expert opinion

Do we need recommended dietary intakes for phytochemicals?

May 15, 2013

Jeffrey B. Blumberg, PhD, Friedman School of Nutrition Science and Policy, Tufts University, Boston, USA

“Fruits and vegetables contain a wide variety of phytochemicals such as carotenoids and polyphenolic flavonoids. As systemic oxidative stress of cell membranes, DNA, and proteins can contribute to the aging process and risk of various degenerative conditions, the antioxidant functions of phytochemicals may contribute to their protective effect against chronic diseases. Indeed, evidence of specific biological functions of various phytochemicals, e.g., anti-inflammation and anti-carcinogenesis, is accumulating. In addition, phytochemicals have been reported to be involved in direct modulation of signal transduction. In particular, carotenoids such as lycopene and lutein have been suggested to control redox sensitive molecular targets and platelet-derived growth factor, respectively. Accordingly, phytochemicals have been suggested to provide health benefits such as maintaining inflammation balance, providing cardiovascular, neurocognitive, and visual health, and reducing the risk of cancer. Beta-carotene, for example, may reduce risk of cancer and heart disease in physiological doses (1, 2). Adequate lutein and zeaxanthin intakes may protect against age-related macular degeneration(3), and lycopene has been found to be associated with a reduced risk of cancers (4). Flavonoids – a large group of polyphenols present in fruits, vegetables, and beverages including wine and tea as well as tree nuts and whole grains – have been associated with reduced risk of developing cancer and cardiovascular disease (5, 6).

Even though numerous observational studies suggest that diets high in fruits and vegetables play a role in reducing chronic diseases, several intervention trials failed to show a beneficial effect of relatively high doses of single phytochemicals, vitamins or minerals. Thus, it may be the combination of various phytochemicals that are required to exert the biological actions that promote health. For example, antioxidant nutrients can work in a synergistic manner to remove free radicals (7). Although vitamin C (ascorbic acid) is a poor inhibitor of free radical formation, it can effectively recycle vitamin E (alpha-tocopherol) from alpha-toco-pheroxyl radicals. Vitamin E can reduce beta-carotene peroxyl radicals. The combination of vitamin E and beta-carotene has been reported to cooperatively slow down lipid peroxidation in in vitro systems. While 
beta-carotene at physiologic concentrations did not show a protective effect against oxidation in a biological model system, the oxidation was decreased by beta-carotene with the presence of either vitamin E or C. Further, flavanol can directly recycle vitamin E. Importantly, the additive/synergistic interactions between phytochemicals may occur not only with respect to antioxidant activity but also various other biological functions (8).

Scientists are still trying to gain a firm understanding of exactly how phytochemicals function. Food is complex and so is the way our bodies process it; therefore, the research on food conducted in laboratories might not always lead to conclusive answers. While many studies are done with a single nutrient, this cannot be applied very well to what we eat and drink in daily life. We do not only consume single nutrients. It may not be the one single chemical under investigation that has a positive effect but rather a combination of chemicals and how they all work together. A true understanding of exactly how or to what degree phyto-chemicals prevent disease could only result with the investment of a large amount of time and money – but perhaps the answers will never fully be revealed. There is no clear consensus: research cannot tell us which doses and which combinations have the best benefit. As their specific and unique contribution to disease prevention often is not clear, it is difficult to apply recommended dietary intake (RDI) levels.

However, that does not mean there is no promise: current evidence strongly suggests that phytochemicals play many roles in promoting health. They are likely to be featured in future dietary guidelines in which their important role in maintaining optimum health will be stressed. Dietary guidelines are less quantitative than RDIs but no less important, and focus more on qualitative issues. The best way to get a diet rich in phyto-chemicals is to eat a healthy one that includes a lot of fruits, vegetables, and whole grains. Variety is key; there’s no reason to focus on one food. In some cases, there is also a good reason to take dietary supple-ments. However, they are called supplements for a reason; they’re not called dietary substitutes.”

References

  1. Hercberg S. et al. Incidence of cancers, ischemic cardiovascular diseases and mortality during 5-year follow-up after stopping antioxidant vitamins and minerals supplements: a postintervention follow-up in the SU.VI.MAX Study. Int J Cancer. 2010; 127:1875–1881.
  2. Kim J. et al. Intakes of vitamin A, C, and E, and beta-carotene are associated with risk of cervical cancer: a case-control study in Korea. Nutr Cancer. 2010; 62:181–189.
  3. Zhao L. and Sweet B. V. Lutein and zeaxanthin for macular degeneration. Am J Health Syst Pharm. 2008; 65(13):1232–1238.
  4. Guil-Guerrero J. L. et al. Cytotoxicity screening of several tomato extracts. J Med Food. 2011; 14:40–45.
  5. Romagnolo D. F. and Selmin O. I. Flavonoids and cancer prevention: a review of the evidence. J Nutr Gerontol Geriatr. 2012; 31(3):206–38.
  6. Peterson J. J. et al. Associations between flavonoids and cardiovascular disease incidence or mortality in European and US populations. Nutr Rev. 2012; 70(9):491–508.
  7. Yeum K. J. et al. Synergistic interactions of antioxidant nutrients in a biological model system. Nutrition. 2009; 25:839–846.
  8. Bertipaglia de Santana M. et al. Association between soy and green tea (Camellia sinensis) diminishes hypercholesterolemia and increases total plasma antioxidant potential in dyslipidemic subjects. Nutrition. 2008; 24:562–568.