food
- Secondary metabolites
- US FDA, Guidance for Industry: Evidence-Based Review System for the Scientific Evaluation of Health Claims
- Brown, KM; Arthur, JR (2001). "Selenium, selenoproteins and human health: a review". Public health nutrition 4 (2B): 593–9. doi:10.1079/PHN2001143. PMID 11683552.
- Papp, LV; Lu, J; Holmgren, A; Khanna, KK (2007). "From selenium to selenoproteins: synthesis, identity, and their role in human health". Antioxidants & redox signaling 9 (7): 775–806. doi:10.1089/ars.2007.1528. PMID 17508906.
- Qualified Health Claims Subject to Enforcement Discretion, Docket No. 2004Q-0201, US Food and Drug Administration
- US FDA, Qualified Health Claims: Letter of Denial - Xangold Lutein Esters, Lutein, or Zeaxanthin and Reduced Risk of Age-related Macular Degeneration or Cataract Formation (Docket No. 2004Q-0180)
- Krishnadev N, Meleth AD, Chew EY (May 2010). "Nutritional supplements for age-related macular degeneration". Current Opinion in Ophthalmology 21 (3): 184–9. doi:10.1097/ICU.0b013e32833866ee. PMC 2909501. PMID 20216418.
- Cooking and nutrient loss, World's Healthiest Foods
- Liu, RH (2004). "Potential synergy of phytochemicals in cancer prevention: mechanism of action". The Journal of nutrition 134 (12 Suppl): 3479S–3485S. PMID 15570057.
- Rao, AV; Rao, LG (2007). "Carotenoids and human health". Pharmacological research 55 (3): 207–16. doi:10.1016/j.phrs.2007.01.012. PMID 17349800.
- Agarwal, A; Shen, H; Agarwal, S; Rao, AV (2001). "Lycopene Content of Tomato Products: Its Stability, Bioavailability and in Vivo Antioxidant Properties". Journal of medicinal food 4 (1): 9–15. doi:10.1089/10966200152053668. PMID 12639283.
- Dewanto, V; Wu, X; Adom, KK; Liu, RH (2002). "Thermal processing enhances the nutritional value of tomatoes by increasing total antioxidant activity". Journal of Agricultural and Food Chemistry 50 (10): 3010–4. doi:10.1021/jf0115589. PMID 11982434.
 Further reading
- Higdon, J. An Evidence – Based Approach to Dietary Phytochemicals. 2007. Thieme. ISBN 978-1-58890-408-9
- Rosa, L.A. de la / Alvarez-Parrilla, E. / González-Aguilar, G.A. (eds.) Fruit and Vegetable Phytochemicals: Chemistry, Nutritional Value and Stability. 2010. Wiley-Blackwell. ISBN 978-0-8138-0320-3
- Phytochemical Database - United States Department of Agriculture
- Phytochemicals at LPI - Linus Pauling Institute at Oregon State University