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With Polyphenols Aplenty, Berries May Protect From Disease
By Maria Frye

Revered for their dessertlike qualities, edible berries have been part of the human diet for centuries. Romans grew the ancestor of strawberries, which later crossed with varieties in America and Chile around 1750. Europeans have cultivated raspberries since the Middle Ages, and man might have consumed blackberries since the Neolithic Era.

While we have satisfied our palates with these delicious fruits for hundreds of years, researchers have recently started recognizing berries’ health properties, including their phytochemical content. Common berry fruits such as black and red raspberries, blackberries, blueberries, and strawberries are rich sources of phytochemicals, especially the polyphenols or phenolic compounds. Berries are particularly plentiful sources of polyphenols, including flavonoids such as anthocyanins and flavonols, as well as ellagitannins.

Researchers have studied these polyphenols from berries in vitro and in rats and humans. Although epidemiological research has not consistently demonstrated the benefits of these phytochemicals from berries, research in vitro and on rodents have been favorable in revealing how these fruits may provide protection from disease.

Polyphenol Research

Anthocyanins
Anthocyanins are the major polyphenols of black raspberries and blackberries and are abundant in red raspberries and strawberries.1 Anthocyanins also contribute the most of all phenolic compounds to antioxidant capacity in blueberries and strawberries, while ellagitannins contribute the most to and are the major phenolics of red raspberries.1 Likewise, black raspberries contain high concentrations of ellagitannins, particularly in their seeds.1 Blueberries do not have ellagitannins due to a lack of genetic ability to synthesize these phenolic compounds, but they are additionally rich in flavonols and anthocyanins.1

The phenolic structure of anthocyanins contributes to their antioxidant capabilities, such as scavenging reactive oxygen species.2 In vitro experiments and in vivo research of rats have demonstrated the anticarcinogenic effects of anthocyanins from berries.3 Epidemiological investigations of humans, however, have not provided enough evidence of the abilities of anthocyanins in inhibiting cancer.3

Experts have speculated as to why anthocyanin concentrations of berries have anticancer effects in vitro and in rats but not in people.3 One possible reason could be that anthocyanins are poorly absorbed in humans, as the blood concentrations of anthocyanins after consumption in humans are far below the in vitro levels that demonstrate anticarcinogenic effects.4 Even though anthocyanins are absorbed and excreted rapidly in people, the efficiency of their absorption is weak. Research has demonstrated that less than 1% of doses of anthocyanins from berries were absorbed and excreted in humans.3,4 However, epidemiological studies by Weisel and colleagues do suggest that a diet high in anthocyanins could decrease certain amounts of oxidative damage from free radicals.3

Flavonols
Similarly, epidemiological research has not shown a strong relationship between flavonol intake and a decreased risk of cancer. However, other experiments of flavonols and flavones proved favorable in regard to lowering the risk of cardiovascular disease. Scientists examined the relationship of flavonols and flavones to the risk of coronary heart disease in older men.5 They discovered that the men who consumed flavonols and flavones had 51% less risk of coronary heart disease. More testing is necessary to determine the advantages of consuming flavonols, as well as the potential for flavonols to aid memory in humans. In vivo studies of rats have indicated that anthocyanins and flavonols from blueberries might reduce age-related deficiencies in memory.6

Flavonols, particularly quercetin, have a better absorption rate than anthocyanins. Although scientists have not measured the specific absorption rate of quercetin from berries, 52% of quercetin compounds is absorbed from onions yet only about 25% of quercetin is absorbed from apples.1 Scientists believe that bioavailability differs in these foods because of their different structures. Bioavailability can therefore differ greatly among various foods, and ideally each individual food needs to be tested.2

Ellagitannins
The bioavailability of ellagitannins is also not well known.7 They are hydrolyzed into ellagic acid in the stomach or small intestine and are not easily absorbed due to their large structure.7 This could be a possible reason for the lack of epidemiological studies concerning the potential anticancer effects of ellagic acid. One of the few studies examined whether ellagic acid in a purified form could counteract the side effects of chemotherapy in men with advanced prostate cancer.7 Researchers found that ellagic acid only slightly lessened the side effects of chemotherapy, although the specific improvements were not described. One positive outcome, however, was that ellagic acid could help people undergoing chemotherapy maintain their white blood cells, which are normally degraded during treatment, thereby helping to maintain immune function.7

Researchers have specifically studied ellagic acid’s anticancer effects in the lungs, esophagus, liver, and skin.2. In vivo studies of mice have determined that ellagic acid provides antitumor effects.2 Interestingly, researchers observed that the tumor inhibition occurred when ellagic acid was given before, during, and after a carcinogenic dose. When rats consumed ellagic acid only after they were exposed to the carcinogen, no significant effect occurred. This investigation may reveal the importance of consuming phytochemicals not only after one develops a disease but also before to prevent cancer.

Berries Offer Bountiful Benefits
The phytochemicals of berries, specifically anthocyanins, flavonols, and ellagitannins, may have the potential to reduce our risk for chronic disease, especially cancer and cardiovascular disease. Further investigation into the healthful properties of these phytochemicals, particularly from berries, is essential. Additionally, since scientists are still exploring the absorption rate and bioavailability of the phytochemicals of berries, there are no set minimum or maximum intake levels. Yet, since much work has demonstrated positive correlations with the phytochemicals of berries and decreased risk for chronic disease in vitro and in rodents, dietitians should encourage berry consumption. Moreover, berries contain other healthful properties, including fiber, folic acid, and vitamin C, which may prevent chronic disease.

— Maria Frye is a senior at Loyola Marymount University. Hawley Almstedt, PhD, helped edit the article and is an assistant professor in the department of natural science at Loyola Marymount University.

 

References

  1. Zhao Y (ed). Berry Fruits: Value-Added Products for Health Promotion. Boca Raton, Fla.: CRC Press, Taylor & Francis Group; 2007.
  2. Hannum SM. Potential impact of strawberries on human health: A review of the science. Crit Rev Food Sci Nutr. 2004;44(1):1-17.
  3. Wang LS, Stoner GD. Anthocyanins and their role in cancer prevention. Cancer Lett. 2008;269(2):281-290.
  4. Beattie J, Crozier A, Duthie GG. Potential health benefits of berries. Curr Nutr Food Sci. 2005;1:71-86.
  5. Hollman PC, Katan MB. Absorption, metabolism, and health effects of dietary flavonoids in man. Biomed Pharmacother. 1997;51(8):305-310.
  6. Williams CM, Mohesen MA, Vauzour D, et al. Blueberry-induced changes in spatial working memory correlate with changes in hippocampal CREB phosphorylation and brain-derived neurotrophic factor (BDNF) levels. Free Rad Biol Med. 2008;45(3):295-305.
  7. Espin JC, Garcia-Conesa MT, Tomas-Barberan T. Nutraceuticals: Facts and fiction . Phytochemistry. 2007;68(22-24):2986-3008.